Unbalance vs misalignment vs looseness
Three faults that all raise vibration at running speed. How the spectrum, phase and time waveform tell unbalance, misalignment and looseness apart.
Why they get confused
Unbalance, misalignment and mechanical looseness are the three most common faults on rotating machines, and all three show up at running speed, 1x, and its harmonics. An overall vibration reading, or an alarm on ISO 20816 severity, says that something changed but not which of the three it is. The fix is different for each: balancing, alignment or tightening and repair. Diagnosing the wrong one costs a stop and leaves the fault in place.
Side by side
| Sign | Unbalance | Misalignment | Looseness |
|---|---|---|---|
| Spectrum | Dominant 1x, few harmonics | 1x and 2x, 2x often high, sometimes 3x and 4x | Many harmonics of 1x, sometimes half orders (0.5x, 1.5x) |
| Direction | Radial; axial stays low, except on overhung rotors | High axial for angular, high radial for parallel | Often strongest vertically, varies by point |
| Phase | Steady; about 90° between horizontal and vertical on one bearing | About 180° across the coupling | Unstable, changes between readings; about 180° between a loose part and its base |
| Time waveform | Clean sine wave at 1x | Repeatable, one or two peaks per revolution | Erratic, truncated or with impacts |
| Response to speed | Force rises with the square of speed | Changes with load and temperature | Can appear or vanish at certain speeds and loads |
Unbalance
Unbalance means the mass center of a rotor doesn't sit on its axis of rotation: buildup on fan blades, erosion, a lost balance weight, a repaired impeller. The rotor pulls on its bearings once per revolution, so the spectrum shows a single strong 1x peak in the radial direction. The unbalance force grows with the square of speed, which is why a small unbalance becomes a big problem on a fast machine.
Phase confirms it. On a bearing, the horizontal and vertical readings differ by about 90°, and the phase stays stable from one reading to the next. On an overhung rotor, such as an end-suction pump impeller or a fan wheel outside its bearings, unbalance also gives a high axial 1x, which can be mistaken for misalignment.
Misalignment
Misalignment forces two coupled shafts off a common line. It shows up as 2x as well as 1x, with high axial vibration for angular misalignment and high radial 2x for parallel misalignment, and a phase difference of about 180° across the coupling. The full signature, and what else looks like it, is in misalignment vibration spectrum.
Mechanical looseness
Looseness comes in three forms, and each looks different:
- Structural looseness of the base, a cracked frame or a weak foundation: mostly 1x, strongly directional, and easy to take for unbalance. Readings taken down the structure, from the bearing to the base and the foundation, show a large change in amplitude or phase at the loose joint.
- Rocking or loose hold-down bolts: 1x and 2x, sometimes 3x, with about 180° between the machine foot and the base it sits on.
- Rotating looseness, such as a bearing loose on its shaft or in its housing, or worn fits: a long row of 1x harmonics, often up to 10x and beyond, sometimes with half orders like 0.5x, 1.5x and 2.5x. The time waveform looks truncated or erratic, and phase jumps between readings.
Looseness amplifies the other faults. A small unbalance on a loose machine can read as severe vibration, so check for looseness before balancing.
A practical order of checks
- Look at the spectrum: a clean 1x, a strong 2x, or a row of harmonics?
- Compare the axial reading with the radial readings on the same bearing.
- Look at the time waveform: sine wave, repeatable pattern, or impacts?
- Measure phase: horizontal against vertical on one bearing, across the coupling, and from the bearing down to the base.
- Check the machine itself: hold-down bolts, soft foot, cracks, coupling condition, buildup on the rotor.
After the repair, take a new reading. The peak that pointed to the fault should drop, and the reading becomes the baseline for the next comparison.
How Misoki uses this
Prism AI runs this comparison on every measurement: the 1x and its harmonics, axial against radial, the time waveform and, where it is measured, phase. It names the fault, grades its severity and says what to do on the next stop. Findings that matter are reviewed by ISO CAT II/III vibration analysts before they reach you.
Frequently asked questions
01How do you tell unbalance from misalignment?
Unbalance gives a dominant radial 1x, low axial vibration and about 90° phase between the horizontal and vertical readings on a bearing. Misalignment adds a strong 2x, often high axial vibration, and about 180° phase across the coupling. Overhung rotors are the exception: unbalance there also raises axial 1x.
02What does mechanical looseness look like in a spectrum?
Rotating looseness shows many harmonics of running speed, often up to 10x, sometimes with half orders such as 0.5x and 1.5x, and an erratic time waveform. Structural looseness is mostly 1x and directional, and shows up as a big change in amplitude or phase across the loose joint.
03Should I balance a machine that shows high 1x vibration?
Only after ruling out looseness, misalignment and resonance. Looseness amplifies unbalance, and balancing a loose or misaligned machine rarely lasts. Confirm with phase readings first.
Know which fault it is before the stop.
Send us a week of vibration data and we walk you through what Prism finds.