Copyright Goodheart-Willcox Co., Inc.
Chapter 6 Cylinders 129
Excavator manufacturers make use of this inherent principle and maxi-
mize it on the boom, stick, and bucket controls. For example, when digging in
diffi cult applications that require high break-out forces, the boom, stick, and
bucket cylinders will be extending, which provides maximum cylinder force.
See Figure 6-6. However, after the excavator has loaded the bucket, high speed
is more desirable, and the extra force is not necessary. Therefore, to dump the
bucket, to extend the stick, or to extend the boom, the excavator will retract a
hydraulic cylinder for the fastest cylinder speeds at a reduced force.
Double-Acting, Double-Rod Cylinders
When manufacturers want a cylinder that delivers the same speed and same
force in both directions, a second rod is added to the cylinder. See Figure 6-7. The
most common application for double-acting, double-rod cylinders is steering.
Rams
A ram is a single-acting cylinder that uses a cylinder rod that is the same
diameter as the cylinder’s piston. Rams are used in applications that require
long stroke and must maintain the rigidity of the rod. Some combines use rams
to lift the header. See Figure 6-8.
Figure 6-7. A double-rod cylinder is used for some steering applications so that, as the operator steers, the forces
and speeds are equal in both directions.
Figure 6-6. Excavators optimize differential cylinders by retracting cylinders when speed is more important than
force, and extending cylinders when force is more important than speed. A—Force is important during digging and
lifting, so the cylinders are extended. B—Speed is important during dumping, so cylinders are retracted.
A B