Machine Design : Couplings, Cotters, Knuckle & Belt Drives

By Shivendra Pratap|Updated : February 7th, 2022

Power transmission by belts:

  • Belt drive is combination of two pulleys on each shaft with an endless belt wrapped around them having some initial tension.
  • Power is transmitted from the driver pulley to the belt and from the belt to the driven pulley with the help of friction.
    • Friction between belt and pulley surface limits the maximum power that can be transmitted. Once limiting value is exceeded, slipping of belt occurs. 
    • Rectangular cross-section belts are known as flat belts while V-belts have trapezoidal cross-section.
    • Flat belts are used to transmit the moderate amount of power between shafts less than 6m apart.
    • V-belts are used along with pulleys having a similar cross-section as that of the belt. To further increase the power transmission capacity, multiple V-belt system (having more than one V belt & grooves in the system) is used


  • (i). There are two types of drives—rigid and flexible.

    (ii). Gear drives are called rigid or non-flexible drives where there is direct contact between the driving and driven shafts through the gears.

    While in flexible drives, there is an intermediate link such as belt, rope or chain between the driving and driven shafts. Due to link being flexible, the drives are termed as the ‘flexible’ drives. Thus, Belt drives are the flexible drives.

    Advantages and disadvantages of Belt drives:

    Advantages: Belt drives offer the following advantages compared with other types of drives:

    (i). They can be used for power transmission between the axes of driving and driven shafts having considerable distances between them. 

    (ii). The belt drive operation is smooth and silent.

    (iii). Simple design and low initial cost.


    (i).  Have large dimensions and thus occupying space.

    (ii). Due to belt slip, the velocity ratio is not constant.

    (iii). They impose heavy loads on shafts and bearings.


    (i). Flat belts

    (ii). V-belts


    (i). Open belt drive (O.B.D.):

    OBD ⇒ Direction of rotation are same  ⇒ like internal gear                           

    (ii). Cross belt drive (C.B.D.):

    CBD ⇒ Direction of rotation are opposite = like external gear

    (iii). Compound belt drive:

    Compound ⇒ To obtain higher speed reduction (compound gear train).

    Open belt drive (O.B.D.):

    Let D1 = Diameter of driven pulley

    D2 = Diameter of driven pulley or follower pulley

    C = Centre distance between two parallel shafts

    V1 = Linear velocity of driven pulley

    V = Linear velocity of belt

    V2 = Linear velocity of driven pulley,




    ⇒ This drive is not suitable for smaller center distance it should be medium.

     Due to centrifugal force:             

    Total tension in tight side = T1 + TC

    Total tension in slack side = T2 + TC

    T1 + TC ≤ Tmax


    Cross belt drive:



    Length of the belt:


    Comparison between open belt drive & cross belt drive:



    If slip and belt thickness is taking into consideration.



    Initial tension (To):

    • Initial tension is the tension develop in the belt when it is in the stationary cond.
    • It is provided in the belt by taking a length of the belt less than actual required length.
    • In presence of initial tension, power transmission capacity of belt drive increases hence it is useful.

    Centrifugal Tension (Tc):

    Centrifugal tension is the addition tension develop in the belt in presence of centrifugal force acting on the belt.

    Tmax = T1 + Tc




    (ii). T1 = 2Tc




    V-belt are used to transmit power between two parallel shaft which are at smaller centre distance and rotating in same direction.

     Cross section = Trapezoidal cross section


    Ration of belt tensions: 



    Practical application:

    (i). air compressor     

    (ii). Automobile radiator fans

    (iii). M/c tools like shaper, milling M/c etc.


    Effect of creep:

    (i). Creep results in the less angular velocity of the driven pulley than that calculated by considering the ratio of diameters of pulleys.

    (ii). The efficiency reduction by 1 to 2% due to creep.

    Knuckle joint: 

    • Knuckle joint is used to connected two rods whose axes either coincide or intersect and lie in one plane.
    • The knuckle joint is used to transmit axial tensile force. The constructions of this joint permits limited angular movement between rods, about the axis of the pin.

    Applications of knuckle joints are as follows:

    • Joints in value mechanism of a reciprocating engine.
    • Fulcrum for the levers.
    • Joints between the links of a bicycle chain.

    Advantages of Knuckle joints:

    • The joint is simple to design and manufacture.
    • Few parts in the knuckle joint reduces cost and improves reliability.
    • The assembly or dismantling of the parts of a knuckle joint is quick and simple.

    Cotter Joint:

    • It is used to connect two co-axial rods subjected to either axial tensile' or axial compressive force.
    • It is not suitable to connect rotating shafts and transmitting torque.

    Applications of cotter joint are as follows: Its applications include:

    (i). Piston rod and crosshead joint in a steam engine.

    (ii). the piston rod and the tail or pump rod joint.



    These are mechanical components used for connecting two rotating shafts and transmitting torque from one shaft to the other shaft.

    Example: connection between the  electric motor output shaft  and input shaft of a hydraulic pump.

    Types of Shafts Couplings

    Rigid Couplings:

       Perfectly aligned shafts are connected using the couplings. Couplings are simple and inexpensive.

      Rigid Couplings are of following types:

    • Sleeve or Muff Coupling
    • Clamp or Split-muff or Compression Coupling
    • Flange Coupling

    Flexible Couplings

    • Flexible couplings are used to connect two shafts having lateral or angular misalignment. Flexible elements provided in flexible coupling absorb shocks and vibrations.

    Flexible Couplings are of the following types:

    • Bushed pin type Coupling 
    • Oldham Coupling
    • Universal Coupling

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