Extrusions tend to cause the moving parts to bind, the O-ring seal to
fail, and particles of the O-ring seal to contaminate the fluid.
Backup rings can also be used with lower pressure systems to extend
When installed, a backup ring is placed on the side of the O-ring
not subjected to pressure.
In cases where the O-ring is subject to
pressure from both sides, two backup rings are used, one on each side
of the O-ring.
The use of V-rings is rather limited in hydraulic
systems; however, they are used in some shock struts.
A V-ring can
seal in only one direction and can be used to seal surfaces
regardless of whether there is movement between the parts.
U-Rings. Similar to V-rings in design and function, U-rings are
used to seal pistons and shafts on some master brake cylinders.
Cup Seals. Another type of seal used frequently on master brake
cylinders is cup seals.
They are effective in controlling leaks in
only one direction, and when installed the lip of the cup must be
facing the fluid to be contained.
Composite seals made from both rubber and metal are
called oil seals, and they are used to seal hydraulic pump and motor
drive shafts. Their outer body, or case, is made from pressed steel
and is force-fitted into the component housing.
Inside the metal
case is a lipped rubber seal and a spring.
The rubber seal is
securely anchored against movement to the metal case, and the spring
encircles the lip, holding it firmly to the surface it seals and is
commonly referred to as a Garloc Seal.
During installation, the
housing must be free from foreign matter or burrs, and the seals must
be seated squarely with proper special tools.
Wiper Seals. Scrapers or wiper seals are made of metal, leather,
or felt and used to clean and lubricate the exposed portion of piston
When installed and operating properly, wiper seals prevent
dirt from entering the system and aid in preventing piston shafts
Prior to use, all seals must be examined to ensure they are made from
the correct material, in the proper shape and size, and free from
nicks, cracks, rough spots, or other defects.
Immediately prior to
assembly, clean and lubricate the seals and contact surfaces with the
operational fluid of the system.