End face mechanical seal

Mechanical seals or so-called dynamic seals take over the seal rotating shafts against a wall, for example, a machine housing. The main components are two successive sliding components, the spring-loaded seal ring ( in the picture above position a) and a mating ring. One of the two rings sitting fixed in the stationary housing ( stator) ( in the picture above position d ), the other is using Verdrehsicherungsstiften on the rotating shaft attached (rotor). The areas between these two parts are - depending on the type of mechanical seal - mostly flat and are usually made ​​of carbon - graphite materials, metal, ceramic, plastic or resin-bonded carbon.

Classification

Two groups of dynamic seals can be distinguished: axial and radial. As axial seal applies the mechanical seal, with special shapes represent the supply head gaskets (steam head gaskets ) and ball valve seals. The radial seals can be in gap seals ( models with multi-piece rings, clad carbon rings, labyrinth seal ) and contact seals ( oil seals ) divide.

Construction

The opposite axial or radial sealing surfaces rotate relative to each other and form a primary seal gap. Between the sealing surfaces of the surrounding medium produces a liquid or gaseous state of aggregation, depending on the lubricant film. The sealing of the Mechanical seal against the shaft or housing is usually with secondary seals in the form of additional O- rings or seals. To this extent, mechanical seals in principle of five components: the slip ring, the mating ring feathering and one each side seal. Depending on the design of the seals, the number of components can reduce or increase significantly.

Seal ring and mating ring are referred to as mating pairs. The optimum choice of materials for the pair depends primarily on the type of media to be sealed, the rotational speed and the working temperature. The mechanical seals one speaks of so-called hard-hard or hard-soft pairings. It is with hard - soft pairing a combination of slide called, in one of the sliding partners is significantly softer than the other, resulting in better wear, but also has better run-flat in adverse lubrication conditions. A typical combination is a carbon ring with a ring made of silicon carbide. In the hard - hard pairings are both seal rings made ​​of hard and wear resistant material. This pairing is primarily used for abrasive products, but has poorer emergency running properties. For the tightness and wear next to the material combination are especially the absolutely flat design of the sliding surfaces and the surface roughness is crucial. They are therefore usually lapped, polished, and possibly provided with a super finish.

The choice of suitable material pairs should be done in close cooperation and careful consultation with the manufacturer of mechanical seals or the component manufacturers. The main applications for mechanical seals include pump for domestic hot water, feed water, car radiators or air conditioning systems, agitators for wet and dry operation, centrifuges or water turbines. Today, mechanical seals for shaft diameters from about 5 to 630 mm, pressures of about 0.01 to 450 bar, temperatures of about -200 to 450 ° C and sliding speeds of up to 150 m / s are available.

Materials for sliding rings

As a slip ring or ring counter a number of different materials come into question, which can be divided into four main groups:

• Carbon

• Ceramic materials

• Metallic materials

• Plastic

Wear of the seal faces

Since hydrazine was banned as a corrosion inhibitor because of its health risks, occur in the mechanical seals of boiler feed pumps on previously unknown corrosion. As a reason, the electro-corrosion has been found, because the relative motion between rotating and stationary seal ring leads to static charges, which are not derived by the very low conductivity of the boiler water of less than one micro -Siemens per cm [ ĩS / cm] in contrast to earlier. Within a short operating times, sometimes a few hundred hours of operation, fingertip -sized pieces from the sliding and / or the counter-ring break out and lead to rapid increase in leakage current, which is on the one hand because of the loss of efficiency, on the other hand, because of the no longer guaranteed operating reliability unacceptable and to a repair standstill with the leads involved and unfortunately well known high costs, mainly due to production loss. To solve this problem, for example, worked with wavy surface profile of the sliding surfaces or sliding surfaces coated with crystalline nature-identical, but artificially made diamond. After 26.680h in a long-term experiment with diamond-coated seal rings at the Institute of Hydraulic Machinery Graz University of Technology has shown that there is an excellent condition with almost no corrosion images of the mechanical seal after such a long period of operation.