hydraulic seals

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SKF, ECOPUR, H-ECOPUR, S-ECOPUR, X-ECOPUR,XH-ECOPUR, FLUOROTREL, LUBRITHANE, SEAL JET,SPECTRASEAL, TEFL ATHANE and X-SLIDE are registeredtrademarks of the SKF Group.

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SKF Group 2014The contents of this publicat ion are the copyright of the pub-lisher and may not be reproduced (even extracts) unless priorwrit ten permission is granted. Every care has been taken to en-sure the accuracy of the information contained in this publicationbut no liabilit y can be accepted for any loss or damage whetherdirect , indirect or consequentia l arising out of the use of the in-

formation contained herein.

PUB SE/P1 12393/2 EN August 2014

This publicat ion supersedes publication 5397.

Certain image(s) used under license from Shutterstock.com

SKF mobile appsSKF mobile apps are available from both Apple App Store andGoogle Play. These apps provide useful information and allowyou to make crit ical calculations, providing SKF KnowledgeEngineering at your fingertips.

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SKF hydraulic seals general technical information . . 21

Piston seals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Rod and buffer seals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

Wiper seals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

Guide rings and guide strips. . . . . . . . . . . . . . . . . . . . . . 249

O-rings and back-up rings . . . . . . . . . . . . . . . . . . . . . . . 291

Other fluid power applications. . . . . . . . . . . . . . . . . . . . 347

Product index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352

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Unit conversions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Foreword. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5This is SKF. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8SKF the knowledge engineering company. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Introduction to fluid power. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Hydraulic cylinders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Cylinder classif ication. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Hydraulic seals and guides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14SKF hydraulic seals and guides capabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Customized sealing solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16SKF SEAL JET system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Criteria for seal selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1 SKF hydraulic seals general technical information . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Specifications for effective sealing systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Gap extrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36Installation and assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

2 Piston seals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45Product tables2.1 MPV, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 592.2 DPV, inch sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 612.3 LPV, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 632.4 CPV, inch sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

2.5 GH, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 GH, inch sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 702.6 APR, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 APR, inch sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 792.7 LCP, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 LCP, inch sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 852.8 LTP, inch sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 872.9 CUT, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 912.10 SCP, inch sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 952.11 MD-L, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 992.12 UNP, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

UNP, inch sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

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3 Rod and buffer seals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .111Product tables3.1 S1S, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1233.2 ZBR, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 ZBR, inch sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

3.3 SIL, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1353.4 PTB, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 PTB, inch sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1493.5 STD, inch sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1653.6 DZ, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 DZ, inch sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1803.7 DZR, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

DZR, inch sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1863.8 RBB, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 RBB, inch sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1913.9 S9B, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193

S9B, inch sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1963.10 RSB, inch sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

4 Wiper seals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .207Product tables4.1 PA, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2174.2 MCW, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 MCW, inch sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2234.3 PAD and PADV, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2274.4 DTW, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 DTW, inch sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232

4.5 DX, inch sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2374.6 HW, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 HW, inch sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243

5 Guide rings and guide strips. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249Product tables5.1 WAT rod or piston guide rings, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2565.2 WAT rod or piston guide rings, inch sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2625.3 RGR rod guide rings, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2765.4 PGR piston guide rings, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280

6 O-rings and back-up rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .291Product tables6.1 OR O-rings in static radial sealing, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3006.2 OR O-rings in static radial sealing, inch sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3166.3 STR Back-up rings in static radial sealing, metric sizes . . . . . . . . . . . . . . . . . . . . . . . . 3246.4 STR Back-up rings in static radial sealing, inch sizes by dash-number,

metric dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3286.5 STR Back-up rings in static radial sealing, inch sizes by dash-number . . . . . . . . . . . 338

7 Other fluid power applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347

8 Product index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352

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Unit conversions

Unit conversions

Quantity Unit Conversion

Length inch 1 mm 0.03937 in. 1 in. 25,40 mm

foot 1 m 3.281 ft. 1 ft. 0,3048 myard 1 m 1.094 yd. 1 yd. 0,9144 mmile 1 km 0.6214 mi. 1 mi. 1,609 km

Speed,velocity

foot per second 1 m/s 3.28 ft/s 1 ft/s 0,30480 m/sfoot per minute 1 m/s 196.8504 ft/min 1 ft/min 0,00508 m/smile per hour 1 km/h 0.6214 mph 1 mph 1,609 km/h

Force pound-force 1 N 0.225 lbf. 1 lbf. 4,4482 N

Pressure,stress

pounds per square inch 1 MPa 145 psi 1 psi 6,8948 103Pa1 N/mm2 145 psi1 bar 14.5 psi 1 psi 0,068948 bar

Temperature degree Celsius tC= 0.555 (tF 32) Fahrenheit tF= 1,8 tC+ 32

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Foreword

This catalogue contains the standard assort-ment of SKF hydraulic seals and guides typic-ally used in hydraulic cylinders. To provide thehighest levels of quality and customer service,

these products are available worldwidethrough SKF sales channels. For informationabout lead times and deliveries, contact yourlocal SKF representative or SKF AuthorizedDistributor.

This catalogue reflects SKFs state-of-the-art technology and production capabilities asof 2014. The data contained within may differfrom that shown in earlier catalogues becauseof redesign, technological developments, orrevised calculation methods. SKF reserves the

right to continuously improve its products withrespect to materials, design and manufactur-ing methods, some of which are driven bytechnological developments.

Getting started

Introduction to fluid powerprovides an over-view over hydraulic cylinder types and classi-fication, a reference to SKF customized solu-tions and standard assortment of hydraulic

seals and guides. The first chapter containsgeneral technical information. Seal counter-surface finish properties, materials, fluidmedia and gap extrusion are discussed indetail. Furthermore, product storage andinstallation information is included.

Guidance values

Since several factors simultaneously affectthe sealing system and seal performance, allstated values in graphs and tables in this pub-

lication should be considered as guidelinesonly and not as absolute values for practicalapplications.

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Colour coding for material

The product illustrations in this catalogue usea colour coding system to indicate the materialgroup, but they do not match the actual mate-rial colours. For information about the actual

material colours, refer to Materials ( page 26).Consistent colour coding for material groups

Thermoplastic polyurethane elastomers(TPU), moulded articles

Thermoplastic polyurethane elastomers(TPU), machined articles

Rubbers

Rigid thermoplastics

Unfilled PTFE and other engineeredplasticsFilled PTFE

Phenolic resins

Thermoplastic polyester elastomers(TPC)

Metals

Units of measurement

This catalogue is for global use. Therefore, the

predominant units of measurement are inaccordance with ISO 80000-1. Imperial unitsare used whenever necessary. Unit conver-sions can be made using the conversion table( page 4).

For easier use, some values are provided inboth metric and imperial values. Values aretypically rounded. Therefore, the two valuesdo not always match according to the conver-sion formula.

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This is SKF

From one simple but inspired solution to a mis-alignment problem in a textile mill in Sweden,and fifteen employees in 1907, SKF has grownto become a global industrial knowledge leader.

Over the years, we have built on our expertise inbearings, extending it to seals, mechatronics,services and lubrication systems. Our know-ledge network includes 46 000 employees,15 000 distributor partners, offices in morethan 130 countries, and a growing number ofSKF Solution Factory sites around the world.

Research and development

We have hands-on experience in over fortyindustries based on our employees knowledge

of real life conditions. In addition, our world-leading experts and university partners pioneeradvanced theoretical research and developmentin areas including tribology, condition monitor-ing, asset management and bearing life theory.Our ongoing commitment to research anddevelopment helps us keep our customers atthe forefront of their industries.

SKF Solution Factorymakes SKF knowledgeand manufacturingexpertise available locallyto provide unique solu-tions and services to ourcustomers.

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Meeting the toughest challenges

Our network of knowledge and experience,along with our understanding of how our coretechnologies can be combined, helps us createinnovative solutions that meet the toughest of

challenges. We work closely with our customersthroughout the asset life cycle, helping them toprofitably and responsibly grow their businesses.

Working for a sustainable future

Since 2005, SKF has worked to reduce thenegative environmental impact from our oper-ations and those of our suppliers. Our continu-ing technology development resulted in theintroduction of the SKF BeyondZero portfolio ofproducts and services which improve efficiency

and reduce energy losses, as well as enable newtechnologies harnessing wind, solar and oceanpower. This combined approach helps reducethe environmental impact both in our oper-ations and our customers operations.

Working with SKF IT andlogistics systems andapplication experts, SKFAuthorized Distributorsdeliver a valuable mix ofproduct and applicationknowledge to customersworldwide.

