Short Answer:

Hammer blow is the periodic vertical force produced by the unbalanced rotating masses of a locomotive wheel when it moves at high speed. It acts on the railway track due to the centrifugal force of the rotating parts. The hammer blow increases with the speed of rotation and the amount of unbalanced mass.

In simple words, hammer blow is the vertical impact given by the wheel on the track when the rotating parts are not properly balanced. This effect causes vibration, wear of track, and discomfort during motion, hence it should be minimized by proper balancing.

Detailed Explanation :

Hammer Blow

Hammer blow is an important concept in the study of the balancing of locomotives and rotating masses. It refers to the vertical force exerted by a locomotive wheel on the rail due to the centrifugal effect of the unbalanced rotating mass. When a wheel is rotating, any unbalanced mass produces a centrifugal force acting radially outward. The vertical component of this force acts alternately upwards and downwards on the rail as the wheel rotates, creating a repeated impact known as a hammer blow.

The hammer blow appears once during each revolution of the wheel, and its magnitude varies depending on the mass unbalanced, wheel speed, and radius of rotation. Excessive hammer blow leads to dynamic instability and damage to both the wheel and the track.

1. Origin of Hammer Blow

In a locomotive, the rotating and reciprocating parts of the engine (such as crank, connecting rod, and piston) produce forces during operation. To reduce vibrations, partial balancing is done by adding a counterbalance mass to the wheel. However, when this counterbalance mass rotates, it also produces a centrifugal force.

This centrifugal force has a vertical component that changes direction as the wheel rotates. When this component acts downward, the wheel presses harder against the track, and when it acts upward, it tends to lift the wheel from the track. The alternating action of these vertical forces produces a series of impacts on the rail called hammer blow.

Hence, the hammer blow is a vertical unbalanced dynamic force transmitted to the track due to the rotating unbalanced mass.

2. Expression for Hammer Blow

Let,

•  = balance mass attached to the wheel
•  = radius of rotation of the balance mass
•  = angular velocity of the wheel

The centrifugal force due to the rotating balance mass is given by:

The vertical component of this force at any instant is:

The maximum hammer blow occurs when , i.e., when the wheel crank is at 90° or 270° position. Thus,

This maximum vertical force acts alternately on both sides of the wheel once every revolution and represents the magnitude of the hammer blow.

3. Factors Affecting Hammer Blow

The hammer blow depends on several factors, including:

• a) Unbalanced Mass (m₍b₎): Greater unbalanced mass produces a higher hammer blow.
• b) Radius of Rotation (r): A larger radius increases the centrifugal force, hence increasing hammer blow.
• c) Speed of Rotation (ω): Since centrifugal force varies with the square of the angular speed, hammer blow increases rapidly with speed.
• d) Balance Weight Design: Improper counterbalance placement or overbalancing can result in excessive hammer blow.

Therefore, while balancing locomotives, engineers must carefully determine how much rotating mass should be balanced to reduce hammer blow without creating other unbalanced forces.

4. Effects of Hammer Blow

Hammer blow has several harmful effects on both the locomotive and the railway track:

• a) Track Damage: The continuous upward and downward impacts damage the rails and sleepers.
• b) Vibration: It produces strong vibrations in the locomotive body, making operation uncomfortable.
• c) Wheel Wear: Uneven loading causes wear on the wheel surface and axle bearings.
• d) Loss of Adhesion: When the upward force is large, the wheel may lift slightly, reducing traction between wheel and rail.
• e) Maintenance Cost: Repeated hammer blows increase the maintenance cost of both engine and track.

These negative effects make it essential to control hammer blow by proper balancing techniques.

5. Methods to Reduce Hammer Blow

To minimize hammer blow in locomotives, the following measures are taken:

• Partial Balancing of Reciprocating Parts: Only a part of the reciprocating mass is balanced to avoid excessive rotating unbalance.
• Reducing Rotating Mass: Using lightweight components reduces the centrifugal force generated.
• Limiting Speed: Hammer blow increases rapidly with speed, so locomotive speed should be within safe limits.
• Improved Wheel Design: Modern wheel designs with accurate counterweight placement help reduce imbalance.
• Regular Maintenance: Proper inspection and balancing of rotating parts prevent excessive unbalanced forces.

By combining these methods, engineers can keep the hammer blow within safe limits for smooth and safe operation.

6. Importance in Locomotive Design

In locomotive design, hammer blow plays a critical role. The amount of counterbalancing used must be carefully chosen. If the counterbalance is too small, the unbalanced reciprocating forces remain large; if it is too large, the hammer blow becomes excessive. Hence, a compromise is made between horizontal balance and vertical hammer blow.

Engineers design locomotives so that the maximum hammer blow at full speed does not exceed a certain fraction of the static wheel load (usually one-twentieth of the total wheel load). This ensures that the wheel always remains in contact with the track and prevents derailment.

Conclusion

Hammer blow is the vertical dynamic force produced due to the unbalanced rotating masses of a locomotive wheel. It acts alternately upward and downward on the track, causing vibration and damage. The hammer blow increases rapidly with speed and unbalanced mass, and therefore must be controlled through careful design and balancing. By maintaining proper counterbalance, limiting speed, and using accurate wheel design, engineers ensure safe and stable locomotive operation with minimum wear and vibration.