Copyright Goodheart-Willcox Co., Inc.
138 Hydraulic Systems for Mobile Equipment
Cylinder Rod
The cylinder’s rod must also be considered during a rebuild. If the rod has
cracks, nicks or is bent, it will cause leakage. The rod’s surface can be rechromed.
If repair parts are unavailable because the cylinder is too old, a hydraulic repair
shop can fabricate a rod, or even an entire cylinder, using materials they have
on hand.
Cylinder Loads, Dampening, and Speeds
As machines are manufactured, the type of cylinder load, potential shocks,
and cylinder speeds must all be factored into the machine’s design. Loads
exerted on cylinders are classifi ed as thrust loading and tension loading. A
thrust load occurs anytime a cylinder must push a load. This is also known as
compression loading. A tension load occurs any time a cylinder pulls a load. It
is also known as shear loading.
Cylinder Dampening
Hydraulic cylinders can exert shock loads to machines. Shock loads occur
when the cylinder’s piston harshly hits the cylinder barrel as it reaches the end
of travel. The shock loads jar machine components and linkages causing poten-
tial damage. Manufacturers use different techniques to reduce the shock loads
placed on the machine:
• Cushioned cylinders.
• Orifi ces.
• Accumulators.
• Programmable kick-outs.
A cylinder cushion consists of an internal plunger that blocks off a larger
portion of the cylinder’s fl ow as the cylinder reaches the end of travel. See
Figure 6-21. When the plunger reaches its cavity, the cylinder’s remaining oil
must fl ow through the cushioned oil passage. A threaded adjustment is often
placed in the cavity providing some adjustment to the cushion.
Cylinder cushions can be designed as fi xed, variable, cushioned in just
one direction, or cushioned in both directions. A reverse fl ow check is used
when the cylinder is actuated in the opposite direction. It routes oil to the
piston allowing the fl uid to act on the entire piston area, not just the area of
the plunger.
An orifi ce can be used to dampen the stroke of a cylinder. The schematic
in Figure 6-22 shows a cylinder that is cushioned and dampened. The orifi ce
restricts and limits the rush of fl uid into the cylinder’s inlet. Only after the cyl-
inder reaches its end of travel will the cylinder’s supply pressure reach the full
system pressure. The result is a dampened cylinder. The orifi ce can be located
at the DCV or it can be integrated at the cylinder.
Accumulators are frequently used in mobile machinery to dampen a cyl-
inder’s shock loads. Chapter 13 will explain accumulator fundamentals and
applications.