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Our knowledge your successSKF Life Cycle Management is how

we combine our technology plat-forms and advanced services, andapply them at each stage of theasset life cycle, to help our cus-tomers to be more successful,sustainable and profitable.

Working closely with you

Our objective is to help our customers improveproductivity, minimize maintenance, achievehigher energy and resource efficiency, and opti-mize designs for long service life and reliability.

Innovative solutions

Whether the application is linear or rotary or a

combination, SKF engineers can work with youat each stage of the asset life cycle to improvemachine performance by looking at the entire

application. This approach doesnt just focus onindividual components like bearings or seals. Itlooks at the whole application to see how eachcomponent interacts with each other.

Design optimization and verification

SKF can work with you to optimize current ornew designs with proprietary 3-D modelling

software that can also be used as a virtual testrig to confirm the integrity of the design.

SKF the knowledgeengineering company

S

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atio

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DesignanddevelopM

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comm

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SKF Life CycleManagement

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BearingsSKF is the world leader in the design, developmentand manufacture of high performance rolling bearings,plain bearings, bearing units and housings.

Machinery maintenanceCondition monitoring technologies and maintenanceservices from SKF can help minimize unplanned down-time, improve operational efficiency and reduce main-tenance costs.

Sealing solutionsSKF offers standard seals and custom engineered seal-ing solutions to increase uptime, improve machine reli-ability, reduce friction and power losses, and extendlubricant life.

MechatronicsSKF fly-by-wire systems for aircraft and drive-by-wiresystems for off-road, agricultural and forklift applica-tions replace heavy, grease or oil consuming mechanicaland hydraulic systems.

Lubrication solutionsFrom specialized lubricants to state-of-the-artlubrication systems and lubrication management

services, lubrication solutions from SKF can help toreduce lubrication related downtime and lubricantconsumption.

Actuation and motion controlWith a wide assortment of products from actuatorsand ball screws to profile rail guides SKF can workwith you to solve your most pressing linear systemchallenges.

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Fluid power systems transfer and utilizemechanical power through a working fluid.Energy is transmitted, stored and used throughthe transfer and pressure of fluids. There are

two main types of fluid power system:

hydraulic using liquid, such as oil, for theworking fluid

pneumatic using gas, such as air, for theworking fluid

Introduction to fluid power

BA

Fig. 1

Double-acting hydraulic cylinder

End cap Piston Cylinder bore Piston rodHead

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Hydraulic cylinders

A hydraulic cylinder is a linear actuator used topush or pull a load, or to selectively resistmotion under load, by means of fluid pressure.

Double-acting cylinders, the most commontype, are able to push and pull (fig. 1). Highpressure fluid pumped into the extend cham-ber (port A) acts on the piston to push thepiston rod out, thereby extending the lengthof the cylinder. Inversely, to retract the pistonrod and reduce the length of the assembly,high pressure fluid is pumped into the retractchamber (port B) and acts on the oppositeside of the piston.

Other cylinder types are shown in figure 2.

A cylinder which can only push but not pull isreferred to as a single-acting cylinder. Highpressure fluid is pumped into the extendchamber (port A) and acts on the piston topush the piston rod out. An external force isrequired to return the cylinder to the retractedposition. A typical application for a single-act-

ing cylinder is a lift truck, where the load of thefork pushes the cylinder back. Multi-stage cylin-ders (also referred to as telescoping cylinders)have two or more piston rods in a coaxialarrangement to achieve greater extended

length compared to the retracted length.

A

A

A

A B

Fig. 2

Cylinder types

Single-acting plunger

Single-acting piston

Single-acting multi-stage

Double-acting multi-stage

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Cylinder classification

The type of cylinder and the application inwhich it is used are two of the main criteriawhen selecting the appropriate seals and

guides. Applications are referred to as lightduty, medium duty or heavy duty applications.These classif ications are somewhat subjectivebut the duty levels are typically characterizedby the following criteria.

Light duty cylinders

Light duty cylinders, e.g. cylinders used forstationary equipment indoor in a factory envir-onment, may be characterized by:

system pressures up to 160 bar (2 300 psi) temperatures up to 70 C(160 F) rare pressure peaks in excess of system

pressure minimal side loads acting on guides environment with moderate temperature

fluctuations and relatively free ofcontaminants

Medium duty cylinders

Medium duty cylinders, e.g. cylinders in agri-

culture off-highway equipment, may be char-acterized by:

system pressures up to 250 bar (3 625 psi) temperatures up to 90 C (195 F) moderate pressure peaks in excess of sys-

tem pressure moderate side loads acting on guides environment with temperature fluctuations

and typical external contaminants such asdust and moisture

Heavy duty cylinders

Heavy duty cylinders, e.g. cylinders in off-highway earthmoving or forestry equipment,may be characterized by:

system pressures of 400 bar (5 800 psi)ormore

temperatures exceeding 90 C (195 F)withpeaks in excess of 110 C (230 F)

regular pressure peaks in excess of system

pressure heavy side loads acting on guides, usually

due to heavy components and/or highaccelerations

tough environment with wide temperaturefluctuations and typical harsh externalcontaminants

Hydraulic seals and guidesHydraulic cylinder seals are used to seal theopening between various components in thehydraulic cylinder. Figure 3shows an exampleof the seal and guide components of a heavyduty cylinder. There are two main types ofhydraulic seals in the system:

Dynamic sealsThey seal between components in relative

motion. In a hydraulic cylinder the rod seal-ing system seals dynamic reciprocatingmotion between the piston rod and head,while the piston sealing system sealsdynamic reciprocating motion between thepiston and cylinder bore.

Static sealsThey seal between components fixedtogether without relative motion. Hydrauliccylinders use static seals in various loca-tions depending on the design and construc-

tion. The most common are static sealsbetween the piston and piston rod andbetween the head and cylinder bore tube.

Each dynamic seal in a hydraulic cylinder has aspecial function to contribute to the perform-ance of the system:

Piston seal functions

act as pressure barrier and prevent fluidpassing the piston important for control-

ling the cylinder motion or maintaining theposition when at rest

Rod seal functions

act as pressure barrier and keep the operat-ing fluid inside the cylinder

regulate the fluid film which extends withthe surface of the piston rod important toinhibit rod corrosion and to lubricate thewiper seal and the rod seal itself

accept the lubrication film back into the

cylinder when the rod retracts

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Buffer seal functions

protect the rod seal from fluid pressurepeaks in excess of system pressure

attenuate the fluctuations in system pres-sure, thereby improving rod seal perform-

ance by allowing the rod seal to deal withmore constant or gradually changingpressure

act as an internal excluder to keep systemcontaminants, such as metal particles, fromdamaging the rod seal

Wiper seal functions

exclude external contaminants from enter-ing the cylinder assembly and the hydraulicsystem

accept the lubrication film back into thecylinder when the rod retracts

Guide rings (rod and piston) functions

prevent metal-to-metal contact betweencomponents

react the radial load caused by side loads onthe cylinder assembly

keep the piston rod and piston accuratelycentred in the cylinder assembly import-ant for performance of the rod sealing

system and piston sealing system

The subsequent chapters contain additionalinformation regarding the function of eachseal or guide in the system and the selection ofthem.

Fig. 3

Head static sealBuffer seal

Rod guide ringPiston static sealPiston sealPiston guide ring Rod seal Wiper seal

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Fig. 4SKF hydraulic seals andguides capabilitiesThe SKF assortment of hydraulic seals com-prises hundreds of different designs and

material combinations. The products shown inthis catalogue are the more commonly usedseal profiles and sizes for hydraulic cylinders.The flexible SKF manufacturing systems andprocesses, combined with the industrys mostagile and comprehensive design and engineer-ing capability, allow additional profiles andsizes to be added on a daily basis. If applicationrequirements are not served by the commonprofiles or sizes in this catalogue, contact SKFto provide a specif ic offer.

Customized sealing solutions

In addition to the standard assortment ofseries-produced seals and sizes listed in thiscatalogue, SKF designs and manufactureshydraulic seals customized for virtually anyrequirements.

The design and development of high per-formance materials combined with testing andfailure analysis are vital elements for success-

ful seal development. SKF combines theseelements with its extensive application know-ledge, to offer solutions that are based on anunderstanding of sealing systems under vari-ous real conditions. SKF continuously developsnew customized materials and designs andoperates its own testing facilities around theworld to provide optimized sealing solutionsthat meet the sophisticated demands of manyindustries.

Finite element analysis (FEA)FEA is used to simulate the impact of operat-ing conditions, material select ion and sealdesign on seal behaviour ( fig. 4).

