lubrication & journal bearings [compatibility mode]

I/C: KALLURI VINAYAK

Introduction

• Objective of lubrication is to reduce friction,

wear and heating of machine parts which move

relative to each other.

• Lubricant is exactly that substance which does

the above when inserted between moving

surfaces.surfaces.

• Lubrication is needed everywhere, for example,

sleeve bearings, antifriction bearing, cam and

follower, gear teeth, piston in cylinder, crank

shaft and connecting rod bearings.

Introduction

• In a sleeve bearing, a shaft or journal, rotates

within a sleeve or bushing, and the relative

motion is sliding.

• Frequently used in high load, high speed or high

precision applications where ordinary ball

bearings have short life or high noise and vibration.bearings have short life or high noise and vibration.

• In applications requiring low load bearing

capacity, nylon bearings requiring no lubrication,

a powder metallurgy bearing with lubricant built-

in, a bronze bearing with ring oiling, solid

lubricant film or grease lubrication may be

satisfactory

CLASSIFICATION

• Bearings are classified in two ways.

1. Based on type of load carried a. Radial bearings

b. Thrust bearings or axial bearings

c. Radial – thrust bearings

2. Based on lubrication mechanism 2. Based on lubrication mechanism a. Hydrodynamic lubricated bearings

b. Hydrostatic lubricated bearings

c. Elastohydrodynamic lubricated bearings

d. Boundary lubricated bearings

e. Solid film lubricated bearings

Radial bearings

Thrust bearings / axial bearings / Collar bearings

Single collar thrust bearing Multiple collar thrust bearing

Radial – thrust bearings

Types of Lubrication

1. Hydrodynamic

2. Hydrostatic

3. Elastohydrodynamic

4. Boundary

5. Solid film5. Solid film

Hydrodynamic (full-film) Lubrication• Metal-to-metal contact is prevented by a thick

film of lubricant present in between the bearingsurfaces.

• The film pressure is created by the moving surfaceitself by pulling the lubricant into a wedge-shapedzone at a velocity sufficiently high to create thepressure necessary to separate the surfaces againstpressure necessary to separate the surfaces againstthe load on the bearing

• Stability can be explained by the laws of fluidmechanics.

• Lubricant is introduced into the load-bearing area

at a pressure high enough to separate the surfaces

with a relatively thick film of lubricant.

• Lubrication does not require motion of one surface

relative to another.

• Considered in designing where the velocities are

Hydrostatic Lubrication

• Considered in designing where the velocities are

small or the frictional resistance is to be an

absolute minimum.

Elastohydrodynamic Lubrication

• Lubricant is introduced between surfaces that are in

rolling contact, such as mating gears, rolling

bearings and cams etc.

• The mathematical explanation requires the

Hertzian theory of contact stress and fluid

mechanics.mechanics.

Boundary lubrication.

• Insufficient surface area, drop in velocity, lessening of

lubricant quantity, increase in bearing load, or increase

in lubricant temperature lead to a decrease in viscosity—

any one of these—may prevent the buildup of a film

thick enough for full-film/ hydrodynamic lubrication.

• Bearings operating in above situations are called boundary

lubricated bearings.

• Mixed hydrodynamic- and boundary-type lubrication

occurs first, and as the surfaces move closer together, the

boundary-type lubrication becomes predominant.

Solid-film Lubrication

• Necessary when operation is to be at extremely

high temperatures because ordinary minerals oils

degrade;

• Graphite and Molybdenum disulphide are often

used

• Composite bearing materials are being researched• Composite bearing materials are being researched

because liquid lubricants also proved to be

environmentally non-sustainable

Design Considerations

• Values either given or are under the control of

the designer are

1. The viscosity µ

2. The load per unit of projected bearing area, P

3. The speed N

4. The bearing dimensions r, c, β, and l

• The dependent variables (designer cannot control • The dependent variables (designer cannot control

these except indirectly by changing one or more

of the above group) are

1. The coefficient of friction f

2. The temperature rise T

3. The volume flow rate of oil Q

4. The minimum film thickness h0

PETROFF’S EQUATION:

�Imagine the film as composed of a

series of horizontal layers and the

force F causing these layers to

deform or slide on one another just

like a deck of cards

�Intermediate layers have velocities

that depend upon their distances y

from the stationary surface

Contd0.

Stable Lubrication

�McKee brothers explained

the difference between

boundary (unstable) and

hydrodynamic (stable)

lubrication in an actual test

of friction by reference to

Fig.

�Region to the right of line B A defines stable lubrication�Region to the right of line B A defines stable lubrication

because variations are self-correcting.

�Region to the left of line B A represents unstable

lubrication.

� Point C represents what is probably the beginning of

metal-to-metal contact as µN/P becomes smaller.

