An O-ring is doughnut-shaped. O-rings are usually molded from rubber compounds; how-ever, they can be molded or machined from plastic materials. The O-ring is usually fitted into a rectangular groove (usually called a gland) machined into the mechanism to be sealed. An O-ring seal consists of an O-ring mounted in the gland so that the O-rings cross section is compressed (squeezed) when the gland is assembled (fig. 7-6).

An O-ring sealing system is often one of the first sealing systems considered when a fluid closure is designed because of the following advantages of such a system:

As stated previously, O-rings are used in both static (as gaskets) and dynamic (as packing) applications. An O-ring will almost always be the most satisfactory choice of seals in static applications if the fluids, temperatures, pressure, and geometry permit.

Standard O-ring packings are not specifically designed to be used as rotary seals. When infrequent rotary motion or low peripheral velocity is involved standard O-ring packings may be used, provided consistent surface finishes over the entire gland are used and eccentricities are accurately controlled. O-rings cannot compensate for out-of-round or eccentrically rotating shafts. As rotary seals, O-rings perform satisfactorily in two application areas:

1. In low-speed applications where the surface speed of the shaft does not exceed 200 ft/min

2. In high-speed moderate-pressure appli-cations, between 50 and 800 psi

The use of low-friction extrusion-resistant devices is helpful in prolonging the life and improving the performance of O-rings used as rotary seals.

O-rings are often used as reciprocating seals in hydraulic and pneumatic systems. While best suited for short-stroke, relatively small diameter applications, O-rings have been used successfully in long-stroke, large diameter applications. Glands for O-rings used as reciprocating seals are usually designed according to MIL-G-5514 to provide a squeeze that varies from 8 to 10 percent minimum and 13.5 to 16 percent maximum. A squeeze of 20 percent is allowed on O-rings with a cross section of 0.070-inch or less. In some reciprocating pneumatic applications, a floating O-ring design may simultaneously reduce friction and wear by maintaining no squeeze by the gland on the O-ring. When air pressure enters the cylinder, the air pressure flattens the O-ring, causing sufficient squeeze to seal during the stroke. If the return stroke does not use pneumatic power, the O-ring returns to its round cross section, minimizing drag and wear on the return stroke.

As a maintenance person or supervisor working with fluid power systems, you must be able to positively identify, inspect, and install the correct size and type of O-ring to ensure the best possible service. These tasks can be difficult since part numbers cannot be put directly on the seals and because of the continual introduction of new types of seals and obsolescence of others. (Naval Ships Technical Manual, chapter 078, contains a table that cross-References obsolete and current O-ring specifications for ship applications.) O-rings are packaged in individually sealed envelopes. O-ring seals manufactured to govern-ment specifications are marked according to the requirements of the specific military specification and standard. The required marking for each package is as follows:

NOTE: Keep preformed packings in their original envelopes, which provide preservation, protection, identification, and cure date. When you select an O-ring for installation, carefully observe the information on the package. If you cannot positively identify an O-ring, discard it. The part number on the sealed package provides the most reliable and complete identification.

A standardized dash number system for O-ring sizes is used in many military and industrial specifications. The O-ring size is identified by a dash number rather than the actual dimensions for convenience. The basis for the dash numbers is contained in Aerospace Standard AS568. For nongasket O-rings (packing), the dash numbers are divided into groups of one hundred. Each hundred group identifies the cross section size of the O-rings within the group (table 7-2). The 900 series dash numbers contained in AS568 identify all the presently standardized straight thread tube fitting boss gaskets. With the exception of -901, the last two digits of the dash designate the tube size in 16ths of an inch. For example, the -904 size is for a 1/4-inch tube.

The critical dimensions of an O-ring are its ID, its cross sectional diameter (W), and the height and width of the residual molding flash (see fig. 7-7).

Nominal dimensions have been used to describe O-ring sizes, although this practice is rapidly being replaced by the use of dash numbers. The actual inside diameter of a seal will be slightly less than the nominal ID, but the actual OD will

be slightly larger than the nominal OD. For example, an AS568-429 O-ring is described in nominal dimensions as 5 inches ID by 5-1/2 inches OD by 1/4-inch W. Actual dimensions are 4.975 inches ID by 5.525 inches OD by 0.275 inches W.

Material and performance requirements for O-rings are often identified in military specifications. The dimensions of these O-rings will usually be found in accompanying slash sheets (which bear the specification number and are a part of the specification) or will be identified by various drawings and standards that relate to the specification. Included among the specifications are Air Force-Navy Standards (AN), Mili-tary Standards (MS), and National Aerospace Standards (NAS). If the specification does not identify sizes, the sizes should be identified by the AS568 dash number. Usually, you can use drawings, technical manuals, and allowance parts lists (APLs) to identify replacement O-rings. (Notes 2 and 3 of table 7-1 list some of the frequently used military specifications).