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SKF SEAL JET system

With the innovative production system SKFSEAL JET virtually any kind of seal for anyconceivable application can be manufacturedin any dimension and design. SKF operates

more than 200 machining centres worldwideand aims to be located as close to the customeras possible. This reduces manufacturing anddispatch time to a minimum. With local andglobal application engineering teams sup-ported by research and development centres,machined seals (fig. 5) produced with SKFSEAL JET machines (fig. 6) can be custom-ized to meet the most stringent sealingrequirements of many industries. Key factorsof the SKF SEAL JET systems (fig. 7) are:

partnership with customers from the designphase to serial production with customizedsolutions

prompt manufacturing of seals and compon-ents up to 4 000 mm (157 in.)in diameter asone piece and larger with short deliverytimes

diameters up to 10 000 mm (395 in.)andlarger by using a special welding technique

virtually unlimited seal designs

extensive range of sealing materials includ-ing materials certified to or complying witha broad range of industry standards andregulations (FDA, NSF, KTW, NORSOK,NACE, etc.)

Fig. 5

Machined seals

Fig. 7

SKF SEAL JET system

Fig. 6

SKF SEAL JET machine

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Criteria for seal selection

The selection of the right seal profile andmaterial for a given application requiresconsideration of many factors. This catalogue

supports the selection of the right seal fortypical hydraulic seal applications and existingcylinder designs. For any application factorsoutside of the ordinary, or to specify sealingsystems in new hydraulic cylinder designs, acertain amount of expertise, beyond the con-tent of this catalogue, may be required. Thehydraulic seal experts at SKF can assist inselecting the right sealing system for newcylinder designs.

Before seals can be selected certain appli-

cation parameters and information should becollected. The following most common appli-cation considerations are nearly alwaysrequired when selecting hydraulic seals:

fluid pressure range,the range of operating fluid system pres-sure, as well as frequency and severity ofpressure peaks

temperature range,the range of the fluid and cylinder assembly,

both when operating and at rest speed,

the stroking speed of the reciprocating pis-ton rod

fluid media,the type and viscosity of f luid used in thesystem

hardware dimensions,the rod and bore diameters, seal groovedimensions and gaps (if already specif ied),cylinder overall length and stroke length as

well as surface f inish specifications (ifalready specified)

application of the cylinder,the type of equipment the cylinder will beused on and how the cylinder will operate inthe equipment as well as installat ion, dutycycles and environmental factors (externaltemperature, contaminants)

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SKF hydraulic seals generaltechnical information

Specifications for effectivesealing systems. . . . . . . . . . . . . . . . . . . 22Counter-surface finish properties . . . . . 22

Recommendations for dynamic

sealing surfaces . . . . . . . . . . . . . . . . . 24Recommendations for static sealingsurfaces . . . . . . . . . . . . . . . . . . . . . . . 25

Materials . . . . . . . . . . . . . . . . . . . . . . . . . 26Thermoplastic elastomers . . . . . . . . . 26Rubbers . . . . . . . . . . . . . . . . . . . . . . . . 27PTFE . . . . . . . . . . . . . . . . . . . . . . . . . . 27Rigid thermoplastics andthermosets . . . . . . . . . . . . . . . . . . . . . 27

Hydraulic fluids . . . . . . . . . . . . . . . . . . . . 31

Gap extrusion . . . . . . . . . . . . . . . . . . . . 34

Storage. . . . . . . . . . . . . . . . . . . . . . . . . . 36Storage conditions . . . . . . . . . . . . . . . . . 36Shelf life . . . . . . . . . . . . . . . . . . . . . . . . . 37

Installation and assembly. . . . . . . . . . 38Seal housing grooves . . . . . . . . . . . . . . . 38

Corner radii . . . . . . . . . . . . . . . . . . . . . 38Groove edge radii . . . . . . . . . . . . . . . . 39

Installation chamfers . . . . . . . . . . . . . . . 40

Installing rod seals . . . . . . . . . . . . . . . . . 40Installing piston seals . . . . . . . . . . . . . . . 42Installing wiper seals . . . . . . . . . . . . . . . 43

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Specifications for effectivesealing systemsDesigning hydraulic rod and piston sealingsystems requires careful attention to the

dynamic seal interaction and the principles oftribology to ensure long service life, properseal function, minimal wear, low friction andsmooth operation. Tribology is the study of thedesign, friction, wear and lubrication of inter-acting surfaces in relative motion. The mostimportant considerations for tribological prop-erties of a dynamic sealing system are:

the seal material the seal profile design

the counter-surface material and finishproperties the hydraulic fluid, which is the lubrication

for the dynamic sealing surfaces

For information about seal profile design, seethe relevant product chapter.

Counter-surface finish properties

The surface properties of the cylinder bore

and the piston rod have a great influence on

the function and service life of the seal.Parameters for specifying a surface finish aredefined by ISO 4287. The most common sur-face roughness parameter specif ied is Ra(inunits m or in.), i.e. the arithmetic mean

deviation of the surface profile. This valuedoes not, however, completely describe howthe surface can be expected to affect the seal.The reason for this is that two surfaces withthe same values of Rabut with different sur-face profile characteristics can lead to differ-ent lubrication film thickness, resulting invarying seal performance and level of wear.

The material ratio curve (Abbott-Firestonecurve) provides more information about thesurface profile characteristics. It describes the

ratio of the material-filled length to the evalu-ation length at a given cutting depth expressedin percent ( fig. 1). The slope at the begin-ning of the curve represents the peaks in theprofile, which are causing initial wear on theseal. The slope at the end of the curve repre-sents the valleys in the profile, which serve aslubricant reservoirs.

0 50 100

Fig. 1

Surface profile

Evaluation length

Material ratio curve(Abbot-Firestone curve)

Cutting depth [m]

Material ratio [%]

22

General technical information


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Table 1shows some surface profiles. Toensure a surface finish with a suitable profileand characteristics for effective dynamic seal-ing, SKF recommends a defined combinationof the following surface parameters:

Ra arithmetic mean roughness Rz mean peak-to-valley height Rmax maximum peak-to-valley height Rmr material ratio at a given depth

SKF recommends evaluating Rmrat a cuttingdepth of 0,5 Rzbased on cref= 0%( diagram 1).

Table 1

Seal counter-surface examples

Surface profile Ra Rz Rmax Rmr Material ratio curve

m m m %

Eroded surface

2,4 7,3 10,4 40

Machined surface

2,5 9,5 10,2 20 25

Ground and polished surface

0,09 1,2 1,8 90

0 50 100Rmr[%]

0,5 Rz

Cref (0%)

Diagram 1

Cuttingdepth [m]

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Recommendations for dynamic sealingsurfaces

The dynamic sealing surfaces on the pistonrod and in the cylinder bore ( fig. 2) requiresimilar, but somewhat different surface

finishing.Piston rod

The recommendations for the piston rod seal-ing surface ( table 2) assume that typicalmaterials and processes are used to manufac-ture the piston rod by induction hardening acarbon steel rod, grinding, hard-chromiumplating and then polishing it to achieve thespecified diameter and finish.

For alternative rod materials and coatings,

other surface specifications, f inishing methods,seal materials and designs may be required.Examples of such alternative rod coatingsinclude:

various hard metals applied by high velocityoxygen fuel (HVOF)

ceramic coatings of various compositionsand finishes

stainless steel iron nitride

For recommendations for sealing systems withalternative rod materials and coatings, contactSKF.

Table 2

Recommendat ions for the dynamic sealing surface onpiston rods(induct ion hardened and chrome plated rods)

Thermoplasticsand rubbers

PTFEmaterials

Ra 0,05 to 0,3 m 0,05 to 0,2 m(2 to 12 in.) (2 to 8 in.)

RZ 0,4 to 2,5 m 0,4 to 2 m(15 to 100 in.) (15 to 80 in.)

Rmax 0,4 to 2,5 m (15 to 100 in.)

Rmr 50 to 95%(cutting depth 0,5 Rzbased on cref= 0%)

Hardness 50 HRC

Hardening

depth

~1,2 to 2,5 mm (~0.05 to 0.1 in.)

Chromiumlayer thickness

~20 to 30 m (~800 to 1 200 in.)

Fig. 2

Cylinder bore dynamic surface Piston rod dynamic surface

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Cylinder bore

The recommendations for the cylinder boresurface ( table 3) assume that typicalmaterials and processes are used to machinethe bore by either honing or roller burnishingto achieve the specif ied diameter and finish.

Table 3

Recommendations for the dynamic sealing surface oncylinder bores(honed or roller burnished tubes)

Thermoplastics, rubbers and PTFEmaterials

Ra 0,05 to 0,2 m(2 to 8 in.)

