MECH 3440-Notes 2 - Belt Drives (1 of 2)
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APPLICATIONS of V Belt?
- • blowers and fans, HVAC apps
- • conveyors
- • production machinery
- • mining and crushing machinery
- • air compressors
- • cars - belts, bearings, lubrication
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.
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
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)
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
or other synthetic materials.
- Sheave (pulley) materials are typically,
- - cast iron,
- - pressed steel,
- - or die-cast alloys(rpm limitations must be monitored)
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.
• A wide variety of belts are available.
• V-belts are the most widely used.
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
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
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
Typical Belt Section and Groove Geometry?
Relationship center distance C and the sheave diameters D1 and D2 ?
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)
Distance Between Two Sheaves?
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