The George W. Woodruff School of Mechanical Engineering ME3180 1 ME 3180B - Mechanical Engineering...

1The George W. Woodruff School of Mechanical EngineeringThe 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


ME3180ME3180 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.


ME3180ME3180 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.


ME3180ME3180BEARINGS (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


ME3180ME3180 Rolling-Element Bearings

To introduce rolling-element bearings brief review of sliding bearings may be helpful.

Sliding bearings commonly involve steel shafts supported by bearing surfaces 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 completely separate surfaces. On the other hand, sliding bearings used with engine crankshafts experience hydrodynamic lubrication during normal operation; therefore oil film completely separates surfaces.

Rolling-element bearings involve separation of shaft and outer member by 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.


ME3180ME3180 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 may dampen noise from other sources.Sliding bearings are cheaper than ball or roller bearings for simple applications where minimal lubrication provision is required. Where sliding 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 than operating with small clearance). This is important in applications requiring precise positioning of rotating member.


ME3180ME3180Rolling-Element Bearings (Cont’d)

Rolling-element bearings are ideally suited for applications involving high starting loads. For example, use of roller bearings to support rail car axles 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.


ME3180ME3180 Rolling-Element Bearings (Cont’d)

• 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 with

4L

D


ME3180ME3180 Classification of Rolling-Element Bearings

Figure 10-20 Classification of Rolling-Element Bearings Courtesy of NTN Corporation


ME3180ME3180Classification of Rolling-Element Bearings (Cont’d)

Figure 10-20 Classification of Rolling-Element Bearings Courtesy of NTN Corporation (Cont’d)


ME3180ME3180Types and Characteristics of Rolling Bearings (Courtesy, NSK Corp.)


ME3180ME3180Types and Characteristics of Rolling Bearings (Courtesy, NSK Corp.) (Cont’d)


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FIGURE 11-1 Nomenclature of Ball Bearing. (Courtesy of New Departure-Hyatt Division, General Motors Corporation.)


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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 14.7. Tapered 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.)


ME3180ME3180 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.)


ME3180ME3180 Shield and Seal Bearings (Cont’d)Seals:• Having rubbing contact;• Provide greater lubricant retention and protection against

contamination;• With suitable seals, it is feasible to grease/lubricate bearing for life at

the 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)


ME3180ME3180REAL 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 shown and other heavy applications.

The George W. Woodruff School of Mechanical Engineering ME3180 1 ME 3180B - Mechanical Engineering...

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