Short Answer:

Hammer blow is the unbalanced vertical force caused by the rotating parts of a locomotive, such as the crank and connecting rod. When the wheel rotates, part of the reciprocating mass is balanced by adding extra weight to the wheel, but this causes an upward and downward force on the track known as hammer blow. It produces vibration and stress on both the wheel and the railway track.

In simple words, hammer blow is the dynamic vertical force created due to unbalanced rotating masses in a locomotive wheel. This effect makes the wheel strike the rail repeatedly, which can damage the track and reduce the life of mechanical components. Engineers try to minimize hammer blow by properly balancing the moving parts of the engine.

Detailed Explanation:

Hammer Blow

Hammer blow is a mechanical effect observed in reciprocating steam engines and locomotives due to unbalanced forces acting on the wheel and the track. In a locomotive, certain parts such as pistons, connecting rods, and cranks move back and forth during operation. When these moving parts are balanced, counterweights are added to the driving wheels to reduce unbalanced horizontal and vertical forces. However, while balancing the reciprocating mass, some portion of it must remain unbalanced to avoid other unwanted vibrations. This partial balancing creates a vertical unbalanced force that acts on the wheel and the track, known as the hammer blow.

In other words, hammer blow is the result of the vertical component of the centrifugal force produced by the unbalanced rotating mass on the wheel. As the wheel rotates, the unbalanced mass exerts an alternating upward and downward force. When this force acts on the track, it behaves like a hammer striking the rail at every revolution of the wheel, hence the name “hammer blow.”

Causes of Hammer Blow

The main cause of hammer blow is the partial balancing of reciprocating masses. When a portion of the reciprocating mass is balanced by adding counterweights, the rotating parts produce a centrifugal force. This centrifugal force has both vertical and horizontal components. The vertical component acts alternately upward and downward during wheel rotation, generating a periodic impact on the track. This repeated action is responsible for the hammer blow.

If the counterweight is large, the hammer blow increases. On the other hand, if the counterweight is too small, horizontal unbalanced forces increase. Therefore, engineers must choose a balance between reducing horizontal forces and keeping hammer blow within safe limits.

Effects of Hammer Blow

Hammer blow has several negative effects on the operation and life of locomotives and railway tracks. Some of the main effects are:

1. Track Damage: The repeated vertical force strikes the rail surface like a hammer, leading to wear, cracks, and even deformation of the track.
2. Wheel Damage: The wheel rim may also experience fatigue and uneven wear due to continuous impacts.
3. Vibration: The alternating vertical forces cause strong vibrations in the locomotive, which affect comfort and stability.
4. Reduced Efficiency: Excessive vibration and shock reduce the energy efficiency of the locomotive and may require frequent maintenance.
5. Safety Concerns: In extreme cases, large hammer blows may lift the wheel off the rail momentarily, causing instability or derailment.

Mathematical Expression

The magnitude of hammer blow can be represented as the vertical component of the centrifugal force produced by the balancing mass.
If:

•  = balancing mass,
•  = radius of rotation,
•  = angular speed,
  then the centrifugal force =  .
  The vertical component, which produces hammer blow, varies as the wheel rotates and can be expressed as:
  where   is the crank angle. This shows that hammer blow changes direction and magnitude as the wheel turns.

Reduction of Hammer Blow

To minimize hammer blow, several design measures are taken:

1. Proper Balancing: Only a suitable portion of the reciprocating mass is balanced to limit both horizontal and vertical unbalanced forces.
2. Lightweight Parts: Using lighter pistons and connecting rods helps reduce the reciprocating mass, which lowers the unbalanced forces.
3. Improved Design: Modern locomotives use better wheel and axle designs that distribute the balancing mass more effectively.
4. Speed Limitation: Operating within a safe speed range ensures that centrifugal forces do not increase excessively.

Practical Importance

In railway engineering, controlling hammer blow is very important for smooth and safe train operation. Tracks and wheels are costly components, and repeated hammering reduces their service life. Proper balancing of locomotives improves ride comfort, safety, and mechanical reliability. Therefore, hammer blow is always considered in the dynamic design of locomotives to maintain an efficient and durable system.

Conclusion

Hammer blow is the vertical unbalanced force caused by the partial balancing of reciprocating masses in a locomotive. It acts alternately upward and downward on the rail, producing hammer-like impacts. Although balancing helps reduce horizontal vibrations, it introduces this vertical effect, which must be controlled carefully. Reducing hammer blow ensures smoother operation, longer life of rails and wheels, and overall safety of the railway system.