RZ 0,4 to 2 m(15 to 80 in.)

Rmax 0,4 to 2,5 m(15 to 100 in.)

Rmr 50 to 95%(cutting depth 0,5 Rzbased on cref= 0%)

Fig. 3

Counter-surfaces of static seals exposed to pressureRa 0,8 m (30 in.)

Axia l faces of all groovesRa 3,2 m (125 in.)

Seat diameters of guide rings and wiper seals

Ra 1,6 m (63 in.)

Recommendations for static sealing surfaces

Static sealing surfaces must also have aproper surface finish to enable the desiredsealing performance ( fig. 3).

The counter-surfaces of static seals

exposed to pressure, including the static sideof dynamic rod and piston seals, should have aroughness Ra 0,8 m (30 in.).

Seat diameters of guide rings and wiperseals should have a roughness of Ra 1,6 m(63 in.). Axial faces of all grooves should havea roughness of Ra 3,2 m (125 in.).

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Materials

Materials play a major role in the performanceand lifetime of seals. Generally, hydraulic sealsare exposed to a variety of application andworking conditions, such as a wide tempera-

ture range, contact with various hydraulicfluids and the outside environment as well ashigh pressures and contact forces. The appro-priate seal materials have to be selected toachieve a reasonable service life and serviceintervals. A wide variety of seal materials fromfour major polymeric material groups isavailable:

thermoplastic elastomers, such as polyure-thane (TPU) and thermoplastic polyester

elastomers (TPC) rubbers, such as nitrile rubber (NBR) andhydrogenated nitrile rubber (HNBR), fluoro-carbon rubbers (FKM, FPM)

polytetrafluoroethylene (PTFE) and itscompounds

rigid thermoplastics and thermosets andtheir composites

Many different material properties should beconsidered to support and maintain the seal-

ing function over the expected seal service life,for example:

good elasticity over a wide temperaturerange, especially at low temperatures

excellent compression set and stress relax-ation behaviour to keep the sealing force forthe requested operating period

adequate hardness and flexibility to avoidleakage and allow easy installation

superior gap extrusion resistance to cover

the increased pressures of fluid powerequipment

adequate working temperature range good chemical compatibil ity to cover a wide

assortment of hydraulic fluids such as min-eral and synthetic oils, biodegradable andwater-based fluids or fire-resistant fluids

excellent tribological properties, i.e. lowfriction values and high wear resistance toachieve a high efficiency and avoid earlyfailures especially when sealing against

rough counter-surfaces

In addition to these considerations, the struc-ture and morphology of polymeric materials

make selection and specif ication of seal mater-ials much more complicated than the standardmaterials used in mechanical engineering (e.g.aluminium or steel). Mechanical properties ofpolymeric materials are strongly influenced by

time, temperature, load and rate of motion.Highly complex intermolecular processesaffect the stress relaxation and retardationphenomena. Furthermore, the tribology con-ditions of the system (e.g. friction and wear)has a strong influence on the seal materialbehaviour and vice versa. Therefore, state-of-the-art sealing systems can only be developedby close cooperation between material expertsand product designers, supported by advanceddesign tools like non-linear FEA and extensive

seal testing capabilities.SKF has a global material developmentorganization that closely cooperates with theproduct development and testing functions.SKF is uniquely suited to develop, simulate,test and manufacture tailor-made materialsfor specific customer needs.

The following tables list the most commonmaterials used by SKF for serial production ofhydraulic seals. A wide variety of additionalseal materials are available for special hydrau-

lic seals or other seal applications.

Thermoplastic elastomers

SKF has a long history in developing and sup-plying special polyurethane grades for sealingpurposes. SKF manufactures the well-knownECOPUR family of polyurethanes includingH-ECOPUR for outstanding chemical andhydrolysis resistance, X-ECOPUR for extrusionresistance or S-ECOPUR for low friction andresistance to wear.

Polyurethanes combine the elastic prop-erties of elastomers with the processability ofthermoplastic materials. Seals made of poly-urethanes provide excellent wear and pres-sure resistance and avoid leakage. Due totheir elasticity and flexibility, they are easy toinstall. Special sealing polyurethane gradeshave a superior compression set and relaxa-tion performance as well as temperature sta-bility compared to commodity industrialgrades.

Table 4( page 28)lists commonthermoplastic elastomers.

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Rubbers

Rubbers are widely used in the seals industryfor rotary shaft seals, static sealing elementssuch as O-rings and energizers, as well asdynamic seals in the fluid power industry.

Depending on the chemical composition, rub-bers can cover a wide temperature range upto 200 C (390 F)and more and withstand awide variety of hydraulic fluids. NBR elastomersin a hardness range from 70 to 90 Shore A (shA)are the most commonly used rubbers in thefluid power industry. For higher temperaturesand more aggressive hydraulic fluids, SKF rec-ommends HNBR or FKM elastomers.

Table 5 ( page 29) lists common rubbers.

PTFEPTFE is a polymer with very unique proper-ties. Due to its chemical composition, it is theplastic material with the highest chemicalresistance and the lowest coefficient of fric-tion, however, it has some restrictions interms of mechanical properties and wear.Therefore, PTFE is very often modified byadding various organic and/or inorganic fillersto improve specif ic properties such as wear orextrusion resistance.

One of the most important characteristics ofPTFE is the low coeff icient of friction that pro-vides outstanding start-up behaviour as wellas minimized stick-slip phenomenon. There-fore, PTFE is the preferred material in applica-tions that require accurate posit ioning ofhydraulic cylinders. Due to the increased mod-ulus of elasticity compared to rubbers andpolyurethanes, PTFE seals can usually not beinstalled by simple snap-in procedures andrequire special tools and procedures for

installation.Table 6( page 29) lists common PTFE

materials.

Rigid thermoplastics and thermosets

Rigid thermoplastics and thermosets and theircomposites are characterized by much higherhardness and stiffness as well as reducedelasticity compared to polyurethanes, rubbersor PTFE. Therefore, they are used for com-ponents where mechanical strength is more

important than flexibility, such as guide rings,anti-extrusion rings or special piston sealarrangements for heavy duty applications.

SKF offers rigid thermoplastics and com-posites in a wide variety of homogenous (un-filled or virgin) grades (e.g. polyacetal orpolyamide), filled grades (e.g. glass fibre rein-forced PA) and fabric-reinforced composites

(e.g. phenolic resin with cotton fabric, PF). Forextreme conditions, SKF can provide highperformance materials such as PEEK(polyetheretherketone).

Table 7( page 30) lists common rigidthermoplastic and thermosets.

WARNINGAt temperatures above 300 C (570 F)allfluoro elastomers and PTFE compoundsgive off dangerous fumes. If there is con-tact with your skin or if the vapours areinhaled, seek medical advice immediately.

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Table 4

Thermoplastic elastomers (TPU and TPC)

Mater ial code Mater ial descr iption Hardness Colour 1) Example usesShore A Shore D

(shA) (shD)

LUBRITHANEU-1003

TPU 95 n black Rod seals, pis ton seals, wiper seals

LUBRITHANEU-1004

TPU 55 n black Wiper seals

LUBRITHANEU-1023

TPU 93 n blue Rod seals, buf fer seals, piston seals,wiper seals

LUBRITHANEU-1029

TPU 94 n light blue Rod seals, buffer seals, piston seals,wiper seals

PU54/199 TPU 97 54 n blue Piston seal slide rings

W93 TPU 93 n white Press-in wiper seals

Y95A TPU 95 n yellow Back-up rings for O-rings

395A TPU 98 n black Back-up rings for O-rings

B93 TPU 93 n dark blue Rod seals

ECOPUR TPU 95 48 n green Premium U-cup seals, w iper seals andchevron packings

X-ECOPUR TPU with increasedhardness and extrusionresistance

97 57 n dark green Rod and buffer seals

X-ECOPUR PS TPU with increasedhardness and extrusionresistance

98 60 n green Piston seals for heavy duty hydraulicapplications

H-ECOPUR TPU with increasedchemical resistanceagainst biodegradeableand water based fluids

95 48 n red Rod and piston seals for applicationsrequiring hydrolysis resistance,increased chemical resistance orconformance with FDA regulations

XH-ECOPUR TPU with increasedchemical resistance andhigher hardness

97 60 n dark red Rod and piston seals for applicat ionsrequiring hydrolysis resistance,increased chemical resistance orconformance with FDA regulations

S-ECOPUR TPU with improvedtribologicalcharacteristics

95 48 n dark grey Rod and piston seals for waterhydraulics, non-lubricated pneumaticsor dry-running conditions

FLUOROTRELF-1504

TPC 55 n turquoise Energized rod seals, piston seals, anti-extrusion rings

TPC-ET72/075 TPC 72 n black Piston seal support rings

1) The colour examples may deviate from the actual colour of the material.

