Advances in the design and materials used in rotating shafts and their corresponding seals have extended the life and reliability of vehicles and power equipment. To minimize fluid leakage and friction in rotary dynamic seals, however, these designs have a critical dependence on shaft surface texture and machine lead angle. Without proper control of these factors seals can prematurely fail.
Surface roughness, for instance, affects both the lifetime and effectiveness of a seal. The lip of a new seal makes contact with the shaft and abrades as the shaft rotates. If the shaft is too rough, the seal abrades quickly and will begin to leak. If the shaft is too smooth the seal will not bed correctly and will also leak. The shaft must be rough enough that the initial lip wear will allow a small film of fluid (typically between 1µm and 3µm thick) to enter the shaft/seal interface. When this occurs, the seal begins riding on a thin layer of liquid and further wear ceases. The fluid's meniscus at the seal's outside edge prevents subsequent fluid leakage.
Machine lead also affects the seal's effectiveness. All machining and polishing processes for shafts leave some degree of residual grooving on the shaft's surface. If these grooves are at an angle (called the machine lead, lead angle, or simply lead) to the shaft's axis they can move the fluid within the shaft/seal interface. Angled one way they pull the fluid out past the seal, creating a slow leak. Angled the other way they push the fluid in the interface zone back into the housing, depleting the film layer and thus leading to excessive wear and early failure. Current standards for shafts and seals call for shaft leads less than 0.05° (see Table 1).
Table 1- Standards covering shaft and seal design and test call for lead angles less than 0.05° and roughness ranges that allow fluid films to develop.