Good Morning Everyone,
This morning I want to discuss the lowly seal. It is one of those often under-appreciated items in the engine that can make for a very bad day should one fail or worse, work its way out of its installed position. This is very important for aircraft applications where the loss of any critical fluid can mean that you are heading for a forced approach and a potential off field landing.
There are two ways in which a shaft seal can fail on you. The rubber seal material can deteriorate and allow fluid to pass. This is the engineered way it is supposed to fail. The designers of the seal want it to wear over time and begin to leak slowly. This mode of failure usually retains the majority of fluid and gives the operator opportunity to catch the problem before all of the fluid is lost.
The other benefit is that it still acts as a splash barrier to retain as much fluid as possible. In this example, I am speaking about simple seals that are utilized in non-pressurized applications.
The other failure mode for a seal is for it to spin with the shaft and work itself out of its installed position. Should this happen you would have a majority of the fluid escaping the system which can lead to a catastrophic failure due to loss of lubrication.
In aircraft applications we need to consider the possibility of the latter scenario and take appropriate precautions against the loss of the seal and subsequent loss of the lubrication fluid of the system.
There are two ways to accomplish this: The first way works if you have enough depth in the cavity where the seal is installed. I have seen applications where a 1/4″ seal is driven into a race that is 1/2″ deep. This allows for a slot to be milled in the remaining 1/4″ for a internal cir-clip to be installed.
The second method of retaining the seal is to make a retaining plate to go around the shaft that is slightly larger than the shaft but smaller than the outer diameter of the seal. There is one of these on Lycoming engines that retains the main crankshaft main bearing.
Whenever you are installing a new system or are maintaining an existing system, you should know about and appreciate the lowly seal. It is important to recognize the symptoms that seals exhibit when they are starting to fail and to understand applications where it is imperative that the seal be positively retained by either of the two methods mentioned above.
Have a Great Day!