Another method of preventing leakage in fluid systems is by use of O-ring seals. Figure 9-64 shows an O-ring seal with two cross-sectional views. An O-ring is a doughnut-shaped, circular seal (view A of fig 9-64) that is usually a molded rubber compound. An O-ring seal has an O-ring mounted in a groove or cavity (usually called a gland).

Figure 9-64.-O-ring seal with two cross-sectional views.

When the gland is assembled (view B of fig 9-64) the O-ring cross section is compressed. When installed, the compression of the O-ring cross section enables it to seal low fluid pressures. The greater the compression, the greater is the fluid pressure that can be sealed by the O-ring. The pressure of the O-ring against the gland walls equals the pressure caused by the recovery force of the compressed O-ring plus the fluid pressure.

The fluid pressure against the walls of the gland and the stiffness of the O-ring prevent fluid from leaking past the O-ring. If the downstream clearance is large, the O-ring is forced into this clearance (view C of fig 9-64). The stiffness of the O-ring material prevents the O-ring from being forced completely through the downstream clearance unless that clearance is abnormally large or the pressure is excessive.

0-rings are commonly used for sealing because of their simplicity, ruggedness, low cost, ease of installation, ease of maintenance, and effectiveness over wide pressure and temperature ranges.

Failure of an 0-ring can sometimes begin with the removal of an old 0-ring. If you incorrectly remove an 0-ring with pointed or sharp tools, you can scratch or dent critical surface finishes that can result in seal failure.

Before installing a new 0-ring, inspect the sealing surfaces for any abrasions and wipe them free of any dust, dirt, or other contaminants. Before installation, inspect the 0-ring for any damage. If faulty, discard it.

When you install the 0-ring, lubricate it. In most cases it is already coated with the system fluid or petrolatum grease. Do not stretch the 0-ring more than twice its original size during installation, and do not roll or twist it into place. This may leave a permanent twist in the 0-ring and reduce its effectiveness and shorten its life.

When installing an 0-ring, take extreme care to avoid forcing it over sharp edges, corners, and threaded sections. You should use some type of sleeve or cover to avoid damaging the 0-ring.

FASTENERS

The proper use of fasteners is very important and cannot be overemphasized. Many shipboard machinery casualties have resulted from fasteners that were not properly installed. Machinery vibration, thermal expansion, and thermal contraction will loosen the fasteners. At sea, loosening effects are increased by the pitch and roll of the ship. You are familiar with such standard fasteners as nuts, bolts, washers, wingnuts, and screws. In this section we will discuss some of the new developments in fastener technology, such as the various types of locknuts, which you may not be familiar with.