Here are four greasy facts: Most bearings should not be overgreased. Some bearings, however, should be overgreased. All greases are not created equal. The color of grease is meaningless 99% of the time. These four facts, gleaned from mechanical and lubrication engineers, make sense once they’re explained.
Overgreasing can be bad. Grease performs two functions in bearings: lubrication and cooling. The lubrication aspect is obvious. Cooling occurs when warm grease transfers heat to the bearing housing or surrounding metal. If a bearing is packed too tight with grease it inhibits movement of grease within the bearing, decreasing its ability to cool.
The grease intervals listed in the owner’s manual for zerks on a machine are matched to the specific operating conditions and design of each bearing. Lube bearings according to the manual, and then only until resistance is felt.
Take note: When a mechanic "packs a bearing," it’s more accurate to say he’s "filling" a new bearing so rollers or balls are well-lubricated at start-up. "Packed" bearings are usually installed in housings or bearing blocks with cavities adjacent to the bearings so excess grease can purge once the bearing is in operation.
Overgreasing can be good. The exception to the "never overgrease" rule is bearings that aren’t necessarily harmed by overgreasing. One-way seals in needle-bearing caps on U-joints allow grease to purge once the cap is full. In theory, it’s possible to pump grease until clean grease purges from all four caps, but most engineers recommend stopping when fresh grease appears at one seal, or when the sound of grease popping from a seal is heard.
Conventional ball bearings sometimes have "bleed holes" in their seals. Stop pumping grease to those bearings when grease purges from the bleed hole. If there is no bleed hole, stop adding grease when resistance is felt.
Greases differ. The National Lubricating Grease Institute (NLGI) rates greases according to their thickness and resistance to flow. NLGI 000-grade grease is near-liquid, while NLGI 6 grease is wax-solid at room temperature. Between those extremes, cotton picker grease is rated at NLGI 00, corn head gearbox grease at NLGI 0 and multipurpose grease used on farms at NLGI 2.
Engineers match the NLGI grade of grease to operating conditions of specific bearings. If grease is too thick it might not flow properly; if it’s too thin, it might drain away.
Greases also differ by their thickeners and additives. Thickeners—lithium-complexes, clays and polyurea—hold lubricants in greases in suspension so they don’t drain away.
Lithium-complex thickeners account for more than 60% of greases sold worldwide. They are economical and work under a wide range of temperatures, pressures and rpms.
Clay thickeners resist heat well but can cake and leave a thick hard residue after all their lubricants have bled away.
Polyurea-complex thickeners offer all-round performance under a variety of temperatures and pressures. They’re often used by auto and farm equipment manufacturers as "factory fill" during assembly.
Molybdenum and graphite are additives, not thickeners. Moly is used in extreme pressure situations where rotation is limited: ball joints, tractor loader pins and front axle pivots. Graphite is added to ensure there is a final layer of protection when extraordinary pressures squeeze all the lubricant from a grease’s thickener.
Grease color is cosmetic. Go ahead and coordinate your grease color to match the paint on your machinery. Colorations of grease are purely cosmetic. The "normal" color of grease is wheat-
like. The two exceptions are graphite and moly greases, which are darkened by those additives.
A set of aluminum-alloy seal installers provides a range of tapered disks to match common seals and bearings. Select a disk that matches the outside diameter of a seal and strike the interchangeable handle to evenly seat seals without distorting their malleable edges. The beveled faces on the disks are for installing tapered bearing races. The soft aluminum prevents damage to hardened races during installation. Price: $30 to $50.
Be sure to visit Dan’s "In The Shop" blog at www.FarmJournal.com, where he’ll share more tips and insights. Send comments and story suggestions to firstname.lastname@example.org.
- Early Spring 2013