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Table 5

Rubbers

Material code Material descript ion Hardness Colour1)

Example uses Shore A

A-8501 NBR 70 n black Rod seals, piston seal s, energizersA-8504 NBR-LT

(low-temperature grade)70 n black Rod seals, piston seal s, energizers

A-8526 NBR 90 n black Piston seal energizers

C-7021 HNBR 70 n black Rod seals, piston seal s, energizers

C-7022 HNBR 80 n black Rod seals, piston seal s, energizers

V-7501 FKM 70 n black Piston seals

V-7503 FKM 90 n black Rod seals, piston seal s, energizers

N70/015 NBR 70 n black Piston seal energizers

N70/6052 NBR 70 n black O-rings and energizers

N80/047 NBR 80 n black Piston seals

N80/198 NBR 80 n black Energi zer s for heavy duty applicat ions

SKF Ecorubber-1 NBR 85 n black U-cup seals , chevron packings, machinedseals

SKF Ecorubber-2 FKM 85 n brown U-cup seals , machined seals , chevronpackings, wiper seals

SKF Ecorubber-H HNBR 85 n black U-cup seals , chevron packings, machinedseals


Table 6

PTFE materials

Mater ial code Material descript ion Hardness Colour 1) Example uses Shore D

292 PTFE+ 40 % bronze+ MoS2

62 n brown-grey Piston seal slide rings, guide strips

SKF Ecoflon 1,100

PTFEunfilled

57 n white Anti-extrusion rings, chevron sets,O-rings, food-compatible products (FDAcertificate)

SKF Ecoflon 2,702

PTFE+ 15% glass fibres+ 5% MoS2

60 n grey Anti-extrusion rings, chevron sets,O-rings

SKF Ecoflon 3,741

PTFE+ 40% bronze

65 n brown Anti-extrusion rings, chevron set s,O-rings

SKF Ecoflon 4 PTFE

+ 23% hard carbon+ 2% graphite

65 n black Anti-extrusion rings, chevron sets,O-rings


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Table 7

Rigid thermoplastics and thermosets

Material code Material description Colour 1) Example uses

707 PA 6 n black Anti-extrusion rings

P-2501 PA 6.6 + 30% glass fibres n black Split piston seal slide rings

P-2506 PA 6.12 n black Anti-extrusion rings

P-2518 POM n black Anti-extrusion rings

P-2551 PA 6 + 40% glass fibres n dark grey Guide rings and split piston seal slide rings

P-2552 PA 6 + 40% glass fibres + PTFE n black Guide rings

PA66/011 PA 6.6 + 20% glass fibres n black Split piston seal slide rings

POM/076 POM n red Guide rings in compact sets (light and mediumduty cylinders)

PF Phenolic resin with cotton fabric n brown Guide rings

SKF Ecomid PA 6 n black Anti-extrusion rings, guide rings

SKF Ecotal,728

POM n black Ant i-ex trusion rings, guide rings and scrapingwiper seals

SKF Ecopaek PEEK n beige Anti-extrusion rings, guide rings

SKF Ecowear 1000 UHMWPE (ultra-high-molecular-weight polyethylene) n

white Anti-extrusion rings, guide rings

SKF Ecotex Polyester resin with polyesterfabric and graphite filler n

l ight orange Guide r ings


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Hydraulic fluids

Fluids used in hydraulic systems serve mul-tiple functions for the system performance:

transfer power by flow under pressure act-

ing on moveable parts lubricate surfaces in contact and relativemotion hydraulic cylinder components andseals, as well as other system componentssuch as pumps and valves

prevent corrosion of components cool the system, by carrying heat from areas

of high load, motion or turbulence and spreadit to the entire volume of the system includingreservoir tanks and cooling equipment

clean the system by carrying contaminants

and wear particles to filter bodies or settlingareas

The fluids used in hydraulic systems come invarious chemical compositions and viscositygrades as suited to specif ic applications.

Viscosity is a measurement of the thicknessof a fluid or the resistance to flow. Seal per-formance is affected by the viscosity of thefluid and changes to the viscosity during use.Most typical hydraulic fluids exhibit decreased

viscosity with increasing temperature andincreased viscosity with increasing pressure.

The most commonly used media in hydraulicsystems are mineral oil based fluids with vari-ous additives. However, a variety of alternativefluids may be encountered in special applica-tions. For example, biodegradable fluids suchas synthetic (HEES) or natural esters (HETG)and polyalphaolef ines (PAO) may be used toreduce environmental impact in the event ofaccidental spills. Flame retardant f luids based

on water or synthetic esters may be safelyused in confined spaces or where the hydraulicsystem is used in close proximity to ignitionsources. The data, specif ications and recom-mendations in this catalogue are for commonmineral oil fluids. For guidance on specif ica-tions of sealing systems for alternative fluids,contact SKF.

The chemical composition of hydraulic fluidscan impact the seal life and performancedepending on compatibility with the seal

material(s). Absorption and reaction of theseal material(s) with non-compatible fluidscan cause, for example:

changes in seal material volume increasedswelling or decreased shrinking and theirrespective impacts on seal contact force andfriction

hardening and embrittlement of the seal

material softening, loss of strength or dissolving ofthe seal material

degradation of the polymer chains or cross-linking, causing the material to fatigue orlose resilience

discoloration of the seal material

Generally, these changes are accelerated byhigher temperature. To avoid these changesand the resulting damage to seal function and

life, careful consideration should be taken toensure compatibility between the fluid and allseal materials, as well as the temperature andmechanical loads on the seal material. SKF hasa long history and extensive database of testresults concerning compatibility of variousseal materials and fluids, as well as unparal-leled expertise in developing materials to meetcustomers needs for chemical resistance ofseal materials.

Table 8( page 32) summarizes the com-

patibility rating for the most important fluidsand materials used in the fluid power industry.For materials not listed, contact SKF. Table 8provides general guidelines for new, clean flu-ids. Fluids vary by manufacturer, additives andcontaminant levels. Materials vary by specificcompound. The guidelines cannot substitutefor testing the compatibil ity of a seal in theactual fluid and under actual operating condi-tions. Temperatures higher than specified intable 8can lead to degradation of the basic

fluid or its additives. This can cause deterior-ation of the seal material. For applicationswhere higher temperatures are required,contact SKF.

In addition to the specif ied hydraulic fluid,seal materials can be attacked by exposure toother fluids from other parts of the machinery(e.g. greases, fuels, coatings), environmentalfactors (e.g. humidity or radiation) anddegradation and reaction with the fluids,additives and contaminants in the system

producing additional chemicals.

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1) Ethylene-propy lene rubber for reference only not common for hydraulic cylinders2) For filled PTFE, compatibility of filler must be considered separately (e.g. bronze not recommended for water-based fluids).3) Exposure to water-based fluids or moisture causes swelling.4) Contact SKF

Hydraulic fluids and seal material compatibility

Fluids Thermoplastic elastomersECOPUR,LUBRITHANE

H-ECOPUR,XH-ECOPUR

Ether-based TPU TPC( 95 shA)

TemperatureNormal High Normal High Normal High Normal High

Hydraulic fluids 60 C( 140 F)

100 C( 210 F)

60 C( 140 F)

100 C( 210 F)

60 C( 140 F)

100 C( 210 F)

60 C( 140 F)

100 C( 210 F)

Mineral oils HL, HLP, HVLPA B A A A B/C A A/B

ATF(automatic transmission fluids) A B A A A B/C A B

HETG(triglycer ides, rape seed oil) A B/C A A A C A B/C

HEES(synthetic esters) A B/C A A A C A B/C

HEPG(polyalcylene glycols) B D A C B/C D C D

HEPR(polyalphaolefines) A B A A A B/C A B

Fire resistant fluids,water-based

40 C( 105 F)

60 C( 140 F)

40 C( 105 F)

60 C( 140 F)

40 C( 105 F)

60 C( 140 F)

40 C( 105 F)

60 C( 140 F)

WaterB D A A A B A B

HFA-fluids(oil in water) B D A A B B/C A B

HFB-fluids(water in oil) B D A A B D A B

HFC-fluids(water-glycol) C D A B/C B B/C C D

Fire resistant fluids,water-free

60 C( 140 F)

100 C( 210 F)

60 C( 140 F)

100 C( 210 F)

60 C( 140 F)

100 C( 210 F)

60 C( 140 F)

100 C( 210 F)

HFD-R fluidsD D D D D D D D

HFD-U fluids(polyol and carboxylic esters) B D A A B D A B/C

Mineral greases A B A A A B A A

A ExcellentB GoodC LimitedD Not recommended

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Table 8

Rubbers Thermoplastics and thermosets PTFE2)

