Bearings
Every rotating shaft has to be held in place by something, and that something has to let it spin freely while still carrying whatever load is pushing or pulling on it. Bearings are the quiet, unglamorous components that make this possible — and getting them right or wrong is the difference between a machine that runs for decades and one that seizes within weeks.
What a bearing actually does
A bearing is a component designed to allow relative motion between two parts of a machine — most commonly a shaft rotating inside a fixed housing — while minimising the friction between them and still supporting whatever load is applied. Without a bearing, a rotating shaft would simply drag directly against its housing: metal sliding on metal, generating heat, wearing both surfaces away, and soaking up mechanical energy that should be doing useful work instead.
This links directly back to rotating machinery covered earlier in this module. A flywheel, or any rotating shaft in a real machine, doesn't spin in isolation — it's always supported by bearings at its mounting points, and the frictional torque those bearings introduce is exactly the kind of loss that has to be accounted for when analysing how a real rotating system behaves, rather than the idealised frictionless rotation often assumed in first-pass calculations.
Plain bearings and rolling-element bearings
There are two broad families of bearing design, distinguished by how they handle the interface between the moving and stationary parts.
- Plain bearings (also called sleeve bearings or bushes) are the simplest design: just a smooth cylindrical sleeve, usually made from a relatively soft metal alloy or a low-friction composite, fitted between the shaft and the housing. The shaft slides directly against the inner surface of the sleeve, relying on a thin film of lubricant to keep the two solid surfaces from making direct contact. They are cheap, simple, mechanically robust, and tolerant of shock loading, but they generally produce higher friction than the alternative, especially at low sliding speeds before a full lubricant film has built up.
- Rolling-element bearings use small hardened balls (ball bearings) or cylinders (roller bearings) placed between the shaft and the housing, usually held in place by a cage so they stay evenly spaced. Instead of the shaft sliding against a fixed surface, the balls or rollers roll between an inner and outer race as the shaft turns. This converts what would otherwise be sliding friction into much lower rolling friction, and rolling-element bearings are correspondingly the standard choice wherever low friction and precise, long-lived rotation matter.
Why rolling friction is so much lower than sliding friction
The physical reason rolling friction is generally far lower than sliding friction comes down to what's happening at the contact point. In sliding friction, two solid surfaces are dragged continuously across one another — microscopic high points on each surface catch, deform, and shear against each other the whole time relative motion continues, constantly dissipating energy.
In rolling contact, by contrast, each point on the ball or roller touches the race only briefly: the material at the contact patch deforms slightly under the load as it arrives, and then recovers elastically as it rolls away again, with very little sliding actually occurring between the two surfaces at the contact itself. Because the deformation is mostly elastic (energy is stored and then returned, rather than dissipated as heat through continuous shearing), far less mechanical energy is lost per revolution than would be lost dragging the same surfaces across each other. This is the same basic reason it takes much less effort to roll a heavy barrel across a floor than to slide it.
A bearing that wastes even a small fraction of a percent of the power passing through it doesn't sound like much in isolation — but a machine that runs continuously, for years, converts that small continuous loss into a genuinely significant amount of wasted energy and heat over its lifetime. This is exactly the same idea as the distinction between indicated power and brake power covered in the thermodynamics topic: friction losses at bearings are one of the concrete mechanical loss mechanisms that make a real engine's brake power lower than its indicated power, and reducing bearing friction is one of the most direct ways engineers improve overall mechanical efficiency.
The loads a bearing has to carry
Bearings aren't just about reducing friction — they also have to physically support the forces acting on the shaft, and different applications load a bearing in different directions.
- Radial load acts perpendicular to the axis of the shaft — for example, the weight of a shaft and whatever is mounted on it, pressing down through the bearing into its housing. Most general-purpose bearings are primarily designed to carry radial load.
- Axial load (also called thrust load) acts along the axis of the shaft, tending to push the shaft lengthways rather than sideways — for example, the reaction force on a propeller shaft, or the load on a shaft driving a screw mechanism.
Some bearing designs are optimised to carry radial load only, some are built specifically as thrust bearings to carry axial load, and some (such as certain angled rolling-element designs) are built to handle a combination of both simultaneously. Choosing the right bearing type for a given application means matching its load-carrying capability to the actual combination of radial and axial forces the shaft will experience in service.
Key takeaways
- A bearing allows relative motion (typically rotation) between two parts of a machine while minimising friction and supporting load.
- Plain bearings are a simple sleeve or bush relying on a thin lubricant film to separate sliding surfaces — cheap and robust, but higher friction.
- Rolling-element bearings use balls or rollers that roll rather than slide between the shaft and housing, giving much lower friction.
- Rolling friction is lower than sliding friction because rolling contact deforms and recovers mostly elastically, rather than surfaces continuously dragging and shearing against each other.
- Even small reductions in bearing friction compound significantly over continuous long-term running, directly improving a machine's mechanical efficiency.
- Bearings must support radial load (perpendicular to the shaft) and/or axial (thrust) load (along the shaft), and different designs are optimised for one, the other, or both.
- Bearing friction is one of the concrete real-world loss mechanisms behind the gap between indicated power and brake power in a real machine.