1. Use two center holes for positioning and clamping

Generally, the important outer circular surface is used as the coarse benchmark for positioning, and the central hole is machined, and then the central hole at both ends of the shaft is used as the fine benchmark for positioning; As far as possible, the datum shall be unified, overlapped and mutually referenced, and multiple surfaces shall be installed and processed at one time. The central hole is a unified positioning benchmark and inspection benchmark for workpiece processing. Its own quality is very important, and its preparation is relatively complex. It is often positioned with the support journal, and the central taper hole is turned (drilled); Then position with the center hole and finish turning the outer circle; Outer circle positioning and rough grinding of taper hole; Positioning with the center hole and fine grinding the outer circle; Finally, the outer circle of the bearing journal is used for positioning, and the taper hole is finely ground (scraped or grinded), so that the accuracy of the taper hole can meet the requirements.

2. Positioning and clamping with external surface

If it is impossible to locate hollow shaft or short shaft with center hole, the outer circular surface of shaft can be used to locate, clamp and transmit torque. Generally, three jaw chuck, four jaw chuck and other general-purpose fixtures are used, or various high-precision automatic centering special fixtures, such as liquid plastic thin-wall centering fixture, diaphragm chuck and so on.

3. Positioning and clamping with various plugs or pull rod mandrels

When machining the outer surface of hollow shaft, various plugs or pull rod mandrels with central holes are often used to install the workpiece. Plugs are commonly used for small taper holes; Pull rod mandrel with plug is often used for large taper hole.

The following methods are generally adopted for bearing installation:

(1) Press fit

When the inner ring of the bearing is tightly fitted with the shaft and the outer ring is loosely fitted with the bearing seat hole, the press can be used to press the bearing on the shaft first, and then the shaft together with the bearing can be installed into the bearing seat hole. During press fitting, an assembly sleeve "copper or mild steel" made of soft metal material shall be placed on the end face of the bearing inner ring.

The inner diameter of the assembly sleeve shall be slightly larger than the diameter of the journal, and the outer diameter shall be slightly smaller than the edge of the bearing inner ring to avoid being pressed on the cage. When the outer ring of the imported bearing is tightly fitted with the bearing seat hole and the inner ring is loosely fitted with the shaft, the bearing can be pressed into the bearing seat hole first. At this time, the outer diameter of the assembly sleeve should be slightly smaller than the diameter of the seat hole.

If the bearing ring is tightly fitted with the shaft and seat hole, the inner ring and outer ring should be pressed into the shaft and seat hole at the same time during installation. The structure of the assembly sleeve should be able to press the end faces of the bearing inner ring and outer ring at the same time.

(2) Heating fit

By heating the bearing or bearing pedestal, the installation method of changing the tight fit into the loose fit by thermal expansion. It is a common and labor-saving installation method. This method is suitable for the installation of bearings with large interference. Before hot assembly, put the bearing or separable bearing ferrule into the oil tank and heat it evenly to 80 ~ 100 ℃, then take it out of the oil and install it on the shaft as soon as possible. In order to prevent the inner ring end face and the shaft shoulder from not fitting tightly after cooling, the bearing can be tightened axially after cooling. When the outer ring of the bearing is tightly fitted with the light metal bearing, the hot fitting method of heating the bearing seat can avoid scratching the mating surface.