More Greasy Guidelines
Jan 25, 2012
Here are some more tidbits related to greases and lubricating farm machinery:
-The color of grease--aside from moly-based greases--has little relation to performance. The normal color of grease is wheat-colored, maybe slightly amber. Moly-additives make greases blackish. Blue greases, green greases and red greases are the result of additive dyes that are pretty much cosmetic.
-a thick, tacky grease is not inherently "greasier" than a slimier grease that drools oil. There are different National Lubricating Grease Institute (NLGI) ratings that range from "000" to 6," with 000-rated grease being nearly fluid ,and 6-rated grease being nearly solid. The key is matching the thickness of grease to where it's used. Oil in grease is what provides lubrication; thickness of the carrier merely influences whether or not the grease stays in bearings at specified heat and rpms. There are situations where a thick, tacky grease wouldn't "flow" between the balls or rollers in a bearing and the bearing would starve for lubrication, and there are situations where grease that's too liquid would drain out of the bearing. Engineers calculate the rpm and temperatures bearings will operate under, and owner's manuals list the type of grease those engineers believe will provide the best lubrication.
-too much grease can be nearly as harmful as too little, especially in sealed bearings. Part of the function of grease is to cool bearings--grease absorbs heat from the balls and races, gets slung or moved to the relatively cooler outside of the bearing where it cools, then gets moved back to the warmer portions to absorb more heat as it provides lubrication. Pumping a bearing with grease until it won't take any more grease risks limiting grease movement within the bearing. Engineers call it "churning," where grease just sits and churns in one spot, and eventually overheats. Once again, engineers have analyzed the specific needs of bearings and determined the optimum frequency to grease them, to refill and compensate for grease loss due to vaporization and drainage. If the owner's manual says to grease a bearing only every 50 hours, greasing it every day could potentially overgrease that bearing and contribute to reduced lifespan.
-The exceptions to the risk of overgreasing are bearings with "weep holes" and the needle bearings in u-joints. As noted in an earlier post, u-joint seals are "one-way" seals, and designed to purge grease. Bearings with weep holes are similar. In those cases it's acceptable to pump grease until you see grease purge from the bearing.