Understanding Velocity in Vibration Analysis

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Dynamic balancer “Balanset-1A” OEM

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Velocity is the rate of change of displacement with respect to time — in plain terms, a measure of how fast a vibrating component is moving. Of the three primary vibration parameters — displacement, velocity and acceleration — velocity is the one most widely used to judge the overall health and vibration severity of general rotating machinery across the most common diagnostic frequency range. It sits in the middle of the trio both literally and practically: one mathematical step away from displacement and one from acceleration.

1. Why Velocity is the Standard for Severity

Velocity has become the default parameter for general-purpose vibration monitoring for several connected reasons:

• Best indicator of destructive energy: the energy that fatigues a machine is most directly related to velocity. A given velocity level corresponds to a fairly consistent level of severity across a wide span of machine speeds and types, which is why limits can be written once and applied broadly.
• “Flat” frequency response: across the most critical band for machine diagnostics — roughly 10 Hz to 1,000 Hz, or 600 to 60,000 CPM — velocity gives the most balanced view. It is almost equally sensitive to low-frequency faults such as unbalance and to higher-frequency faults such as misalignment, making it an excellent all-round single number.
• Basis for international standards: the global machinery-vibration standards — chiefly ISO 20816, which superseded the long-serving ISO 10816 — use RMS velocity as the primary metric for acceptance limits and alarm levels across the different machine classes. The familiar zone A/B/C/D boundaries in ISO 20816-3 are quoted in mm/s RMS.

2. Units and Measurement

Common Units

Vibration velocity is normally expressed in one of two units:

• mm/s (millimetres per second): the SI unit, used in most of the world.
• in/s (inches per second): the imperial unit, common in the United States.

Velocity is almost always measured and trended as an RMS value, because RMS best represents the energy content of the signal. Where a peak figure is quoted instead, it should be labelled clearly, since converting between the two assumes a sinusoid; a vibration unit converter handles the arithmetic and keeps mm/s, in/s and dB consistent.

How is it Measured?

Velocity can be obtained in two principal ways:

1. Directly, with a velocity transducer: an electrodynamic velocity sensor generates a voltage in direct proportion to vibration velocity. These rugged moving-coil pickups were once common but have largely been superseded by accelerometers.
2. By integrating an accelerometer signal: the dominant method today. A robust accelerometer measures acceleration, and the data collector or monitoring system electronically performs the integration that converts it to velocity. This combines the wide frequency range and durability of an accelerometer with the diagnostic advantages of the velocity parameter.

3. Velocity’s Role in Diagnostics

A high overall velocity level tells you a machine has a problem, but not what the problem is. The diagnostic step is to examine the velocity spectrum and see which frequencies are feeding the high overall value:

• High velocity at 1× RPM (running speed) points to unbalance.
• High velocity at 2× RPM points to misalignment.
• A series of velocity peaks at running-speed harmonics indicates mechanical looseness.

This is precisely the workflow a field instrument follows. A portable two-channel analyser such as the Balanset-1A measures the overall velocity at each bearing, then breaks it into a spectrum so the engineer can read off the 1×, 2× and harmonic content — and, where the culprit is unbalance, proceed directly to correcting it in the machine’s own bearings.

4. Velocity Compared with Displacement and Acceleration

No single parameter is best everywhere; each dominates a different part of the frequency range:

• Displacement is best for very low-frequency motion — shaft orbits, structural movement and clearances — and is the natural choice for proximity-probe measurements on journal bearings.
• Velocity owns the broad mid-band where most rotating-machinery faults live, making it the everyday parameter for overall severity.
• Acceleration is best at very high frequencies, where it emphasises early bearing and gear faults that velocity would under-weight.

You can move between the three by integration (acceleration → velocity → displacement) and differentiation in the other direction. Even so, for a “big-picture” view of a machine’s dynamic health in its normal operating range, velocity remains the single most valuable parameter — and a quick way to benchmark a reading against ISO zones is the vibration severity chart.

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