Bearing Vibration: A Glossary Of Terms

Vibration in an aircraft can be very normal, as it will inevitably be caused by the many moving parts in an aircraft. In 2015 there were 24,142,000 general aviation flight hours logged, and operators who logged any of those hours need to be aware of unusual vibration and how it impacts the entire aircraft. Bearing vibration exhibits very low amplitude in the beginning, making it difficult to differentiate from background vibration. Before operators use vibration analysis techniques to detect bearing vibration, they should know these common terms.

• Cycle: The entire oscillation movement of the outer surface of an outer bearing ring that is vibrating. As a bearing ring vibrates, the outer surface moves upwards to its upper limit, then downwards to its lower limit, and then to its starting point in the middle. As long as the bearing is rotating, the vibration cycle will repeat.
• Displacement: The measure of oscillation in a vibrating object.
• Peak to peak displacement is the measurement between the upper and lower limits created during the cycle.
• Frequency: The number of vibration cycles in a given time. The cycle frequency is typically expressed as cycles per second (CPS) or Hertz (Hz), both of which mean the same thing.
• Vibration Acceleration: A calculation that indicates the vibratory force by multiplying mass by acceleration. Force is more damaging at higher frequencies, making it very important to monitor vibration acceleration. It is an especially useful measurement when a bearing experiences vibration frequencies above 2,000 Hz, as that is considered a high frequency. Vibration acceleration is measured in G and converted to decibels (dB).
• Vibration Velocity: A calculation that gives an indication of the severity of the vibration. It is measured by multiplying the displacement by the frequency. When a bearing component is moving a certain distance (displacement) at a particular rate (frequency), then it must be moving at a certain speed. As the vibration velocity measurement increases, the bearing will make more noise. The measurement indicates the fatigue potential caused by bearing vibration. A Bearing Vibration Tester is used to measure vibration velocity in microns per second. The readings are then separated into three frequency bands: low (50 to 300 Hz), medium (300 to 1,800 Hz), and high (1,800 to 10,000 Hz).

In order to maintain a high-quality aircraft, it is very important to monitor the vibration of a bearing. Too much vibration is potentially damaging to the bearing and the equipment it is used in, shortening the life of both. Monitor vibration and your aircraft will be in good condition.