Preventive methods: How to detect
torsional vibrations
The amplitude of torsional vibration can be destructive and since most machinery is not equipped to measure it, the first indication of a problem is typically failure of a drivetrain component. However, there are some methods for detection that can be utilized to determine the presence of torsional vibrations. Flexible shaft couplings often times provide tell-tale signs that can be detected visually upon disassembly or, in some cases, during operation. The flexible element(s) of the coupling (i.e. disc pack, gear teeth, grid, elastomeric blocks, etc.) which are meant to accommodate for shaft misalignment while transmitting the load are the components of the coupling that are primarily susceptible to damage which is visible. Additionally, shaft sections, keyways, and bolted flange connections can present visible damage; but this usually occurs in addition to flexible element damage due to the higher torque capacities of those interfaces and components.
For drivetrains which include a gearbox, audible noises may be detected when torsional vibrations are present. This can sound like ‘chattering” or looseness in the gear mesh. However, without a more detailed inspection, or actual measurements, the loads seen during these events cannot be determined. Note that radial vibration is sometimes measurable on gearboxes when torsional vibration is severe.
Fretting is the result of the relative movement of two surfaces subjected to a load. Torsional vibration can produce fretting wear in bolted flange connections, splines, and at hub/shaft interfaces. During inspection, couplings and shafts should be carefully inspected for signs of fretting as it tends to reduce the fatigue life and is a common crack initiation point. For ferrous materials, surfaces with fretting wear usually contain reddish-brown areas which are oxidized wear particles as well as some areas that appear polished as the wear particles begin to serve as a lapping compound. Photograph 2a shows a fretting wear pattern around a bolt hole of a coupling flange connection caused by relative movement of the flange and washer. Photograph 2b shows a different bolt hole with a crack that has begun to propagate and after initiation in the fretting zone.
Visual inspection should also be aimed at finding surface cracks in the load-carrying components. Attention should be paid to areas where the stress can become concentrated by sharp corners or an abrupt change in geometry such as steps in a shaft, keyways, welded joints, etc. The limitation of visual inspection is that small and sub-surface cracks are not visible to the naked eye. For critical equipment susceptible to torsional vibration, inspection methods for crack detection should include either dye penetrant or magnetic particle inspection (MPI). Dye penetrants are effective at quickly identifying surface cracks, but are not capable of detecting sub- surface cracks and can be difficult to use in small corners. MPI is typically the best method to detect cracks in ferrous materials. Examples of cracks detected using MPI in the root of gear teeth and a coupling component are shown in Photographs 3a and 3b, respectively.
Photographs 2a and b: Showing fretting wear around bolt hole from dynamic contact with mating washer (left) and crack initiated in fretting zone (right).
Photographs 3a and b: MPI crack indications in the root of gear teeth (left) and crack that propagated through a flexible coupling component (right).
66 | ISSUE 109 | SEP 2024 | THE REPORT
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