The Crucial Role of Torsional Stress in Shaft Design: Understanding its Impact

Torsional stress is a critical aspect of shaft design, as it can lead to elastic or plastic deformation, ultimately causing failures in various rotating components such as shafts, couplings, fans, gears, engine dampers, and compressor oil pumps. Understanding the impact of torsional stress is essential for ensuring the reliability and safety of rotating equipment.

Torsional Vibration: The Root Cause of Shaft Failures

To comprehend the impact of torsional stress, it is crucial to analyze torsional vibration, which involves speed fluctuations of various components and the twisting of shaft sections while the machinery is rotating. Excessive torsional vibration can lead to failures at a 45-degree angle to the shaft axis, which may not be apparent until after a failure occurs.

Torsional vibration can be caused by several factors, including:

1. Unbalanced Loads: Uneven distribution of loads on the shaft can create torsional vibrations, leading to stress concentrations and potential failures.
2. Resonance: If the torsional natural frequencies (TNFs) of the system coincide with the excitation frequencies, it can result in amplified torsional vibrations, causing significant stress on the shaft.
3. Sudden Load Changes: Abrupt changes in load, such as the engagement or disengagement of a clutch, can induce torsional vibrations that can damage the shaft.
4. Gear Mesh Excitation: The periodic engagement and disengagement of gear teeth can create torsional vibrations, especially in high-speed gear systems.
5. Combustion Pulsations: In internal combustion engines, the cyclic nature of the combustion process can generate torsional vibrations that can be transmitted through the crankshaft.

Calculating Torsional Stress: The Fundamental Equation

To calculate torsional stress, the formula t = Tr/J is used, where:

• t is the torsional stress (in Newton-meters or Pascals)
• T is the transmitted torque (in Newton-meters)
• r is the distance from the center of rotation (in meters)
• J is the polar moment of inertia area (in square meters)

It is important to note that there are several assumptions required for the torsion equation to effectively analyze the torsional stress of an object, such as:

1. The material is the same throughout the body.
2. The load is uniformly distributed along the shaft axis.
3. The torque does not exceed the elastic deformation of the material.
4. The shaft cross-section is circular.
5. The length of the shaft does not change during loading.

Violating these assumptions can lead to inaccurate torsional stress calculations, potentially resulting in underestimated or overestimated stress levels.

Torsional Vibration Analysis: A Critical Design Tool

Torsional vibration analysis (TVA) is a crucial step in the design process, as it helps determine the separation margin from torsional natural frequencies (TNFs) or verify previous calculations. If a torsional problem is identified, the system may need to be modified to address the issue.

The TVA process typically involves the following steps:

1. Mass-Elastic Model Development: Create a mathematical model of the system, including the masses, stiffnesses, and damping characteristics.
2. Model Normalization: Adjust the model parameters to match the measured torsional vibration data, ensuring the accuracy of the analysis.
3. Torsional Natural Frequency Calculation: Determine the TNFs of the system, which are the frequencies at which the system can experience resonance.
4. Separation Margin Evaluation: Assess the separation between the TNFs and the excitation frequencies to ensure the system operates away from resonance.
5. Proposed Solution Evaluation: If a torsional problem is identified, use the TVA software to evaluate potential solutions, such as modifying the system’s mass-elastic properties or introducing damping mechanisms.

Torsional vibration analysis software, such as ANSYS, NASTRAN, or custom-developed tools, can be used to perform these calculations and simulations, providing valuable insights into the system’s torsional behavior.

Torsional Vibration Measurement: Ensuring Reliable Data

Torsional vibration measurements play a crucial role in increasing the reliability and safety of rotating equipment. To measure torsional vibration, specialized equipment is required, such as:

1. Torsiograph: A device that measures the angular oscillation of a shaft, providing direct torsional vibration data.
2. Encoder: An instrument that measures the angular velocity of a shaft, which can be used to calculate torsional vibration.
3. Laser Vibrometer: A non-contact measurement tool that can detect the angular velocity or displacement of a shaft’s surface, enabling torsional vibration analysis.

In addition to these direct measurement techniques, telemetry systems like the TorqueTrak 10K can measure shear torsional strain, which can then be converted to stress and/or torque by knowing the geometry and material properties of the shaft section.

Accurate torsional vibration measurements are essential for validating the mass-elastic model used in the TVA process, ensuring the reliability of the analysis and the subsequent design decisions.

The Importance of Torsional Stress Consideration in Shaft Design

Understanding and accounting for torsional stress is critical in shaft design to prevent elastic or plastic deformation, failures, and to increase the reliability and safety of rotating equipment. Proper calculation, analysis, and measurement of torsional stress and vibration are essential in the design process.

Neglecting torsional stress can lead to a range of issues, including:

1. Shaft Failures: Excessive torsional stress can cause the shaft to fail, leading to costly downtime and potential safety hazards.
2. Coupling Failures: Torsional vibrations can cause excessive stress on couplings, leading to premature wear and failure.
3. Gear Damage: Torsional vibrations can induce high dynamic loads on gear teeth, leading to pitting, scoring, and other types of gear failure.
4. Bearing Failures: Torsional vibrations can create uneven load distributions on bearings, leading to accelerated wear and failure.
5. Fatigue Cracking: Cyclic torsional stresses can cause fatigue cracking in shafts, couplings, and other rotating components.

By understanding the impact of torsional stress and incorporating appropriate design measures, engineers can ensure the long-term reliability and safety of rotating machinery.

Conclusion

In summary, torsional stress is a critical aspect of shaft design that must be thoroughly understood and accounted for to prevent failures and ensure the reliable operation of rotating equipment. By analyzing torsional vibration, calculating torsional stress, performing torsional vibration analysis, and conducting accurate torsional vibration measurements, engineers can design shafts and related components that can withstand the demanding operational conditions they will face.

Reference:
– Torsional Stress: Understanding Its Impact on Shaft Design
– Torsional Vibration Analysis Guide
– Shaft Design Considerations