Bearings made of high-performance polymers tend to be underestimated, especially in terms of the permitted temperatures.
The literature very often provides specifications on the long-term service temperature. The long-term service temperature is the highest temperature under long-lasting heat exposure, which the plastic withstands without mechanical load for a certain period without the variation of the material's tensile strength below or above a specified value. This standardized test however yields only a less relevant characteristic value, as bearings are almost always subjected to a load. More revealing are the application temperatures of the materials.
Picture 08: Material tests are possible up to 250° C.
The lower application temperature is the temperature limit under which the material becomes so stiff and hard that it is too brittle for normal applications. The upper long-term application temperature is the temperature limit that the material can tolerate for a long period without significantly altering its properties.
The upper short-term application temperature is the temperature limit above which the material becomes so soft that it can withstand only very low outer loads.
In this regard "short-term" is to be understood as a period of few minutes. The danger of the bearings creeping out of the bore is already existent when the bearing moves axially, or the forces act on the bearing axially. In these cases, a special fastening is required in addition to the press-fitting.
Table 04 shows the temperature limit from which a fastening is to provided for the bearing in the bore even with low axial forces. The greater the forces, the more reasons to engage such a fastening.
Temperature and load
Figures 02 and 03 show the maximum recommended surface pressure [p] of the iglidur® bearings through the temperature. With increasing temperature, this value decreases continuously.
When using the bearing, it should be noted that due to friction the bearing temperature could be higher than the ambient temperature.
coefficient of thermal expansion
The coefficient of thermal expansion of polymers is 10 to 20 times higher compared to metals. In contrast to metals it also does not act linear on plastics. The coefficient of thermal expansion of the iglidur® bearing is an important factor for the required bearing clearance. The shaft does not clamp in the bearing within the limits of the respectively intended application temperatures. The coefficients of thermal expansion of the iglidur® bearings were tested for important temperature ranges and are stated in the individual sections in their respective material tables.