Bushing and Plain Bearings Press or Shrink Fit Design and Application

Press fit or shrink fit bushing design and installation is a common method of retaining bearings by use of interference between the bushing and the bushing hole. Bushings are available in standard sizes with both inside and outside diameters surfaces finished, and are commonly available in many sizes.

Typically, bushings are designed and manufactured 0.002 to 0.003 inch over nominal on outside diameter sizes for sizes 3 inches or less. Diameters greater than 3 inches, the outside diameters are designed 0.003 to 0.005 inch over the nominal size. Because interference material is built into the bushings, and the bushing is normally manufactured to tolerances of +/- .0002. The effective interference of the press fit is designed and facilitated within the target hole which the bushing is installed.

The process of press or shrink fitting the bushing into the hole, causes the bearing reduce in size due to the compressive forces applied. Additionally, the material which the bushing has been pressed into distorts some as well. In general, this diameter changes are equal to 70 to 100 per cent of the amount of the material interference or fit. Unfortunately, precisely or mathematically predicting the size change due to the effective compressive forces applied during installation has not been shown to be accurate. Typical installation compression forces and effective diameter changes may be modeled or characterized by fabricating several test cases and measuring the range and variations of the actual fit. Often, bushings are designed with additional diametrical material requiring a post press fit installation machine process to accurately ream to the desired diameter.

Shrink fits are facilitated by chilling the bushing using a suitable median, such as liquid nitrogen, dry ice, or placing the bushing within a freezer. Cooling down the bushing is preferred rather than heating the target housing. Normally, the bushing is of significantly less mass than the target housing, though heating the housing and simultaneously cooling the bushing is no uncommon .

When a bushing or bearings are pressed into a housing, the force driving the bushing / bearing should be applied equally or uniform to the end of the bearing to avoid galling, peening or damaging the bushing / bearing. The mating surfaces of both the bushing and housing should be thoroughly cleaned and imperfections as well as burrs removed.