What is transmission of power by shafts?

Short Answer:

Transmission of power by shafts means the transfer of mechanical energy from one place to another through the rotational motion of a shaft. A shaft is a rotating machine element that transmits power and torque between the source (like an engine or motor) and the machine that uses it (like a pump, wheel, or generator).

In simple words, power transmission by shafts occurs when torque is applied to a shaft, causing it to rotate and deliver power to another mechanical part. Shafts are used in machines, vehicles, turbines, and industrial equipment to transmit power efficiently through twisting or torsional motion.

Detailed Explanation :

Transmission of Power by Shafts

In mechanical systems, power transmission is the process of transferring energy from one mechanical component to another. When the transmission is achieved through rotating shafts, it is called transmission of power by shafts.

A shaft is a long, cylindrical component designed to carry torque and rotational motion. It acts as a connecting link between the driving machine (such as an engine or motor) and the driven machine (like a pump, gear, fan, or generator). The shaft transmits the mechanical power generated by the driver to the driven equipment through rotation.

Basic Principle of Power Transmission by Shafts

The principle of power transmission by shafts is based on torsion. When torque is applied to a shaft, it twists along its longitudinal axis. This twisting action transfers mechanical energy through the shaft to the driven machine.

The transmitted power depends on three main factors:

  1. The torque acting on the shaft.
  2. The angular speed (rotational speed) of the shaft.
  3. The angle of twist within the elastic limit of the material.

The relationship between torque, speed, and power can be expressed mathematically.

Power Transmission Formula

The power transmitted by a rotating shaft is given by the formula:

Where,

  •  = Power transmitted (watts or kW)
  •  = Rotational speed of the shaft (revolutions per minute, rpm)
  •  = Torque transmitted by the shaft (N·m)

Explanation:

  • The torque  causes the shaft to rotate.
  • The angular speed  (in rad/s) is given by .
  • Therefore, power .

This means that power transmitted by a shaft is directly proportional to both torque and speed. If the shaft speed or torque increases, the transmitted power also increases.

Shear Stress Due to Power Transmission

When a shaft transmits power, it is subjected to torsional stress. The torque produces shear stress on the outer layers of the shaft material.

The maximum shear stress (τ) in the shaft is given by:

Where,

  •  = Shear stress (N/m²)
  •  = Torque (N·m)
  •  = Diameter of the shaft (m)

This formula is valid for solid circular shafts. For hollow shafts, the stress distribution changes slightly, but the principle remains the same — torsion causes shear stress within the material.

The shaft diameter must be chosen carefully to ensure that the induced shear stress does not exceed the allowable limit of the material, ensuring safety and durability.

Angle of Twist

Along with shear stress, a shaft also experiences angular deformation when torque is applied. The angle of twist (θ) can be determined using the torsion equation:

Where,

  •  = Torque applied (N·m)
  •  = Polar moment of inertia of the shaft (m⁴)
  •  = Modulus of rigidity (N/m²)
  •  = Angle of twist (radians)
  •  = Length of the shaft (m)

This equation ensures that the shaft’s material and size are adequate to transmit the required torque without excessive twisting.

Types of Shafts Used for Power Transmission

  1. Transmission Shafts:
    Used to transfer power between the source and the machine components. Examples include line shafts, countershafts, and overhead shafts.
  2. Machine Shafts:
    Form part of the machine itself, such as crankshafts and camshafts in engines.
  3. Axle Shafts:
    Used in vehicles to transmit power from the gearbox to the wheels.
  4. Spindle Shafts:
    Short shafts used in machines like lathes and milling machines to hold cutting tools or workpieces.

Elements Used for Power Transmission

To effectively transmit power by shafts, several mechanical elements are attached to or supported by the shaft:

  1. Couplings: Connect two shafts to transmit torque.
  2. Keys and Keyways: Prevent relative motion between the shaft and attached components like pulleys or gears.
  3. Bearings: Support the rotating shaft and reduce friction.
  4. Pulleys, Gears, and Sprockets: Transfer power from one shaft to another through belts, chains, or gear teeth.

Each element plays a vital role in ensuring smooth, safe, and efficient power transmission.

Design Considerations for Power Transmission Shafts

When designing shafts for power transmission, the following factors must be considered:

  1. Material Strength:
    Shaft material should have high strength, rigidity, and resistance to fatigue. Common materials include carbon steel, alloy steel, and stainless steel.
  2. Torque and Power Requirements:
    The shaft must safely carry the torque corresponding to the transmitted power.
  3. Speed of Rotation:
    High-speed shafts must be balanced carefully to prevent vibration.
  4. Length and Support:
    Shaft length and the number of bearings influence bending and torsional stresses.
  5. Safety Factor:
    Adequate safety margin must be included to prevent failure under unexpected load conditions.

Applications of Power Transmission by Shafts

  1. Automobiles:
    Drive shafts transmit torque from the engine to the wheels.
  2. Industrial Machines:
    Shafts transfer power from motors to various mechanical systems such as conveyors, compressors, and turbines.
  3. Marine Systems:
    Ship propeller shafts transmit power from the engine to the propeller.
  4. Power Plants:
    Turbine shafts transmit power to generators for electricity production.
  5. Agricultural Machinery:
    Shafts connect tractors to harvesting or tilling equipment.

Advantages of Power Transmission by Shafts

  • Simple and effective method of transferring torque.
  • High efficiency with proper lubrication and alignment.
  • Capable of transmitting large amounts of power.
  • Suitable for long distances when combined with couplings or belts.
Conclusion

The transmission of power by shafts involves transferring mechanical energy through the rotational motion of a shaft under the action of torque. The power transmitted depends on the torque and speed of rotation. Shafts are fundamental components in almost every machine, enabling power flow between engines and driven devices. Proper design considering shear stress, angle of twist, and material strength ensures efficient and safe operation. Hence, understanding the principles of torsion and shaft design is essential for reliable power transmission in mechanical systems.