If there is a constant in agriculture it’s that it’s a dirty field. And in today’s highly mechanized agricultural world, dirt can hobble and kill. It’s a continual fight to make complex equipment run smoothly in the constant presence of soil, dust, and other grit.
Ironically, petroleum-based lubricants can be their own worst enemy. “Wet” lubricants — intended to reduce friction — can capture solids in dirty environments, increasing friction and abrasion of moving parts. As lubricants load up with particulates, lubricity declines; viscosity rises. Relative motion of mating parts takes more energy, generates more heat — but the lubricant can’t flow so well, so it is less able to transfer heat. The whole thing can become just a mess.
For engines, where temperatures and part-to-part speeds are high, a wet lubricant is essential. But many motion-control components including linear, rotating, and oscillating shafts, linkages, and some clutches, operate in more relaxed conditions. You may need low-friction interfaces between exposed moving parts, but it is often highly desirable to avoid wet lubricants.
Plastic bearings can be effective dry, low-friction interfaces. Bearings in a variety of engineered plastics have improved significantly in the past decade and, in many parameters, they now outperform metal bearings. Plastic bearings are used successfully in original equipment and in the aftermarket.
Grain drills — case in point
Great Plains Mfg. Inc. of Assaria, Kans. builds a range of agricultural equipment, including 2 and 3-section folding grain drills. A tractor pulls a drill, which meters seed into the soil through pairs of sharp steel discs. The weight of each folding section is partly supported by gage wheels which supply mechanical power to feeder cups by way of shafts, a chain drive, a clutch, and a gearbox. The feeder cups meter grain out of the seed hoppers.
When a grain drill is being transported on the road, or is turning around at the end of a pass during seeding, the disc openers must be lifted above ground level. To do this, the operator actuates a hydraulic mechanism that raises the implement’s ground-engaging portions (including seed hoppers and metering mechanisms). At the same time, the seed-metering mechanism must be disengaged. If not, power from the gage wheel, which always contacts the ground, would continue to cause seed dispersal. As ground-engaging portions are lifted, a mechanical linkage automatically disengages the clutch to stop seed metering.
The drive side of the clutch receives power from a chain drive, driven by a shaft off the gage wheel. The driven side of the clutch runs a jackshaft, which connects to a gearbox. The gearbox powers the main feeder cups, and optional smallseeds and fertilizer attachments.
Great Plains had initially specified a wrap-spring clutch. “The wrap-spring clutch is very easy to produce,” says Michael McClure, Great Plains’ engineering manager for grain drills. “It has relatively few and simple parts. But we had continual problems with it.” Besides the wrap spring itself, the clutch contained two main components: a stepped shaft/hub part into which the jackshaft fits, and a drive plate/bearing part to which the drive sprocket bolts. “We had both parts made of powdered metal,” Mr. McClure explains. “The tools were simple single-cavity molds, so the parts were fairly inexpensive to produce.”
The company soon found it to be false economy. Powdered metal parts are porous, and they are first immersed in a hot oil bath where they absorb oil. In operation, shaft rotation draws a small quantity of oil from pores to the surface, much like sintered-bronze plain bearings do. However, Mr. McClure notes that “...the clutch is used in a very rugged environment. It’s totally out in the open, with no shielding. It’s very dirty and subject to rain, dust, and mud.”
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Great Plains found that the oil film readily trapped these contaminants if not maintained regularly. “The two halves would become bonded together, and the clutch would fail to disengage,” Mr. Mc- Clure comments. The seeding mechanism was therefore running whenever the grain drill was moving, causing unwanted metering of seed and fertilizer.
Rather than modify the existing clutch, Great Plains decided to completely redesign, replacing the wrapspring clutch altogether. It settled on a cast-iron jaw clutch for its inherent reliability, positive engagement and disengagement, and ease of maintenance, according to Mr. McClure. He made sure the bearing surface of the new clutch was designed to run without wet lubricants.
Pressed into a bore in the drive plate of the jaw clutch, Figure 1, is a glassfiber- reinforced plastic bearing that supports the driven spindle. The flanged bearing by igus Inc. is of “G300” material, which resists abrasion and has a maximum load rating of 11,600 psi. The matrix material itself is unaffected by most weak acids, organic solutions, alkalis, petroleates, and agricultural chemicals. Nominal bearing dimensions are 1¼-in. ID by 113/32-in. OD on the sleeve, with a flange 111/16 in. in diameter by 5/64 in. thick. Overall length is 7/8 in.
The plastic bearings are designed to run with no wet lubricant. A dry lubricant is compounded into the matrix material, providing a coefficient of friction between 0.09 and 0.22 without susceptibility to contamination. The bearing has a PV (Pressure × Velocity) value of 28,600 psi-fpm — well above the needs of the grain-drill clutch application.
The drive plate, which houses the plastic bearing, turns at speeds to 200 rpm, rotating around the spindle, which is stationary when the clutch is disengaged. According to Mr. McClure, the bearing is subject to a significant amount of cantilever pressure, constant vibration, and occasional shock loading. But with the new clutch design in production for more than a year, no bearing has failed. In fact, bearings show nearly no sign of wear, fretting, or deformation. “Servicing the new clutch has been minimal with zero bearing failures,” he says. He also notes that Great Plains now uses the clutch on seven models of grain drill. He adds that the jaw clutch design is easier to assemble and disassemble than the wrapspring design, making service work quicker and less costly.
Carsten Blase is Vice President, igus Inc., East Providence, R.I.