Lubrication 2012

7/27/2019 Lubrication 2012

1/21

FLUID FILM LUBRICATION

Reproduced mainly from

Introduction to Tribology byBharat Bhushan


2/21

Gas-Lubricated Bearings

Kingsbury (1897).

The gas is generally air.

This avoids the need for a liquid lubricationsystem, simplifies the bearing design, and

reduces maintenance.

Gas bearings are also called aerodynamic orself-acting gas bearings.


3/21

Applications

Gyroscopes where precision and constanttorque are required,

Machine tool spindles,

Turbomachinery Dental drills

Food

Textile machinery and Tape and disk drives as part of magnetic

storage devices


4/21

The gas is compressible and the change in

density as a function of pressure cannot be

neglected in the solution of Reynolds

equation.

Due to low viscosity of gas, the film thickness,

pressures, and load capacities of gas bearings,

which are proportional to the fluid viscosity,

are much lower than with a liquid.


5/21

The frictional force is reduced in roughly the

sameproportion; therefore, the value of frictionforce is very low.

However, the coefficient of friction iscomparable to that of liquid bearings.

Since the energy dissipated by friction losses islow, the temperature rise is low in the gasbearings


6/21


7/21


8/21

Elastohydrodynamic Lubrication (EHL)

EHL is a subset of HD lubrication

Elastic deformation of the contacting solids plays a significant role

The film thickness in EHD lubrication is thinner (typically 0.5 -5 um)

In isolated areas, asperities may actually touch.

Therefore, in liquid lubricated systems, boundary lubricants thatprovide boundary films on the surfaces for protection against any solid

solid contact are used.

Bearings with heavily loaded contacts fail primarily by a fatiguemode that may be significantly affected by the lubricant


9/21


10/21

EHL is most readily induced in heavily loadedcontacts(such as machine elements of low

geometrical conformity), where loads act overrelatively small contact areas

EHL phenomena also occur in some low elastic-

modulus contacts of high geometricalconformity, such as

lip seals,

conventional journal and thrust

bearings with soft liners,

and head-tape interface in magnetic

recording tape drives


11/21

In EHL-the simultaneous solutions of the

Reynolds equation, the elastic deformation

equation, and the equation relating viscosity

and pressure.

Thermal and shear rate effects also become

important and need to be taken into account


12/21

In EHL, adhesive wear occurs during start-

stop operations and corrosive wear of the

bearing surfaces can also occur as a result of

interaction with the lubricant

In well-designed heavily loaded bearings,

fatigue wear is most common.


13/21


14/21


15/21

In the absence of boundary lubricants andgases (no oxide films), friction may becomevery high ( > I).

All self-acting bearing interfaces duringcontact start-stops (CSS), before a fluid film as

a result of HD or EHL is developed, operate inthe boundary lubrication regime.


16/21

The failure in boundary lubrication occurs by

adhesive and chemical (corrosive) wear

Boundary lubricants form an easily sheared

film on the bearing surfaces, thereby

minimizing adhesive wear and chemical wear

The important physical properties of the films

are the melting point, shear strength and

hardness.


17/21

Under these conditions, the lubricant viscosity isrelatively unimportant and the physical and

chemical interactions of the lubricant with thesolid bodies control friction and wear

Even a monolayer of adsorbed molecules may

provide some protection against wear.

Lubrication in some situations can be achieved bythe use of multimolecular lubricant films.

Monolayer lubrication is referred to asBOUNDARY LUBRICATION and multimolecularlubrication is referred to as MIXED LUBRICATION.


18/21

Occurs under high-load and low-speedconditions in machine components such as

bearings, gears, cam and tappet interfaces,

and piston ring and liner interfaces.

Boundary lubrication forms a last line of

defense


19/21

The boundary films are formed by physical adsorption,chemical adsorption, and chemical reaction; for typical

examples, see Fig. NEXT.

The physisorbed film can be of either monomolecular(typically < 3 nm) or polymolecular thickness.

The chemisorbed films are monomolecular

In general, the stability and durability of surface filmsdecrease in the following order:

chemical reaction films, chemisorbed films, andphysisorbed films.


20/21


21/21

Properties of the solid surface that are

desirable for good lubrication

A solid should have a strong tendency formolecules to adsorb on the surface.

Consequently, metals tend to be the easiest surfaces to

be lubricated.

The solid surfaces should have a high wetting sothat the liquid lubricant wets the solid easily.

For better lubrication, the surface should bereactive to the lubricant under test conditions sothat durable, chemically reacted films can form.

Documents

Lubrication 2012