Cooling the Bush v Heating the FrameWithout a doubt, cooling the bush for a shrink fit is far better than using heat to expand the frame. For very large or long bushes a combination of both may be required to obtain enough clearance, but clearly, using cooling to do the majority of work is the best option. The main advantages of cooling include:
- It is quicker: A bush, even a large one, can be cooled in liquid nitrogen in a few minutes. By contrast, trying to heat a frame to give the same degree of clearance can take a quarter of an hour or more.
- Cooling is more controllable: Cooling mediums such as liquid nitrogen or dry ice are always at the same temperature, giving predictable results. Controlling the temperature of a frame using heat is much more difficult. There is always the danger of over-heating the frame causing distortion or softening of the bush when it is fitted.
- Cooling is more appropriate in a hot climate: When working on equipment that is already hot (machines left in the sun can easily reach 100°C), the effectiveness of cooling increases. In contrast, it becomes harder to gain the same clearance by heating.
- Cooling is safer: Although liquid nitrogen is capable of causing burns, a treated bush only stays cold for a few minutes. In contrast, a heated frame can remain dangerously hot for over half an hour.
Step 1: PreparationThere are a few things to arrange or consider before you start the job. These include:
- Ensure that you have internal and external micrometers appropriate to the range of the bush sizes you are fitting. If you are unable to obtain these, good quality verniers will suffice.
- Be aware of the effect of room temperature on your measurements. Ensure that the job and measuring equipment are at the same temperature. Remember that equipment lying in the sun can easily reach 100°C and equipment left out overnight can cool down to zero degrees or below in some locations.
- Obtain a regular supply of liquid nitrogen. Failing this, use dry ice, a freezer, or even a fridge.
Step 2: MeasureIn starting the job the first thing to do is to check and measure the bores and bushes you are about to fit. Look for ovality, burrs and taper in the bores. Ideally, all bores should be line bored.
Write down the sizes of all the holes and the OD's of all the bushes using a check sheet.
The interference or "crush" between the bush and the bore will depend on the size of the bush and the material. Different manufacturers apply different standards, but as a general guide, steel bushes will have a crush of about 1 thou per inch of diameter, (1 micron per millimeter). A higher crush is required for bushes that are shorter relative to their diameter and for some thin-walled bushes.
Step 3: CalculateThe next step is to calculate the shrinkage or expansion required to fit the bushes.
When fitting, the bush needs to shrink, or the frame needs to expand, (or both), until the interference is eliminated. Then shrink or expand a bit more to ensure clearance. The "bit more" is usually another 1 to 3 thou per inch (1 to 3 micron per millimetre). The lesser amount applies to smaller, shorter bushes and the high clearances needed for larger or longer bushes. Remember that too much clearance when fitting bushes is rarely a problem.
Here is an example for a 5" nominal (125mm) steel bush:
|Action||Imperial||Metric||Notes & Methods|
|Measure frame diameter||5.000"||125.000mm||Take the smallest size|
|Measure bush diameter||5.005"||125.125mm||Take the largest size|
|Calculate Interference||0.005" (5 thou)||0.125mm||Bush diameter minus frame diameter|
|Shrinkage required||0.001 (1 thou/inch)||0.001 (1 micron/mm)||Divide interference by frame diameter|
|Add working clearance||0.001 (1 thou/inch)||0.001 (1 micron/mm)|
|Total clearance required||0.002 (2 thou/inch)||0.002 (2 microns/mm)|
Once the amount of clearance required is determined, the next step is to decide the best fitting technique. To make this choice easy we have provided a chart which allows you to see at a glance the options available. The chart works for steel or bronze bushes of any size.
Step 4: Cool, Heat or Both?
Using the chart for the example above, shrinking the steel bush in liquid nitrogen, with the frame at say, 30°C, gives a shrinkage of 2.4 thou per inch. If however, the bush was very long, a slightly higher shrinkage may be required, and heating of the frame to around 90°C may be required.