Metal plain bearings have been the industry standard for many years, but the use of plastic bearings is steadily increasing, especially in contaminated environments. Developed over the last 25 years, these self-lubricating plastic bearings offer performance advantages over metal, namely improved lubricity, elasticity, and corrosion resistance. As a result, more engineers turn to self-lubricating plastic bearings.
Maintaining lubricity
All bearings must be lubricated, not only to reduce friction and wear, but, in the case of plain bearings, to prevent them from seizing the shaft which they support. Metal bearings rely mainly on oil and grease lubricants. But in dirty environments, these wet lubricants trap solid contaminants, causing a loss of lubricity or slipperiness. This increases friction and accelerates wear between shaft and bearing, a condition that is especially critical in low-load, low-speed, stop-and-start, and linear applications. As the lubricants become loaded with dust or dirt, they also lose their ability to dissipate heat, and friction rises significantly. Inevitably, the moving parts lock together or seize, causing failure and downtime.
By contrast, self-lubricating plastic bearings contain a mix of dry lubricants, which are not easily fouled by solid contaminants. In operation, movement between shaft and bearing causes microscopic abrasion of the dry lubricant, filling and smoothing the shaft surface to reduce friction.
If the shaft surface is too smooth, it inhibits the abrasion of the dry lubricant. As a result, the lubricant is ineffective and stick-slip can occur. For this reason, don’t use plastic bearings with chrome-covered shafts. If stick-slip occurs, roughening the shaft with fine sandpaper usually produces the necessary abrasion of dry lubricant to restore free movement.
Most plastic bearing materials expand when exposed to heat and moisture. But this factor is only significant when the running clearance between the bearing and shaft is less than 0.001 in. Plastic bearing manufacturers recommend larger clearances to eliminate potential problems, such as excessive wear or seizing of the shaft, from expansion of the bearing material.
Elasticity
Plastic bearings have less strength and load capacity than metal types. But their resilience enables them to absorb vibration and moderate shock loads that would cause metal bearings to squeak. In addition, plastic bearings can be more forgiving of misalignment. They bend and return to their original shape when the misalignment force is released.
With plastic bearings, especially those that contain no reinforcing fibers, the material can deform under edge loading without affecting bearing performance. The lubrication capabilities of the bearing material are effective even during misalignment. Plastic materials that contain reinforcing fibers possess more load capacity but are less able to deform and return to their original shape.
Combating corrosion
Plastic bearings do not corrode as a result of moisture. Some of the plastic materials commonly used in these bearings, such as PEEK, Torlon, and PTFE (Teflon), offer resistance to many chemicals, even etching acids, sulfuric acid, and chlorine gas in concentrations up to 65%.
Other bearing materials, such as nylon and Delrin, contain a blend of thermoplastic alloys combined with reinforcing composite fibers to provide high strength for applications involving extreme pressure and heat. The lubricants imbedded in the plastic material can’t be washed away or eaten away by chemicals, as can occur with wet lubricants.
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Because these dry lubricants resist high temperatures, they are typically used in ovens and textile machines, as well as semiconductor and heavy machinery manufacturing equipment. These bearings have a coefficient of friction between 0.090 and 0.18 during operation with steel shafts. In applications such as chemical pumps, acidic fluids can provide additional lubrication to the bearings, further reducing friction and wear.
Bearings made of PEEK (polyaryletherketone) plastic resist acid in chemically aggressive environments, such as found in battery and semiconductor manufacturing applications.
Outdoor applications
Fiber-filled plastic bearings designed for outdoor applications resist corrosion and weather. A hard crystalline surface, combined with the use of dry lubricants, increases the bearing’s resistance to wear and abrasion. Also the fibers in this material resist water absorption and prevent swelling of the bearing.
Various types of plastic bearings have been developed for use in automotive and aeronautical manufacturing, agricultural and gardening equipment, construction industry equipment, and pneumatic and hydraulic cylinders that are used in dirty, dusty, highly contaminated environments.
More benefits
Other advantages of plastic bearings include quiet operation and light weight. Because plastic is non-conductive (unless other materials such as carbon or metal are added), it acts as an insulator, for added safety in certain applications.
Plastic bearings solve seeding woes
Plastic bearings solved a sticky problem with two and three-section folding grain drills manufactured by Great Plains Manufacturing of Assaria, Kansas. Pulled by a tractor, these machines plant seeds, driving them into the soil with pairs of sharp steel discs. An operator actuates a clutch to stop the seeding at the end of a row or when the equipment is moved to another location.
The problem arose when dust and gritty contaminants fouled the oil that lubricates metal bearings in the clutch. The oil became gummy, preventing clutch operation, so the seed and fertilizer continued to spill on the ground after the operator tried to disengage the clutch. The only solution was to replace the fouled oil — very inconvenient in a farmer’s field. The contaminants trapped in the oil also caused the bearings to wear out about every six months.
The manufacturer redesigned the clutch, incorporating a flanged plastic bearing impregnated with a dry lubricant that is unaffected by rain, dust, mud, and other contaminants.
According to Michael McClure, Great Plains Manufacturing engineering manager, the clutch bearing is subject to cantilever pressure (edge loading due to tilting between shaft and bearing), constant vibration, and shock loading. But after a year of use in the grain drills, the plastic bearings showed no sign of wear, fretting, or deformation.
Helping mowers clear new ground
In another outdoor application, plastic bearings serve as drive-axle bushings on a self-propelled sickle-bar mower manufactured by Garden-Way Inc. of Troy, N.Y. The mower clears terrain that is overgrown with brush and operates in a dirty, dusty, sometimes wet environment. The 160- lb mower is supported by two 1-in. ID plastic bearings that are impregnated with a dry lubricant. The company expects the bearings to last the lifetime of the mower.
Cutting mail sorter downtime
Oil contamination problems with mail sorting machines prompted the U.S. Postal Service to replaced bronze journal bearings with dry-lubricated plastic bearings on 998 mail sorting machines. Before the switch, belt drive pulleys on the machines routinely failed — despite regular servicing — due to premature bearing wear. Dust and airborne paper particles from the mail were absorbed by the lubricating oil, forming a gummy substance that allowed increased friction and wear.
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The machines operate with five belt-driven pulleys, each containing a pair of bronze journal bearings. The pulleys turn relatively slowly, at 450 rpm, and exert only about 10 lb force on the bearings. This contributed to the lubrication problem, because the light loading failed to draw enough oil onto the shaft to prevent wear.
Each of the mail sorters, operating in postal centers around the country, required about ½-hr each week to clean and oil the original bronze bearings.
As a test, workers fitted one machine with plastic bearings in place of the bronze bearings. The plastic bearings have the same OD as the metal ones they replaced, but their ID is slightly larger, which makes it easier for the heat to escape so they operate cooler. The plastic material also absorbs vibration and shock loading encountered in the mail sorting operation.
Previously, the metal bearings failed after a month of operation, and that sometimes caused damage to the pulleys.
But, the plastic bearings showed no wear after 6 months of 12.5-hrper- day service. Based on these preliminary results, engineers estimate that the plastic bearings will operate at least 12 times longer than the bronze bearings they replaced, according to Mike Lucero, a technician in the Sacramento USPS mail forwarding center.
The postal service replaced the bronze bearings on all of the machines with plastic bearings, and expects a significant cost savings due to reduced maintenance. In addition, the plastic bearings run quieter than the metal ones, which squeaked and squealed continuously, Mr. Lucero notes. “With these bearings, this is going to be a labor-free device now,” he comments.
Carsten Blase is vice president, igus Inc., East Providence, RI.
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