NBR, HNBR FKM EPDM1) PA, PF POM PEEK

Temperature TemperatureNormal High Normal High All All All All All

60 C( 140 F)

100 C( 210 F)

60 C( 140 F)

100 C( 210 F)

A A A A D A A A A

A A A A D A A A A

A/B A/B A A D A A A A


A A/B A/B C/D A A A A A


40 C( 105 F)

60 C( 140 F)

40 C( 105 F)

60 C( 140 F)

A A A A A C3) A A A

A A A B D C3) A A A

A A A A D C3) A A A

A A A/B B/C A C3) A A A

60 C( 140 F)

100 C( 210 F)

60 C( 140 F)

100 C( 210 F)

D D A/C4) A/C4) A A A A A

A/B C A A D A A A A

A A A A D A A A A

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Gap extrusion

The process by which seal material is forcedinto the clearances between components iscalled gap extrusion. The dimension of this

clearance gap is referred to as the extrusiongap, or e-gap ( fig. 4).The resistance of a given seal component to

gap extrusion is mainly determined by thematerial composition and quality. Materials ofgreater hardness and stif fness typically alsohave improved resistance to extrusion. There-fore, full-face anti-extrusion or back-up ringsof materials harder than the seal material maybe used to prevent seal extrusion into thee-gap ( fig. 5).

Pressure is the main driver of extrusion, butthe e-gap size and application temperatureare also major factors. Diagram 2shows thepressure resistance of different materials as afunction of temperature. The values weremeasured on an SKF test rig. The tests werecarried out with a rectangular sample, dimen-sions 38,7 x 49 x 5 mm under static pressureand an extrusion gap of 0,3 mm. The pressurevalues were taken at an extrusion length of0,5 mm ( fig. 4). While these sample values

illustrate the differences in extrusion resist-ance for standard grades of typical seal mate-rials, there are many variations of each basiccomposition that impact the extrusion resist-ance of seals. In addition, the profile designand the seal friction affect extrusion. Formaximum allowable pressure, temperatureand e-gap of each seal profile, refer to theprofile data for each profile in the relevantchapters.

The maximum e-gap in a hydraulic cylinder

occurs when the cylinder components are atthe maximum radial misalignment of compo-nents. This misalignment is affected by:

external forces acting upon the cylinderassembly (e.g. acceleration forces, frictionalmoments from rotation of cylinder endconnections)

the weight of the cylinder components(especially when used horizontally)

deformation of cylinder components (rod

flexing, guide ring radial deformation underforce)

the tolerance stack up of multiple cylindercomponents

Fig. 4

Extrusiongap

Fig. 5

Gap extrusion

Extrusion length

2 400

2 000

1 600

1 200

800

400

0 40 60 80 100

SKFEcotal

X-ECOPUR(TPU57D)ECOPUR

SKFEcorubber-1

Diagram 2

Pressure [bar]

Temperature [C]

TPU back-up ring,hardness 90 shA

NBR seal,hardness 70 shA

TPU seal,hardness 95 shA

Rigid POM full-faceanti-extrusion ring

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Therefore, it is necessary to calculate thee-gap at the maximum misalignment at min-imum material conditions of the cylinder andguide components.

For rod seals, the maximum e-gap should

be calculated with the following conditions( fig. 6):

guide ring groove at maximum diameter D rod at minimum diameter d guide ring cross section at minimum thick-

ness t (considering tolerances and any radialdeformation of the guide ring under load)

rod seal housing throat at maximumdiameter h

For piston seals, the maximum e-gap shouldbe calculated with the following conditions( fig. 7):

bore at maximum diameter D guide ring groove at minimum diameter d guide ring cross section at minimum thick-

ness t (considering tolerances and any radialdeformation of the guide ring under load)

piston seal housing at minimum outsidediameter OD

The maximum allowable e-gap is provided inthe profile data for each rod seal and pistonseal profile in the relevant chapter. The e-gapcan be kept within these limits by specifyingand controlling the tolerances of dimensionsdescribed above and shown in figs. 6and 7.

dD

e

h

t

Fig. 6

dD

e

OD

t

Fig. 7

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Storage

During storage, the properties of elastomerproducts can be damaged either by chemicalreactions or by physical processes. Chemical

reactions are basically caused by the influenceof heat, light, oxygen, ozone or contaminationby chemicals. The physical processes, whichare called physical ageing, are either due tothe influence of external stresses leading tocracks and permanent deformation, or due tothe migration of plasticizers, which makes thematerial more britt le and can lead to deforma-tion of the parts.

Therefore, elastomer products only main-tain their characteristics for several years

without major changes, if they are properlystored. The ageing behaviour of elastomerproducts and their reaction on storage condi-tions depend considerably on their chemicalstructure. Unsaturated elastomers (e.g. nitrilerubber) age more quickly under improperstorage conditions than saturated elastomers(e.g. fluorocarbon rubber).

Storage conditions

Elastomer products should be stored inaccordance with the following recommenda-tions, which are in line with the recommenda-tions provided in ISO 2230 or DIN 7716.

Rubber and plastic products should bestored in a cool and dry room. The relativehumidity should be < 65%. Storage tem-perature should be around 15 C (60 F)andnot exceed 25 C (75 F). If the storage tem-perature is below 15 C (60 F), care shouldbe taken during handling of stored productsbecause they may have stif fened. Theyshould be warmed up slowly at ambienttemperature.

The storage room must not contain anyozone-producing devices, such as electricmotors or high-voltage devices.

Rubber and plastic products should not beexposed to direct sunlight or artif icial lightwith a high UV content (bulbs are preferredto neon lamps).

Rubber products should not be exposed todrafts. They should be stored in airtightpackaging. The package material must notcontain plasticizers. Polyethylene is the

most suitable package material. Contact between rubber products of differ-

ent compositions should be avoided. Contact between rubber and plastic prod-

ucts and chemicals or dangerous metals(e.g. copper, manganese) should be avoided.

Rubber and plastic products should bestored as tension-free as possible, i.e. theparts should not be subject to tension, pres-sure or bending. Rubber products, espe-cially seals, must not be hung on nails or

tightly folded or rolled for reasons of space.

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Table 9

Shelf life recommendations in years

Material Typicalshelf life

Possibleprolongation

TPU(standard) 5 2ECOPUR, LUBRITHANE 5 2H-ECOPUR, XH-ECOPUR 10 2TPC 5 2

NBR 6 3HNBR 8 4FKM, FPM 10 4EPDM 8 4MVQ(silicone) 10 5

PTFE 15 5

PA, POM, PF 8 4PEEK 15 5

UHMWPE 10 5

Shelf life

When stored under the conditions mentionedabove, elastomer products retain their typicalproperties for several years ( table 9).

The typical shelf life may be prolonged

based on the actual product conditions at theend of the typical shelf life. Trained and expe-rienced experts can approve a prolonged stor-age period based on a visual inspection of rep-resentative samples. The samples should notreveal any permanent distortion, mechanicaldamage or surface cracking. The materialshould not show any signs of hardening orsoftening nor any kind of tackiness.

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Installation and assembly

Seal housing grooves

The type of seal housing determines themethod of installation, required equipment

and the degree of diff iculty. There are fourmain types of seal housings.

Closed housing grooves

Closed housing grooves are the most commonseal housings ( fig. 8). They require themost planning and effort to ensure that theseal is installed properly without damage. Notall seal cross section sizes and material combi-nations can be installed into this type of sealhousing.

Open housing grooves

Open housing grooves allow the seal to bepressed in with minimal deformation and aretherefore a good choice when the seal design,material or size prevent installation into aclosed or stepped housing. Some seals, suchas press-in wiper seals, have a metal sleevethat retains the seal in an open groove bypress forces ( fig. 9), whereas other sealsmay require a snap ring ( fig. 10). Plastic

snap rings, such as RI for rods or RR for piston,are available from SKF on request. Openhousing grooves require specif ied edge radiior chamfers to prevent seal damage when theseal enters the housing groove or passes thesnap ring groove.

Fig. 8

Closed housing groove

Fig. 9

Open housing groove

Fig. 10

Open housing groove with snap ring

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Split two-piece closed grooves

These grooves incorporate two separablemachine components to provide an opengroove when the seal is installed and a closedgroove when the machine is fully assembled

( fig. 11).Stepped grooves

Stepped grooves allow seals to be installedwith less deformation ( fig. 12). Snap-inwiper seals are a common example in hydrau-lic cylinder applications.

