All bearings are designed with tolerances to allow the moving parts to move, but how are wear limits determined? In other words, after time, the components will start to wear out and the bearing will become looser. Looking at some aircraft maintencance manuals, they provide wear limits in the radial and axial directions. But how are these wear limitis determined? Is there a particular calculation that is done or is an arbitruary number chosen?
General ideas only. In the case of journal bearings with forced lubrication (oil pressure), increasing clearance results in oil pressure drop. Ideally, journal bearings hold an oil film which supports the rotating shaft and minimizes metal/metal contact. Automobile engines run for hundreds of thousands of miles using this strategy. Radial ball bearings will tolerate excess clearance (depending on the load) but the shafts they support often will not. In cases where oil seals are used, excess shaft play damages seals and permits leaking. In the case of motor or pump shafts, bearing slack permits rotor/stator contact or impeller/involute contact. Rotating masses are never perfectly balanced or axially aligned. Bearing slack multiplies unbalanced force and consequent load.
