Q11: Tell me about rotary seals and how they work?
ANSWER: The first rotary shaft seal units were made from leather - the hat shaped until being clamped between metal pressings to form the outer casing.
Not only were those heavy and expensive to make, but they were true ‘brute force’ seals relying on heavy, wide contact between lip and shaft backed up by a powerful coiled garter spring to prevent oil leakage. If it was not for the fact that leather allowed a certain amount of lubricant to find its way to the interface, the lips would have burned up at much lower speeds than they did. Even so, the peripheral speeds which could be handled were relatively low and heavy loading resulted in high shaft wear.
The introduction of synthetic rubber in 1938 did not improve matters much since the seal design followed the principles of leather.
Seal manufacturers began to research the function of oil seals and it was shown that a seal acted like a bearing - an oil film being generated between lip and shaft. It was demonstrated that so long as a certain oil film thickness was not exceeded the meniscus which formed at the air side of the interface would not break and leakage was avoided. Relatively light loads were required to achieve this situation and so friction was greatly reduced with lower running temperatures and much longer seal life.
For the first time, seals could be designed which would work successfully without trial and error. The heavy metal cased construction has long since been replaced by a bonded seal using a single metal pressing with an outer layer of rubber to give a better fluid-tight joint with the machine housing.
It will be appreciated that any damage to the shaft where the seal runs will cause leakage because the optimum oil film thickness will be exceeded locally. Even a slight scratch can cause leakage so it is of the utmost importance to provide the right finish on the shaft. To maintain the oil film thickness within its required limits means that the seal lip must follow any shaft movement. This becomes difficult when the shaft is subject to eccentric running or vibration at high speeds. An application where such conditions exist is the rear end of an automotive crankshaft.
Some years ago, fluid seal manufacturers developed what was termed hydrodynamic or positive action features to help the seal cope with such conditions. These features consisted of raised ribs on the air side of the sealing lip which ‘bedded-in’ to form tiny projections to catch any oil leaking from the seal and return it to the oil film under the lip. Such features introduced great reliability to rotary shaft seals for difficult automotive applications hitherto impossible to achieve.
Although initially the positive action features were unidirectional a fact not of any consequence in many applications, duo-positive features soon became available making it possible to offer a standard range of seals of this type, which could be offered for any application without knowledge of the direction of shaft rotation. |
Teflon