MACHINE Bearing Note 1

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The George W. Woodruff School of Mechanical Engineering

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ME 3180B - Mechanical Engineering Design - Spring 2005

Bearings No.1Lecture Notes




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BEARINGS

Bearing Types:

• Rolling Element Bearings




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BEARINGS

Bearing Types:

•Sliding / Journal / Plain Bearings




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Sliding / Journal / Plain Bearings

Involves two moving parts, usually steel, cast iron having a sliding

contact with bronze, babbitt, or nonmetallic polymer (nylon, teflon).

Applications for Journal Bearings

• Transmission shafts, links, pins, engine cranshafts and connecting-rod of

automobile, engines, lawn mower, wheels, garden carts, children’s tricycles.




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BEARINGS (Cont’d)

Lubrication

All bearing types have to be lubricated or greased occasionally.

• Why do we lubricate or grease bearings?

– To maintain separation of surfaces.

– To produce low resistance to motion, i.e., low coefficient of friction.

– Remove heat.




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Bearings Cont’d

– Notes:

Babbitt metal is “any of white alloys composed primarily

of tin or lead and lesser amounts of antimony, copper,

and other metals, and used for bearings.”

Materials with inherently low coefficients of friction,

such as nylon and teflon, are often used in light-load

applications as bearing materials, with no other

lubrication.




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BEARINGS (Cont’d)

Comparison of Rolling and Sliding Bearings

• Rolling element bearings have a number of advantages over sliding

contact bearings and vice versa. Hamrock[26] lists following

advantages of rolling over sliding bearings:

– Low starting and good operating friction, μstatic≈ μdynamic (Starting

torque is considerably lower. It is, in fact, not much greater than

running torque.)

– Can support combined radial and thrust (axial) loads

– Less sensitive to interruptions of lubrication – No self-excited instabilities




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Bearings Cont’d

Comparison of Rolling and Sliding Bearings Cont’d

– Good low-temperature starting

– Can seal lubricant within bearing and “lifetime-lubricate”

– Typically require less space in axial direction

– Bearings are internationally standardized making for more

economical designs and easier torque.

– They can be arranged so as to position very precisely parts they

connect, both radially and axially.




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BEARINGS (Cont’d)

Comparison of Rolling and Sliding Bearings

• Following are disadvantages of rolling bearings compared to

hydrodynamic conformal sliding bearings[26]:

– Rolling bearings may eventually fail from fatigue

– Require more space in radial direction

– Poor damping ability

– Higher noise level

– More severe alignment requirements

– Higher cost

– Higher friction




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Rolling-Element Bearings

To introduce rolling-element bearings brief review of sliding bearings may

be helpful.Sliding bearings commonly involve steel shafts supported by bearingsurfaces of wear-compatible materials such as bronze or tetraflouroethylene (TFE), as duPont’s Teflon.

Good for low speed applications (lawn mower wheels, garden carts,

children’s tricycles, etc.). Sliding bearings involve use of oil but lubricant does not completelyseparate surfaces.

On the other hand, sliding bearings used with engine crankshafts experience hydrodynamic lubrication during normal operation; thereforeoil film completely separates surfaces.

Rolling-element bearings involve separation of shaft and outer memberby balls or rollers, and ensuing friction is called rolling friction.




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Rolling Element Barings

A major advantage of rolling-element bearings is low starting friction.

Sliding bearings can achieve comparable low friction only with full-film

lubrication (complete surface separation). This requires hydrostatic

lubrication with costly auxiliary external supply system, or

hydrodynamic lubrication, which cannot be achieved during starting.

Rolling-element bearings are known as “anti-friction” bearings. This term

is perhaps unfortunate because these bearings do not in all cases provide

lower friction than fluid-film bearings. With normal operating loads,

rolling-element bearings (without seals) typically provide coefficients of friction between 0.001 and 0.002.




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Rolling-Element Bearings (Cont’d)

Rolling-element bearings take up more radial space around shaft, but plain bearings usually require greater axial space.

Rolling-element bearings generate and transmit a certain amount of noise,whereas fluid-film bearings do not normally generate noise and maydampen noise from other sources.

Sliding bearings are cheaper than ball or roller bearings for simple

applications where minimal lubrication provision is required. Wheresliding bearings require forced lubrication system, overall cost of rolling-element bearings may be lower.

Another advantage of ball and roller bearings is that they can be“preloaded” (mating bearing elements are pressed together rather thanoperating with small clearance). This is important in applications requiring

precise positioning of rotating member.




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Rolling-element bearings are ideally suited for applications involving

high starting loads. For example, use of roller bearings to support rail caraxles eliminates need for extra locomotive to get long train started.

On the other hand, fluid film bearings (slide bearings) are well suited for

high rotating speeds with impact and momentary overloads. Higher

the rotating speed, more effective the hydrodynamic pumping action.Also, fluid film effectively “cushions” impact, as duration involved is too

short for film to be squeezed out.

High rotating speeds are generally disadvantageous to rolling-element

bearings because of rapid accumulation of fatigue cycles and high

centrifugal force on rolling elements.