Design Constraint: 6107.1 −

×≥P

Nµ

Thick Film Lubrication

An eccentricity ratio,

c

e=ε

ε−=⇒−= 100

c

hech

Significant Angular Speed

It has been discovered that the angular speed N that is

significant to hydrodynamic film bearing performance is

fbj NNNN 2−+=

The Relations of the Variables• Albert A. Raimondi and John Boyd, of

Westinghouse Research Laboratories, used aniteration technique to solve Reynolds’hydrodynamic equation

• charts are used to define the variables for length-diameter (l/d) ratios of 1:4, 1:2, and 1 and for beta angles of 60 to 360◦.angles of 60 to 360◦.

• The charts appearing in text book are for full journal bearings (β = 360◦) only.

• For other categories, referA. A. Raimondi and John Boyd, “A Solution for the Finite Journal

Bearing and Its Application to Analysis and Design, Parts I, II, and III,” Trans. ASLE, vol. 1, no. 1, in Lubrication Science and Technology, Pergamon, New York, 1958, pp. 159–209.

Fig. 12.12Viscosity Charts: I

viscosity used in the

analysis must

correspond to T .correspond to Tav.

Viscosity Charts: II

viscosity used in the

analysis must

correspond to T .

Fig. 12.13

correspond to Tav.

The remaining charts from Raimondi and Boyd relate several bearing

design variables to the Sommerfeld number. These variables are

– Minimum film thickness

– Coefficient of friction

– Lubricant flow

– Film pressure

Raimondi and Boyd Charts:

Film–pressure distribution notation

W = bearing load (N)

N = speed (rps)

h0 = minimum film-thickness (mm)

e = eccentricity (mm)

P = film pressure (MPa)

Pmax= max fill pressure (MPa)

Φ= position of the minimum film thickness

θpo = terminating position of the lubricant film

θpmax = the position of maximum film pressure. Fig. 12.15

Film–pressure distribution notation

Chart for minimum film-thickness variable and eccentricity ratio.

Fig. 12.16

c

e=εh0 = minimum film-thickness (mm)

e = eccentricity (mm), c= radial clearance (mm)Eccentricity ratio,

Chart for the position of the minimum film thickness h0.

Fig. 12.17

Chart for coefficient-of-friction variable;

Fig. 12.18

Chart for flow variable

Fig. 12.19

Chart for determining the ratio of side flow to total flow.

Fig. 12.20

Chart for determining the maximum film pressure.

Fig. 12.21

Chart for the terminating position of the lubricant film

and the position of maximum film pressure.

Fig. 12.22

Problem:

A full journal bearing has a journal diameter of 40 mm,

with a unilateral tolerance of −0.025 mm. The bushing

bore has a diameter of 40.08 mm and a unilateral

tolerance of 0.075 mm. The bearing is 40 mm long. The

journal load is 2.2 kN and it runs at a speed of 1800

rev/min. Using an average viscosity of 25 mPa.s, find

the minimum film thickness, eccentricity, position ofthe minimum film thickness, eccentricity, position of

minimum film thickness, coefficient of friction, the

torque to overcome the friction, the power loss to

friction, total volumetric flow rate of lubricant, side flow

rate of lubricant, the maximum film pressure, and the

location of maximum and terminating pressures, for the

minimum clearance assembly.

Temperature Rise Dimensionless Variable

Fig. 12.24

Trumpler’s Design Criteria

• A throat of at least 200 µ is necessary to pass the debris

particles from the ground surface. To achieve this,

• To avoid the degrading of lubricant properties at high

temperatures, therefore

• In starting under load there is metal to metal contact,

mmdh 00004.000508.00 +≥

CT 0

max 121≤

• In starting under load there is metal to metal contact,

abrasion, and the generation of wear particles between

journal and bushing, which, over time, can change the

geometry of the bushing,

• Design load factor is used for different load applications

except in starting load calculation.

MPald

W st 068.2≤

2, ≥dnfactordesign

PROBLEM

• A full journal bearing has a shaft diameter of 80.00

mm with a unilateral tolerance of −0.01 mm. The

l/d ratio is unity. The bushing has a bore diameter

of 80.08 mm with a unilateral tolerance of 0.03

mm. The SAE 30 oil supply is in an axial-groove

sump with a steady-state temperature of 60◦C. The

radial load is 3000 N. Estimate the average film radial load is 3000 N. Estimate the average film

temperature, the minimum film thickness, the heat

loss rate, and the lubricant side-flow rate for the

minimum clearance assembly, if the journal speed

is 8 rev/s.

PROBLEM (12.11):

• A full journal bearing has a shaft diameter of 80.00

mm with a unilateral tolerance of −0.01 mm. The

l/d ratio is unity. The bushing has a bore diameter

of 80.08 mm with a unilateral tolerance of 0.03

mm. The SAE 30 oil supply is in an axial-groove

sump with a steady-state temperature of 60◦C. The

radial load is 3000 N. The rise in film temperature radial load is 3000 N. The rise in film temperature

is 100C and estimate the minimum film thickness,

the heat loss rate, and the lubricant side-flow rate

for the minimum clearance assembly, if the journal

speed is 8 rev/s.

(Refer viscosity chart II)