Understanding Rubbing in Rotating Machinery

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Portable balancer & Vibration analyzer Balanset-1A

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Vibration sensor

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Optical Sensor (Laser Tachometer)

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Balanset-4

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Magnetic Stand Insize-60-kgf

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Reflective tape

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Dynamic balancer “Balanset-1A” OEM

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Rubbing is the friction contact and relative sliding motion between rotating and stationary components in a machine. The term emphasises the continuous friction aspect of rotor-to-stator contact, distinguishing it from the light, intermittent contact or single impacts that may also occur. Rubbing generates friction forces, releases significant heat through friction work, and produces a distinctive vibration signature dominated by backward whirl, sub-synchronous components and thermal effects. It is one of the most dangerous faults a rotating machine can develop, because it can escalate to failure within minutes.

“Rubbing” and “rotor rub” are often used interchangeably. In practice, rubbing tends to stress the friction and thermal side of the contact, while rotor rub is the broader umbrella covering every form of contact — from light scraping to hard impacts.

1. The Friction Mechanics of Rubbing

Coulomb Friction Model

Rubbing obeys the principles of dry (Coulomb) friction:

• Friction force: F = μ × N, where μ is the coefficient of friction and N is the normal force pressing the surfaces together.
• Direction: the friction force always opposes the relative motion between the contacting surfaces.
• Typical coefficients: steel on steel μ ≈ 0.3–0.5; steel on seal material μ ≈ 0.2–0.4.
• Heat generation: essentially all of the friction work is converted to heat at the contact.

Tangential and Normal Forces

During contact, two force components act on the rotor:

• Normal force: pushes radially inward on the rotor at the rub point.
• Friction force: acts tangentially, opposing rotation.
• Resultant force: the combination tends to slow the rotor and deflect it backward, against the direction of spin.
• Torque increase: the friction dissipates power, raising the drive torque the machine must supply.

2. Characteristic Vibration Patterns

Backward Whirl

The single most distinctive feature of rubbing is backward (reverse) whirl:

• The friction force creates a tangential component that drives the orbital motion backward.
• The shaft orbit traces opposite to the direction of shaft rotation.
• The whirl frequency is typically sub-synchronous — less than 1× running speed.
• Common frequencies appear at fractional orders: 0.5×, 0.33×, 0.25×.
• The orbit shape is often irregular or visibly distorted.

Spectrum Characteristics

• Sub-synchronous peaks: multiple peaks below 1×, frequently at fractional harmonics.
• Synchronous component: the 1× synchronous peak may rise as rub forces add to it.
• Higher harmonics: 2×, 3×, 4× harmonics appear from the non-linearity of intermittent friction.
• Broadband noise: the noise floor across the whole spectrum lifts.
• Unstable spectrum: peaks come and go or shift frequency from one measurement to the next.

Time Waveform Features

• Impulsive events or spikes each time contact initiates, visible in the time waveform.
• Clipping or flattening at the peak deflections, where the stator physically limits travel.
• An irregular, non-sinusoidal overall shape.
• Beat patterns produced by several frequencies coexisting.

3. Thermal Effects of Rubbing

Heat Generation

Friction converts mechanical energy directly into heat:

• Rate: the power dissipated equals friction force × sliding velocity.
• Magnitude: a light rub may release 10–100 watts; a heavy rub, kilowatts.
• Concentration: that heat is dumped into a very small contact area.
• Temperature rise: local surface temperatures can exceed 500 °C in severe cases.

Thermal Bow Development

The danger of rubbing lies in a heat–vibration feedback loop:

1. The initial rub deposits heat on one side of the shaft.
2. Asymmetric heating bends the shaft into a thermal bow.
3. The thermal bow increases the shaft’s deflection.
4. Greater deflection drives more severe rubbing.
5. More rubbing generates yet more heat.
6. This positive feedback can lead to rapid, runaway failure.

Because each turn of this loop deepens the next, rubbing is regarded as a form of self-excited vibration and a route into outright rotor instability.

Secondary Thermal Effects

• Bearing heating: heat conducts along the shaft into the bearings.
• Oil degradation: excessive temperatures break down the lubricant.
• Material changes: phase transformations or metallurgical changes in the heat-affected zones.
• Thermal stress: can initiate cracks in the thermally stressed regions.

4. Detecting Rubbing in the Field

Vibration Monitoring

• Sub-synchronous alarms: alert on peaks at 0.3–0.5× running speed.
• Orbit monitoring: automated orbit analysis flags the appearance of backward whirl.
• Spectral changes: algorithms detect the sudden arrival of multiple harmonics.
• Waveform clipping: detection of the non-sinusoidal distortion that contact produces.

Recognising these patterns is exactly what a portable analyser is for. Working in the machine’s own bearings at operating speed, a two-channel instrument such as the Balanset-1A captures the time waveform and 1× amplitude and phase, so a technician can see the impulsive clipping and the fractional-order energy that mark a rub, and then check whether residual unbalance or misalignment is the underlying driver before any teardown.

Temperature Monitoring

• Bearing temperature sensors with rapid-rise alarms.
• Infrared temperature monitoring of exposed shaft sections.
• Temperature-differential monitoring — top versus bottom of a bearing.
• Rate-of-change alarms, for example greater than 5 °C per minute.

Additional Indicators

• Torque increase: power consumption rises as friction loads the drive.
• Speed fluctuation: small speed variations from the varying friction torque.
• Acoustic emission: high-frequency sound from the contact, detectable by acoustic emission sensors.
• Visual inspection: wear debris, discolouration and visible scoring.

5. Responding to a Rub

Immediate Actions

1. Reduce severity: decrease speed or load if it is safe to do so.
2. Monitor closely: keep continuous watch on vibration and temperature.
3. Prepare for shutdown: have an emergency shutdown ready.
4. Emergency stop: trip the machine if vibration or temperature is escalating.
5. Allow cooldown: run the turning gear or allow natural cooling before inspection, so a thermal bow can relax.

Investigation

• Inspect for physical evidence of contact.
• Measure clearances at the suspected rub locations.
• Check for thermal bow or permanent shaft bow.
• Identify the root cause — excessive vibration, insufficient clearance, and so on.

Corrective Actions

• Increase clearances: machine out damaged areas or replace components.
• Address the root cause: balance the rotor, correct alignment, fix the bearing issue that allowed contact.
• Replace damaged parts: seals, bearing components and shaft sections as needed.
• Verify clearances: confirm adequate clearance at every location before restart.

Rubbing is one of the most serious vibration-related faults in rotating machinery. Its capacity to escalate rapidly through thermal feedback demands immediate recognition, a prompt and disciplined response, and thorough correction — because the alternative, in critical equipment, is catastrophic failure.

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