How Do I Measure Wear on a Plastic Bushing, and When Should I Replace or Take It Out of Service?

What does "worn" mean for a plastic bushing?

Wear at the inner diameter increases the diameter, and that is what produces play. A plastic bushing is solid polymer with no layered structure or backing to expose, so the whole wall is available as wear zone and there is no sudden "wear-through" point to mark the end of life. For that reason a plastic bushing has no universal wear limit. The limit is the maximum clearance the application permits.

That limit depends on the job, and the range is wide. A precision control valve tolerates only a few hundredths of a millimetre of wear. A farm pivot on a shaft above 50 mm can accept a clearance of well over a millimetre. Allowable wear is subjective, and each application has its own requirement; a plastic-bearing maker's lifetime calculator, for instance, defaults to 0.25 mm and offers five choices of maximum wear.

So the question to answer is not "how worn is it?" but "how much play can this joint tolerate before it stops doing its job?"

How do you measure a plastic bushing?

Use the right tool, at the right time, on the right parts.

Do not use calipers on a plastic bushing. The jaws press into the soft material and compress it, so repeated readings disagree.

Measure it installed. A plastic bushing is made slightly oversize because it is press-fitted. It can be measured properly only after it has been pressed into its housing, and a bushing measured out of its housing will read larger than the final figure (Q1-04).

Measure the shaft too. Clearance is the bore minus the shaft. In some joints the shaft wears more than the bushing, so a worn shaft can give you the play even when the bushing looks fine (Q1-05).

Use a go/no-go pin gauge for a quick check. Drop a pin the size of the intended shaft into the installed bushing. If it falls through under its own weight, it is a "go". If it sticks, it is a "no-go". Small peaks and valleys in the bore matter little, because they smooth out as the embedded lubricant is released.

How do you work out the clearance?

With tolerances:

• minimum clearance = minimum bore after press fit − maximum shaft diameter
• maximum clearance = maximum bore after press fit − minimum shaft diameter

For example, with illustrative numbers: an installed bore range of 25.020 to 25.110 mm and a shaft of 24.940 to 24.960 mm gives a minimum clearance of 25.020 − 24.960 = 0.060 mm and a maximum of 25.110 − 24.940 = 0.170 mm. That is the starting window.

Later, you measure the installed bore at 25.400 mm with a 24.950 mm shaft. The clearance is now 25.400 − 24.950 = 0.450 mm. Compare it with the limit you set for the joint, not with a figure from a different machine.

You can also feel for play at the machine. Unload the joint as far as you can, move it by hand or with a light known force, and read the movement with a dial gauge at a known distance from the bushing centre. A movement of 0.3 mm at 100 mm from the centre is an angle of atan(0.3 ÷ 100) = 0.17° (K01-Q1-09).

When should you replace it?

When the clearance reaches the limit you set, or when something else says the bushing is finished.

Set the limit before you need it. Decide what the joint must do. A joint that positions a part needs a small clearance. A joint that only carries a load slowly through a pivot can tolerate more. Use the manual for your machine, or your own record.

Replace sooner when you see:

• A cracked or broken bushing, or one that has softened or melted (Q1-09).
• A swollen bushing that has closed up on the shaft (Q1-03).
• A bushing that has turned in the housing, which means the press fit is lost (Q1-04).
• A worn or scored shaft. A new bushing on a worn shaft will wear out quickly. Replace or refinish the shaft as well.

A stuck bushing is removed by the machine maker's procedure.

What should you record?

Keep a record that works for any pivot: the exact pivot position, the operating hours since installation, the usual work, how it is lubricated, and when the change was first noticed. Keep what you observe separate from what you think caused it, until measurements support the diagnosis.

"Wearing out early" is best defined in measurable terms: a shorter interval than comparable joints doing similar work, or an unexpectedly rapid change in measured clearance. Judging life also needs a wear limit, because a statement that a bushing should last several years cannot be assessed without operating hours, workload and a limit. A clearance measured at each service gives you the trend (Q1-06).

What if you are replacing it and do not know what it was made of?

A worn bore may no longer represent the original size, and appearance cannot reliably identify a polymer grade. When you ask for a replacement, say whether the old bushing worked well or needed a different fit, wear life or retention method, and give the shaft and housing details.

FAQ