ME 3180B - Mechanical Engineering Design - Spring 2005

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ME 3180B - Mechanical Engineering Design - Spring
2005

• Bearings No.1
• Lecture 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,
  childrens tricycles.

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BEARINGS (Contd)

• 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 Contd

• 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 (Contd)

• Comparison of Rolling and Sliding Bearings
• Rolling element bearings have a number of
  advantages over sliding contact bearings and vice
  versa. Hamrock26 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 Contd

• Comparison of Rolling and Sliding Bearings Contd
• 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 (Contd)

• Comparison of Rolling and Sliding Bearings
• Following are disadvantages of rolling bearings
  compared to hydrodynamic conformal sliding
  bearings26
• 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 bearing surfaces of
  wear-compatible materials such as bronze or
  tetraflouroethylene (TFE), as duPonts Teflon.
• Good for low speed applications (lawn mower
  wheels, garden carts, childrens 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.

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Rolling-Element Bearings (Contd)

• 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.

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Rolling-Element Bearings (Contd)

• 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.

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Rolling-Element Bearings (Contd)

• 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

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Classification of Rolling-Element Bearings
Figure 10-20 Classification of Rolling-Element
Bearings Courtesy of NTN Corporation
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Classification of Rolling-Element Bearings
(Contd)
Figure 10-20 Classification of Rolling-Element
Bearings Courtesy of NTN Corporation (Contd)
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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.) (Contd)
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Types and Characteristics of Rolling Bearings
(Courtesy, NSK Corp.) (Contd)
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Types and Characteristics of Rolling Bearings
(Courtesy, NSK Corp.) (Contd)
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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.) (Contd)(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.) (Contd)
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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.) (Contd)
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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.) (Contd)(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.) (Contd)
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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. (Contd)
• (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.)

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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.)
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Shield and Seal Bearings (Contd)

• 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.)
(Contd)
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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.) (Contd)
• (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.