Interference fit
By R. David Whitby, Contributing Editor | TLT Worldwide May 2023
Thermal expansion and contraction are often used to create this.
A very good example of interference fitting is the replacement of worn tires on railway engine and rolling stock wheels.
There are many ways in which parts of a machine can be fitted together to ensure that they don’t move relative to each other. One way is creating an interference fit, also known as a pressed fit or a friction fit. This is because the two parts are held together by friction.
Two main methods are used to create an interference fit: force or thermal expansion and contraction.
Force can range from a sharp tap with a hammer, for a light fit, to a large hydraulic press, for an almost permanent fit. A hydraulic press may apply a force of a few newtons for small parts to several kilonewtons (tons) for large parts, without damaging critical components. When fitting a shaft into a hole, it is common for its edges to be chamfered (bevelled). The chamfers form guides for the pressing action, thereby helping to distribute the force evenly around the circumference of the hole. This enables the compression to occur gradually, making the pressing operation smoother, more easily controlled and requiring less force.
Thermal expansion and contraction are often used to create an interference fit. Most materials expand when heated and shrink when cooled. An enveloped component can be cooled before assembly, allowing it to be slid easily into its mating component. As it warms up, it expands to form a tight fit. Obviously, machining tolerances must be controlled carefully to ensure that the eventual fit is neither too loose nor too tight. The degree of cooling and the extent of contraction and expansion also must be determined prior to assembly.
The alternative method is to heat one component so that it expands and can then be allowed to shrink as it cools. This method is called shrink fitting. Axles on railway carriages and wagons are often assembled this way.
A very good example of interference fitting is the replacement of worn tires on railway engine and rolling stock wheels. Replacing a whole railway wheel due to a worn contact surface can be expensive, so many wheels were fitted with a replaceable steel tire. This is a hoop of steel fitted around the center of a steel wheel. The tire is fabricated with a shoulder on its outer face to locate it on the wheel center and a groove on the inside diameter of the flange face. Its inside diameter is machined to be slightly less than the diameter of the wheel center on which it is mounted, resulting in an interference fit.
The tire is heated in an oven to a controlled temperature, so it expands to a predetermined extent. The wheel center, sometimes already mounted on the axle, is lowered into the tire, which has its flange side up. As the tire cools, a retaining ring (a shaped steel bar rolled into a hoop) is fitted into the groove. Hydraulically operated rolls swage the groove down on to the retaining ring.
The tire is held in place mainly due to its interference fit. It also is kept in place, if the interference fit is insufficient, by the shoulder on the outside and the retaining ring. Loss of the shrink fit can be caused by machining errors or by severe drag braking of the wagon or carriage down a gradient. Worn tires are removed by machining out the retaining ring and heating the tire to relax the interference fit.
In some railways, the use of tires is becoming obsolete, particularly where wagons are only used over short distances or where rail freight usage is relatively low. Here, tires do not wear out much, so they don’t need renewing very often. It can be cheaper to fit a one-piece (“monoblock”) wheel. These are lighter and provide better integrity, because there is no tire to come loose. Additionally, the time and reduced productivity of having to inspect and replace tires and wheels is another cost for modern railways. Consequently, monoblock wheels can be more economical.
With thermal expansion and contraction interference fits, cooling is sometimes preferable for metals, because it is less likely to change the properties of the metal than is heating. Assembling a hardened gear onto a shaft is a case in point, where there is a risk of heating the gear too much and changing its surface properties.
David Whitby is chief executive of Pathmaster Marketing Ltd. in Surrey, England. You can reach him at pathmaster.marketing@yahoo.co.uk.