When do you need to consider thermal expansion in linear guide systems?

Most materials used in linear systems have a positive coefficient of linear thermal expansion — that is, when their temperature is increased, they expand in length, and when their temperature is decreased, they contract in length.

One notable exception is Kevlar, the aramid from DuPont, which is sometimes used for tensile cords in toothed belts. Kevlar has a negative coefficient of linear thermal expansion…it contracts when heated and expands when cooled.

The tendency of a material to expand or contract with temperature change is given by its coefficient of linear thermal expansion (CLTE), α, which expresses the material’s rate of expansion (ΔL), per unit length (L0), per degree temperature change (ΔT).

ΔL = change in length

α = coefficient of linear thermal expansion

L0 = original length

ΔT = change in temperature

Because the rate of expansion is very low, the CLTE is often expressed as parts per million per degree C (ppm/°C) or parts per million per degree F (ppm/°F). However, the SI unit for CLTE uses the kelvin temperature scale and is simply expressed as 1/K or K^(-1).

Sources of heat that influence linear bearing temperature can be both external and internal. The most obvious source of heating (or cooling) is the ambient environment. But any moving parts that experience friction — including ball or lead screws, rack and pinion sets, gearboxes, and even motors — generate heat. And much of this heat is transferred directly to the machine or to the surface on which the guide rails are mounted. And the guides themselves also generate heat internally, due to preload and friction between the bearing and the guide rail or shaft.

The coefficient of linear thermal expansion expresses the material’s rate of expansion (ΔL), per unit length (L0), per degree temperature change (ΔT).

Since it’s nearly impossible to construct a linear system from just one type of material, it’s important to understand how different rates of thermal expansion for various components can lead to inaccuracies, poor performance, and even failure of the system.

Read more to learn how thermal expansion affects different types of linear bearings.