Machine Design

Hydraulic squeeze bushings lets shafts slide, lock, and slide

A new catalog on Ambush squeeze bushings includes sizing guidelines and a selection chart. Squeeze bushings are available for shaft diameters from 0.25 to 12 in. and metric sizes from 10 to 300 mm.

Hydraulic squeeze bushings are used to briefly clamp moving shafts to fixed housings in a wide range of applications, especially machine tools. Type ASA squeeze bushings only resist axial forces. They are commonly used in fixturing and reciprocating shaft applications. For instance, a shaft might firmly hold a workpiece in place on a table until the part is machined. Removing pressure from the bushing allows pulling the shaft back and releasing the piece. Model AST squeeze bushings, however, resist torque and axial forces. One or two holes (for dowel or roll pins) in the mounting cap resist torque and ensure the squeeze bushing does not move.

The bushing clamps only to the shaft, not the bushing OD. Axial loads transmit through the bushing ends while its anchored flange resists torque.

Bushings are made of alloy bronze or spring steel with plastic molded over a slotted squeeze surface. Seals are provided at each end to resist hydraulic pressure up to 5,000 psi depending on diameter.

Hydraulic pressure acts upon a molded sleeve between the seals. Pressure compresses (squeezes) the bushing radially over the entire length between the seals. This squeeze action locks the bushing to the shaft without moving the shaft. Accurate clamping can be accomplished in fixtures, machine tools, and general applications.

A few design hints include providing a polished chamfer on the shaft to permit assembly without pinching O-rings. Tapped holes in the flange makes for easy removal of model AST squeeze bushings. The machine-housing wall should be thick enough to resist the same pressure used to clamp the squeeze bushing. Also, lubrication oil should not be trapped on the ID of the squeeze bushing because it reduces clamping forces. Provide for air bleeding at the highest point of the bushing's OD relief pocket and at the hydraulic lines. Locate a 4-mm oil inlet or bleed holes not closer than s + 12 mm from each end, where s = shoulder width of the bushing.

Bushings must be preloaded to prevent axial movement under load. Preload must be 0.0002 to 0.0003 × length (inches). If the preload is too high, the bushing will not function properly. Shoulders on each end of the bushings help support hydraulic and thrust forces. Maximum clearance is 0.25 mm for diameters and 0.1 mm for axial locations.

This information provided by Advanced Machine & Engineering Co., Rockford, Ill.

TAGS: Hydraulics
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