MECH 3440-Notes 2 - Belt Drives (1 of 2)

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MECH 3440-Notes 2 - Belt Drives (1 of 2)
2012-01-31 13:27:49
Mechanical Equipment BCIT

Belt Drives
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  2. APPLICATIONS of V Belt?
    • • blowers and fans, HVAC apps
    • conveyors
    • production machinery
    • • mining and crushing machinery
    • • air compressors
    • • cars - belts, bearings, lubrication
  3. V Belt Types?

    Belts and chains...
    = Belts and chains represent major types of flexible power transmission elements

    = Belts and chains transmit power between shafts that are widely separated as compared to gear drives which are relatively close and must be precise.

    = Belt drives have an inherently adjustable center distance and need not be as precise as geardrives.
  5. Power Transmission Options?
    • 1. direct coupling
    • 2. v belts/flat belts
    • 3. synchronous belts
    • 4. chains
    • 5. line shafting/miter gears
    • 6. open gearing
    • 7. gear reduction units
  6. Belt Drive vs. Coupling?
    • • more latitude in where the motor is placed
    • • belts will slip or burn off if machine jams
    • • belts are quicker/easier to install
    • • belt drives are less efficient
    • • belt drives can be more dangerous
    • • belt drives are susceptible to ambient conditions (heat and abrasion)
  7. Grip of the belt on the sheave?
    = V belt drives are friction drives and the amount of power that a V belt can transmit is directly related to the grip of the belt on the sheave

    • = Amount of grip depends on:
    • area of contact (arc length, side areas)
    • – materials of belt and pulley
    • tension in the belt
    • – condition of the components

    = V-Belts are typically rubber but belts could be cotton, leather, nylon or other synthetic materials.
  8. SHEAVE?
    • Sheave (pulley) materials are typically,
    • - cast iron,
    • - pressed steel,
    • - or die-cast alloys(rpm limitations must be monitored)
  9. Belt drive rotational speeds?
    = Belt drive rotational speeds are relatively high and the linear speed of a belt is usually 2500 to 7000 feet per minute.

    = Belt drives at low speed (HP = T x RPM) have tension forces which are too high for typical belt cross sections.

    = At very high speeds, the dynamic effects such as centrifugal forces, belt whip, and vibration reduce the effectiveness of the drive and its life.
  10. • A wide variety of belts are available.
    • V-belts are the most widely used.
  11. Advantages of V-Belts?
    • 1. optimum power transmission mechanism for high speeds
    • 2. absorb shocks to cushion motors and bearings against load fluctuations
    • 3. vibration and noise levels are low
    • 4. maintenance and replacement are quick and easy
    • 5. lubrication free
    • 6. flexibility of drive location
    • 7. large speed ratios possible
    • 8. easy to change speed ratios
  12. Disadvantages of V-Belts?
    • 1. low speeds result in unacceptable belt tensions and belt failure
    • 2. speed ratio/centre distance restrictions exist due to requirement for large contact surface
    • 3. relatively inefficient – as low as 93%
    • 4. not good for hot environments (kilns, etc)
    • 5. OHL (overhung load) accommodation is required
  13. Basic V-Belt Geometry?
    • • The size of the sheave is indicated by its pitch diameter which is slightly smaller than the outside diameter of the sheave.

    • The speed ratio between the driving and the driven sheaves is inversely proportional to the ratio of the sheave pitch diameters

    • The linear speed of the pitch line of both sheaves is the same and equal to the belt speed, vb.
  14. Typical Belt Section and Groove Geometry?

  15. Relationship center distance C and the sheave diameters D1 and D2 ?
  16. Angle of Contact of the Belt on each Sheave?

    • These angles are important because commercially available belts are rated with an assumed contact angle of 180 deg
    • • This will only occur if the drive ratio is 1:1 (no speed change).
    • • The angle of contact on the smaller of the two sheaves will always be less than 180deg, lowering the power rating.
    • Differences from 180deg are accounted for in the Dodge Catalog by the use of the arc-length correction factor. (p.574)
  17. Distance Between Two Sheaves?