Much of the vibration measurements, assessments and understanding of machine dynamics is focused around linear vibrations, with the typical units of acceleration, velocity and displacements, combined with frequency as the signature to decode the causes.
The widely used approach for such assessments are by quantifying housing and bearing interface vibrations and at instances by measuring direct shaft vibrations, in case of turbines and generators.
Relatively, much less attention is given to angular vibrations which are as important for understanding the health of any rotating system, particularly when a drive and driven systems are involved with varying loads and torques. Engines, compressors, turbines, motor and gear drives are all the typical cases of such equipment which undergo changing forces resulting in angular vibrations, which is also called tortional vibrations.
Basic notation of twist and torsional movements in rotating system
Torsional vibrations occur when a drive system—either due to its design or because it is subjected to loads beyond its capacity—prevents the driven system from synchronizing smoothly, leading to a back-and-forth jittery motion in the angular direction of rotation.
Crankshaft tortional angle mapping
The typical units of angular vibrations is in radians per second (or degrees); a lower number indicates very cohesive movement of the drive and driven systems and thus lower torsional vibrations in the system.
Even with optimized design of drive and driven systems, there are all probabilities of rotary systems to experience torsional vibrations; repeat start-stop, sudden changing in torque and loads, weaking of kinematics can all lead to this condition.
To minimize torsional vibrations, particularly at the interface of the drive and driven systems, various types of coupling configurations are introduced. Depending on the extent of power transfer requirements and the transverse loads that cause tortional vibrations, a combination of visco-elastic couplers and in some cases, fluid couplings are used to minimize the torsional forces and their resulting vibrations.
Examples of torsional vibration couplings
Continued torsional vibrations cause higher fatigue in the rotating systems, specifically at the joints where power transfer happens. Based on the material properties, temperature and load variance, the residual life of such systems tend to decrease exponentially, sometimes also with unwarranted failures of the drive / driven shafts / couplers.
Typical torsional failures / Modelling torsional vibrations
Instrumentation and methodologies exist to measure and quantify torsional vibrations; however, most methods require a physical sensing device to be introduced at the interface of the drive / driven system to assess the values, meaning that, considerable intervention is required to setup the apparatus for such measurements. Additionally, the torsional vibration sensors needs to fit to the shaft / drive sizes and as such it is a challenge to have a universal fitment to suit all types of machinery.
Advancements in technology have led to the development of strain-based torsional vibration measurement devices that can be mounted directly onto rotating components—on both the drive and driven sides—without requiring system disassembly, unlike earlier sensor types. These compact, matchbox-sized sensors are rigidly attached to the shaft(s) and measure electrical signals corresponding to the relative strain between the input and output sides of the drivetrain. The data is wirelessly transmitted to a receiver, which displays the torsional vibration values. These devices are accurate, portable, and versatile, making them suitable for a wide range of applications
Wireless torsional vibration measurement system
These new generation devices requires some key parameters as input such as shaft diameter, material details, young's modulus etc., and once these are provided correctly, the measurements are easy, reliable and time saving.
NV Dynamics, certified by Bureau Veritas as an agency to conduct and noise and vibrations assessments on ships and vessels, use these state of the art instrumentation to conduct torsional tests of ship engine shafts. We also use them in many land based applications across various industry drives, couplers and other power transfer mechanisms. Get in touch to know more.