Shrinking Steel Tyres onto Cast Iron Wheels


It is sometimes desirable to fit a steel tyre to a cast iron wheel. Several reasons for doing this are the repair or refurbishment of damaged or badly worn wheels, or concern (often unjustified) over possible excessive wear of cast iron wheels on steel track.

Type Profile
Typical tyre profile

 

Type Profile
Detail magnification

The usual way, and typical profile, of fitting tyres is shown in the diagram at the left. The main wheel body is turned with a flat parallel rim, with a small bevel on the outside edge. This bevel ensures that the tyre, when fitted, sits in the correct position and gives a lttle bit of clearance for the inside corner of the tyre.

The tyre has a small lip turned on the inside. This lip does not have to be very big, say approx 0.8mm (0.03") thick and 1.5mm (0.06") deep for 5"g (or 1mm thick and 2mm deep for 7¼"g). The reason for this lip is not, as sometimes thought, to prevent the tyre from moving on the wheel during service - there is more than enough strength in the shrink-fit to hold the tyre in position, but to provide a precise location for the tyre during fitting. The tyre is VERY hot and difficult to handle when fitting, and the tyre just sits in the correct position on the rim and stays put and square while the tyre cools (and shrinks into place).

If the lip on the outide of the tyre is not wanted, it can be removed in the lathe in a second machining operation after the tyre is fitted if desired.

Drawings not to scale, for descriptive purposes only.

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Type Profile
Typical tyre profile

 

Type Profile
Detail magnification

An alternate profile for fitting tyres is shown at the left where a small lip or shoulder is machined on the rim of the tyre, and a corresponding groove profile on the wheel. Dimensions of the lip etc are similar to above.

Small bevels on the wheel rim and the tyre lip are still needed to ensure correct positioning of the tyre on the wheel. Note carefully on the drawing.

This profile does not require a second machining operation after tyre fitting.

Drawings not to scale, for descriptive purposes only.

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How much interference fit is required for a shrink-fit tyre?
The old golden rule still applies: 1 thou (0.001") per inch plus an additional thou. [e.g. on a 6" dia wheel, the tyre would be 0.007" smaller dia]

If you are metric this means 25 micron (0.025mm) per 25 mm plus 25 micron. [i.e. on a 150 mm dia wheel, 0.200mm up to 0.225mm is about right.]

Turn the wheels and the tyres, and check, and re-check, your measurements three times. Ensure all parts are free of burrs and sharp edges etc, and are well finished. Just a normal smooth tool finish, but not polished, is best.

Fitting the tyres
Heat the tyre to 280°C to 300°C [540~570°F] (full purple to dark blue colour), and do NOT heat the wheel - just keep the wheel at room temperature. A normal kitchen gas stove is all that is required to heat the tyre. Just make sure the heating is even for the whole tyre.
A full steel colour temperature chart is available here.

Place or mount the wheel hub in a suitable heat resistant position with the wheel face horizontal and facing up, and drop the hot tyre on. Don't panic -  it will seem as though there is too much for it contract! Just wait a bit and watch it shrink into place before your eyes! Wait for the whole assembly to cool down before handing with bare hands.

If ever there is a need to remove the tyre, just cut carefully with a hacksaw through the tyre until almost cut completely through. Then use a cold chisel or similar in the cut to perform the final separation. The tyre will then just become loose with a 'pop' and fall off. (Note: Heating the tyre again to expand it usually doesn't work very well as the whole wheel tends to heat up and expand as well, defeating the purpose of heating the tyre.)