Mechanisms: Bearings

They lie at the heart of every fidget spinner and in every motor that runs our lives, from the steppers in a 3D printer to the hundreds in every car engine. They can be as simple as a lubricated bushing or as complicated as the roller bearing in a car axle. Bearings are at work every day for us, directing forces and reducing friction, and understanding them is important to getting stuff done with rotating mechanisms.

Rolling-Element Bearings

Roller bearing. Source: Machine Design

What first pops to mind when using the term “bearing” is probably something from the broad category of rolling-element bearings, like roller bearings or ball bearings. We’ve all seen them before — tough steel balls or cylinders trapped between two grooved tracks. Legend has it that Leonardo da Vinci invented ball bearings in the 1500s, but like many of his drawings, no practical implementation would be attempted for hundreds of years until advancements in material science and engineering caught up with his original vision.

Rolling-element bearings rely on rolling elements constrained between inner and outer races to reduce friction and manage loads. There are two main loads for bearings: radial and axial. Radial loads on bearings are perpendicular to the long axis of the shaft. The force exerted by the shaft of a giant electric motor’s rotor on the case is an example of a radial load. Ball bearings are great at handling radial loads, but roller bearings, with more surface area along the roller than the point contact of a ball, are even better.

Neither is particularly good at handling axial loads, or loads parallel to the long axis of the shaft, though. An axial load might be a CNC router’s spindle being pressed up into the motor during an overly ambitious plunge cut with an end mill. Because the grooves in the inner and outer races of these bearings can’t be very deep, there’s not much for the rolling element to press against in the axial direction, resulting in poor performance in that dimension.

Tapered roller bearing by Silberwolf, CC BY 2.5

Tapered roller bearings are better at handling axial loads, somewhat at the cost of some radial load handling. Tapered roller bearings are exactly what they sound like — roller bearings where the inner race is shaped like a shallow cone, with a matching taper to the outer race. Tapered roller bearings handle high axial loads, but only in the direction that tends to jam the inner race into the taper of the outer race. If axial loads in both directions need to be handled, tapered bearings are usually used in opposing pairs.

There are, of course, lots of variations on the theme of rolling-element bearings. Sometimes there are two parallel races inside a single bearing for better load handling, or even opposed tapered elements so a single bearing can handle axial loads better. Extra parts like shields and seals can keep grit out or lubricants in. And while most roller and ball bearings are made from steel, other materials ranging from wood to plastic and even ceramics have been used. Ceramic ball bearings have some interesting properties:

Hydrodynamic Bearings

A much simpler type of bearing is the hydrodynamic bearing, or plain bearing. A shaft rotating in a hole is the simplest example, although with direct metal-on-metal contact, such bearings are not long for the world in most practical cases. Most hydrodynamic bearings rely on the properties of fluids to achieve the goals of reducing friction and managing loads. Grease or oil are the most common fluids used as in hydrodynamic bearings; the lubricant forms a film between a shaft and the bearing surface that prevents metal-on-metal contact and is capable of transmitting a huge amount of radial load because of the incompressibility of fluids.

Oil-impregnated bronze bushing from National Bronze

One common hydrodynamic bearing is the sintered bronze bearings found in many hobby motors and servos. Bronze particles are pressed and heated into a porous cylindrical shape with a bored out center. The motor’s shaft passes through the bore, and oil that has soaked into the bore provides the fluid film needed to reduce friction. Plain bearings are also found in internal combustion engine crankshafts, both at the ends of the connecting rods and between the crankshaft and crankcase, and are usually lubricated indirectly through splashing or by oil pumped under pressure through small galleries. In general, plain bearings like these have a layer of softer metal between the rotating parts to prevent damage if the engine is starved for oil, and to facilitate replacement during an overhaul.

Of course, this barely scratches the surface of the field and completely misses fascinating bearings like magnetic bearings where the metal-on-metal contact is prevented by magnetic repulsion, or fluid bearings that use a pressurized gas or a liquid to hold a shaft apart from a journal. But these are at least a few of the most common bearings, and a little of the engineering behind the mechanisms.

Featured image source: GMN Paul Müller Industrie GmbH & Co. KG

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