Copyright Goodheart-Willcox Co., Inc.
Chapter 6 Cylinders 145
Figure 6-29. A double-acting cylinder cannot drift back into the barrel due to a bad piston seal if the rod is pointing
upward. The amount of oil that is displaced from the cap side cannot fit into the smaller cavity in the rod side.
One misconception related to cylinder drift is that it is usually the result
of a bad piston seal. However, a failed piston seal can cause a cylinder to drift
only if the cylinder rod is pointing toward the ground. In Figure 6-29, the cyl-
inder’s rod is pointing upward. For this example, consider the cap side of the
cylinder has an area of 10 in2 and the cylinder’s ring area is 9 in2. If the piston’s
seal had a failure or was even removed, before the cylinder could drift back
into the barrel one inch, it would require displacing 10 in3 of oil from the cap
side, and it would have to place that 10 in3 in a 9 in3 cavity in the rod side of the
cylinder. As a result, a cylinder that has the rod facing upward cannot drift due
to a piston seal failure.
However, if the double-acting cylinder has the rod facing toward the
ground, a piston seal failure can cause the cylinder to drift. See Figure 6-30.
Presuming the cylinder has the same areas, 10 in2 on the cap side and 9 in2 on
the rod side, if the cylinder moved one inch, then 9 in3 of oil from the rod side
would easily fi t in a 10 in3 cavity on the cap side, and the cylinder would be
allowed to drift. As the cylinder moved, the voided 1 in3 cavity on the cap side
would drop to a vacuum state.
The dozer in Figure 6-31 has two double-acting cylinders used to raise
the blade. The blade has been removed. Notice the cylinder on the left has
leaked down and the cylinder on the right remains secure in its barrel.
Chapters 21 and 22 will detail equipment and methods used for diagnosing
cylinders.