Machine Design (Belt Drive)

MACHINE DESIGN

BELT DRIVE

Drives:  It is a machine tool which is used to transmit Power or Rotatory Motion from one shaft to another or one system to another.

There are two types Drives: 

1. Rigid drives or Non-flexible drives (such as gear)
2. Flexible drive (such as Belt, chain and rope)

Belt Drive: Belt, Chain and rope drives are called 'flexible' drives. In flexible drives, there is intermediate links such as belt, ropes or chain between the driving and driven shafts. Since this link is flexible, the drives are called 'flexible' drives.

 Advantage of Belt Drive:

• Belt drives can transmit power over considerable distance between the axes of driving and driven shafts.
• The operation of belt drive is smooth and silent.
• They can transmit only a definite load, which if exceeded, will cause the belt to slip over the pulley, thus protecting the parts of the drive against overload.
• They have the ability to absorb the shocks and damp vibration.

Disadvantage of Belt Drive:

• Belt drives have large dimensions and occupy more space.
• The velocity ratio is not constant due to belt slip.
• They impose heavy loads on shafts and bearings.
• There is considerable loss of power resulting in low efficiency. 
• Belt drives have comparatively short service life.

Difference between Flat belt and v-belt drive:

Desirable Properties Of Belt Material : 

• The belt material should have high coefficient of friction with the pulleys. 
• The belt material should have high tensile strength to withstand belt tensions.
• The belt material should have high wear resistance.
• The belt material should have high flexibility and low rigidity in bending in order to avoid bending stresses while passing over the pulley. 

Different Between Open Belt Drive And Closed Belt Drive :

Length Of Open Belt Drive/Cross Belt Drive :

• Open Belt Drive

Here,   αs  = Wrap angle for small pulley (degree) 

              αb = wrap angle for big pulley (degree) 

              D = Diameter of big pulley (degree) 

              D = Diameter of small pulley (degree) 

              C = Center distance (degree) 

Belt Drive :

• For Open Belt
  

• For Closed Belt

α = Angle of wrap for belt (radians)

P1 = Belt tension in the tight side 

P2 = Belt tension in the loose side (N) 

m = Mass of the one meter length of belt (kg/m) 

v = Belt velocity (m/s)

 f = Coefficient of friction

• Power Transmitted = (P1 - P2) Velocity
• Condition for maximum power

• The maximum permissible tension in the belt should be three times the tension due to centrifugal force (Pmax = 3Pc)
• Alternatively, the tension in the tight side of the belt should be twice the tension due to centrifugal force (P1 = 2Pc)

Reason Of Power Loss In Belt Drive:

• Power loss due to belt creep on the pulley.
• Power loss due to internal friction between the particles of the belt in alternate bending and unbending over the pulley.
• power loss due aerodynamic resistance to the motion of pulleys and belt.
• power loss due to friction in bearing of pulleys.