Mechanical seals is being used increasingly on fluid pumps to replace packed glands and lip seals. Pumps with mechanical seals perform more efficiently and generally perform more reliably for extended periods of time. Mechanical seals are provided to prevent pumped fluids from leaking out along the drive shafts.
The controlled leakage path is between two flat surfaces associated with the rotating shaft and the housing respectively. The leakage path gap varies as the mechanical seals faces are subject to varying external loads which tend the move the mechanical seals faces relative to each other.
The mechanical seals requires a different shaft housing design arrangement compared to that for the other type of mechanical seals because the mechanical seals is a more complicated arrangement and the mechanical seals does not provide any support to the shaft.
In order for the mechanical seals to perform over an extended time period with low frictional the faces are generally hydrodynamically lubricated. The fluid film will need to carry substantial load. If the load becomes to high for the film surface contact will take place with consequent bearing failure. This lubricating film is generally of the order of 3 micrometres thick or less. This thickness is critical to the required sealing function. Mechanical seals often have one face of a suitable solid lubricant such that mechanical seals can still operate for a period without the fluid film.
There are a number of mechanical seal types, each providing advantages for specific applications: the pusher type, the non-pusher, unbalanced, balanced, conventional, and cartridge type.