There are three types of contact bearings: sliding, rolling and flexural. The simplest bearing in the world is the flexural bearing, such as the rubber bushing at each end of a leaf spring. Though firmly attached to the spring and the chassis, it still allows relative movement by flexing. It's life and range of motion are limited, but modern materials have made it more capable and more common.
Before they invented wheels, prehistoric men invented roller bearings when they used logs to roll heavy objects along the ground. When they used the logs as axles for wheels, they invented sliding bearings – and probably lubrication, too.
The bronze bushing was a major improvement, because even when machined smooth, bronze still has microscopic voids in its surface that can be impregnated with oil, making it self-lubricating. To day many self-lubricating bushings are made of extremely low-friction plastics, and they carry both sliding and rotating loads. At low speeds and/or loads, they have a long service life, but as speed or load increase, sliding bearings generate more friction.
Rolling element bearings are also called anti-friction bearings. They were rediscovered and refined for a low-power machine that needs fric-tion reduction and precision, a ship's clock.
There are three parts to rolling element bearings: an inner race, an outer race, and the rolling elements themselves, either balls or rollers.
"Full complement" bearings fill all the space between the races with rolling elements. These can carry big loads for their size and are used in low-speed applications, such as steering mechanisms. In more refined designs, the rolling elements are in a cage that separates them so they don't rub against each other. These are used for continuous rotation and higher speeds.For a given number of rolling elements, roller bearings can carry more load than ball bearings because the load is spread along a line instead of concentrated at a point. This means wheel bearings can be smaller, but if that load line is 90 degrees to the axle, the bearing carries only radial loads (perpendicular to the axle), and another bearing is needed for axial loads generated in turns.
A double row of preloaded ball bearings carries both loads, but they're expensive and complex to make. In the 1890s, Henry Timken invented the tapered roller bearing, with tapered rollers and cone-shaped inner and outer races. With the rollers at an angle to the axle line, a single less expensive bearing can support both radial and axial loads. This was a major development.
Another major development was the self-aligning ball bearing invented by SKF in 1907. The inner race rides on a shaft, and the outer race is mounted in a housing. Two rows of caged balls each ride in their own track in the inner race, but they share the spherical bearing surface of the outer race. This bearing can tolerate slight misalignment between the shaft and the housing, and it has the lowest friction of all rolling bearings.
All contact bearings need lubrication to reduce friction, inhibit corrosion and carry away heat. However, in non-contact bearings, the lubricant also carries the load. A prime example is a piston engine crankshaft rotating in plain bearings.
No matter what the crank-to-bearing clearance is, when the engine is running, the clearance is greater at the top as the piston pushes down on the crank. Oil is dragged from the larger clearance into the smaller clearance, and oil pressure in the smaller space increases enough to counter the load pushing down on the crank, holding the crankshaft up off of the bearing.
This is called hydrodynamic lift, because it's generated by fluid drag, not by the engine's oil pump. In theory, if the engine is never stopped and the oil remains thick enough to withstand the pressure, the bearing will never wear, because it's that thin film of oil that supports the entire load.
As speed increases, the lubricant in non-contact bearings generates more friction. But magnetic bearings generate absolutely no friction at any speed, and they can be used in extremely harsh environments where other bearings would not survive.
Electromagnetic bearings use a sensor to measure the air gap between the bearing elements, and a computer operating closed-loop current controls maintains that gap. They're usually custom-built for the application because they're big and complex, and larger applications need external power and a cooling system. But they offer performance advantages that mechanical bearings cannot match.
Magnetic bearings have been used to support the rotor in vacuum pumps that get too cold for lubricants to work, and they've been used to support trains in Europe and Asia, where Japan's MagLev (magnetic levitation) Train has topped 360 mph.
And just as better bearings made it possible to produce even better bearings, computer-controlled electromagnetic bearings make it possible to produce even better computer chips.
It's been a long way since the log.
