1. 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.
2. 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
3. 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.
4. 2. Sealing a closed container while allowing relative
movement of some part, such as diaphragms, bellows, and
boots
Application of a
diaphragm seal
5. 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
6. 4. Sealing around a rotating shaft: lip seal; wipers and
scrapers; and face seal,
8. 5. Protection of rolling-element bearings supporting shafts
to keep contaminant, from the balls and rollers
Seal for ball bearing
9. 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.
10. 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.
12. 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.
13. 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.
14. 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.
15. 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.
16. 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.
