Plain bearings offer a simple way to combine linear and rotary motions in a single device.
When a machine component requires a combination of linear and rotary motions, designers normally opt for two separate support assemblies, each with its own set of bearings. If the movements must be precise, the designers often select rolling-element bearings for both types of motion. Though often overlooked, plain bearings may offer a simpler, less expensive solution for such applications – especially those involving limited rotary motion.
Plain bearings (also called plane bearings) can slide on a shaft in a linear direction or pivot in a rotary direction. They resist contamination, and often provide a degree of precision matching that of rolling-element bearings. They're also simple devices that have no moving parts.
One of the types commonly used for linear and rotary motion consists of a metal outer shell with a composite liner bonded inside. The liner is self-lubricating, typically containing PTFE or Teflon reinforced by filler materials to improve durability. Such bearings handle loads up to 3,000 psi (plain bearing load capacity is normally rated in pressure). Their coefficient of friction ranges from 0.15 to 0.25.
Other versions that accommodate both types of motion include bronze bearings and solid plastic bearings. Bronze bearings handle loads as high as 40,000 psi. They need lubrication, relying on heat to release oil from their pores. This makes them better suited to rotary applications, which develop the temperatures needed to release the oil. Also they have a higher coefficient of friction, 0.3 to 0.35.
Solid plastic bearings are a lowcost alternative for light applications, having a load capacity less than 3,000 psi. They tend to deform or cold flow at high speeds, loads, and temperatures. And moisture causes them to swell. Their coefficient of friction is 0.15 to 0.25.
Where they work
Plain bearings generally work well for applications that consist mainly of linear motion (continuous or oscillating) combined with a pivoting motion, or rotation of less than 360 deg. Such applications are found in automotive assembly, packaging, printing and converting, pharmaceutical, and paper industries. In one example, a packaging machine moves a bag from one station to another, then rotates to unload the filled bag.
Other applications combine linear motion (either continuous or oscillating) with high-speed rotary motion. This category is better served by rolling-element bearings because they have less contact area between bearing and shaft and so generate less heat during rotary motion.
Now lets see how plain bearings handle some specific applications.
An engineer at a General Motors assembly plant bemoaned the fact that his rolling-element bearing assembly had failed again. The assembly provides both linear and pivot motion for a pallet stop in a conveyor system. A circulating ball bushing on a 3/4-in. diameter shaft allowed linear travel of the shaft followed by a 180- deg pivot. Side loading during the pivot caused the ball track within the bearing to break and the individual balls to fall out. Compounding the problem, the conveyor was exposed to dirt, dust, and excess lubricants.
Searching for an answer, the engineer found that plain bearings offer two key advantages for this application. First, unlike ball bearings, they don't need to be protected from the contamination inherent in this application. Second, catastrophic bearing failure is unlikely. Ball bearings can fail suddenly, damaging other components. But the liner in a plain bearing wears gradually, so you can replace it before it fails.
The plant switched to plain bearings, replacing the $160 dual rollingelement bearing assembly with a single plain bearing costing about $16. The new bearings have worked in this application for two years without a hitch.
Paper mill dryers
Thermo Electron, a major supplier of "doctor assemblies" in paper mills, has adopted plain bearings for paper mills around the world because of their ability to handle shock loads, vibration, and contamination.
A doctor assembly consists of a blade that extends the width of a paper machine -- up to 20 ft -- and is supported by a large shaft (2 to 8-in. diameter) with two bearings. These bearings let the razor-sharp blade oscillate back-and-forth to scrape or "doctor" excess paper and contaminants from a dryer drum. The bearings also let the blade assembly pivot away from the drum for frequent blade cleaning and replacement.
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These doctor assemblies, many of which weigh thousands of pounds, commonly use ball bushings. With this type of bearing, each ball concentrates the load at the point where it contacts the shaft. But a special type of plain bearing, called a doctor bearing, distributes the load across a wide surface, giving it up to four times the load capacity of ball bushings traditionally used in these applications.
Because of its resilience and large contact area, the composite liner in a doctor bearing damps machine vibration and shock loads, preventing fretting corrosion and brinelling of the shaft as well as tears or breaks in the paper web.
Paper mills are typically hot, humid, and dusty. Contaminants in the form of adhesive mist, paper dust, and lubricants can work their way past conventional bearing seals to cause wear, stick slip, and sudden failure. However, a doctor bearing has double-lip seals that fit snugly on the shaft to exclude contaminants. The seals continually push excess lubricant, paper dust, and adhesive aside at both ends of the bearing's linear stroke, creating "contamination doughnuts" around the shaft.
In the pharmaceutical industry, packaging machines run at high speed, and they are exposed to a corrosive environment and frequent washdowns. One large pharmaceutical manufacturer near Chicago recently switched to plain bearings to solve an ongoing maintenance problem on a highspeed bag-filling line involving both linear and pivot motion. Bags move along the production line at high speeds, are filled and sealed. Then the machine pivots to unload the finished bags, transferring them to another operation.
The bag filler originally incorporated 2-in. diameter recirculating ball bushings running on 400 series stainless steel shafts. Constant washdown was causing the stainless steel shafts to corrode. The ball bushings required constant lubrication to protect them from the washdown, but excess lubricant threatened to contaminate the production process. Further, any misalignment in the pivot action caused concentrated loads that broke the ball bushings.
The plant replaced the shafts with 300 series stainless steel ones, which have more corrosion resistance, but are too soft for use with ball bushings. The bushings were replaced by plain bearings with a Teflon-based composite liner that is compatible with stainless steel shafts. The new bearings are selflubricating and corrosion resistant, so lubrication is no longer needed to protect against washdowns. The threat of catastrophic failure is eliminated due to the predictable wear rate of plain bearings.
Carrying the load
Depending on the material used, a plain bearing can carry more load than a recirculating ball bushing, the most commonly applied rolling-element bearing for linear motion. There are two reasons:
• Surface area between the bearing and shaft is larger than that of ball bearings, which have point contact.
• Only one or two tracks in a recirculating ball bushing carry the load at a given time. A ball bushing must be oriented so that enough ball tracks can be positioned to carry the load, whereas a plain bearing can be mounted in any orientation.
Watch your speed
Speed affects a plain bearing liner most through heat buildup. This isn't critical in most linear applications because heat dissipates over the length of travel. But short strokes or high speeds, typically over 140 fpm, may generate enough heat to cause thermal expansion and tightening of the bearing ID on the shaft. Designers can adjust for this effect with a compensated bearing, which has a slightly larger ID to allow for thermal expansion.
In rotary applications, the generated heat stays in one location and builds up. As the bearing ID closes due to thermal expansion, friction against the shaft increases, creating more heat. Such cases may require cooling or a switch to rollingelement bearings.
Mark Huebner is a specialized products engineer at Pacific Bearing Co., Rockford, Ill.