Bearing Instabilities & Oil Whirl/Whip

Where Module 4.1's Warning Comes Into Play

Module 4.1 flagged subsynchronous vibration as a red flag that can't be explained by unbalance or misalignment — both of which always produce vibration at or above running speed. This module covers the most common cause of subsynchronous vibration: oil film instability in the journal bearings themselves.

Oil Whirl — A Self-Excited Instability

Oil whirl is a self-excited instability where the shaft, riding on its hydrodynamic oil film (Module 2.4), begins orbiting within the bearing clearance at roughly 0.4 to 0.48 times running speed — squarely in the subsynchronous range flagged in Module 4.1. A key characteristic: oil whirl frequency tracks proportionally with running speed. As shaft speed changes, whirl frequency changes right along with it, maintaining that same roughly 0.4-0.48x ratio.

Oil Whip — When Whirl Locks Onto a Critical Speed

Oil whip occurs when oil whirl's frequency happens to approach the rotor's first critical speed (Module 2.1). Rather than continuing to track proportionally with running speed as whirl does, the vibration frequency locks onto and stays near that critical speed even as actual running speed continues increasing further. This "locking on" behavior is what makes oil whip particularly dangerous — once locked near the critical speed, vibration amplitude can grow rapidly, because the machine is now sustaining near-resonant conditions continuously rather than only briefly passing through critical speed during a normal startup ramp.

What Makes Whirl More Likely

Two factors particularly increase susceptibility to oil whirl. Excessive bearing clearance — from wear, incorrect assembly, or babbitt damage (Module 2.4) — gives the shaft more room to develop unstable orbital motion within the oil film, directly connecting bearing condition monitoring to vibration diagnostics: a developing whirl problem can be a downstream symptom of bearing wear not yet caught through direct clearance measurement. Light bearing loading — where static load is low relative to bearing design capacity — also increases susceptibility, since a stronger static load helps stabilize the oil film's position and resist the onset of whirl. This is why whirl is more commonly seen on lightly loaded machines or bearings, sometimes appearing transiently during specific low-load operating conditions.

Confirming Whirl/Whip on an Orbit Plot

An orbit plot (Module 4.2) during oil whirl or whip typically shows a distinctive shape — often described as a smaller loop superimposed on or offset from the primary running-speed orbit, reflecting the two different frequencies (running speed and whirl/whip) present simultaneously. This distinctive orbit shape, combined with the subsynchronous frequency content itself, provides much stronger confirmation of oil whirl/whip than either the orbit plot or the frequency spectrum alone.