A maintenance tech walks the line with a grease gun every quarter and gives every bearing a few pumps. Some bearings run fine. Others start running hot within weeks, and a few fail with grease bursting past the seals. The grease schedule is not the problem. The amount, the compatibility, and the way the grease goes in are.
What the grease is actually doing
Grease is oil held in a thickener. The oil bleeds slowly out of the thickener to lubricate the rolling elements and the race. The thickener stays in place and acts as a reservoir. This is why a bearing needs only a fraction of its free volume filled with grease. The grease is not a liquid bath; it is a slow-release carrier.
Pack a bearing full and the rolling elements have to churn through grease every revolution. Churning generates heat, which breaks down the grease, drives the oil out of the thickener, and eventually leaves a dry, caked residue. The bearing fails not from lack of lubrication but from too much of it.
How much to put in
The general rule for electric motor bearings is to fill the bearing about one-third full of grease, and to leave the housing cavity roughly half full. The bearing needs grease on the elements; the housing needs some free space so excess can move away from the rotating parts.
Calculate the charge in grams with the common formula: bearing outside diameter times bearing width times a factor around 0.005. A 6208 bearing, 80 mm outside diameter and 18 mm wide, takes roughly 7 grams. A grease gun can deliver 1 to 3 grams per stroke depending on the gun, so two or three strokes is often enough. Counting strokes matters because “a few pumps” on a high-output gun can over-fill a small bearing.
When you regrease through a fitting, run the motor slowly if it is safe to do so. Old grease purges out through the relief as the new grease enters. Stop when fresh grease appears at the relief, then let the motor run for ten to twenty minutes to expel excess before putting it back under load.
How often to regrease
Bearing manufacturers publish regreasing intervals based on speed, load, temperature, and bearing type. A simple reference uses the bearing speed factor, the bore in millimeters times the speed in rpm. At low factors and moderate temperatures, intervals can run a year or more. At high speeds, heavy loads, or temperatures above 70 C, intervals shrink to months.
Temperature is the biggest accelerator. Grease life roughly halves for every 10 to 15 C rise above moderate operating temperatures. A bearing running at 90 C needs grease several times as often as one at 50 C. If the motor sits in a hot area near an oven or on top of hot machinery, use the actual measured housing temperature rather than an assumed ambient.
Don’t ignore idle machines. Grease separates and dries out during long shutdowns. A motor stored for six months should be regreased before startup, and equipment that runs only seasonally needs attention before the first run rather than on a calendar interval counted during idle months.
Mixing greases is a real failure mode
Greases use different thickeners: lithium, polyurea, calcium sulfonate, and others. Two greases that are not compatible can soften, harden, or separate when mixed. A motor filled from the factory with polyurea grease, regreased with a lithium complex grease, can end up with a mixture that does not hold oil. The bearing runs dry even though the housing looks full.
Check the motor nameplate or the manual for the original grease. If you do not know what is inside, clean the bearing and housing completely before switching to a new grease. Don’t pump a new product on top of an unknown one and assume they will blend.
Signs of a greasing problem
A bearing running hot right after greasing, with grease leaking past the seals, is over-packed. Stop, purge the excess, and reduce the charge next time. A bearing that runs dry and noisy on schedule has lost its grease life or is seeing temperatures beyond the grease rating. A bearing with caked, blackened grease has been overheated; the grease is spent and the bearing may already be damaged.
Use the correct grease for the duty. High-temperature greases handle hot bearings but may run stiff at low temperature. Water-resistant greases suit washdown areas. A general-purpose grease at the right quantity and interval beats a premium product applied wrong.
Sealed bearings and automatic lubricators
Sealed or shielded bearings are factory-lubricated for life and have no regreasing fitting. They work well in clean environments at moderate loads. In heavy, hot, or dirty duty, the “life” of the lubricant can be shorter than the mechanical life of the bearing, and there is no way to replenish it. Don’t assume a sealed bearing is maintenance-free in a severe application; it just fails without warning when the internal grease is spent.
Automatic lubricators mount on the bearing and dispense grease over a set period. They are useful on bearings that are hard to reach or where a precise, frequent charge is better than a large quarterly dose. They do not remove the need to calculate the total amount; they just spread it out. A lubricator set to empty too fast over-greases the bearing just like a heavy hand on the gun. Use them where access is poor, and verify the setting against the calculated bearing charge.
Centralized lubrication systems serve many bearings from one pump. They are valuable on large machines, but blocked or broken feed lines mean one bearing runs dry while the system appears to run. Check the injectors and the line pressure; a pump that cycles is not proof that every bearing received grease.
Bottom line
Fill bearings one-third full, calculate the charge by bearing size, and purge old grease while running. Set the interval from the speed factor and the measured temperature, not from a fixed quarterly route. Never mix unknown greases. Most greasing failures are over-greasing failures; a disciplined, measured approach prevents far more bearing damage than carrying a grease gun to every motor on the floor.
