Shrink Fitting

Applications
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Access helpful Application Notes after brief registration:
Shrink fitting an assembled wrist pin into a connecting rod
Shrink Fitting Steel Tube
Shrink Fitting of Bearings
Shrink Fitting Steel Flange over a Graphite Pump Seal
Shrink Fitting Camshaft Gears
Fitting a Carbon Graphite Ring Into A Steel Band
Fitting Steel Parts Into an Aluminum Fuel Pump Housing
Fitting a Steel Shaft Into a Camshaft Gear
Fitting a Cast Iron Rocker Arm
Shrink Fit a Motor Shaft and Roller

Overview

Shrink fitting is a procedure in which heat is used to produce a very strong joint between two pieces of metal, one of which is inserted into the other. Heating causes one piece of metal to contract or expand on to the other, producing interference and pressure which holds the two pieces together mechanically.

Most applications involve a shaft with a given outside diameter and another part such as a gear, steering knuckle or washer which has a bore hole. There are several forms of this type of joint, characterized by the amount of tolerance (space) between the two parts. The shrink fitting process is used for the interference fit type joint.

By heating the mass around the bore hole uniformly, it is possible to significantly expand the size of the hole. The shaft is then easily inserted into the expanded hole. Upon cooling, the mass around the hole shrinks back to its original size and frictional forces create a highly effective joint.

Calculations For Shrink Fitting

Using Induction

Modern induction heating systems provide the speed, accuracy and repeatability necessary to produce quality shrink-fitted joints. With a properly designed work coil, induction heating can uniformly heat the bore hole, producing repeatable expansion. With the remote heat station capability of solid state induction heating power supplies, shrink fitting can be done on the production line.

Typical RF power supplies for metal to shrink fitting range from 1 to 20kW, depending on the parts and application requirements.

Setup

For induction shrink-fitting applications a pancake or helical-shaped coil is placed inside the metal part that is to be expanded. RF power is applied until the part reaches the appropriate temperature. The induction coil is then quickly removed from the hole and the shaft that is to be inserted is then slid into position. The assembly is then allowed to cool. Pressure develops as the heated component contracts. Depending on the mass of the material to be expanded, a pancake-shaped coil is sometimes employed.

Materials Needed

Metal parts: metal housing material with a tight tolerance opening; insert for housing (tight tolerances on the outer diameter are also important)
Alignment fixture (optional)
Power Supply & Induction Coil: Fast, precise heating works best.

Common Problems

If the inner diameter and outer diameter are not held at tight tolerances, the shrink fit will not be possible.
There can be no burs on the joining surfaces.
If the two metal parts are not properly aligned, the shaft will not slide into the correct position.

Tricks Of The Trade

Allow a long soak time for the metal housing to ensure that the whole mass has reached uniform temperature and expanded.
Employ an alignment fixture to smoothly guide the shaft into the housing.