In mechanical design, temperature changes are an indispensable factor. Parts can expand or contract at different temperatures, which may alter the originally precise tolerances and even affect the normal operation of machinery. If temperature factors are not adequately considered during design, issues such as loose assembly, excessive interference fit, jamming, or mechanical failure may occur. This article will explore how temperature changes impact tolerances and provide reasonable design strategies.

1. How does temperature change affect part size?

The size of all materials will change with temperature. The core factor affecting the size change is the coefficient of linear expansion (Thermal Expansion Coefficient, α), which is calculated as follows:

among:

ΔL: Length change

L0: Original length

α: Linear expansion coefficient of the material (unit: 1/℃1/℃1/℃)

ΔT: temperature change (unit: ℃)

The influence of tolerance fit on temperature change

1. The interference fit may become a clearance fit

In high-temperature environments, both the shaft and the hole will expand. If the shaft expands more than the hole, the original interference fit may turn into a loose clearance fit, causing parts to slip or even fail. For example, in the case of an aluminum bearing sleeve fitted into a steel housing, since aluminum has a much higher coefficient of thermal expansion than steel, the fit may loosen as temperatures rise.

2. The clearance fit may become an interference fit

On the contrary, in a low temperature environment, both the shaft and the hole will contract. If the contraction of the shaft is less than that of the hole, the original clearance fit may turn into an interference fit, making assembly difficult. For example, in the installation of aircraft engine bearings in cold environments, the bearing housing hole may contract, preventing proper installation.

3. Expansion under high temperature environment leads to jamming

In some high temperature applications (such as heat treatment equipment, engines), if the two parts expand differently, it may lead to limited relative motion between the parts. For example, the fit of a piston and cylinder, if not designed properly, the piston may interfere with the cylinder wear after temperature rise.

4. Thermal stress caused by temperature difference affects the strength of the structure

If the temperature distribution of a part is uneven, thermal stress (Thermal Stress) can occur, causing deformation or even cracking of the part. For example, in the flange connection of a high-speed train gearbox, if the temperature difference is too large, bolts and flanges may experience stress concentration due to different coefficients of thermal expansion, leading to fatigue failure.

3. How to consider temperature factors in design?

✅1. Choose the right materials

In an environment with large temperature changes, materials with similar expansion coefficients should be selected as far as possible to reduce the variation of coordination.

For example, the proportion of steel and aluminum is more stable than that of steel and aluminum.

In extreme temperature environments (such as aviation and space), low expansion alloys (such as INVAR alloy, which has a very low coefficient of expansion) can be used.

✅2. Use temperature compensation design

In important parts, expansion gap or compensation structure can be set.

For example, the machine tool spindle usually uses floating bearings or thermal compensation rings to avoid high temperature deformation affecting the machining accuracy.

✅3. Calculate and correct the fit tolerance

According to the working temperature range, calculate the expansion of parts and adjust the tolerance fit appropriately.

For example, for high temperature working interference fit, a slightly larger interference can be selected at room temperature to offset the effect of high temperature expansion.

✅4. Use special assembly process

Cold assembly: for parts with interference fit, the shaft can be cooled first (such as liquid nitrogen cooling) and then installed into the hole, and the tightening is achieved by the expansion of temperature recovery.

Hot assembly: For parts that need to be tightly assembled, the hole can be heated first to make it expand, and then the shaft is installed. After cooling, an interference fit is formed.

Case analysis: temperature compensation design of rail joint

✅ Background: The rails of high-speed railways expand in summer and contract in winter. Without reasonable temperature compensation design, track deformation or fracture may occur.

✅ Rx:

Using Continuous Welded Rail (CWR) seamless lines, temperature stress is evenly distributed through the track fixing device to reduce deformation.

Expansion joints (Expansion Joint) are set at rail joint to allow the track to expand and contract freely with temperature changes.

Select appropriate track materials to ensure structural stability in temperature difference environment.

✅ Temperature changes can affect tolerance fits, leading to looseness, jamming, or mechanical failure; temperature factors must be considered in design. ✅ Different materials have different coefficients of thermal expansion; appropriate material combinations should be selected to avoid dimensional mismatch due to temperature changes. ✅ Using temperature compensation designs, such as adjusting fit tolerances, reserving expansion gaps, and employing special assembly techniques, can effectively reduce temperature effects. ✅ In engineering practice, the rational use of thermal expansion properties can optimize assembly processes and enhance mechanical reliability.