The George W. Woodruff School of Mechanical Engineering ME ME 3180B - Mechanical Engineering Design - Spring 2005 Bearings No.1 Lecture Notes

The George W. Woodruff School of Mechanical Engineering ME BEARINGS Bearing Types: Rolling Element Bearings

The George W. Woodruff School of Mechanical Engineering ME BEARINGS Bearing Types: Sliding / Journal / Plain Bearings

The George W. Woodruff School of Mechanical Engineering ME 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.

The George W. Woodruff School of Mechanical Engineering ME 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.

The George W. Woodruff School of Mechanical Engineering ME 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.

The George W. Woodruff School of Mechanical Engineering ME 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, μ static ≈ μ dynamic (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

The George W. Woodruff School of Mechanical Engineering ME 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.

The George W. Woodruff School of Mechanical Engineering ME 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

The George W. Woodruff School of Mechanical Engineering ME 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.

The George W. Woodruff School of Mechanical Engineering ME 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 and

The George W. Woodruff School of Mechanical Engineering ME 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.

The George W. Woodruff School of Mechanical Engineering ME Rolling-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.

The George W. Woodruff School of Mechanical Engineering ME 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

The George W. Woodruff School of Mechanical Engineering ME Classification of Rolling-Element Bearings Figure Classification of Rolling-Element Bearings Courtesy of NTN Corporation

The George W. Woodruff School of Mechanical Engineering ME Classification of Rolling-Element Bearings (Cont’d) Figure Classification of Rolling-Element Bearings Courtesy of NTN Corporation (Cont’d)

The George W. Woodruff School of Mechanical Engineering ME Types and Characteristics of Rolling Bearings (Courtesy, NSK Corp.)

The George W. Woodruff School of Mechanical Engineering ME Types and Characteristics of Rolling Bearings (Courtesy, NSK Corp.) (Cont’d)

The George W. Woodruff School of Mechanical Engineering ME Types and Characteristics of Rolling Bearings (Courtesy, NSK Corp.) (Cont’d)

The George W. Woodruff School of Mechanical Engineering ME Types and Characteristics of Rolling Bearings (Courtesy, NSK Corp.) (Cont’d)

The George W. Woodruff School of Mechanical Engineering ME FIGURE 11-1 Nomenclature of Ball Bearing. (Courtesy of New Departure-Hyatt Division, General Motors Corporation.)

The George W. Woodruff School of Mechanical Engineering ME FIGURE 11-2 Various Types of Ball Bearings.

The George W. Woodruff School of Mechanical Engineering ME FIGURE 20.8 Types of Ball Bearings. (Courtesy, NSK Corp.) (a) Radial Bearings.

The George W. Woodruff School of Mechanical Engineering ME FIGURE 20.8 Types of Ball Bearings. (Courtesy, NSK Corp.) (Cont’d) (b) Thrust Bearings.

The George W. Woodruff School of Mechanical Engineering ME FIGURE (i, j Courtesy Thompson Industries, Inc.)

The George W. Woodruff School of Mechanical Engineering ME FIGURE (i, j Courtesy Thompson Industries, Inc.) (Cont’d)

The George W. Woodruff School of Mechanical Engineering ME FIGURE Cylindrical Roller Bearings. (Courtesy Hoover- NSK Bearing Co.)

The George W. Woodruff School of Mechanical Engineering ME FIGURE Cylindrical Roller Bearings. (Courtesy Hoover- NSK Bearing Co.) (Cont’d)

The George W. Woodruff School of Mechanical Engineering ME FIGURE Spherical Roller Bearings. (Courtesy Hoover- NSK Bearing Co.)

The George W. Woodruff School of Mechanical Engineering ME FIGURE Tapered Roller Bearings. (Courtesy Hoover- NSK Bearing Co.)

The George W. Woodruff School of Mechanical Engineering ME FIGURE Roller-Bearing Types. (Courtesy, NSK Corp.) (a) Radial Bearings.

The George W. Woodruff School of Mechanical Engineering ME FIGURE Roller-Bearing Types. (Courtesy, NSK Corp.) (Cont’d) (b) Thrust Bearings.

The George W. Woodruff School of Mechanical Engineering ME FIGURE Needle Roller Bearings. (Courtesy, The Torrington Co.)

The George W. Woodruff School of Mechanical Engineering ME FIGURE Needle Roller Bearings. (Courtesy, The Torrington Co.) (Cont’d)

The George W. Woodruff School of Mechanical Engineering ME FIGURE 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.)

The George W. Woodruff School of Mechanical Engineering ME FIGURE 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 can be achieved. (Courtesy, NSK Corp.)

The George W. Woodruff School of Mechanical Engineering ME 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.)

The George W. Woodruff School of Mechanical Engineering ME 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)

The George W. Woodruff School of Mechanical Engineering ME 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 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 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

The George W. Woodruff School of Mechanical Engineering ME FIGURE 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.)

The George W. Woodruff School of Mechanical Engineering ME FIGURE Large split bearings find use in heavy machinery. (Courtesy, FAG Bearings Corp.) (a) Bucketwheel excavator for daily stripping rate of 240,000 m 3.

The George W. Woodruff School of Mechanical Engineering ME FIGURE 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.