spin
An abnormal condition in journal bearings where the bearing seizes to the rotating shaft and rotates inside the journal, destroying both the shaft and the journal.
spin: when a bearing fails and rotates on the shaft
Spin occurs when a journal bearing loses its film of lubricant and seizes to the rotating shaft, causing the bearing itself to rotate inside the stationary journal housing. Instead of the shaft turning freely within a fixed bearing, the bearing and shaft turn together as one unit. The result is rapid, destructive friction between the bearing and the journal bore, generating extreme heat and scoring both surfaces beyond repair.
The immediate cause is always loss of hydrodynamic lubrication. When adequate oil film cannot form or breaks down, metal-to-metal contact replaces the fluid layer that normally separates bearing and shaft. Insufficient oil supply, contamination, overheating, inadequate clearances, or shaft misalignment can all trigger this failure. Once seizure begins, friction and temperature spike in seconds. The bearing shell, typically made of soft white metal or copper-lead alloy, wears away rapidly while the hardened steel shaft becomes scored with grooves and patches of transferred bearing material.
Why spin happens and what follows
Spin is particularly common in sleeve or plain journal bearings operating at high loads or speeds where lubrication margins are tight. The bearing may initially slip slightly on the shaft, generating heat that further degrades the oil; resistance increases until stick-slip transitions to continuous rotation. Once spin begins, nothing stops it without shutting down the machine. The bearing is destroyed, and the shaft surface is damaged enough that replacement is often necessary rather than repair.
Prevention relies on maintaining adequate oil supply and temperature control, ensuring proper bearing clearance (typically 0.0005 to 0.002 inches per inch of shaft diameter), and monitoring pressure and temperature. Machine operators watch for unusual heat, noise, or vibration near the bearing. In some designs, thermostats or cutoff switches detect dangerous temperatures and trigger an alarm or shutdown before catastrophic damage occurs.
The term spin distinguishes this failure mode from simple wear, corrosion, or mechanical looseness. It implies a locked, rotating condition that is almost always terminal for the bearing and damaging to the shaft. Once spin has occurred, the bearing shell must be replaced and the journal bore may need reaming or boring to accept an oversized bearing shell.