Detecting Torsional Vibration

Vibration can generally be divided into two categories – linear and torsional.  Linear, the type of vibration most of us are familiar with, is the shaking and the movement of equipment we can easily feel and hear.  As a rule, linear vibrations are related to some type of mechanical problem, such as imbalance or misalignment, or advanced wear of gears or bearings.  Linear vibrations tend to get worse as speeds increase. Automobile tire imbalance is a good example of linear vibration.  As speed increases, the vibration increases in both frequency and amplitude – in other words – the faster the tire turns the worse the imbalance becomes.

Torsional vibrations are much harder to identify, because they often cannot be felt.  Torsional vibrations usually exit only at certain speed/load conditions and often “come and go” with speed changes. This exists to some extent in all rotating systems, but is not normally a problem unless the system is driven or “excited” by a frequency source which is at, or near, the natural frequency of the system (resonance). Systems operating at resonance can generate forces equal to several times the expected forces.

Torsional analysis can predict the speeds at which a system may become resonant, allowing designers to “retune” the system with various methods such as torsional couplings and mass dampeners. Torsional analysis is typically performed by the system packager or a torsional coupling manufacturer.  The damage from linear vibration is usually no surprise. For example, a severely out of balance pulley could easily fail a shaft bearing.

Torsional vibrations generally cause unexpected damage. For example, a gear may wear or break in a fraction of the time it should have taken. Or, a shaft may break unexpectedly even though it is oversized.  When there seems to be no explanation for component failures – consider torsional vibrations. One particularly alarming torsional condition is full reversing in which applied torque actually passes back and forth through zero.  If a gearbox is subjected to a full reversing torque, a severe rattle can occur as the gear teeth load and unload. The noise generated by this action can be quite loud. Keep in mind that the noise might be the result of an existing severe torsional condition and not a defective gearbox.

If you suspect torsional or linear vibrations as a possible problem, contact Cotta Engineering for a discussion of the problem and possible diagnosis.