Corner radii

The corner radii inside the groove should besized to avoid inadvertent contact with with

the adjacent portion of the seal. Static sidecorner radius recommendations are providedin the product tables of the relevant chapter.

Groove edge radii

All outside groove edges and any other edgesthat may come into contact with the seal dur-ing installation or use should be broken with asmall radius. Otherwise, the sharp edge maydamage the seal. Unless otherwise specified,all outside groove edges should have a radius

of approximately 0,2 mm (0.008 in.).

Fig. 11

Split two-piece closed housing groove

Fig. 12

Stepped housing groove

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Installation chamfers

Installation chamfers should be designed intothe cylinder bore and onto the assembly end ofthe piston rod to ensure that the seal can eas-ily transition from its free state diameter into

its installed diameter. The installation chamfershould also be blended into the cylinder boreor piston rod diameter with a generous radius.The chamfer angle and minimum length rec-ommendations are provided in the producttables of the relevant chapter.

Installing rod seals

The method of installation and the possiblegroove types for rod seals depend on the

materials, seal design and ratio between thediameter and cross-sectional height. Table 10provides general recommendations for pro-files made of rubber or TPU with a hardness 95 shA. PTFE or other harder materials mayrequire a smaller radial depth S or even opengrooves. The recommendations in table 10are not a substitute for careful installationtests in the particular application.

Installing rod seals in closed grooves

Rod seals can often be installed into closedgrooves through carefully bending the profilesimilar to a kidney shape and then insert ing itinto the groove. It is very important to avoidsharp bending.

Thin and flexible profiles can be installed byhand ( figs. 13 aand b). Installation toolsfor TPU rod seals help to install prof iles ofgreater section thickness ( figs. 14 ato f).After installation, the seal may need to bereshaped to a round form using a cone-shaped

tool.For PTFE seals, small diameter seals or for

serial assembly, special assembly tools( fig. 15, page 42) may be required to savetime or avoid seal damage. For additionalinformation about such special installationtools, contact SKF.

Table 10

General recommendations for selecting the type ofinstallation for rubber and TPU materials(hardness 95 shA)

Diameter/radialdepth ratio

Type of installation

d/S 6 Open groove6 < d/S 10 Closed groove, installation with tool

d/S > 10 Closed groove, installation by hand

d

S

Fig. 13 a

Fig. 13 b

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Fig. 14a

Tool, rod seal and housing

Fig. 14c

Seal properly folded

Fig. 14e

Seal positioned forinstallation

Fig. 14b

Tool positioned for graspingthe seal

Fig. 14d

Inserting theseal into thehousing

Fig. 14f

Seal properly installed

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Installing piston seals

Piston seals installed in closed grooves mustbe expanded or stretched into position. Sealswith step cut slide rings such as CUT or SCP( Piston seals with rigid split slide rings,page

54) are relatively easy to expand into position.Non-split prof iles should not be expanded to amaterial deformation of more than 20% forTPU or 30% for rubbers. Otherwise, the per-manent deformation would be too large. Heat-ing the seal, e.g. in an oil bath, decreases therequired expansion force, but cannot increasethe maximum material deformation.

Piston seals with a TPU slide ring can usu-ally be installed by hand or with simple tools( fig. 16). PTFE seals or those with thicker

radial sections may require special assemblytools to save time or avoid seal damage( fig. 17). For additional information aboutsuch special installation tools, contact SKF.

The recommendations cannot substitute forcareful installation tests in the particularapplication.

Fig. 15

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Fig. 16

Fig. 17

Fig. 18

Installing wiper seals

Snap-in wiper seals, which are installed instepped grooves (fig. 12, page 39), aretypically of a small radial section per diameterand close to the end of the cylinder head com-

ponent. Therefore, installation by hand isusually possible.Press-in wiper seals require special equip-

ment and careful planning for ease of installa-tion without damaging the wiper seal or hous-ing. Assembly tools adapted for each press-inwiper seal size should be used in conjunctionwith appropriate steady force in a hand oper-ated press. Installation by impact (e.g. strikingthe assembly tool with a hammer) is notadvised. The press assembly tool should be

configured to stop when the wiper seal hasbeen pressed flush with the groove edge( fig. 18). Pressing beyond flush can dam-age the wiper seal.

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Piston seals

Profile overview. . . . . . . . . . . . . . . . . . 46

Basics. . . . . . . . . . . . . . . . . . . . . . . . . . . 48Materials . . . . . . . . . . . . . . . . . . . . . . . 48

Gap extrusion . . . . . . . . . . . . . . . . . . . 48Piston guidance . . . . . . . . . . . . . . . . . 48Piston drift . . . . . . . . . . . . . . . . . . . . . 49Sealing between piston and rod . . . . . 49

Double-acting piston seals. . . . . . . . . 50Piston seals with polyurethaneslide rings . . . . . . . . . . . . . . . . . . . . . . . . 50

MPV profiles . . . . . . . . . . . . . . . . . . . . 50DPV profiles . . . . . . . . . . . . . . . . . . . . 51LPV profiles . . . . . . . . . . . . . . . . . . . . 51

CPV profiles . . . . . . . . . . . . . . . . . . . . 51Piston seals with PTFE slide rings . . . . . 52

GH profiles . . . . . . . . . . . . . . . . . . . . . 52APR profiles . . . . . . . . . . . . . . . . . . . . 52

Piston seals incorporatinganti-extrusion rings . . . . . . . . . . . . . . . . 53

LCP profiles . . . . . . . . . . . . . . . . . . . . . 53LTP profiles . . . . . . . . . . . . . . . . . . . . . 53

Piston seals with rigid split slide rings . . 54CUT profiles . . . . . . . . . . . . . . . . . . . . 54SCP profiles . . . . . . . . . . . . . . . . . . . . 54

Piston seals with integrated guide rings 55MDL profiles . . . . . . . . . . . . . . . . . . . 55

Single-acting piston seals. . . . . . . . . . 56UNP profiles . . . . . . . . . . . . . . . . . . . . 56

Single-acting piston seals indouble-acting cylinders . . . . . . . . . . . . . 56Rod seals used as single-actingpiston seals . . . . . . . . . . . . . . . . . . . . . . . 56

Profile data2.1 MPV . . . . . . . . . . . . . . . . . . . . . . . . 58 Metric sizes . . . . . . . . . . . . . . . . . . 592.2 DPV . . . . . . . . . . . . . . . . . . . . . . . . 60

Inch sizes . . . . . . . . . . . . . . . . . . . . 612.3 LPV. . . . . . . . . . . . . . . . . . . . . . . . . 62 Metric sizes . . . . . . . . . . . . . . . . . . 632.4 CPV . . . . . . . . . . . . . . . . . . . . . . . . 64 Inch sizes . . . . . . . . . . . . . . . . . . . . 652.5 GH . . . . . . . . . . . . . . . . . . . . . . . . . 66 Metric sizes . . . . . . . . . . . . . . . . . . 67 Inch sizes . . . . . . . . . . . . . . . . . . . . 702.6 APR . . . . . . . . . . . . . . . . . . . . . . . . 76 Metric sizes . . . . . . . . . . . . . . . . . . 77 Inch sizes . . . . . . . . . . . . . . . . . . . . 79

2.7 LCP . . . . . . . . . . . . . . . . . . . . . . . . . 82 Metric sizes . . . . . . . . . . . . . . . . . . 83 Inch sizes . . . . . . . . . . . . . . . . . . . . 852.8 LTP . . . . . . . . . . . . . . . . . . . . . . . . . 86 Inch sizes . . . . . . . . . . . . . . . . . . . . 872.9 CUT . . . . . . . . . . . . . . . . . . . . . . . . 90 Metric sizes . . . . . . . . . . . . . . . . . . 912.10SCP. . . . . . . . . . . . . . . . . . . . . . . . . 94 Inch sizes . . . . . . . . . . . . . . . . . . . . 952.11MD-L . . . . . . . . . . . . . . . . . . . . . . . 98 Metric sizes . . . . . . . . . . . . . . . . . . 99

2.12UNP . . . . . . . . . . . . . . . . . . . . . . . . 102 Metric sizes . . . . . . . . . . . . . . . . . . 103 Inch sizes . . . . . . . . . . . . . . . . . . . . 105

More piston seals. . . . . . . . . . . . . . . . . 108More PTFE slide ring piston seals . . . . . 108SPECTRASEAL . . . . . . . . . . . . . . . . . . . . 108Customized machined seal profiles . . . . 108

