Short Answer:

Torsional vibrations are the twisting oscillations that occur in rotating shafts or components when they experience torque variations. These vibrations happen when different parts of a rotating system twist back and forth around its axis due to uneven loading or periodic torque changes. Torsional vibration is commonly found in engines, turbines, and rotating machinery.

When the applied torque changes continuously, the shaft alternately twists and untwists. This causes cyclic stresses and can lead to mechanical failures, fatigue, or noise. Therefore, controlling torsional vibration is very important for the smooth and safe operation of rotating systems.

Detailed Explanation :

Torsional Vibrations

Torsional vibration is a type of mechanical vibration in which the rotating shaft or any circular component experiences twisting motion about its own longitudinal axis. It occurs due to the alternation of torque acting on the shaft during its operation. Unlike linear vibrations that involve up and down or side-to-side motion, torsional vibrations involve angular displacement of the shaft sections.

When torque is applied to a rotating shaft, it twists slightly depending on the stiffness of the material. If this torque changes periodically—such as in engines or compressors—the shaft experiences a series of twisting and untwisting actions. This repetitive twisting creates oscillations that are called torsional vibrations.

The shaft stores strain energy during twisting and releases it during untwisting, much like a spring. If the frequency of these torsional oscillations matches the natural frequency of the shaft, resonance occurs, leading to large amplitude vibrations. This can cause serious damage such as fatigue cracks, excessive noise, and even failure of the shaft or coupling.

Causes of Torsional Vibrations

1. Fluctuating Torque:
   In machines like internal combustion engines, turbines, and compressors, the torque is not constant. Each power stroke produces a variation in torque, leading to twisting motion in the shaft.
2. Sudden Load Changes:
   When the load on a rotating system changes rapidly, it creates a mismatch between input and output torque, resulting in torsional oscillation.
3. Elasticity of Shaft Material:
   The shaft behaves like a torsional spring. Due to its elastic nature, when torque is applied, it twists and returns, creating oscillations if the torque fluctuates.
4. Misalignment or Unbalanced Forces:
   If components like couplings or gears are not properly aligned, they create alternating torque forces that excite torsional vibrations.
5. Resonance Condition:
   When the operating frequency matches the shaft’s natural frequency, the amplitude of torsional vibration increases drastically, leading to resonance failure.

Effects of Torsional Vibrations

• Fatigue Failure: Repeated twisting can produce cyclic stress, which leads to fatigue cracks in the shaft.
• Noise and Vibration: Torsional vibrations can cause unpleasant noise and vibration throughout the machinery.
• Damage to Couplings and Bearings: The oscillating torque may exceed the design limits of couplings and bearings, causing their early wear or failure.
• Loss of Power Transmission Efficiency: Vibrations may reduce the smooth transfer of torque, decreasing overall system efficiency.

Control and Prevention of Torsional Vibrations

1. Torsional Dampers:
   These devices absorb excess vibrational energy and reduce oscillation amplitude. They are commonly used in engine crankshafts.
2. Flexible Couplings:
   Flexible couplings allow slight angular movements between connected shafts, reducing transmitted vibrations.
3. Proper Balancing and Alignment:
   Maintaining proper balance and alignment minimizes unbalanced torque and prevents excitation of torsional modes.
4. Avoiding Resonance:
   The system should be designed so that the natural frequency of the shaft does not match any operating frequency.
5. Increasing Shaft Stiffness:
   Using materials with higher torsional rigidity reduces twist and limits vibration amplitude.

Applications where Torsional Vibrations Occur

• Engines and Crankshafts: Periodic torque from combustion causes twisting motion.
• Propeller Shafts in Ships: The changing load from water resistance induces torsional vibration.
• Turbines and Generators: Variable load and torque cause shaft oscillations.
• Gear Trains: Irregular loading between gears leads to torsional excitation.

Example

Consider a crankshaft in a multi-cylinder engine. Each cylinder produces power at a different instant, creating uneven torque. This torque fluctuation makes the crankshaft twist back and forth around its axis. If the twisting frequency equals the shaft’s natural frequency, the amplitude becomes very high, which can lead to crankshaft breakage.

Hence, engineers use torsional dampers and carefully design crankshafts to control and reduce these vibrations.

Conclusion:

Torsional vibrations are twisting oscillations of rotating shafts caused by variable torque. They can lead to severe mechanical damage, noise, and loss of efficiency if not controlled. Understanding the causes, effects, and control methods helps in designing safe and durable machinery. Using dampers, flexible couplings, and proper balancing are common practices to minimize torsional vibration in mechanical systems.