Hydraulic seals appear to be simple objects when held in the hand but, in use, they are highly complex; precision parts must be treated carefully if they are to do their job properly.

Hydraulic seals are used in the following two main applications:

1. Static seals are used to seal fixed parts. Static seals are usually gaskets, but also may be O-rings or packings.

2. Dynamic seals are used to seal moving parts.

Dynamic seals include shaft and rod seals and compression packings. A slight leakage in these seals is acceptable for seal lubrication.

SEALING MATERIALS.- There have been many different materials used in the development of sealing devices. The material used for a particular application depends on several factors: fluid compatibility; resistance to heat, pressure, and wear resistance; hardness; and type of motion.

The selection of the correct packings and gaskets and their proper installations are important factors in maintaining an efficient fluid power system. The types of seals to be used in a particular piece of equipment are specified by the equipment manufacturer.

Seals are made of materials that have been carefully chosen or developed for specific applications. These materials include polytetra-flouroethylene (PTFE), commonly called TEFLON TM , synthetic rubber, cork, leather, metal, and asbestos. The most common types of materials are discussed in the following paragraphs.

Cork.- The physical properties of cork make it ideally suited as a sealing material in certain applications. The compressibility of cork seals makes them well-suited for confined applications in which little or no spread of the material is allowed. The compressibility of cork also makes a good seal that can be used under various pressures and allows the gasket to be cut to any desired thickness to fit any surface, while still forming an excellent seal. Cork is generally recommended for use where sustained temperatures do not exceed 275 F.

Synthetic Rubber.- The materials used in synthetic rubber seals are either neoprene or nitrile-butadiene base. These seals are available in a wide range of density, tensile strength, and elongations. Many factors contribute to make synthetic rubber ideal for seals. The elasticity of the material makes it easier in many applications. Since synthetic rubber seals are virtually impermeable in their compressed state, they require less sealing load than many other types of gaskets. Synthetic rubber seals are used in a variety of applications and are capable of functioning in temperature ranges as wide as -65 F to +300 F.

Leather. -Leather is a closely-knit material that is generally tough, pliable, and relatively resistant to abrasion, wear, stress, and the effects of temperature changes. Because it is porous, it is able to absorb lubricating fluids. This porosity makes it necessary to impregnate leather for most uses. In general, leather must be tanned and treated to make it useful as a sealing material. The tanning processes are those normally used in the leather industry. It is generally resistant to abrasion regardless of whether the grain side or flesh side is exposed to abrasive action. Leather remains flexible at low temperatures and can be forced with comparative ease into contact with metal flanges. When leather is properly impregnated, it is impermeable to most liquids and some gases. Leather is capable of withstanding the effects of temperatures ranging from -70 F to +220 F.

Metal.- One of the most common metal seals used in Navy equipment is copper. Flat copper rings are sometimes used as gaskets under adjusting screws to provide a fluid seal. Copper is easily bent and requires careful handling. In addition, copper becomes hard when used over a long period of time and when subjected to compression. It is advised that when a component is disassembled, the copper sealing rings should be replaced.

In some fluid power actuating cylinders, metallic piston rings are used as packing. These rings are similar in design to the piston rings used in engines.

TYPES OF SEALS.- Fluid power seals are usually typed according to their shape or design. These types include T-seals, O-rings, quad-rings, and U-cups, and so on. Some of the most commonly used seals are discussed in the following paragraphs.

T-Seals. -The T-seal has an elastomeric bidirectional sealing element resembling an inverted letter T. This sealing element is always paired with two special extrusion-resisting backup rings, one on each side of the T. The basic T-seal configuration is shown in figure 3-44, view A. The backup rings are single turn, bias cut, and are usually made of TEFLON TM , nylon, or a combination of TEFLON and nylon. Nylon is widely used for T-seal backup rings because it provides excellent resistance to extrusion and has low friction characteristics.

The special T-ring configuration adds stability to the seal, eliminating spiraling and rolling. T-seals are used in applications where large clearances could occur as a result of expansion.

O-Rings. -An O-ring is doughnut-shaped. O-rings are usually molded from rubber compounds; however, they can be molded or machined from plastic materials. The O-ring is usually fitted into a rectangular groove that is machined into the item to be sealed.

An O-ring sealing system is often one of the first sealing systems considered when fluid closure is

Figure 3-44.- T-seals.

designed because of the following advantages of such a system:

Small space requirement
No adjustment required
No critical torque in clamping
Low distortion structure

Ease in maintenance
Ease of installation

Simplicity
Ruggedness
Low cost

Effectiveness over wide pressure and temperature ranges

O-rings are used in both static (as gaskets) and dynamic (as packing) applications. An O-ring has always been the most satisfactory choice of seals in static applications when the fluids, temperatures, pressure, and geometry permit.

Quad-Rings. -The quad-ring is very similar to the O-ring, the major difference being that the quad-ring has a modified square type of cross section, as shown in figure 3-45. Quad-rings are molded and trimmed to extremely close tolerances in cross-sectional area, inside diameter, and outside diameter. Quad-rings are ideally suited for both low pressures and extremely high pressures.

U-CUPS.- The U-cup (fig. 3-46) is a popular packing due to its ease of installation and low friction. U-cups are used primarily for pressures below 1,500 psi; but, they can be used for higher pressures with the use of backup rings. When more than one U-cup is installed, they are installed back to back or heel to heel. This back-to- back installation is necessary to prevent a pressure trap (hydraulic lock) between two packings.

Figure 3-45.- Quad-ring.

Figure 3-46.- Typical U-cup seal.