Bearing regrease
How much grease and how often — quantity from the bearing size, interval from size and speed. Over-greasing kills as many bearings as neglect; this puts a number on both.
Bearing regrease · grease quantity & relubrication interval
Gp = 0.005·D·B g · tf = K[14×10⁶/(n√d) − 4d]·f, f = 0.5^((T−70)/15) for T > 70 °C
What this gives you
Two numbers that decide whether a rolling bearing lives out its rated life or dies early from the grease: how much to add at each relube, and how often to add it. The replenishment charge is Gp = 0.005 × D × B grams, with the outside diameter D and width B in millimetres — a small top-up, not a full re-pack. The relubrication interval is the SKF-style estimate tf = K × [ 14×10⁶ ÷ (n · √d) − 4d ] hours, where d is the bore in mm, n the speed in rpm, and K a service factor of 1 for a clean horizontal shaft or 0.5 for vertical, dirty, hot or wet duty.
Why bore and speed, not just size
The interval falls as the product n·√d rises, because it is surface speed at the raceway — not load — that works the oil out of the grease and ages the thickener. A big slow bearing and a small fast one can want relube on completely different schedules even at the same duty. That is why the formula leans on bore and rpm: double the speed and the interval roughly halves; go to a larger bore at the same rpm and it shortens too. The 4d term is a correction that pulls the interval down for larger bearings, where the grease path is longer and heat builds more easily.
Field note — more grease is not more protection
Over-greasing fails as many bearings as neglect. Pack a shielded or lightly-loaded bearing full and the churning grease has nowhere to go; it heats, the base oil bleeds off, and the housing runs hot enough to soften the remaining grease into a useless carbon crust. That is what the small Gp charge is for — a replenishment, roughly a third of the free space, on the interval above. If the motor has a grease relief plug, pull it and run the bearing a few minutes after greasing so the excess purges instead of packing against the shield. Halve the interval (K = 0.5) for vertical shafts, wash-down, dusty air, or housing temperatures above about 70 °C.
Worked example
A 6208-size bearing — bore 40 mm, OD 80 mm, width 18 mm — on a 1,750 rpm horizontal motor: the charge is 0.005 × 80 × 18 = 7.2 g, and the interval is 14×10⁶ ÷ (1,750 × √40) − 160 = about 1,105 hours, roughly 6.6 weeks of continuous running. Move the same bearing to a vertical or dirty duty and K drops to 0.5, so it wants that 7.2 g every 552 hours instead. Treat both as a planning starting point — then let bearing temperature and grease condition fine-tune the schedule.