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• Radial (for carrying radial loads)

• Thrust, or axial-contact (for carrying axial loads)

• Angular-contact (for carrying combined radial and axial loads) Note: Needle Roller Bearings is special type of cylindrical roller bearing

with4

L

D




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Classification of Rolling-Element Bearings

Figure 10-20 Classification of Rolling-Element BearingsCourtesy of NTN Corporation




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Types and Characteristics of Rolling Bearings (Courtesy, NSK Corp.)




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Types and Characteristics of Rolling Bearings (Courtesy, NSK Corp.) (Cont’d)




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Types and Characteristics of Rolling Bearings (Courtesy, NSK Corp.) (Cont’d)




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FIGURE 11-2 Various Types of Ball Bearings.




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FIGURE 20.8 Types of Ball Bearings. (Courtesy, NSK Corp.)

(a) Radial Bearings.




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FIGURE 20.8 Types of Ball Bearings. (Courtesy, NSK Corp.) (Cont’d)

(b) Thrust Bearings.




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FIGURE 14.10 (i, j Courtesy Thompson Industries, Inc.)




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FIGURE 14.10 (i, j Courtesy Thompson Industries, Inc.) (Cont’d)




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FIGURE 14.5. Cylindrical Roller Bearings. (Courtesy Hoover-

NSK Bearing Co.)




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FIGURE 14.5. Cylindrical Roller Bearings. (Courtesy Hoover-

NSK Bearing Co.) (Cont’d)




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FIGURE 14.6. Spherical Roller Bearings. (Courtesy Hoover-

NSK Bearing Co.)




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FIGURE 20.11 Roller-Bearing Types. (Courtesy, NSK Corp.)

(a) Radial Bearings.




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FIGURE 20.11 Roller-Bearing Types. (Courtesy, NSK Corp.) (Cont’d)

(b) Thrust Bearings.




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FIGURE 14.9. Needle Roller Bearings. (Courtesy, The Torrington Co.)




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FIGURE 14.9. Needle Roller Bearings. (Courtesy, The Torrington Co.)

(Cont’d)




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FIGURE 20.10 Special Adaptations of Ball Bearings.(a) Automotive fan and water-pump shaft bearing. Here inner race is shaft itself. (Courtesy, New Departure Hyatt, General Motors.)




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FIGURE 20.10 Special Adaptations of Ball Bearings. (Cont’d)

(b) Cutaway view of ball screw. Endless row of balls separates nut from

screw and allows rotary motion to be converted to linear movement

with minimum of friction. This scheme is used in many vehicle

steering mechanisms. (Courtesy, Thompson Saginaw Ball Screw Co.)(c) Linear ball-bearing guide. These units also utilize endless rows of

balls. Coefficients of friction as low as 0.002 can be achieved.

(Courtesy, NSK Corp.)

Shi ld d S l B i




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Shield and Seal Bearings

Self lubricating bearings must have seals or shields to keep oil or grease in,and protection against contamination.

Shields

• Close-fitting but nonrubbing thin washer;• Protect bearing against all but very small foreign particles and help retain

lubrication.

FIGURE 14.4

Bearings with shields.

(Courtesy New Departure-Hyatt

Division, General

Motors Corp.)

Shield and Seal Bearings (Cont’d)




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Shield and Seal Bearings (Cont d) Seals:

• Having rubbing contact;

• Provide greater lubricant retention and protection againstcontamination;

• With suitable seals, it is feasible to grease/lubricate bearing for life atthe time of assembly;

• Disadvantage:

– Introduce frictional drag and subject to wear.

FIGURE 14.4

Bearings with seals.

(Courtesy New Departure-Hyatt

Division, General

Motors Corp.) (Cont’d)




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REAL WORLD APPLICATIONS

Tapered roller bearings are intended to take considerable thrust loads and are

most often used in pairs. A common example is front-wheel bearings of rear-

drive vehicles, as shown in Fig. 20.12.

Front-wheel-drive cars generally have enclosed double-row ball bearings.

No bearing has infinite life. In many applications down time associated with

changing bearings can be very costly, and design must take into account this

important economic factor.

Imagine, for example, cost of having excavator shown in Fig. 20.13 idle, and labor

involved in disassembling major parts for bearing change.

Bearings on bucket shaft and its drive are split double-row spherical roller

bearings, such as shown in Fig. 20.13.




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FIGURE 20.12

Section through front-wheel hub and suspension showing

wheel bearings. Since road reaction (for straight-ahead driving) is

vertical through center of tire, in-board bearing takes nearly all of it

and is therefore larger. (Courtesy, Jaguar Cars, Ltd.)




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FIGURE 20.13

Large split bearings find use in heavy machinery. (Courtesy,

FAG Bearings Corp.)

(a) Bucketwheel excavator for daily stripping rate of 240,000 m3.

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FIGURE 20.13

Large split bearings find use in heavy machinery. (Courtesy, FAG

Bearings Corp.) (Cont’d)

(b) All components of this bearing are in two parts. Note bolt for

joining roller-cage parts. Bearings like this are used in excavator

MACHINE Bearing Note 1

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