Seals are packing materials used to prevent leaks in liquid-powered systems. A seal is any gasket, packing, seal ring, or other part designed specifically for sealing. Sealing applications are usually static or dynamic, depending if the parts being sealed move in relation to one another. Sealing keeps the hydraulic oil flowing in passages to hold pressure and keep foreign materials from getting into the hydraulic passages. To prevent leakage, use a positive sealing method, which involves using actual sealing parts or materials. In most hydraulic components, you can use nonpositive sealing (leakage for lubrication) by fitting the parts closely together. The strength of an oil film that the parts slide against provides an effective seal.
Static Seals. Pipe-threaded seals, seal rings used with tube fittings, valve end-cap seals, and other seals on nonmoving parts are static seals. Mounting gaskets and seals are static, as are seals used in making connections between components. A static seal or gasket is placed between parts that do not move in relation to each other. Figure 2-39 shows some typical static seals in flanged connections.
Dynamic Seals. In a dynamic sealing application, either a reciprocating or a rotary motion occurs between the two
parts being sealed; for example, a piston-to-barrel seal in a hydraulic cylinder or a drive-shaft seal in a pump or motor.
O-Ring (Figure 2-40). An O-ring is a positive seal that is used in static and dynamic applications. It has replaced the flat gasket on hydraulic equipment. When being installed, an O-ring is squeezed at the top and bottom in its groove and against the mating part. It is capable of sealing very high pressure. Pressure forces the seal against the side of its groove, and the result is a positive seal on three sides. Dynamic applications of an O-ring are usually limited to reciprocating parts that have relatively short motion.
To remove an O-ring seal, you need a special tool made of soft iron or aluminum or a brass rod (Figure 2-41). Make sure that the tool’s edges are flat and that you polish any burrs and rough surfaces.
Backup Ring (Figure 2-42). Usually, made of stiff nylon, you can use a backup ring with an O-ring so that it is not forced into the space between the mating parts. A combination of high pressure and clearance between the parts could call for a backup ring.
Lathe-Cut Seal. This seal is like an Oring but is square in cross-section rather than round. A lathe-cut ring is cut from extruded tubes, while an O-ring must be individually molded. In many static applications, roundand square-section seals are interchangeable, if made from the same material.
T-Ring Seal (Figure 2-43). This seal is reinforced with back-up rings on each side. A T ring seal is used in reciprocating dynamic applications, particularly on cylinder pistons and around piston rods.
Lip Seal (Figure 2-44). This a dynamic seal used mainly on rotating shafts. A sealing lip provides a positive seal against low pressure. A lip is installed toward the pressure source. Pressure against a lip balloons it out to aid in sealing. Very high pressure, however, can get past this kind of seal because it does not have the backup support that an O-ring has.
Sometimes, double-lip seals are used on the shafts of reversible pumps or motors. Reversing a unit can give an alternating pressure and vacuum condition in the chamber adjacent to a seal. A double-lip seal, therefore, prevents
oil from getting out or air and dirt from getting in.
Cup Seal (Figure 2-45). This is a positive seal that is used on hydraulic cylinder pistons and seals much like a lip seal. A cup seal is backed up so that it can handle very high pressures.
Piston Ring (Figure 2-46). A piston ring is used to seal pressure at the end of a reciprocating piston. It helps keep friction at a minimum in a hydraulic cylinder and offers less resistance to movement than a cup seal. A
piston ring is used in many complex components and systems to seal fluid passages leading from hollow rotating shafts. It is fine for high pressures but may not provide a positive seal. A positive seal is more likely to occur when piston rings are placed side by side. Often, a piston ring is designed to allow some leakage for lubrication.
Face Seal (Figure 2-47, page 2-34). This seal has two smooth, flat elements that run together to seal a rotating shaft. One element is metallic and the other is nonmetallic. The elements are attached to a shaft and a body so that one face is stationary and the other turns against it. One element is often spring-loaded to take up wear. A face seal is used primarily when there is high speed, pressure, and temperature.
Packing. Packing is a type of twisted or woven fiber or soft metal strands that are packed between the two parts being sealed. A packing gland supports and backs up the packing. Packing (Figure 2-48) can be either static or dynamic. It has been and is used as a rotating shaft seal, a reciprocating piston-rod seal, and a gasket in many static applications. In static applications, a seal is replacing a packing. A compression packing is usually placed in a coil or layered in a bore and compressed by tightening a flanged member. A molded packing is molded into a precise cross-sectional form, such as a U or V. Several packings can be used together, with a backup that is spring-loaded to compensate for wear.
Seal Materials. The earliest sealing materials for hydraulic components were mainly leather, cork, and impregnated fibers. Currently, most sealing materials in a hydraulic system are made from synthetic materials such as nitrile, silicone, and neoprene.
Leather Seals. Leather is still a good sealing material and has not been completely replaced by elastomers. It is tough, resists abrasion, and has the ability to hold lubricating fluids in its fibers. Impregnating leather with synthetic rubber improves the leather’s sealing ability and reduces its friction. Leather’s disadvantages are that it tends to squeal when it is dry, and it cannot stand high temperatures.
Nitrile Seals. Nitrile is a comparatively tough material with excellent wearability. Its composition varies to be compatible with petroleum oils, and it can easily be molded into different seal shapes. Some nitrile seals can be used, without difficulty, in temperatures ranging from -40 degrees Fahrenheit to +230° F.
Silicone Seals. Silicone is an elastomer that has a much wider temperature range than some nitrile seals have. Silicone
cannot be used for reciprocating seals because it is not as tough. It tears, elongates, and abrades fairly easily. Many lip-type shaft seals made from silicone are used in extreme temperature applications. Silicone O-rings are used for static applications. Silicone has a tendency to swell since it absorbs a fair volume of oil while running hot. This is an advantage, if the swelling is not objectionable, because a seal can run dry for a longer time at start-up.
Neoprene. At very low temperatures, neoprene is compatible with petroleum oil. Above 150 degrees, it has a habit of cooking or vulcanizing, making it less useful.
Nylon. Nylon is a plastic (also known as fluoro-elastomer) that combines fluorine with a synthetic rubber. It is used for backup rings, has sealing materials in special applications, and has a very high heat resistance.