CH. 8: ROLLING CONTACT BEARING

Dr. Abdulazim H. Falah

Rolling Contact Bearing

Also Called “antifriction bearing” or rolling bearing. To reduce the friction effect, a rotating shaft should

be always mounted on bearings.

Rolling Contact Bearing

There are two types of bearing: Rolling contact bearing: where load is transmitted

through component in rolling without slipping. Journal bearing (sleeve bearing): in this case the shaft

(journal) is separated from its support by a this film of oil lubrication (oil or grease).

With rolling bearings, We do not design the bearing but rather we select a bearing according to our design requirement

Uses and Characteristics of Rolling Element Bearing Rolling element bearings are designed to permit relative motion between two machine parts,

usually a rotating shaft and a fixed frame, while supporting the applied loads. Its startup torque and operational friction losses are typically much lower than for plain bearings.

Advantages of rolling element bearings High reliability with minimum maintenance Minimum lubrication required Good for low speed operation Low starting friction and low power loss due to frictional drag Can readily support radial, thrust, or combined radial and thrust loads Small axial space required Nearly universal interchangeability among manufacturers Can be preloaded to eliminate internal clearance, improve fatigue life, or increase bearing

stiffness Increase in operational noise level warns of impending failure

Types of Rolling Element Bearings

Rolling element bearings may be broadly classified as either ball bearings or roller bearings. Within each of these broad categories there are a host of geometrical configurations commercially available.

Rolling element bearings are almost universally standardized by the American Bearing Manufacturers Association (ABMA), American National Standards Institute (ANSI), and the International Standards Organization (ISO).

Bearing Construction

Contact-rolling bearings usually consist s of four parts:

An inner ring An outer ring The balls The cage or separator.

Type of Bearings

Type of Bearings

Ball Bearing

Ball bearings are made in a wide variety of types and sizes:

Single-row radial ( carry mostly radial loads, but can also carry axial loads).

Angular contact bearing (Will take both axial and radial load)

Axial thrust bearing ( When load is directed entirely along the axis, thrust type of bearing should be used).

Self-aligning bearing (will take care of large amount of misalignment).

An increase in radial capacity may be secured by using rings with deep grooves, or by employing a double-row radial bearing.

Various types of ball bearings

ROLLER BEARINGS

Types Roller Bearings

Roller bearing are usually used when shock and impact are present, or when large bearing are needed.

Tapered roller bearing can carry a large axial load. The magnitude depends on the angularity of the rollers. The radial load will also produce a thrust component.

Roller bearing in general can be applied only where the angular misalignment caused by shaft deflection is very small

Spherical roller bearing has excellent load capacity and carry a thrust component in either direction. high misalignment

Types of roller Bearings

Ball vs. Roller Bearings

Roller bearings are stiffer and have a higher load capacity that comparably sized ball bearings. This is due to the type of contact, line contact for rollers vs. point contact for balls.

Ball bearings have a lower friction. This also is a function of contact type.

Ball bearings can often be operated at higher speeds.

Most ball bearings can take modest axial load for “free”. Only tapered rollers can take axial loads.

Ball bearings are less expensive than roller bearings.

Bearing Life

When a bearing is in operation, contact stresses occurs on the inner ring, rolling elements and outer rings.

If the bearing is clean, lubricated, sealed against dust and operates at reasonable temperature, then metal fatigue will be the only cause of failure.

Bearing Life: is measure of the “number of revolutions of the inner ring (outer ring is fixed)” or “ number of hours of use (at standard speed) until first evidence of fatigue.

Bearing Life

According to ABMA, “ Rating Life”, or “minimum Life”, or “ L10”, or “B10” or “ C10” life is the number of revolution (or hours at fixed speed) that 90% of a group of bearings will achieve or exceed before failure criterion develops.

L= life, B = Bearing, and C = Catalogue. Each term represents the period of time after which 10 % of a group of bearings fail.

Bearing Dimensions

FIGURE 14.11Shaft and housing shoulder dimensions.

Bearing Dimensions

•Bearing manufacturers’ catalogues identify bearings by number, give complete dimensional information, list rated load capacities, and furnish details concerningmounting, lubrication, and operation•The letter L stands for Light bearing.•L08 means the bearing bore is 5 times the last two digits in symbol i.e. 08 x 5 = 40 mm.

Bearing Dimensions

Bearing Rated Load Capacities

Bearing Rated Load Capacities

Life Requirement

Required bearing life could differ from that given in a catalogue

It was found that ball bearing life varies inversely with the third power of the load.

For rolling element bearing, the exponent is between 3 and 4. (usually 3.5)

Life Requirement

Reliability Requirement

Median life or average life “L50” (50% failure and 50% survival) is 5 times L10 (90% survival)

Life adjustment reliabilities greater than 90% The rated bearing life for any given reliability

(greater than 90 percent) is thus the product, KrLR. Incorporating this factor into Eq. 14.1 gives

Reliability Requirement

FIGURE 14.13Reliability factor Kr .

Influence of Axial Loading

If the thrust load Ft isconsidered, then pureradial equivalent load Feis used instead of radialload. For angular contactball bearings, contactangle, , is either 15,25, or 35o

Shock Loading (Application Factor Ka)

Summary

Useful Table

Example

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