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Suspension System

By on May 16, 2013

The track irregularities and their interaction with wheel generate vibration of varying amplitude and frequency. These vibrations may cause damage to different parts of the vehicle if not contained and also return it back to the track.

These vibrations are contained by suspension arrangement.

Elements of suspension system are

  • Elastic Element (Helical Spring)
  • Damper,
  • Constraints
  • Bump-stops

Suspension arrangements are of two types namely

Primary Suspension

  • This is in between Axle Box and Bogie consisting of Springs and dampers
  • Primary suspension is invariably provided for all class of rolling stock including wagons
  • Vibration problems generally starts from speed above 80 kmph
  • At higher speed, vibration acceleration increases with square of the speed

WAM4 and WAG5 class of locomotive are provided with CO-CO bogie design with primary suspension only with eight springs per bogie/three wheel sets with 4 each provided with either inner spring alone and balance 4 with inner spring along with friction dampers as shown in the figure below:


Secondary Suspension

  • This is in between Bogie and Vehicle either through a  bolster or bolster-less.

Elements of Suspension System

Elastic Elements (Helical, Air, Rubber  Springs)

  • These are components which return to their original position when forces causing the affect are removed.
  • It absorbs and release impulse energy in dynamic state to limit the sudden impact of perturbation.
  • Equalise the vertical loads between wheels
  • Stabilise the motion of vehicle on track ( Self excited lateral oscillation i.e. hunting of wheel sets is dangerous)
  • Reduce the dynamic forces and accelerations due to track irregularities.
  • Capability of springs depends upon force and deflection.
    • F=-kx
    • It can be linear (uniform rate) and non-linear (rate reduces or increases  with increased deflection)
  • Types of Elastic Element
    • Helical Springs are very common, cheap and robust but little damping. They are cylindrical or conical shape. cylindrical are generally used for railway application. Springs are generally used in parallel with other spring  ( like outer and inner spring) in which the spring constant of each adds to give the final spring constant. Unequal height of inner and outer spring constant provides combination of spring constant property as shown graphically below. Paste picture below of Spring characteristics diagrams online
    • Leaf Springs were used in 4 wheeler but no more used. It also provide damping but unpredictable  through inter-leave spring. It is widely used in road transport. Advantage is stiffness of elastic and damping in a single component. Damping is provided by friction between leaves.
    • Rubber Metal Springs consists of the rubber blocks interleaved reinforced with by steel plate. It damps high frequency vibration and reduction in maintenance cost as it eliminates wearing friction. It is primarily used for primary suspension. The elastic property of rubber can be exploited to make springs that can carry significant load in both compression and shear.
    • Air Spring commonly used in passenger coaches for secondary suspension, small mass, excellent noise and vibration isolation, ability to maintain constant ride height for varying  load condition. Air springs are often provided in series with rubber interleaved spring to provide relief if by chance air spring get deflated.


  • Vehicle weight is supported on springs and dampers limit high amplitude vibration on response to any perturbation dissipating the energy.
  • Damping is provided either by friction or viscous liquid. Mechanical energy is converted into heat
  • Friction damping results due to relative slip between two rigid bodies. It depends on coefficient of friction, load, contact area and relative speed but each depending on many variable factors.  Friction force is proportional to friction coefficient, pressure between surfaces P, and contact area A and can be represented as
  • Viscous damping is achieved when a liquid flows through an orifice converting mechanical energy into heat and proportional to velocity or amplitude and frequency of vibration. Used in passenger coaches

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