A bearing fails by heat as much as by wear, because every watt of friction becomes heat in the bearing: the power lost equals the friction force times the sliding speed, so doubling the speed at the same load and friction coefficient doubles the heat. Lubricated bronze on steel drags at roughly 2-6% of the load pressing it, while the same pair dry drags at 10-20%.
Part of Bearings and Bushings
Friction in a bearing is a force that turns shaft power into heat; how much heat depends on load, surface speed and whether an oil film separates the metal.
Friction Does Its Damage As Heat
Friction is usually described as lost effort, and it is, but a bearing does not stop working because effort is lost. It stops because the lost effort lands in a thin film of oil between two pieces of metal. The heat softens the lubricant, expands the shaft, and thins the film that was keeping the surfaces apart. A bearing that is fine at a walking-pace shaft speed can seize at four times the speed on exactly the same load, because the load did not change but the heat quadrupled.
Three quantities govern this, and they are the same three every other article in this tree comes back to: the load pressing the surfaces together, the sliding speed at the surface of the shaft, and their product, which is the heat. A bearing has a safe load, a safe speed, and a safe combination of the two that is lower than either alone suggests.
What You Need
- The shaft diameter in metres and its speed in revolutions per minute
- The load the bearing carries, in newtons (1 kg of weight is about 9.81 N)
- A friction coefficient for the material pair (a coefficient of 0.02-0.06 is a friction force of 2-6% of the load, and 0.10-0.20 is 10-20%), from the table in Bearing Materials: bronze on steel is 0.10-0.20 dry and 0.02-0.06 lubricated; steel on steel is 0.50-0.80 dry and 0.05-0.15 lubricated
- A note of what the lubricated figures assume: an oil or grease film actually present on the surface, and a housing that can shed the heat into the surrounding air
Method: Estimate The Heat Before You Build
Step 1: Surface speed. Speed at the shaft surface is v = pi x D x n / 60, with D in metres and n in rpm, giving metres per second.
Step 2: Friction force. F = coefficient x load.
Step 3: Heat. Power lost = F x v, in watts. All of it becomes heat in the bearing.
Step 4: Compare the two regimes. Run the sum twice, once with the lubricated coefficient and once with the dry one. The gap is the size of the risk if the oil runs out.
Worked Example
A 50 mm shaft carries 2,000 N (about 204 kg) at 100 rpm in a bronze bearing on a steel shaft.
- Surface speed: 3.1416 x 0.05 x 100 / 60 = 0.262 m/s
- Lubricated, coefficient 0.02-0.06: friction force 40-120 N, so 40 x 0.262 = 10.5 W up to 120 x 0.262 = 31.4 W
- Dry, coefficient 0.10-0.20: friction force 200-400 N, so 52 W up to 105 W
Now run the same shaft at 300 rpm. Surface speed is 3.1416 x 0.05 x 300 / 60 = 0.785 m/s, three times as fast, and the lubricated heat becomes 40 x 0.785 = 31.4 W up to 120 x 0.785 = 94.2 W. Same load, three times the heat.
These figures assume the coefficient stays where the table puts it. It does not: as a bearing heats, the oil thins, and the coefficient climbs toward the dry range. That feedback loop, not the first number, is what seizes bearings.
How many watts a particular housing can shed into still air is not given anywhere in this wiki, so do not trust any calculation that claims a safe watt figure. Measure it instead: run the machine and watch for heat, as described below.
Common Mistakes
- Using the lubricated coefficient for a bearing that is only occasionally oiled. The 2-6% figure applies while a film is present. Between oilings, the bearing runs closer to the dry range.
- Treating speed as irrelevant because the load is light. Heat is load times speed. A light load at high speed can out-heat a heavy load at crawling speed.
- Checking temperature by hand on a running machine. See the safety note.
- Trusting a speed or load limit with no stated lubrication. A limit quoted without saying how the bearing is oiled and cooled is a number without a condition. Ask what it assumes.
Safety
Never reach across a turning shaft or belt to feel a bearing
A line shaft, a belt or a flywheel carries enough stored energy that your own reach is the whole safety system. Stop the machine, name where the stop is and who can reach it, and wait for it to coast down before putting a hand on a housing. A bearing too hot to hold for about 3 seconds is already overheating (see Plain Bearings); shut it down, do not test it again by hand.
What’s Next
- Learn next: Sliding vs Rolling Friction - why a ball bearing drags much less than a bushing
- Combine with: Lubrication Theory - how an oil film changes the coefficient as speed rises