Copyright Goodheart-Willcox Co., Inc.
130 Hydraulic Systems for Mobile Equipment
As previously mentioned, single-acting cylinders are retracted using an
outside force, something other than fl uid pressure. When rams are used to lift
a header, the weight of the header must retract the rams. If the feeder house
(the housing that feeds crop from the header into the combine and is used
for lifting the header) has extra rams added to gain extra lift force, and if the
header is removed from the feeder house, the feeder house might be too light
to overcome the back pressure in the return circuit. This concern is especially
true if the hydraulic system is an open-center system with a fi xed-displacement
pump. Chapter 16 will explain open-center hydraulic systems in greater detail.
To overcome this problem, some manufacturers use double-acting cylinders
in place of single-acting rams. Note that the double-acting cylinders can also
improve a cylinder’s retraction speed, which can be critical if the machine is
operating at fast speeds.
Telescoping Cylinders
A telescoping cylinder is a linear actuator that contains up to six cylinder
tubes that extend to provide a very long cylinder stroke. During retraction, the
tubes (also known as sleeves) collapse into a compact cylinder. The cylinder
can retract to a length that is 20–40% of the overall extended stroke, compared
to the traditional cylinder, which can only be retracted to 50% of the cylinder’s
overall length.
Components
The individual components consist of a barrel, one to four intermediate
stages, and a fi nal stage called a plunger. See Figure 6-9. When retracted, the
barrel houses all of the collapsed stages.
Ram
Figure 6-8. Some combines use single-acting rams to raise the feeder house, which lifts the combine’s header.
Notice that the ram only has one hydraulic hose.