SEALS Seals protect critical components by excluding contaminants or by retaining fluids inside the housing of a machine Seals are an important part of machine design in situations where the following conditions apply: 1.Contaminants must be excluded from critical areas of a machine. 2. Lubricants must be contained within a space. 3. Pressurized fluids must be contained within a component such as a valve or a hydraulic cylinder.
The parameters affecting the choice of sealing system, the materials used, and the details of its design 1. The nature of the fluids to be contained or excluded. 2. Pressures on both sides of the seal. 3. The nature of any relative motion between the seal and the mating component,. 4. Temperatures on all parts of the sealing system. 5. The degree of sealing required: level of leakage permissible 6. The life expectancy of the system. 7. The nature of the solid materials against which the seal must act: corrosion potential, smoothness, hardness, wear resistance. 8. Ease of service for replacement of worn sealing elements
Common conditions in which seals must operate and the types of seals used. 1.Static conditions such as sealing a closure on a pressurized container: elastomers O-rings; T -rings; hollow metal O- rings; and epoxies, silicones, and butyl caulking.
2. Sealing a closed container while allowing relative movement of some part, such as diaphragms, bellows, and boots Application of a diaphragm seal
3. Sealing around a continuously reciprocating rod or piston, such as in a hydraulic cylinder: lip seal; U-cup seal: V-packing; and split ring seals
4. Sealing around a rotating shaft: lip seal; wipers and scrapers; and face seal,
Mechanical face seal
5. Protection of rolling-element bearings supporting shafts to keep contaminant, from the balls and rollers Seal for ball bearing
6. Sealing the active elements of a pump to retain the pumped fluid: face seals and V-packing 7. Sealing infrequently moved elements (a fluid-flow control valve): compression packings and V-packings. 8. Sealing between hard, rigid surfaces (a cylinder head and the block of an engine): resilient gaskets. 9. Circumferential seals (the tips of turbine blades, and on large, highspeed rotating elements): labyrinth seals; abradable seals; and hydrostatic seals.
SEAL MATERIALS Most seal materials are resilient to permit the sealing points to follow minor variations in the geometry of mating surfaces. The hollow metal O-rings, the shape of the seal allows the flexing of hard materials to occur. Face seals require rigid, hard materials that can withstand constant sliding motion and that can be produced with fine accuracy, flatness, and smoothness.
Elastomers Resilient seals such as O-rings, T-rings, and lip seals are made from synthetic elastomers
The prevalent requirements materials for seals Weather resistance: silicone, fluorosilicone, fluorocarbon, ethylene propylene, polyurethane, polysulfide, polyester, neoprene, epichlorohydrin, and PNF. Petroleum fluid resistance: Polyacrylate, polyester, PNF, nitrile, polysulfide, polyurethane, fluorocarbon, and epichlorohydrin. Acid resistance: Fluorocarbon.
High-temperature operation: Ethylene propylene, fluorocarbon, polyacrylate, silicone, and PNF. Cold-temperature operation: Silicone, fluorosilicone, ethylene propylene, and PNF. Tensile strength: Butadiene, polyester, and polyurethane. Abrasion resistance: Butadiene, polyester, and polyurethane. Impermeability: Butyl, polyacrylate, polysulfide, and polyurethane.
Rigid Materials Rigid materials that can withstand the sliding action and that are compatible with the environment around the seal. Metals: Carbon steel, stainless steel, cast iron, nickel alloys, bronze, and tool steels. Plastics: Nylon, filled polytetrafluoroethylene (PTFE), and polyimide. Carbon, ceramics, tungsten-carbide. Plating: Chromium, cadmium, tin, nickel, and silver. Flame-sprayed compounds.
Packings Packings for sealing shafts, rods, valve stems, and similar applications are made from a variety of materials, including leather, cotton, flax, several types of plastics, braided or twisted wire made from copper or aluminum, laminated cloth and elastomeric materials, and flexible graphite. Gaskets Common gasket materials are cork, cork and rubber compounds, filled rubber, paper, resilient plastics, and foams.
Requirements to shaft with seals The steel shafts should be hardened to HRC 30 to resist scoring of the surface. Tolerance on the diameter of the shaft on which the seal bears should conform to the recommendations to ensure that the seal lip can follow the variations of A shaft diameter (mm),and tolerance (mm) The surface of the shaft over which the seal must pass during installation should be high quality with adequate lubrication to ensure full contact and to reduce friction between the seal and the shaft surface.