Bearings, Seals, Lubrication and Shafting
Bearing fundamentals
Rolling element bearings carry load through balls or rollers; plain bearings carry it on a fluid film and need a steady oil supply. Fit selection follows one rule: the ring that rotates relative to the load direction takes an interference fit, the stationary ring a lighter one. Mounting is done with an induction heater (about 110 °C maximum, and never above the seal or grease limit), an oil bath, or a press acting on the inner ring only - never a hammer directly on the rings or cage. Clearance versus preload changes stiffness and temperature; excessive preload is a common cause of early failure.
Seals
A lip seal fits with its lip facing the medium being retained and the spring on the fluid side. Pre-lubricate the lip, protect it from keyways with a sleeve, and seat it square. Labyrinth and mechanical seals handle speed and pressure that lips cannot.
Lubrication and shafting
Viscosity is chosen for speed and temperature; grease grades follow the NLGI scale. Over-greasing pushes past seals and churns grease into heat. Shafting work involves keys and keyways, fits, taper contact, straightness and runout.
Rule of thumb
Initial grease fill (grams) ≈ 0.005 × bearing outside diameter (mm) × width (mm). Re-grease by condition or schedule, not by habit.
Traps
Fitting the rotating inner ring loosely; heating a sealed bearing until the grease cooks out; installing a lip seal backwards; diagnosing every hot bearing as under-lubricated when preload or misalignment is the true cause.