Mechanical seals touch nearly every aspect of industrialized society. Wherever a rotating shaft moves fluid, mechanical seals play a key role in sealing process fluids in, keeping contaminants out, or both.
A few basic components and principles in mechanical seal design contribute to a working seal at the interface of the rotating shaft and stationary pump/mixer/seal-chamber housing. Mechanical seals are usually end-face seals or rotating-face seals, but in some designs they can be circumferential or even a hybrid of lip-type seals. In either case, the following components are common to all mechanical seals:
1. Rotating primary sealing element: fixed to/driven by the shaft and seals against the stationary primary sealing element.
2. Stationary primary sealing element: fixed to the stationary housing of the pump, mixer or other equipment through which the rotating shaft passes and seals against the rotating primary sealing element.
3. Closing force: biases the primary sealing elements in contact to initiate sealing.
4. Static and/or dynamic secondary seals: seal between the mechanical seal components and the equipment shaft and housing.
The more common end- or rotating-face mechanical seal designs feature mating faces as the primary sealing elements. Rings of ceramic, carbide, carbon or composites of these materials are lapped flat in the range of less than 1 micron on an axial end face. These lapped faces run against each other, one rotating with the shaft and the other staying with the equipment housing.
The sealed fluid migrates between the flat faces and forms a stable fluid film at this interface. During shaft rotation, the face materials heat up, wear and degrade quickly without a lubricating fluid film between them. The sealed fluid creates this thin lubricating film.
In a lip-seal-type mechanical seal, a thin film of sealed fluid also lubricates the sealing interface. Rather than two flat rings, the sealing interface is a polymer material deflected against a hard material. This material could be a hardened, coated or plated metal, ceramic, or carbide face or sleeve. One of these elements rotates with the shaft while the other is stationary with the equipment housing.