Polytetrafluoroethylene (PTFE) is commonly used in sliding bearings. Why?
The choice of sliding surface of bearings is of vital importance
because the sliding surfaces generate frictional forces which are exerted on
the bearings and substructure of the bridge. For instance, PTFE and lubricated
bronze are commonly choices of sliding surfaces for bearings. PTFE is a flurocarbon
polymer which possesses good chemical resistance and can function in a wide
range of temperature. The most important characteristic of this material is its
low coefficient of friction. PTFE has the lowest coefficients of static and
dynamic friction of any solid with absence of stick-slip movement (David J.
Lee). The coefficient of friction is found to decrease with an increase in
compressive stress. However, PTFE do have some demerits like high thermal
expansion and low compressive strength.
In designing the complementary
contact plate with PTFE sliding surface, stainless steel plates are normally
selected where the plates should be larger than PTFE surface to
allow movement without exposing the PTFE. Moreover, it is recommended that
the stainless steel surface be positioned on top of the PTFE surface to avoid
contamination of dirt and rubbish. Lubricants are sometimes introduced to
reduce the friction between the PTFE surface and the upper stainless steel
plate. Hence, the PTFE may be designed with dimples to avoid the lubricant from
squeezing out under repeated translation movements.