Copyright Goodheart-Willcox Co., Inc.
134 Hydraulic Systems for Mobile Equipment
A double-acting piston seal holds fl uid in both directions. It provides
improved sealing over metal seals, but is less durable. O-ring piston seals
can be used in double-acting applications. However, the soft rubber O-ring
requires the use of a stiff plastic backup ring. The more popular type of double-
acting seal uses an outer seal ring made of polytetrafl ourethylene (PTFE), com-
monly known as Tefl on. The Tefl on reduces friction, enabling the seal to easily
slide between the barrel and the piston. However, Tefl on is fairly rigid and
requires the use of a rubber ring underneath the Tefl on ring. The rubber ring,
sometimes known as an expander, places pressure on the sliding Tefl on ring.
In Figure 6-15A, the piston seals have been removed. The red ring is the rigid
Tefl on seal, and the neighboring black ring is the soft rubber ring that holds
pressure against the Tefl on seal as it rides against the barrel. Figure  6-15B
shows the piston with the seal ring installed.
A single-acting piston’s lip seal, U-shaped or V-shaped, is designed to
hold fl uid in one direction. If used in a double-acting cylinder, the piston will
require two lip seals. They are less durable than metal seals, but are less likely
to leak. Fluid pressure forces the lip to seal against the barrel and piston, pro-
viding a snug connection. See Figure 6-16.
Follow the manufacturer’s service literature during assembly. The manu-
facturer might state to coat the seal with petroleum jelly prior to assembly. One
service manual recommended heating a piston seal ring prior to installation by
placing the ring in 180°–200° Fahrenheit (80°C–90°C) water.
Gland
The cylinder’s rod is guided by a gland, which acts like a cylinder rod
bearing or bushing. See Figure  6-13 and Figure  6-17. The gland is usually
removable, providing the technician the ability to disassemble the cylinder.
Cylinder glands are removed with a spanner wrench. A spanner wrench is
designed to grip the gland by attaching to the gland’s key slots or dowel holes.
If a spanner wrench is unavailable, and if the gland has an exterior lip, a large
pipe wrench can be used to grip the gland’s lip and back the gland out of the
cylinder. See Figure 6-18. However, to prevent damage to the gland’s outside
surface, a spanner wrench should be the fi rst choice for removing the gland.
Figure 6-14. This double-acting cylinder piston has a wide wear ring.
Wear ring