Many plain bearings consist only of a metal cylinder with a layer of composite material bonded to its ID. The self-lubricating composite material allows linear and rotary motion between bearing and shaft.

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.

Automotive conveyors

The composite liner in a plain bearing cuts the amount of vibration transmitted from machine to workpiece, and prevents contaminants from damaging the bearing and shaft.
Select figure to enlarge.

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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