Fig. 1 shows a generation model of stress relaxation.
The bushing press-fit into the housing generates a redial stress δr1 between them at the temperature T1 (normal temperature).
As the temperature increases, thermal expansion of the bushing increases the stress δr, and at the same time, decreases the allowable stress δm of the bearing material.
The radial stress δr increases up to the temperature T3. Once it goes beyond the temperature T3, the material yields along with a drop curve of the material's allowable stress δm, decreasing the radial stress δr.
Once cooling starts from the temperature T4, the radial stress δr disappears as indicated by a dotted line in Fig. 1, causing a clearance between the bushing and housing.
Stress relaxation
Describes stress relaxation which is a phenomenon taking place when the bearing is fixed by pressure.
There are different methods to fix the bearing to the housing. Generally, an interference is given to between the bearing and housing to fix it by press-fitting.
Thus, if the bearing is given the interference to be fixed to the housing and exposed to a high-temperature atmosphere, a stress caused by the interference is relaxed. This phenomenon is called “stress relaxation.”
Fig. 1 Generation Model of Stress Relaxation
Stress relaxation is seen more frequently in plastic materials with low service temperature limit, but it also occurs in metallic materials depending on the atmospheric temperature.
If stress relaxation occurs, the press-fit bearing may come off from the housing in the normal temperature condition or the bearing may cling to the shaft and slide on its outer diameter side because of a loss of clearance with the shaft, resulting in malfunctioning.
To prevent trouble by stress relaxation, it is necessary to apply an intermediate fitting so as not to allow an interference between the housing and shaft and install a bearing retainer or whirl-stop. Furthermore, it is necessary to design a clearance, taking thermal expansion of the shaft into account. It is also necessary to select bearings with lower thermal expansion coefficient.
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