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For the most part I believe time and temperature are the factors that make grease harden. Both of those cause the oil to go out of the grease. Recently I heard grease was 80% oil which seems a surprising statistic (I'm still a little skeptical). In general water/moisture is another factor often blamed for things degrading but to my knowledge that doesn't apply to grease hardening.

Then again my knowledge in this area is pretty thin. Take it all with a grain of salt.
Saw this Lube Tips:
Why Does Grease Harden?
"What causes the hardened cake-like material to form in grease- lubricated bearings?"

This is generally due to oil separating from the thickener. While it is normal for small amounts of oil to blend from the thickener over time, a grease with excessive bleed characteristics may harden in the bearing prematurely.

In some cases, too much time has passed between relubrication. The solution would be to shorten the interval, say from one year to six months. It is generally a good rule of thumb to schedule relubrication when half of the original oil-in-soap content is lost.

High temperatures caused by overlubrication can often lead to the caking condition. There are also other causes of high heat.

Whatever the source, the heat can either boil the oil out of the thickener, promote premature bleed, or induce oxidation (thickening) of the base oil. All can cause a hardened residue left in the bearing.

High centrifugal forces in large high-speed bearings can cause oil to separate from the thickener prematurely as well.

Jim Fitch, Noria Corporation
You may want to do a quick "age-profile" on your grease extension tubes. This means using the inside tubing diameter to determine grease volume per foot. Knowing how many pumps are typically delivered, and how often, you can determine how old each slug of grease is as you get closer to the bearing. I've seen extremely long grease extension tubing installed in the interest of safety or "convenience", with the result being grease aging in the tubing to a year or more before it finally reaches the intended target. Also look to see if the tubing goes by any hot areas, which would increase the likelihood of hardening. Tubing can also get crimped by people stepping on it, or where it turns corners. If the type of grease was changed, i.e., someone pumped a different grease one time with a different thickener chemistry, they can react at that interface and cause hardening. Testing the grease by FTIR or other appropriate tests, on both sides of the hardened area can help to identify if they are different greases.

Rich Wurzbach
MRG Power Labs
York, Pennsylvania
Maintenance Reliability Group

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