Why is a plastic bushing different?
A polymer bushing adds variables a metal one does not have: it absorbs moisture (some polymers), it expands with temperature several times more than metal, and it creeps under sustained load. Its inner diameter also reaches the final size only after it is pressed into the housing (Q1-04), so that change belongs in the tolerance sum. Copying the clearance of a steel or bronze bushing is therefore a common cause of binding or looseness in the field.
What fit is a reasonable starting point?
A typical start for plastic bearings is a housing bore of H7 (H8 for larger sizes), a shaft tolerance of h9 or h11, which gives more clearance than for a metal bushing, a press-fit oversize that can reach about 2% of the inner diameter, and larger clearances for materials that absorb moisture, such as nylon (MyBushing lists these as its usual start). It is a starting point, not a rule for every material (Q1-05 covers the shaft).
What does moisture do?
It depends on the polymer. Acetal absorbs little, about 0.2% at saturation. Nylon absorbs far more, about 1.5 to 3.0% by weight at equilibrium in ambient humidity and more when soaked, so the bore has to be sized for the swollen dimension. A bushing machined to a 1.000 inch bore at 50% relative humidity may close to about 0.994 inch after soaking, a change of 0.6% (figures from the Ready Plastics bushing guide). The direction matters: a bushing held in a housing swells inward, so the bore closes. For a press-fit bushing, allow extra clearance so the swelling nylon does not bind or crack.
A dry molded nylon sample also does not represent the condition the part reaches in service, so for a close-fitting nylon sleeve, specify the moisture condition at which it is inspected and functionally tested (Q1-07).
What does temperature do, and why does the housing decide the direction?
Plastic expands and contracts about 4 to 10 times as much as metal for the same temperature change. What that does to the clearance depends on whether the outside of the bushing is held.
A free bushing. If the bushing is loose, its whole body grows. The bore gets larger, and the clearance grows with it. Take a nylon bushing on a 25 mm bore with a coefficient of 9.9 × 10⁻⁵ per K, and a rise of 50 K. The bore grows 25 × 9.9 × 10⁻⁵ × 50 = 0.124 mm. A steel shaft of the same diameter grows 25 × 1.17 × 10⁻⁵ × 50 = 0.015 mm. The clearance opens by about 0.109 mm.
A bushing pressed into a rigid housing. This is the usual case. The outside is held to the housing, which expands only as the steel does. The wall cannot grow outward, so it grows inward, and the bore closes. A simple thin-wall estimate: the outside diameter changes by the housing's expansion, the wall grows freely in thickness, and the bore is the difference.
| Case | Outside diameter | Wall growth | Bore change | Shaft growth | Clearance change |
|---|---|---|---|---|---|
| Wall 2.5 mm (outside 30 mm) | +0.0175 mm | +0.0124 mm | −0.0072 mm | +0.0146 mm | −0.022 mm |
| Wall 5 mm (outside 35 mm) | +0.0205 mm | +0.0248 mm | −0.029 mm | +0.0146 mm | −0.044 mm |
For the same 50 K rise, the clearance goes down by 0.02 to 0.04 mm instead of up by 0.1 mm, and a thicker wall closes it more. The estimate ignores two effects that pull in opposite directions: the Poisson effect of the squeezed wall, which makes the closure larger (an elastic hollow-cylinder calculation with a Poisson's ratio of 0.35 to 0.40 gives a clearance loss of about 0.028 to 0.030 mm for the 2.5 mm wall and 0.054 to 0.057 mm for the 5 mm wall, roughly a quarter to a third more), and creep, which lets it relax over time. Treat the table as the direction and the order of magnitude, not as a prediction.
That is why a pressed-in bushing can bind when it gets hot. It also agrees with the nylon moisture example above, where the bore closes as the material swells. The common remark that a plastic bushing gets loose when hot and tight when cold describes the free bushing, not the pressed-in one. Cold has the opposite effect on a pressed-in bushing: it shrinks away from the shaft and the clearance opens. Get the coefficient for the exact grade (Q1-01), and see Q1-16 for the limits of temperature itself.
How do you set the clearance?
Start from the hottest and the coldest temperature at the bushing, not only the air, and from the wettest condition if the material absorbs water. Take the housing and shaft tolerances from the datasheet for the grade, and add the press-fit effect to the bore (Q1-04). Then check the result at both extremes. For a pressed-in bushing, the tight case is hot and wet, and the loose case is cold and dry. The bushing should neither bind at the first nor rattle at the second.