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Profile overview

Profile Description Additionalinformation

page

Profile data page

MPV Polyurethane slide ring, nitrile rubber ener-gizer; suitable for medium to heavy dutyapplications

50 58(metric)

DPV Polyurethane slide ring, nitrile rubber ener-gizer; fits O-ring dash-number housings;suitable for medium to heavy dutyapplications

51 60(inch)

LPV Polyether-based polyurethane slide ring,nitrile rubber energizer; suitable for light tomedium duty applications

51 62(metric)

CPV Polyurethane slide ring, nitrile rubber ener-gizer; fits O-ring dash-number housings;suitable for light to medium dutyapplications

51 64(inch)

GH PTFE slide ring, nitrile rubber energizer; lowbreakaway friction; suitable for mediumduty applications

52 66(metric andinch)

APR PTFE slide ring with incorporated nitrilerubber X-ring for improved sealing perform-ance, nitrile rubber energizer; suitable formedium to heavy duty applications


LCP PTFE slide ring supported by polyamide

anti-extrusion rings, nitrile rubber ener-gizer; very good gap extrusion resistance;suitable for heavy duty applications and highpressures

53 82

(metric andinch)

LTP Sealing ring made of nitrile rubber, sup-ported by polyamide anti-extrusion rings;good gap extrusion resistance; fits O-ringdash-number housings; suitable for mediumto heavy duty applications and highpressures

53 86(inch)

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Profile Description Additionalinformation

page

Profile data page

CUT Step cut polyamide slide ring, nitrile rubberenergizer; suitable for heavy duty applica-tions and high pressures

54 90(metric)

SCP Step cut polyamide slide ring, oval nitrilerubber energizer; fits wide, shallow inch sizehousings; suitable for heavy duty applica-tions and high pressures

54 94(inch)

MD-L Five-piece compact set consisting of a nitrilerubber sealing ring, integrated polyesterelastomer anti-extrusion rings and poly-acetal guide rings; suitable for medium dutyapplications

55 98(metric)

UNP Polyurethane U-cup profile; single-acting;may be used in double-acting cylinderswhen using two seals in back-to-backarrangement; suitable for medium duty

applications


Rod seals that can be used as piston seals

PTB These rod seal profiles are designed withsimilar inside and outside sealing geometry.Therefore, they can also be used as pistonseals (Rod seals used as single-acting pis-ton seals, page 56).

Rod andbuffer seals,page 111

138(metric andinch)

STD 164

(inch)

DZ 178(metric andinch)

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Basics

Piston seals ( fig. 1) maintain sealing con-tact between the piston and the cylinder bore.Differential pressures acting on the piston to

extend or retract the piston rod can be inexcess of 400 bar (5 800 psi). The pressureacting on the piston seal increases contactforces between the piston seal and cylindersurface. Therefore, the surface propertiesof the sealing surfaces are critical to properseal performance (Counter-surface finishproperties, page 22).

Piston seals are typically classified intosingle-acting (pressure acting on one sideonly) and double-acting (pressure acting on

both sides) seals.

Materials

Depending on the profile and the requiredcharacteristics of its components, a piston sealcan consist of one or several materials. Commonmaterials used for piston seals are thermo-plastic polyurethane (TPU), polytetrafluoro-ethylene (PTFE), polyamide (PA), and nitrilerubber (NBR). The standard materials used fora specif ic profile are provided in the Profile

overview(page 46) and in the relevantprofile sections below.

For additional information, refer to Mater-ials(page 26).

Gap extrusion

External forces acting on the piston rod,reacted by the fluid inside the cylinder, canresult in abrupt pressure peaks. These peakscan be far in excess of the system operatingpressure and may press a piston seal into thegap between the piston and bore, therebycausing damage to the seal and adverselyaffecting seal performance and cylinder oper-

ation. Seal materials must be carefully chosento avoid gap extrusion (Gap extrusion, page34). This risk of gap extrusion can also be min-imized by using seals with anti-extrusion rings(Piston seals incorporating anti-extrusionrings, page 53).

Piston guidance

Guide rings avoid sliding metal-to-metal con-tact between the piston and cylinder bore andaccommodate the radial loads of forces acting

on the cylinder assembly. Although pistonseals are designed to accommodate slightradial motion between the piston and bore,effective guide ring function to accurately cen-tre the piston within the bore is important forpiston seal performance. For additional infor-mation about piston guidance, refer to Guiderings and guide strips(page 249).

More information

Counter-surface finish properties . . . . . 22Materials . . . . . . . . . . . . . . . . . . . . . . . . 26Hydraulic fluids . . . . . . . . . . . . . . . . . . . . 31Gap extrusion . . . . . . . . . . . . . . . . . . . . . 34Storage . . . . . . . . . . . . . . . . . . . . . . . . . . 36Installation and assembly . . . . . . . . . . . 38

Fig. 1

Typical piston seal arrangement for double-actingcylinders

Energizer

Slide ring

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Fig. 3

Piston fixed to the rod by a lock nut and with a static sealand two back-up rings between piston and rod end

Fig. 2

Piston welded to the rod end tap

Piston drift

When the piston rod is at rest and held in pos-ition by fluid, any amount of flow passing thepiston can result in an unintended movementof the piston rod and cause drift. Although pis-

ton seal leakage is a possible source of drift,internal valve leakage, external system leak-age and flow between the piston and rod staticconnections should also be carefully checked.In some applications, a minimal amount offlow passing the piston seal within specif iedlimits is permitted. This accepted flow allowsthe use of piston seals of low friction designsand materials or split slide rings for easyinstallation.

Sealing between piston and rodThe piston can be welded to the rod (fig. 2)if the disassembly of the cylinder can be doneby removing the rod end attachment. The pis-ton can also be fixed to the rod end by a locknut (fig. 3), which enables removing thepiston from the rod during complete disas-sembly of the cylinder. When using a lock nut,static sealing (O-rings and back-up rings,page 291) is required between the piston andthe rod end.

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Double-acting piston seals

Double-acting cylinders are the most widelyused cylinder types. They operate with pres-sure on both sides and, therefore, require

double-acting seal arrangements (fig. 1,page 48).Double-acting piston seals have a sym-

metrical profile (cross section) and identicalsealing functions in both directions. Typically,double-acting piston seals consist of a slidering and an energizer. The deformation of theelastomeric energizer when installed providesadequate force to keep the slide ring indynamic sealing contact with the cylinderbore, while sealing statically against the seal

housing groove.A double-acting cylinder typically has thesame fluid on both sides of the piston. There-fore, a relatively thick lubrication film can bepermitted between the piston seal and thecylinder bore to minimize friction and wear.The transportation of f luid occurring duringdynamic operation is, however, small andinsignificant in most applications.

In some older cylinder designs, O-rings wereused as piston seals. To allow easy replacement

of O-rings with the equivalent piston seals, thehousing dimensions for some double-actingpiston seals are the same as those for dash-number O-rings. Therefore, these housingsare also called O-ring dash-number housings.

SKF supplies double-acting piston seals inmany different profiles and in a wide range ofseries and sizes, which make them appropriatefor a wide variety of operating conditions andapplications.

Piston seals with polyurethane sliderings

Piston seals with thermoplastic polyurethane(TPU) slide rings have a nitrile rubber (NBR)energizer. The wear-resistant slide ring has

a profiled dynamic sealing surface. Its geom-etry is optimized to reduce friction and resistgap extrusion. Notches in the slide ring edgesensure rapid pressurization of the seal to reactto abrupt changes in pressure. These profilescan usually be installed without special equip-ment and are resistant to damage duringinstallation and cylinder assembly.

MPV profiles

MPV profiles (fig. 4) have slide rings

made of X-ECOPUR PS (TPU) developed forpressures up to 400 bar (5 800 psi). They aresuitable for high fluid temperatures and inmedium to heavy duty applications, such asearthmoving equipment. MPV profiles areavailable in metric sizes and some fit sealhousings in accordance with ISO 7425-1.

Fig. 4

MPV

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DPV profiles

DPV profiles (fig. 5) have slide rings madeof X-ECOPUR PS (TPU) developed for pres-sures up to 400 bar (5 800 psi). They aresuitable for high fluid temperatures and for

medium to heavy duty applications such asearthmoving equipment. DPV profiles areavailable in inch sizes where O-ring dash-number housings are used.

LPV profiles

LPV profiles (fig. 6) have a polyether-basedpolyurethane (EU) slide ring that providesresistance to hydrolysis (attack from moisture)and good low temperature flexibility. The O-ringenergizer provides a cost-effective sealing

solution. These profiles are developed forpressures up to 250 bar (3 625 psi)and suit-able for light to medium duty applications,such as agriculture and material handli

hydraulic seals

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