Use a bushing where the shaft is heavy, slow, dirty or shocked, and a ball or roller bearing where it is fast and clean, because rolling friction is only 1/10th to 1/100th of sliding friction but the rolling parts must be hardened and ground far more precisely. The workshop rule of thumb puts the switch at about 500 rpm, though the true limit is heat, which depends on how the bushing is oiled and cooled.
Part of Bearings and Bushings
Choosing between a sliding sleeve bearing and a rolling-element bearing for a given shaft, from speed, load, contamination and what the workshop can make.
A Bushing Slides And A Ball Bearing Rolls, So They Fail In Different Ways
In a bushing the shaft slides against a stationary sleeve. It is simple to make, forgiving of dirt and shock, and heavier in friction. In a ball or roller bearing hard elements roll between two races, so the contact moves instead of dragging. Bearings and Bushings gives the rolling friction as 1/10th to 1/100th of sliding; Roller Bearings gives 10-50 times lower. Those two ranges disagree at the top end, so plan on a tenfold saving and treat anything beyond that as a bonus.
Four differences matter more than the headline ratio.
Starting from rest. A bushing starts with no oil film: the film forms only once the shaft is turning fast enough to drag oil into the wedge (see Plain Bearings). A machine that starts and stops often spends much of its life in the worst regime, boundary lubrication. A rolling bearing starts almost as easily as it runs.
Shock. Bearings and Bushings says bushings tolerate shock; Roller Bearings says rolling bearings do. The second article also warns never to strike the outer race of a mounted bearing because it will dent (brinell) the race. A hard race with a tiny contact patch has no give; a bronze bushing deforms and conforms. For a trip hammer or a crusher, assume the bushing is the safer choice.
Dirt. The leaded phase in bronze traps small particles, which is why the carrier article credits it with aiding break-in. Grit between a ball and its race scores both. Rolling bearings want shields, seals or a clean room.
Who can make it. A bronze bushing is cast and bored on a lathe. A ball bearing needs balls within +-0.01 mm and races ground smooth (see Ball Bearings), which the carrier says should not be attempted until the shop holds +-0.02 mm routinely. Until then, a rolling bearing is something you salvage, not something you make.
What You Need
- The shaft speed in rpm and the load in newtons
- A description of the environment: dusty, wet, clean
- A way of oiling or greasing the bearing that you can keep up (see Lubrication Systems)
- The honest answer to “can we make this part?”
Method: Choose In This Order
Step 1: Estimate the heat. Use the sum in Friction Fundamentals: surface speed, times friction force. If the bushing’s heat is more than its housing can shed, you have found your limit.
Step 2: Check the environment. Dust and shock favour the bushing.
Step 3: Check what you can make. If you cannot hold the dimensions, use a bushing and accept the friction.
Step 4: Check the rule of thumb last. Below about 500 rpm and under heavy load, a bushing; above that, where low friction really matters (generators, spindles), rolling.
Worked Example
The 50 mm shaft from the friction page carries 2,000 N at 300 rpm, surface speed 0.785 m/s. In a lubricated bronze bushing the friction force is 40-120 N (coefficient 0.02-0.06), so the heat is 31.4-94.2 W.
Apply the carrier’s 1/10th to 1/100th ratio to the friction force: 40 / 100 = 0.4 N at the low end and 120 / 10 = 12 N at the high end. Heat is 0.4 x 0.785 = 0.31 W up to 12 x 0.785 = 9.4 W.
That is a saving of a factor of ten or more, but the figure assumes a correctly packed bearing. Over-packing with grease causes churning heat; fill only 30-50% of the internal space (Roller Bearings).
Common Mistakes
- Putting a salvaged ball bearing on a machine that is shock-loaded and unsealed. The races dent, then the bearing runs rough, then it fails.
- Treating 500 rpm as a law. It is a rule of thumb, and a well-oiled bushing on a light load can run faster. The limit is heat, so work it out.
- Assuming rolling bearings need no lubrication. They need less, but a dry rolling bearing wears out fast.
- Using a bushing to save effort in making when the shaft starts and stops many times a day. The film never has time to form.
Safety
A seized bearing does not stop the machine
When a bushing seizes on a driven shaft, the belt, the flywheel and the gear train upstream keep their energy and something has to give: a belt tears, a shaft twists, a key shears. Name the stop (where it is, who can reach it, how long the machine coasts) before the first test run, and never stand in line with a flywheel on a first start.
What’s Next
- Learn next: Lubrication Theory - why a bushing’s friction changes with speed
- Combine with: Bronze Bushings - the usual sliding bearing when you do have bronze