chapter 13 lubrication and sliding bearingsturbolab.hanyang.ac.kr/2021_04_29_teaching...

April. 29. 2021

[Chanwoo Lee]

Turbomachinery Laboratory

Hanyang University, Korea

CHAPTER 13

Lubrication and Sliding

Bearings

Sliding Friction on Wet and Dry Sand

Sliding friction over and between sand

layers is relevant for many problems

ranging from civil engineering to

earthquake dynamics.

In many practical situations, small

amounts of water may be present.

Ancient Egyptian tomb drawings

suggest that wetting the sand with

water may influence the friction

between a sled and the sand (Fig. 1).

13.1 Types of Lubricants

The word bearing, applied to a machine or structure, refers to

contacting surfaces through which a load is transmitted.

When relative motion occurs between the surfaces, it is usually

desirable to minimize friction and wear.

Any interposed substance that reduces friction and wear is a lubricant.

Lubricants are usually liquid but can be a solid, such as graphite, TFE,

or molybdenum disulfide, or a gas, such as pressurized air.

13.1 Types of Lubricants – Viscosity (13.5)

The viscosity of a fluid is a measure of its resistance to gradual

deformation by shear stress or tensile stress.

Viscosity versus Temperature

Liquid lubricants that are oils are characterized by their viscosity, butother properties are also important.

Modern oils usually contain one or more additives designed to causethe oil to flow at lower temperatures—the pour-point depressants;have less variation of viscosity with temperature

Greases are liquid lubricants that have been thickened in order toprovide properties not available in the liquid lubricant alone.

Greases are usually used where the lubricant is required to stay inposition, particularly when frequent lubrication is difficult or costly.

Engine oil Bearing grease

https://kixxman.com/engine-oil-viscosity-1

https://www.bearingtips.com/bearing-lubrication-methods/

13.1 Multi-grade Oils

The temperature range the oil is exposed to in most vehicles can be wide,ranging from cold temperatures in the winter before the vehicle is startedup, to hot operating temperatures when the vehicle is fully warmed up inhot summer weather.

A specific oil will have high viscosity when cold and a lower viscosity at theengine's operating temperature. The difference in viscosities for mostsingle-grade oil is too large between the extremes of temperature.

https://en.wikipedia.org/wiki/Motor_oil#Multi-grade_motor_oil

13.1 Multi-grade Oils

To bring the difference in viscosities closer together, special polymer additivescalled viscosity index improvers, or VIIs are added to the oil.

These additives are used to make the oil a multi-grade motor oil, though it ispossible to have a multi-grade oil without the use of VIIs.

The idea is to cause the multi-grade oil to have the viscosity of the base gradewhen cold and the viscosity of the second grade when hot. This enables one typeof oil to be used all year.

In fact, when multi-grades were initially developed, they were frequentlydescribed as all-season oil.

13.2 Types Sliding Bearings

Sliding bearings require direct sliding of the load-carrying member on

its support, as distinguished from rolling-element bearings (Chapter

14), where balls or rollers are interposed between the sliding surfaces.

Sliding bearings (also called plain bearings) are of two types:

1) Journal or sleeve bearings, which are cylindrical and support radial

loads (those perpendicular to the shaft axis)

2) Thrust bearings, which are generally flat and, in the case of a

rotating shaft, support loads in the direction of the shaft axis.

Journal bearing Thrust bearing

https://www.gtw.cz/en/tilting-pad-journal-bearings/

https://www.machinerylubrication.com/Read/587/thrust-bearings

13.3 Types of Lubrication

13.3 Hydrodynamic lubrication

In hydrodynamic lubrication the surfaces are completely separated by

the lubricant film.

The load tending to bring the surfaces together is supported entirely

by fluid pressure generated by relative motion of the surfaces (as

journal rotation).

Surface wear does not occur, and friction losses originate only within

the lubricant film.

Typical values of coefficient of friction (f) are 0.002 to 0.010.

13.3 Mixed-film lubrication

In mixed-film lubrication the surface peaks are intermittently in

contact, and there is partial hydrodynamic support.

With proper design, surface wear can be mild.

Coefficients of friction commonly range from 0.004 to 0.10.

