Copyright Goodheart-Willcox Co., Inc.
Chapter 6 Cylinders 143
Cylinders can contain pressure sensors that measure a load placed on a
cylinder. The feeder lift cylinders on a combine have a pressure sensor con-
nected in parallel with the lift cylinders. The machine has a fl oat setting, which
measures the load on the cylinders using the pressure sensor. The combine’s
header fl oat is designed to allow the header to fl oat across the ground by means
of sensing the feeder cylinders’ pressure.
Agricultural tractors are frequently equipped with three-point hitches.
The hitches often are confi gured with a three-point hitch rockshaft poten-
tiometer and a hitch pin strain gauge sensor. The two sensors measure the
position of the hydraulically controlled hitch and the draft load exerted on
the hitch. The ECM will raise and lower the hitch’s hydraulic cylinder based
on input from those two sensors plus operator inputs, such as sensitivity and
commanded position.
The ECM that monitors these various cylinder inputs will require a cali-
bration process. The calibration is typically required after a sensor has been
replaced, or if an error has occurred. Some calibrations take place every time
the key has been cycled and the lift function is initiated.
Valving Designed for Cylinders
Machine manufacturers incorporate different valve technologies within
mobile equipment to protect and control cylinders. Designers sometimes inte-
grate valves directly into the cylinder housing. Some examples of pressure-
control valves that can be mounted inside cylinder housings are pilot-operated
check valves, counterbalance valves, thermal-relief valves, or pressure-relief
valves. Chapter 7 will detail the purpose and operation of pressure-control
valves.
Regeneration
One valve technology that is designed specifi cally for a double-acting dif-
ferential cylinder is regeneration. Cylinder regeneration takes place when
an unloaded differential cylinder is extending and its return oil is rerouted
to the cylinder’s inlet. See Figure 6-27. The principle is similar to the way a
positive-displacement pump generates a low pressure in a cavity that has
an expanding volume and a higher pressure in a cavity with a decreasing
volume.
When the two cylinder ports are connected together, the cylinder will
extend due to the larger area on the cap side of the piston. Plus the cylinder will
extend at a faster speed, because it is regenerating use of the oil that is normally
routed to the reservoir.
Regeneration is sometimes used on loader circuits to increase the speed
for dumping a bucket. A DCV in the regeneration position is illustrated in
Figure 6-28. The regeneration portion of the spool is highlighted in yellow.