Torsional analysis is extremely important in determining the vibratory behavior of rotating systems such as engine driven pumps or generators. Torsional vibration considers only the rotational (angular) response of an object, typically a shaft, rod, or driveline under the influence of torque fluctuations.
Torsional vibration occurs when a pulsating or transient torque acts upon an object, causing it to oscillate along its axis of rotation. This can become a big problem when the frequency of pulsation coincides with a torsional natural frequency of the system.
Why is Torsional Vibration Analysis Important?
Torsional Vibration Analysis (TVA) provides a complete understanding of the dynamics of a system includes what is happening torsionally.
This is important because:
- Unless gear rattle or other mechanism alerts the operator to high levels of torque fluctuation, torsional vibration problems often don’t show themselves until catastrophic failure occurs. Consequently, it can be referred to as a “silent killer”.
- Torsional Vibration may result in high levels of transverse vibration; but not always.
Common Problems Associated with Torsional Vibration
Commonly observed in engines, drivetrains, and rotating machinery, torsional vibrations can lead to detrimental effects such as increased wear and tear, reduced efficiency, and structural damage. Understanding and mitigating torsional vibration is crucial in engineering design to ensure the reliability and longevity of mechanical systems, often necessitating the use of dampers and tuned mass systems.
Common failures due to torsional failure include:
- Engine failure.
- Crankshaft bolted joint failure.
- Gear rattle and/or failure.
- Coupling failure.
- Keyway hammering/ shaft failure.
- Premature spline wear and/or failure.
- Bearing failure.
Replacing or modifying the failed part usually does not solve the problem. A systems approach is necessary to evaluate the interaction between components.
Measuring Torsionals
Typically, torsionals have been measured at points along the system by looking at gear teeth (such as the flywheel ring gear) with a magnetic pick up. Points not having a production gear have been measured by either (1) fixing a machined gear to the shaft, or (2) mounting an encoder at the “end” of the system, or (3) utilizing a laser torsional vibrometer.
6D has developed a fourth option which utilizes wrapping magnetic tape around the circumference of a rotating part such as engine dampers or couplings. The tape has precisely spaced windows that allows a signal from a mag pickup to be similar to that from gear teeth.
6D’s torsional analysis software can accommodate the uneven spacing that occurs when the tape “meets itself” after one wrap around the component. The software also can adjust for different mean shaft speeds in the rotating system; useful when auxiliary components are driven through a gear box.
This system is a cost-effective alternative to conventional methods for measuring torsional displacements and phase relationships by engine order of a rotating system.
Torsional Vibration Analysis Steps
The system model is assembled using data provided by component manufacturers and assembled to match unique layout of driveline.
- Inertia of significant components (engine, gearbox, generator)
- Stiffness of connecting components (shafts, couplings, etc.)
- Damping (material damping, viscous dampers, elastomeric couplings, etc.)
- Engine details (cylinder layout, power, firing angles, etc.)
A model is only as good at predicting real system response as is the accuracy of the input data.
- Torsional modeling is a mature technology and has been shown to be useful at predicting the resonant behavior of driveline systems.
- Torsional modeling has been used for nearly 100 years to predict the health and reliability of critical systems.
Common Applications
Engine manufacturers use torsional analysis to assess the behavior of the driveline system into which their engine is being installed. A perfectly good, reliable engine can suffer catastrophic failure if the driveline system is not designed properly. That is, rear main and thrust bearings can failure when driveshaft rigidly connected to flywheel fails.
When input data is not available to perform TVA, measurements on the system must be used to find resonance and ensure that desired operating speeds are within the safe operating zone.
Case Studies
In all of these cases, TVA was not performed until after field failures had begun. Measurements were used to “diagnose” the problem and TVA was used to find eventual solution.
Small Generator
The OEM utilized a pin and bush coupling that they have been “using for years” on a new design. After a significant number of low hour shaft failures, measurements showed they were running near resonance at idle speed. Even though the generator was not pulling any load at this speed, the pulsations at idle were at just the right frequency and enough to generate torque fluctuations at nearly +/- 2000 lbft. The coupling arrangement was redesigned with the help of analytical modeling to result in a reliable system.
Gear Input Shaft
When no torsional coupling is installed between engine and right-angle gear, high levels of pulsation result in gear shaft and driveshaft failures. Performing a TVA at the design stage would have caught this and saved the customer and OEM lots of time and expense.
Welder
No torsional coupling was installed between engine and generator rotor. Like the previous generator example, the new design was “scaled up” from a previous reliable design and was thought to be “ok” until the fretting began after only a few hours of runtime.
Measurements showed extremely high levels of torque oscillation. The solution involved redesigning the taper and changing the engine coupling. All of these things can be avoided by performing a TVA on the initial design to find these hidden problems.
Expanded Expertise
Through its partnership with Structural Dynalysis, 6D has expanded and enhanced its Torsional Vibrational Analysis offerings. Structural Dynalysis opened its doors in 2002 and specializes in structural dynamics, vibration testing & analysis, signal processing and driveline dynamics. In addition to the testing services provided, Structural Dynalysis provides thorough torsional vibration analysis (TVA) with comprehensive reporting tailored to the needs of the customer.
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