Every industry insider knows about self-lubricating sintered bearings. They’ve pretty much transformed every machine architecture, large and small. Before these bushings came along, chunky multi-element bearings were fed by intricate oil carrying lines, or they were periodically greased by busy maintenance technicians. Interestingly, focusing on that self-regulating lubricity feature, sintered bushings are loaded with different types of porosity fillers to serve entirely different machine handling applications.

Graphite-Imbued Sintered Bearings

Much has been said about the oil and liquid additives that inhabit the capillaries of plain bearings. When a machine starts to spin, the laws of physics force a film of oil to coat the smooth ring. On the periphery of this process, however, there are other highly popular options available. One of the more common lubricity facilitation agents in use today is graphite. Loosely described as a slippery form of carbon, the dark grey material is electrically conductive. This form of carbon has a low coefficient of friction, just like  liquid oil, and sintered bearings carrying this lubricating agent tend to display enhanced thermal attributes. For some common applications here, expect to see graphite-sintered bushings used in reciprocating motors and devices that use a lot of oscillating motion. A purely oil-filled bearing would have trouble building up an initial film of friction-cancelling grease in such applications, so a seizure becomes more likely.

Fluid-Occupied Sintered Bearings

Wholly fluidized, although some variants do use a 5% to 10% graphite additive, oily self-lubricants are best suited for light-loading applications, especially those that spin at high velocities. Precision-based fractional horsepower motors often employ this technology. Servo motors, tiny drill motors, and stepper devices favour this design. With energy supplied to the sintered ring, there’s no material “sticking” and no surface tensioning effects to prevent the high lubricity film from taking up a low-to-moderate mass load as it spins at great speed. Incidentally, since it hasn’t been mentioned as yet, graphite-filled capillaries are used when sliding motion is added to the mix. Sliding rails, wear plates, that kind of action, that’s where they’re utilized. Inside devices, expect to find start-friendly sintered graphite bearings inside reciprocating pumps and oscillating drive systems.

For that latter device class, imagine a series of tiny pumps, all of which rely on reciprocal motion. Coated in slippery graphite, they take up the linear energy so that a small medical pump might deliver a precise dose of drugs. One final note: it takes more than a denser pore filling to accommodate all of the above features. The correct bearing material must also be mixed from iron or bronze, from leaded bronze, aluminium, or from some coppery alloy. Only then can these common applications be matched to a suitable bearing or bushing family.