Mechanical Drives: Belt and Chain Systems

Belt and Chain Drives

Objectives

• Describe the basic features of a belt drive system
• Describe several types of belt drives
• Specify suitable types and sizes of belts and sheaves
• Specify the primary installation variables for belt drives
• Describe the basic features of a chain drive system
• Describe several types of chain drives

Areas of Application of Belt Drives

• Belt drives are applied where the rotational speeds are relatively high
• Linear speed of a belt: 762-1981 m/min
• Result: relatively low tensile forces in the belt
• High speed of the electromotor makes belt drives suitable for the first stage of reduction

Advantages of Belt Drives

• Simple
• Economical
• Overload and jam protection are provided
• Noise and vibration are damped out
• Machinery life is prolonged because load fluctuations are cushioned (shock-absorbed)
• Lubrication-free
• Require only low maintenance
• Highly efficient (90–98%, usually 95%)
• Some misalignment is tolerable
• Very economical when shafts are separated by large distances

Disadvantages of Belt Drives

• At lower speeds, the tension in the belt becomes too large for typical belt cross sections, and slipping of up to 3% may occur between the sides of the belt and the pulley that carries it
• At higher speeds, dynamic effects such as centrifugal forces, belt whip, catch of air, and vibration reduce the effectiveness of the drive and its life

Types of Belt Drives

• Flat belts
• Grooved or cogged belts
• Standard V-belt
• Double-angle V-belts
• Wrapped Die cut, cog type Synchronous belt construction
• Poly-rib belt
• Vee – band
• Double angle V-belt

Classification of Belt Drives

• Based on peripheral speed
  • Light drives: transmission of small powers at belt speeds up to 12m/s (agriculture machines, small machine tools, etc)
  • Medium drives: for medium powers at 12 – 24 m/s (machine tools, cars, etc)
  • Heavy drives: for large powers and speed > 24m/s (generators, compressors, main drives)

Design of Belt Drives

• Depends on:
  • Difference in belt tension
  • Coefficient of friction
  • Area of contact
  • Center distance

Belt Section

• bt = F1/ftC1 for V < 10 m/s
• bt =F1/(ft – ρV2)C1 for V > 10 m/s
• ft – allowable stress in a belt (N/mm2)
• C1 – 0.6 – 1.0 - a corrector factor depending upon the angle of center line of drive with horizontal, type of drives

Pulley Size

• D > 50t
• D = 1.2(P/nmax)1/3
• B = 1.2b
• d = 0.005D + 3

V-Belt Drives

• Advantages:
  • Higher velocity ratio up to 7 – 10
  • Provide long life, 3 –5 years
  • Possibility of using small center distance
  • Transmit higher torsional moment at less width and tensions
• Disadvantages:
  • Cannot be used with large center distances
  • Subjected to a certain amount of creep
  • More complex design
  • Belt life above 82oC and below – 15o is shortened
  • Centrifugal force prevents the use at speed above 50m/s and at speed below 5m/s are not economical

Synchronous Belts

• Ride on sprockets having mating grooves into which the teeth on the belt seat
• Cog belts are applied to standard sheaves
• The cogs give the belt greater flexibility and higher efficiency compared with standard belts

Chain Drives

• Used to transmit rotational motion and torque from one shaft to another, smoothly, quietly, and inexpensively
• Provide the flexibility of a belt drive with the positive engagement feature of a gear drive
• Suited for applications with large distances (8m) between the respective shafts, slow speed, and high torque

Types of Chains

• Roller chain
• Multiple-strand roller chain
• Construction of bush-roller chain
• Offset sidebar roller chain
• Silent chain