What does each form do?
Four forms cover most plastic bushing jobs.
| Form | What it carries | Where it suits | The trade-off |
|---|---|---|---|
| Sleeve (cylindrical) | Radial load only | The baseline for rotating, oscillating or reciprocating motion | It does not hold itself against axial movement |
| Flanged (T-shaped) | Radial load through the bore, and light axial load on the flange | Where axial movement must be held, or a thrust face is needed | The flange can break if the hole is not square or the bushing is tilted |
| Thrust washer | Axial load between two faces | Where end-face wear is the main problem | It carries almost no radial load, so it is paired with a sleeve |
| Split | Radial load, with a reduced contact area | Where the shaft cannot be pulled out to fit or replace the bushing | Lower load capacity and sealing than a one-piece bushing |
A flange gives a thrust surface for light axial loads and spreads the load better than a plain end.
Which form for which duty?
Start from the direction of the load.
When the load is radial only, and nothing pushes the bushing along the shaft, a sleeve is the first form to consider.
When the load is radial but the bushing or shaft must also be held axially, a flanged bushing does both in one part. With combined loads you must check two things: the PV on the bore, and the face pressure on the flange. The flange is not a free extra. It has its own limit.
When the load is mostly axial, use a thrust washer, or a flanged bushing with enough flange area, and check the face pressure.
When the machine cannot be taken apart, a split bushing lets you fit it without pulling the shaft. The split reduces contact area, so it carries less than a solid bushing. Place the split 90° away from the main load direction, and do not use split bushings where loads are high.
For oscillating or start-stop motion, PTFE or composite liners are the usual favourite, because they suit that kind of boundary-lubricated duty (Q1-01).
How long and thick should the bushing be?
There is no one-size-fits-all length or ratio. Two things decide it.
Length sets the pressure. The load is carried on the projected area, which is the bore times the length. Take a 2 kN load on a 25 mm bore. A 25 mm long bushing gives 2 000 ÷ (25 × 25) = 3.2 MPa. A 40 mm long bushing gives 2 000 ÷ (25 × 40) = 2.0 MPa. A longer bushing lowers the pressure, which helps against the PV limit (Q1-02).
Length also makes edge loading worse. A misalignment of angle θ opens a gap of L × tan θ between the two ends of a bushing of length L. For 0.5° of misalignment, a 25 mm bushing sees a difference of 0.22 mm between its ends. A 40 mm bushing sees 0.35 mm. Compare that with the running clearance the grade needs (Q1-03). A longer bushing carries more load, but it is less forgiving of tilt.
Wall thickness depends on the material, the clearance and the housing support, and a thicker wall calls for larger tolerances. Copying the nearest catalogue size and changing only the bore can leave too little wall for the load or the retention (Q1-04). Ask the maker for the wall and length data of the grade.
What is edge loading, and where does it come from?
Edge loading means the load is carried on one end or edge of the bushing instead of along its whole length. It is a chief factor in bushing failure, especially in farm equipment, where lateral forces are common.
It comes from three places:
- Tilting or torsional moments applied to the bushing.
- Poor self-alignment where bearing points are in a row. Tolerances and dimensional errors create alignment errors, and those load the bushing at its edge.
- A load applied away from the bushing's centre.
In a flanged bushing, edge loads can create shear forces between the flange and the body, which can separate the flange and push the bushing out of its hole. This is why a bushing that shows damage on one side or a broken flange points to a design problem, not just wear (Q1-09).
How do you avoid edge loading?
Fix the housing first. The hole must be square to the surface, or the flange will not lie flat and can deform or break under stress. The hole must be free of burrs, and a small chamfer is enough. A chamfer that is too large can reduce the load capacity of the flange and the bushing along the running surface.
Then deal with the alignment. Where several bearing points are in a row, check that tolerances do not force the bushing to tilt. Add support where the load is applied away from the bushing's centre.
If the hole cannot be changed, or the tilting moments cannot be absorbed, split the two jobs: a plain cylindrical bushing plus a separate thrust washer for axial support. The parts then take their loads independently. The price is more assembly work and a slightly higher cost.
A field example
A farm trade paper (Western Producer) reports a harvester header maker that fitted one common-size bushing at six pivot points. Those pivots are not fast: the shafts oscillate slowly through about 25°. After the first year the company moved to a different compound that suited the job better, and it reported no problems after three harvest seasons. This is a trade-press report from one maker, and it shows the point of the article: the right form and the right compound depend on the duty (Q1-12).
What changes on a vehicle frame or chassis?
A tube frame or a thin sheet bracket is much less rigid than the machined housing this article assumes. Under load the two brackets that carry a pivot can twist relative to each other, and two brackets welded separately add up their position errors. Both tilt the bushing, which is how edge loading starts (Q1-09).
An illustration: a 1° relative tilt across a 40 mm long bushing opens a gap of 40 × tan 1° ≈ 0.70 mm between its ends, and across a 25 mm bushing about 0.44 mm. Compare that with the running clearance (Q1-03). On a flexible bracket, a shorter bushing or a sleeve plus a separate thrust washer is more forgiving than a long flanged part. Check that the flange seats on a flat, burr-free face, because a thin bracket can dish around the hole.
This applies to non-suspension pivots such as pedals, shifters and levers. For control-arm and other suspension bushings, see our sister site's Bushings Resource Center. Pivots in steering and brake linkages are safety parts, so take the bushing form and the service limit from the vehicle maker.