13.3 Boundary lubrication

In boundary lubrication surface contact is continuous and extensive,

but the lubricant is continuously “smeared” over the surfaces and

provides a continuously renewed adsorbed surface film that reduces

friction and wear.

Typical values of f are 0.05 to 0.20.

Complete surface separation (as in Figure 13.2a) can also beachieved by hydrostatic lubrication. A highly pressurized fluid such asair, oil, or water is introduced into the load-bearing area.

Since the fluid is pressurized by external means, full surfaceseparation can be obtained whether or not there is relative motionbetween the surfaces.

The principal advantage is extremely low friction at all times, includingduring starting and low-speed operation.

Disadvantages are the cost, complication, and bulk of the externalsource of fluid pressurization. Hydrostatic lubrication is used only forspecialized applications.

13.3 Hydrostatic lubrication

13.4 Basic Concepts of Hydrodynamic Lubrication

If the shaft rotating speed is progressively increased, more and more oil adhering

to the journal surface tries to come into the contact zone until finally enough

pressure is built up just ahead of the contact zone to “float” the shaft, as shown in

Figure 13.3c.

When this happens, the high pressure of the converging oil flow to the right of the

minimum film thickness position (h0) moves the shaft slightly to the left of center.

Under suitable conditions, equilibrium is established with full separation of the

journal and bearing surfaces.

This constitutes hydrodynamic lubrication, also known as full-film or thick-film

lubrication. The equilibrium eccentricity of the journal in the bearing is dimension

e, shown in Figure 13.3c.

Hydrodynamic Pressure

https://dyrobes.com/help1800/BePerf/html/bepe7zn7.htm

Attitude angle: In a bearing, the angle betweenthe resultant of the radial loads acting on a rotorand a line connecting the bearing and shaftcenters, measured in the direction of rotation.

Eccentricity: The radial displacement of the rotorjournal centerline from the geometric center ofa fluid-film bearing

Eccentricity ratio: A dimensionless quantityrepresenting the average position of the shaftwithin the bearing compared to the availableclearance.

Bearings enable smooth (low friction) motion between solid surfaces in relative motion

and, if well designed, support static and dynamic loads. Bearing affect the dynamic

performance of machinery

Full film lubricationGas bearings operate at the

three regimes of lubrication +

Dry friction at start up shut

Note that the achievement of hydrodynamic lubrication requires three things.

1. Relative motion of the surfaces to be separated.

2. “Wedging action,” as provided by the shaft eccentricity.

3. The presence of a suitable fluid.

13.5 Viscosity

Analogy between viscosity m of

a fluid (also called dynamic

viscosity and absolute viscosity)

and shear modulus of elasticity

G of a solid.

Figure 13.5a shows a rubber bushing

bonded between a fixed shaft and an outer

housing.

Application of torque T to the housing

subjects an element of the rubber bushing

to a fixed displacement, as shown in

Figure 13.5b.

If the material between the housing and

concentric shaft is a Newtonian fluid (as

are most lubricating oils), equilibrium of an

element involves a fixed velocity, as

shown in Figure 13.5c.

13.5 Viscosity

13.5 Kinematic Viscosity

13.5 Absolute viscosity

13.5 SAE Grades

The Society of Automotive Engineers classifies oils according to viscosity.

Viscosity–temperature curves for typical SAE numbered oils are given in Figure 13.6.

A particular oil may deviate significantly from these curves, for the SAE

specifications define a continuous series of viscosity bands. For example, an SAE 30

oil may be only a trifle more viscous than the “thickest” SAE 20, or only a trifle less

viscous than the “thinnest” SAE 40 oil.

Furthermore, each viscosity band is specified at only one temperature. SAE 20, 30,

40, and 50 are specified at 100°C (212°F) whereas SAE 5W, 10W, and 20W are

specified at -18°C (0°F).

Multigrade oils must conform to a viscosity specification at both temperatures.

For example, an SAE 10W-40 oil must satisfy the 10W viscosity requirement at -

18°C and the SAE 40 requirement at 100°C.

13.5 Viscosity and SAE Number

Thank you!

chapter 13 lubrication and sliding bearingsturbolab.hanyang.ac.kr/2021_04_29_teaching...

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