Bushing Tolerance

Standard bushing tolerances are specified on their respective dimensiontables.

1. Standard Tolerances: Bushing tolerances are typically defined in specific tables associated with the bushing type or manufacturer.

2. Key Components & Recommended Fits (Based on ISO 286-2):

   • Housing Bore: Toleranced to H7. This is a standard hole basis tolerance, providing a specific range for the housing bore diameter.

   • Bushing Inner Diameter (ID): Toleranced to H9 after it has been mounted (pressed) into the housing. This is crucial – the ID changes upon installation due to press-fit compression. H9 provides a larger tolerance range than H7.

   • Shaft: Toleranced between f7 and h8.

     • f7 typically provides a clearance fit (shaft is always smaller than the tightest bushing ID).

     • h8 typically provides a transition or slight clearance fit (shaft can be very close to or slightly smaller than the tightest bushing ID). The choice between f7 and h8 (or others) depends on the required operational clearance.

3. Factors Influencing Tolerances:

   • Material: Tolerances depend heavily on the bushing material (e.g., bronze metal vs. various plastics).

   • Wall Thickness: Thicker or thinner walls can affect how the bushing behaves during installation and operation.

   • Environmental Factors (Especially for Plastics):

     • Moisture Absorption: Plastic bushings can swell when they absorb moisture. This must be accounted for in the tolerance selection to prevent the bushing from becoming too tight on the shaft.

     • Thermal Expansion: Both plastic and metal bushings expand and contract with temperature changes. This is often more pronounced in plastics and must be considered, especially in applications with wide temperature ranges. Low-absorption metal bushings can still face issues if thermal expansion isn't considered with tight tolerances.

4. Purpose of Tolerances:

   • Tolerances define the upper and lower limits of deviation for dimensions (like the housing bore diameter, shaft diameter, and the bushing's inner/outer diameters before and after mounting).

   • These limits, based on standards like ISO 286-2, ultimately determine the smallest and largest possible clearance or interference between the mating parts (shaft/bushing and bushing/housing). This ensures proper function (e.g., free rotation, correct press-fit).

In essence, selecting the correct tolerances for the housing, the shaft, and understanding how the bushing's dimensions change based on material, environment, and installation is critical for achieving the desired fit and performance of a plain bearing assembly. The provided H7/H9/f7-h8 combination is a common starting point for many general-purpose bushing applications.

Bushings and Plain Bearings - Tolerances

TOLERANCE DETAILS

Housing ØH7

Bushing inner-Ø after mountingH9

Shaft tolerancef7 to h8

Tolerances and measuring system The installation dimensions and tolerances of viiplus® bronze metal bushings are dependent on material and wall thickness. For each plastic bushing material, moisture and thermal expansion are critical. metal bushings with low moisture absorption can be obstructed when there is a minimal amount of tolerance.

the upper and lower limits of bore or outside diameter deviations for bearings with Normal tolerances; the upper and lower limits of shaft or housing bore diameter deviations for relevant tolerance classes in accordance with ISO 286­2; the smallest and largest values of …