Glossary term
Rotor Rub
Contact between a rotating shaft or rotor and a stationary component, diagnosed with orbit shape, shaft centerline, vibration spectra, temperature and inspection evidence.
Definition
phenomenonRotor rub is contact between a rotating shaft or rotor and a stationary component such as a seal, bearing, casing, guard, stator or close-clearance element.
Rotor rub occurs when dynamic orbit motion, shaft centerline movement, thermal growth, misalignment, bow, looseness, bearing distress, seal distortion or insufficient clearance allows rotating and stationary parts to touch. The contact may be intermittent, partial, local, full annular or severe enough to cause heating, material transfer, vibration growth, cracked parts or machine trip.
Rotor rub is contact between a rotating part and a stationary part. In rotating machinery this may involve a shaft touching a seal, impeller contacting a casing, rotor blades touching a stator, a coupling guard being struck, or a close-clearance component losing its running margin.
The contact can be brief and intermittent, or it can become a sustained annular rub. A light rub may leave only a change in orbit shape and a small temperature rise. A severe rub can bend a rotor, damage seals or blades, generate high friction heat, excite resonances, start cracks and trip the machine.
Clearance Screen
A useful first screen compares available radial clearance with shaft centerline position and orbit amplitude:
where c_{rem} is remaining clearance, c is radial running clearance, r_c is the distance from clearance center to mean shaft centerline, and a_{orbit} is the relevant dynamic orbit radius.
Contact is plausible when:
Example: a sleeve-bearing machine has radial clearance:
A shaft centerline plot shows mean displacement from the clearance center:
The filtered orbit radius during startup is:
The remaining clearance is:
That is a small margin. If thermal growth or transient vibration adds another 20\ \mu\text{m}, the estimated remaining clearance becomes negative and rub becomes credible.
Friction Heating
Rub severity depends on contact force, sliding speed, material pair, duration and cooling. A simple friction-power estimate is:
where \mu is friction coefficient, N is normal contact force and v is surface speed. Shaft surface speed is:
For a shaft diameter:
and speed:
the surface speed is:
If \mu=0.25 and the contact force is N=120\ \text{N}:
Even a modest contact force can create serious local heating at high speed. That is why rub decisions should use clearance, temperature, orbit and inspection evidence, not vibration amplitude alone.
Diagnostic Evidence
Rotor rub can create several measurement patterns:
| Evidence | Possible meaning | Caution |
|---|---|---|
| flattened or clipped orbit | shaft path is constrained by contact | check probe linear range and filtering |
| broadband vibration increase | impact or friction excitation | can also come from looseness or sensor mounting |
| harmonics of 1x | nonlinear contact or periodic impact | harmonics are not unique to rub |
| subharmonics or intermittent bursts | partial rub or stick-slip behavior | needs time waveform review |
| temperature rise near seal or bearing | friction or changed load path | verify process and lubrication state |
| visible rub marks | direct physical evidence | inspect both rotating and stationary surfaces |
The strongest diagnosis combines orbit plots, shaft centerline position, waterfall spectra, temperature trend and inspection. A single spectrum is rarely enough.
Severity Levels
Rub severity is not determined by one amplitude number. A light intermittent rub may show brief waveform clipping and small temperature change, while a severe rub may produce rapid heating, orbit collapse, material transfer and a protection trip.
| Rub state | Typical evidence | Practical action |
|---|---|---|
| suspected intermittent rub | short bursts, marginal clearance estimate, no damage confirmed | hold sweep and gather raw waveform |
| confirmed light rub | witness marks, stable temperature, limited vibration growth | inspect, correct clearance cause and retest |
| worsening rub | rising temperature, increasing broadband energy, changing shaft centerline | stop and inspect before further speed increase |
| severe rub | smoke, trip, high heat, heavy witness marks or rotor bow | do not restart without engineering release |
This classification is deliberately conservative. A rub can change quickly because contact heats the local parts, thermal growth reduces clearance further, and the rotor orbit can move into a different nonlinear response. The safest decision is usually to stop early while evidence is still interpretable.
Distinction from Related Terms
Rotor rub is not runout. Runout is a geometric or measurement deviation observed relative to a datum. Rub is physical contact during operation. Runout can contribute to a rub by reducing local clearance, but it is not the same failure mode.
Rotor rub is not unbalance response. Unbalance creates a synchronous rotating force. A rub may produce 1x, harmonics, broadband energy, subharmonics or intermittent bursts depending on contact severity and nonlinearity.
Rotor rub is not oil whirl. Oil whirl is a self-excited fluid-film instability. A rub is contact. The two can interact because a large whirl orbit can create contact, and a rub can change bearing or seal conditions enough to alter stability.
Release and Protection Decisions
A rub suspicion is usually a hold condition during commissioning. Continuing a speed sweep may be acceptable only if the evidence shows adequate clearance margin, stable temperature, no worsening orbit distortion and protection limits are active. If the waveform shows impacts, temperature is rising or the shaft centerline is moving toward a clearance boundary, the test should stop.
Useful release evidence includes:
- probe calibration and installed gap;
- shaft centerline origin and clearance assumption;
- raw and filtered orbit plots;
- slow-roll runout and electrical runout checks;
- temperature trend at nearby bearings, seals or casing locations;
- waterfall spectrum and time waveform during the suspected event;
- inspection for witness marks, discoloration, transferred material or debris.
Common Mistakes
Do not dismiss rub because the overall vibration is below a generic alarm level. A local rub can damage a seal or blade before casing vibration becomes large.
Do not diagnose rub only from harmonics. Misalignment, looseness, clipping, gear mesh, waveform distortion and sensor faults can also create harmonics. Rub diagnosis needs contact plausibility and machine evidence.
Do not remove DC shaft position from orbit data without saving it. The mean shaft centerline is often the difference between a harmless orbit and a clearance violation.
Validation Limits
The clearance screen is simplified. Real machines have noncircular clearances, thermal distortion, anisotropic bearings, probe angle errors, casing motion, seal deformation, transient rotor growth and uncertain contact locations. The result should be treated as a decision aid, not a guarantee.
A defensible rub review states the clearance basis, coordinate origin, probe convention, speed, load, temperature, filtering, runout compensation, protection thresholds and inspection evidence. When rub is plausible, the engineering decision should favor stopping and inspecting over trying to tune the spectrum while contact continues.