Why does a plastic bushing need a press-fit?
To stay put. The press-fit seating holds a plain bushing in the housing hole, which stops it slipping out of the bearing point and turning with the shaft. The oversize needed depends on the material, its thermal expansion and its moisture absorption, so each maker sizes its bushings for particular hole fits.
What interference does it need, and what does it do to the bore?
The bushing is made bigger than the hole, by up to about 2% of the inside diameter before fitting, and the bore reaches its final size only after it is pressed into a housing of the recommended tolerance. A housing bore of H7, or H8 for larger sizes, is a typical recommendation (MyBushing). Use the housing tolerance the maker gives for your grade (Q1-03).
What the interference does to the bore can be estimated. A bushing in a rigid housing has its outside held, so the squeeze can only go inward. Take a bushing with a 25 mm bore and a 30 mm outside diameter, pressed in with 0.1 mm of diametral interference (an illustration, not a recommendation). Treating the wall as an elastic hollow cylinder with its outside held and its inside free, the bore closes by 1.05 to 1.06 times the interference, so about 0.105 mm. For a 5 mm wall (35 mm outside diameter) the factor is 1.07 to 1.09. Creep lets the squeeze relax over time, so the closure in service is a little less.
Two practical points follow. The bore after fitting is the nominal bore minus roughly the interference, so the running clearance must be worked out after the press-fit, not from the bore on the drawing of the loose part. And a housing bore that is 0.02 mm too small takes about 0.02 mm out of the bore, which is as large as the clearance changes caused by temperature in Q1-03.
Why does a plastic bushing crack when it is pressed in?
The squeeze is a permanent strain in the wall. In the example above, 0.105 mm of closure on a 25 mm bore is a hoop strain of about 0.4%, held for the life of the part. Anything that adds to it, or that concentrates it, can crack the bushing:
- Too much interference. A housing bore under the maker's tolerance, or a bushing from a different grade or maker than the housing was cut for.
- Sharp edges and burrs. They damage the outside of a soft bushing and start cracks. A missing chamfer also scrapes material off during pressing.
- Hammering or tilting. The bushing goes in unevenly and is loaded unevenly.
- Swelling after fitting. Nylon that takes up moisture grows inward in a closed bore, and a hot pressed-in bushing grows inward too (Q1-03). Leave room in the housing and set the clearance for the wettest, hottest case.
How do you press it in?
Prepare the bore first. It should be within the recommended tolerance, smooth, flat and chamfered, with at least the edge broken to remove burrs. If the maker allows it, cooling the bushing makes it shrink a little and go in more easily. Start it square, with the bushing not tilted in the hole; a guide chamfer on its leading edge helps.
Use an arbor press, not a hammer, which can drive the bushing in unevenly. A rubber mallet is the fallback when there is no other way. Use a flat press head and no centring pin: a pin suits metal bushings, but a plastic bushing is not as round or as coaxial, its guide chamfer is enough, and a pin adds a risk of damage.
How do you check the result?
After the press-fit, not before. A bushing that looks oversize is meant to be. Check the bore with a go/no-go pin gauge: the go side must pass easily through the pressed-in bushing. If it does not, look at these:
- A soft housing. A bore in plastic or aluminium can expand when the bushing goes in, so the bushing ends up over- or under-size. Check the bore dimension and the bore material.
- Scraped material. Material shaved off during pressing means the housing bore needs a chamfer.
- Seizing on the shaft. Too little operating clearance, or temperature or moisture making the bushing swell (Q1-03).
How do you keep it in place?
The press-fit is the retention, and it depends on the squeeze lasting. Heat can undo it. At temperature the squeezed wall creeps and relaxes, and when it cools its outside diameter ends up smaller than the housing bore. That is the usual explanation for a bushing that loses its grip after overheating, and it is consistent with the pressed-in behaviour in Q1-03. In such cases, secure the bushing axially or choose a grade with better creep resistance.
Common ways to do that are a flange on the bushing (Q1-13), a shoulder or retaining ring in the housing, or a stop on the outside face. Gluing is not normally needed. If you plan to bond a bushing, test it first, because a result from another application cannot be carried across without data.
Can you ream it after fitting?
Avoid it. Machining the sliding surface usually raises the wear rate. If the bore is too tight, check the housing, the shaft and the clearance first (Q1-03).