Vibration Units and Transducers

Vibration is oscillating motion, described by its amplitude and its frequency. The same motion can be expressed three ways, and choosing the wrong one hides the fault.

The three units

• Displacement — how far the machine moves. Emphasises low frequencies. Used for shaft relative measurement on sleeve bearing machines and for slow-speed equipment.
• Velocity — how fast it moves. Roughly constant severity across the mid frequency range, which is why velocity is the general-purpose unit for overall machine condition on most industrial equipment.
• Acceleration — rate of change of velocity. Emphasises high frequencies, so it is the unit for bearing and gear defect detection.

A bearing defect that is obvious in acceleration can be almost invisible in displacement. Measuring the right quantity matters more than measuring precisely.

Amplitude conventions

Amplitude is quoted as peak, peak to peak, or root mean square. RMS relates to energy and is the usual basis for overall severity criteria. Comparing an RMS figure to a peak limit produces a wrong conclusion, so always confirm which convention a standard or an alarm level uses.

Transducers

• Accelerometer — the general-purpose sensor. Wide frequency range, rugged, measures casing vibration. Mounting is critical: a stud mount gives the full frequency range, a magnet mount less, and a handheld probe far less. A high frequency fault measured with a handheld probe may simply not appear.
• Velocity transducer — self-generating, used on older permanent installations.
• Proximity (eddy current) probe — measures shaft movement relative to the bearing housing. Essential on sleeve bearing machines, where the casing barely moves even when the shaft is running hard against the bearing. Turbines and large compressors are monitored this way, usually with two probes at right angles plus a phase reference.

Consistency

A trend is only valid if measurements are repeatable. Same location, same direction, same mounting method, same speed, same load, same settings. Permanently marked measurement points are worth the effort they take to establish.