What vibration tells you
Every rotating machine vibrates a little. A healthy motor, pump or fan has a characteristic vibration pattern that stays fairly steady while it runs under similar conditions. When something changes inside, such as a bearing starting to wear, a shaft going out of line or a rotor picking up deposits, the pattern usually changes too. Vibration analysis is the practice of measuring that pattern and interpreting the changes.
It is one of the most widely used condition monitoring techniques because it can often show a developing problem well before the machine becomes noisy, hot or stops. It does not detect every kind of fault, and readings need interpreting, but for rotating equipment it is often among the most informative checks available.
The basic measurements
A vibration sensor, commonly an accelerometer, is attached to or held against the machine, usually near the bearings where forces from the rotating parts are transmitted to the housing. The signal is described in a few ways.
- Amplitude is how strong the vibration is. Higher amplitude generally means more energy going into shaking the machine.
- Frequency is how often the vibration repeats. This is the part that points toward a likely cause.
- Phase compares the timing of vibration at different points and can help separate certain faults from others.
Amplitude may be expressed as displacement, velocity or acceleration. Each tends to be more useful in a different frequency range, and which one is used depends on the machine and the practices of the site.
Frequency spectra
Raw vibration is a jumble of overlapping signals. Analysts commonly convert it into a spectrum, a chart showing how much vibration there is at each frequency. Peaks in the spectrum are compared with the machine’s running speed.
Some common patterns, described in general terms:
- Imbalance often shows up as a strong peak at the shaft’s rotation frequency.
- Misalignment between connected shafts often produces peaks at the rotation frequency and at multiples of it. Shaft alignment methods and the condition of flexible couplings are the usual follow-up checks.
- Looseness, such as loose mounting bolts or a worn fit, tends to produce many harmonics, sometimes with an irregular appearance.
- Bearing problems often show up at higher frequencies related to the bearing’s geometry, and may appear as a rising general noise level in the signal. Knowing the typical bearing wear patterns helps connect a signal to a physical cause.
- Gear problems may appear at the frequency at which gear teeth mesh, with sidebands that reflect wear or damage.
- Flow-related issues in pumps, such as pump cavitation, may show up as broad, random vibration rather than clean peaks.
These are tendencies, not guarantees. Several faults can look alike, and one machine can have more than one at the same time, so a trained analyst usually combines the spectrum with other information about the machine.
Baselines and trends
A single reading says little unless there is something to compare it with. Good practice is to record a baseline when the machine is known to be in good condition, such as after commissioning or a successful overhaul, and then compare later readings under similar load and speed.
Trending matters more than any one number. A reading that is slightly above a general guideline but stable for years may be less concerning than a reading that doubled over a few months. Teams often set alert levels based on guidance for the machine class and on their own history, and then adjust them with experience. Published severity charts and standards give general reference ranges, but a machine’s own history is usually the better yardstick.
How readings are collected
Two approaches are common.
- Route-based measurement. A technician walks a planned route with a portable instrument and records readings at set points, often monthly or on another interval. This is relatively low-cost and works well for a large number of ordinary machines.
- Permanently installed sensors. Sensors feed data continuously or at frequent intervals to a monitoring system. This suits critical machines, hard-to-reach locations or machines where faults can develop quickly. Maintenance sensors of this kind are often part of a wider predictive maintenance effort.
Consistency matters. Using the same measurement points, the same sensor mounting method and similar operating conditions makes trends reliable. Moving a sensor a few inches or measuring at a different load can look like a change in the machine when it is only a change in the method.
Using the findings
A vibration finding is usually a prompt for action, not a verdict. A typical sequence is to confirm the reading, compare it with the baseline, consider operating changes, plan an inspection or correction, and recheck afterward. The recheck matters, because it shows whether the correction worked.
Any inspection or repair that follows should be carried out by trained personnel following the manufacturer’s manual and the site’s lockout tagout procedures. The measurement itself can also involve working near moving parts, so sensor placement and route walking should follow the site’s safety rules.
Limits to keep in mind
- Vibration analysis is best suited to rotating machines. It is less informative for faults that do not change vibration, such as some electrical or process-related problems.
- Poor data, loose sensors or inconsistent methods produce misleading results.
- Interpretation takes training and experience, and software alerts do not replace judgment.
- The value comes from acting on what is found. Readings that are collected but never reviewed do not improve reliability.
Used well, vibration analysis helps a team shift some work from reacting to breakdowns toward planning repairs for a convenient time, which supports overall equipment reliability.