Rolling Contact Bearing

Classification of Bearing:

1. Depending upon direction of force

• Radial Bearing: It support the load which is perpendicular to the axis of the shaft.
• Thrust Bearing: It support the load which act along the axis of the shaft.

2. Depending upon type of friction

• Sliding Contact Bearing: The surface of shaft slides over the surface of bush and these two surfaces are separated by film of lubricating oil.
• Rolling Contact bearing: are also called anti-friction bearing due to its low friction characteristics. In this sliding friction is replaced by rolling friction e.g. bearing used in machine tool spindles, automobile axles gear box.

Rolling element/Antifriction bearing (REB)

• Material for REB: Hardened steel (C45)
• Rolling element can be ball or roller
• Rollers can be straight, tapered or contoured

Types of Rolling Contact Bearings:

• Self realigning ball bearing for hinged condition. It is used to compensate misalignment and can take radial and axial load both.
• Taper roller bearing for axial and radial load.
• Deep grove ball bearing for pure radial load.
• Thrust ball bearing for pure axial load.

Static Load Carrying Capacity:

It is defined as the static load which corresponds to a total permanent deformation of ball and races at the most heavily stressed point of contact equal to 0.0001 of the ball diameter.

Dynamic Load Carrying Capacity:

It is defined as the load that will lead to a life of 1 million revolutions of the inner race.

Equivalent / Actual Bearing Load

The load rating of a bearing is given for radial loads only. Therefore, if a bearing is subjected to both axial and radial load, then an equivalent radial load is estimated as,

\[ P_e = K_s(XVF_r + YF_a)\]

$F_r$ : Radial load

$F_a$ : Axial / thrust load

X : Radial load factor

Y : Thrust load factor

$K_s$ : Shock or service factor

V : Rotation factor with value 1 if inner race is rotating and 1.2 if outer race is rotating;

Note: In case of pure radial load Equivalent load will be equal to radial load and in case of pure axial load it will be equal to axial load.

Bearing Life Under Variable Load

\[\boxed{L_{10} = (\frac{C}{P_e})^n}\]

n =  3 for ball bearing 

n =10/3 for roller bearing

Miner's Rule

If the rolling element bearing is subjected to variable load P1, P2, P3, … for cycles n1, n2, n3,…

\[n_1(\frac{P_1}{C})^n + n_2(\frac{P_2}{C})^n + n_3(\frac{P_3}{C})^n + . . . . = 1\]

Nomenclature

All bearings are designated by four numbers:       P Q R S

P : Type of bearing i.e. single row deep groove or angular contact etc

Q: Indicate the series i.e. 1 for extra light, 2 for light, 3 for medium and 4 for heavy

Last two digits (RS) give the bore of the bearing which comes in multiples of 5 mm above 20 mm.

Example: 6308 number means medium series single row deep groove ball bearing of 8x5 = 40 mm bore

Needle Bearing

It has cylindrical roller having very small diameter and relatively long length. It is also called quill bearing.
\[\frac{L}{d} > 4\]

• This can be used with or without inner and outer race.
• Needle bearing suitable where limited radial space is available.
• It is also suitable for continuous rotation where load is variable or intermittent.
• It is used in oscillatory motion, piston pin bearing, rocker arm, universal joint.
• It is used to replace sleeve bearing because outer diameter is less.
• It has large load carrying capacity compared to their size.

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