The oil film that holds a shaft up in a journal bearing normally does its job quietly, but under the wrong conditions it can drive the shaft into a self-sustaining instability rather than damping it. Oil whirl is that instability: the shaft starts orbiting at a frequency below running speed, pushed around by the oil wedge itself. Oil whip is the far more dangerous escalation, where that subsynchronous motion locks onto a rotor natural frequency and refuses to let go. Both live below running speed in the spectrum, and recognizing them is important because oil whip in particular can wreck turbomachinery quickly.
Oil whirl and oil whip in one line: Oil whirl and oil whip are subsynchronous fluid-film instabilities that occur in journal bearings. Oil whirl is a forward-precession orbit at a frequency somewhat below half of running speed, commonly seen in the region of about 0.42 to 0.48 times running speed, driven by the average velocity of the oil in the bearing. Oil whip is the dangerous escalation where that whirl frequency locks onto a rotor critical speed and stays there as running speed rises, producing large, damaging vibration.
In a journal bearing the shaft rides on a wedge of pressurized oil rather than touching the metal directly, and that oil is being dragged around by the rotating shaft. Because the oil closest to the shaft moves at shaft speed while the oil at the stationary bearing wall does not move, the average velocity of the oil film works out to somewhat less than half of shaft speed. Under certain conditions of load, speed, and clearance, the shaft can begin to be pushed around the bearing by this circulating oil wedge rather than sitting steady in it, and it starts to orbit at roughly the oil's average velocity.
That is oil whirl, and its signature is a vibration component sitting below running speed, commonly in the region of about 0.42 to 0.48 times running speed, which is why it is described as subsynchronous. It is a forward-precession phenomenon, meaning the shaft orbits in the same direction as it rotates, which shows up clearly on a full spectrum as forward-dominated energy at the subsynchronous frequency. In an orbit plot it produces a characteristic looping motion, and on a spectrum it appears as a distinct peak below the running-speed line rather than at a harmonic of it.
What makes oil whirl notable is that it is an instability of the bearing rather than an imbalance or a defect on the shaft, so it does not respond to the usual remedies for those problems. It tends to appear when a lightly loaded journal bearing is running fast enough for the oil dynamics to take over, and it can come and go as operating conditions shift. On its own, moderate oil whirl may be tolerable, but it is a warning that the bearing is on the edge of instability, and it sets the stage for the much more serious condition it can develop into.
Oil whip is what oil whirl becomes when the machine's speed rises far enough. As running speed increases, the subsynchronous whirl frequency, which tracks at a fraction of running speed, climbs along with it. If that rising whirl frequency reaches a rotor natural frequency, a critical speed, the instability can lock onto that natural frequency instead of continuing to track running speed. From that point the vibration stays at the fixed critical-speed frequency even as the machine keeps accelerating, because the rotor's own resonance is now driving the motion.
This lock-on is what makes oil whip so dangerous. Once the instability is sitting on a resonance, the machine is being excited at a frequency it strongly amplifies, so the vibration amplitude can grow rapidly and to large levels rather than staying modest as ordinary whirl might. On a cascade plot the transition is unmistakable: the subsynchronous ridge follows a sloping fraction of running speed during whirl and then goes vertical, holding at a constant frequency as speed continues to rise, which is the visual signature of whip locking onto the critical speed.
The danger to turbomachinery is real and can be quick. The large, self-sustaining orbit of oil whip drives the shaft toward the bearing clearance and can cause heavy rubbing, bearing damage, or worse if it is allowed to persist. Because whip is a resonance-locked instability, simply speeding up further does not escape it the way passing through an ordinary critical speed would, since the vibration stays pinned to the natural frequency. This is why subsynchronous vibration is watched closely on machines with journal bearings, and why its appearance below running speed is treated as a signal to investigate the bearing condition rather than ignored.
Catching oil whirl and oil whip depends on watching the subsynchronous region of the spectrum, below running speed, which is exactly where ordinary overall-vibration alarms pay the least attention. A monitoring system that computes and trends the amplitude in the subsynchronous band, not just the overall level, can flag the appearance of a component in the whirl region before it has a chance to develop into whip. Because whirl is a forward-precession phenomenon, systems that also compute the full spectrum from XY probes can confirm the forward-dominated signature that distinguishes it from other subsynchronous causes.
The transition from whirl to whip is inherently a speed-related event, so it is best seen against a record that includes machine speed. A platform that historizes vibration together with a speed reference lets an analyst reconstruct the cascade behavior and see whether a subsynchronous component tracks running speed as whirl or locks onto a fixed frequency as whip. Merobix brings the vibration and speed signals into a browser-accessible history, so a run-up or a period of instability can be reviewed after the fact, and the subsynchronous behavior examined without a specialist having been on site during the event.
For journal-bearing turbomachinery at remote sites, this continuous record is what makes early detection practical. A subsynchronous component that appears and grows over successive startups is visible as a trend in the stored history, giving warning that the bearing is drifting toward instability before whip actually strikes. Trending the subsynchronous amplitude and keeping the speed context together in one place turns oil whirl from something an analyst only notices during a rare on-site visit into a monitored condition that raises a flag automatically, buying time to address the bearing before the instability escalates.
Oil whirl appears at a subsynchronous frequency somewhat below half of running speed, commonly described as being in the region of about 0.42 to 0.48 times running speed. This is because the shaft is driven by the oil film whose average velocity is a little less than half of shaft speed. It shows up as a distinct peak below the running-speed line on the spectrum, not at a harmonic of running speed.
Oil whirl is a subsynchronous instability where the shaft orbits at a fraction of running speed and that frequency tracks running speed as the machine speeds up. Oil whip is the escalation where the whirl frequency reaches a rotor critical speed and locks onto it, staying at that fixed frequency even as the machine accelerates further. Whip is far more dangerous because it excites a resonance and its amplitude can grow rapidly.
Oil whip locks the vibration onto a rotor natural frequency, so the machine is being driven at a frequency it strongly amplifies, and the amplitude can grow quickly to large levels. Unlike passing through an ordinary critical speed, speeding up further does not escape whip because the vibration stays pinned to the resonance. The large self-sustaining orbit can drive the shaft into the bearing, causing heavy rubbing and bearing damage if it is allowed to persist.
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