Once you understand how grease actually works to lubricate a bearing, it becomes obvious why over-greasing causes so much trauma to both bearings and the grease itself.
Remember, all we want from our lubricant is to provide a little separation in the war zone. Nothing more… nothing less.
If you’re reading the term “war zone” for the first time, we use that term to describe the region of the bearing where all the wear and tear occurs.
Now let’s dispel three myths about greasing bearings.
Myth #1: if some grease is good, then a lot more must be great.
WRONG! Most bearing manufacturers like SKF, FAG, NTN, KOYO, all recommend that the bearing housing cavity only be filled to 30% capacity. Lube departments using a time-based approach to grease replenishment almost always leave their assets in an over-greased state.
Myth #2: More grease will provide better cooling for the bearing.
WRONG! Grease doesn’t provide cooling, air space does. Filling every void with grease chokes the bearing’s ability to dissipate heat generated by even normal friction levels.
Myth #3: If there is a grease nipple on the bearing housing it must be greased.
WRONG! Some motors come with “sealed-for-life” bearings installed. These are meant to be never greased… EVER. Yet someone thought it would be clever to install a grease nipple anyways. You have to know what’s inside your motor because grease is like tooth paste. Once you squeeze the trigger you can’t stuff the grease back inside the tube.
Enough bad practices please. We need a greasing strategy, but more than this, we need a greasing culture. Bad greasing culture eats good greasing strategy for lunch. It only takes one bad actor, often well-intentioned – to destroy an asset.
Grease guns don’t kill bearings… people do.
link from: https://www.bearing-news.com/greasing-myths/