Strategies for lubricating bearings on farm equipment range from fanatical to lackadaisical. Experts in the bearing industry say optimal strategies to lubricate bearings are somewhere in between.
Bob Sutton, an engineer with NTN Bearings, warns that over-greasing bearings can be as damaging as undergreasing.
"A water treatment plant in Houston was having trouble burning out bearings on some of their big pumps,” Sutton says. "The pumps ran 24 hours a day, and their maintenance crews were greasing them religiously three times a day. I removed the grease zerks from the bearings so they couldn't over-grease them, and they haven't lost a bearing in eight years.”
His example is extreme, but it illustrates an engineering fact. "Overfill a bearing and you can overheat it from within because the excess grease creates turbulence, which creates heat,” Sutton says. "Overgreasing can kill a bearing with kindness.”
Owner's manuals are the best resource for proper greasing intervals. Manufacturers' recommended lubrication intervals are based on a bearing's load specifications, average shaft speed, maximum shaft speed, whether it will be exposed to moisture or dust and other variables unique to the bearing's location on a machine. Mathematical formulas and computer simulations allow engineers to determine if one bearing should be greased every 10 hours while another bearing needs to be greased only every 200 hours.
A little can go a long way. Accidental damage can occur even if lubrication intervals are maintained. Overlubrication is again the culprit. "Ideally, a bearing should be greased while it's turning,” Sutton says. "If you grease a static bearing, you're pushing grease into the cavities between the balls [or rollers] closest to the zerk. If you try to force grease around the entire bearing, there's a risk of popping or loosening the seal, which lets in contamination and reduces bearing life.”
Sutton is a sarcastic fan of battery- or air-powered grease guns. "I love them, " he chuckles. "They make it easy to pop seals or overfill bearings. They sell lots of bearings for me.”
He recommends only a few pumps of a hand-operated grease gun per bearing at recommended intervals. "It's doesn't take much grease to keep a bearing happy,” he says. "If you feel resistance, stop greasing so you don't pop a seal.”
Another cause for varying lubrication intervals is that some zerks lubricate components other than bearings. The zerks that supply lubricants to sheaves that slide on bushings demand frequent lubrication. Other zerks feed cavities in gearcases or wheel hubs.
"A wheel hub cavity should never be more than half full of grease, and usually less, to allow for heat dissipation,” says John Munson, vice president of sales for Standard Bearings in Des Moines, Iowa. "The same rule applies to gearcases. If you overfill them, you risk turbulence in the grease, which creates heat. You want air space in the gear case so grease gets slung against the sides by spinning gears.”
As the grease slides down the inside of the housing, it exchanges heat with the cooler metal. The gearcase housing acts as a cooling element. "Packing” a gearcase may reduce cooling because only the grease pressed against the housing receives the cooling effect. Consult owner's manuals for proper gearcase fill levels.
Munson says one of the problems unique to bearings in agricultural equipment is the amount of time they sit idle. Moisture from leaky seals or condensation creates subtle but insidious problems that shorten bearing life.
- Mid-February 2010