Cylinder Drift Problems and Their Solution
FLUID POWER - Design Data Sheet 54
There are certain conditions in a hydraulic cylinder circuit
which may cause a cylinder to slowly drift (creep) when its 4-way
control valve is in its center neutral position. The cause (in the
fluid circuit) for cylinder drift is the unbalance between areas on
opposite sides of the piston. Oil which leaks across the spool of
the 4-way valve, under pressure, acts on unequal areas of the
piston to develop a force unbalance which may cause unwanted piston
movement unless there happens to be sufficient dead load or
reactionary load against the piston rod to restrain piston drift.
Oil leaking internally across the piston seals can also cause drift
as discussed later.
Cylinders which have an oversize or 2:1 ratio piston rod have a
greater tendency to drift than those cylinders with smallest
(standard) diameter rod. Drifting force is equal to system pressure
multiplied times rod area. Therefore, on circuits where drifting
may be a problem, the cylinder should have the smallest diameter
rod which has sufficient column strength.
In this issue we will consider those circuits which may have a
tendency to drift, and will attempt to offer solutions.
Figure 1. If the cylinder is expected to support
a heavy load or reactionary force while stopped, internal spool
leakage of the 4-way valve will permit the cylinder to drift.
Leakage paths inside the valve are shown in dotted lines.
Figure 1. Drift
due to valve spool leakage and a reactionary load.
Figure 2. Cylinder drift from valve spool
leakage can usually be minimized to an acceptable level by placing
a pilot-operated check valve in the cylinder line to prevent
reverse flow caused by load reaction. This check valve must be
piloted from the opposite cylinder line, and this line must be
vented to tank through the spool of the 4-way valve in neutral
position to prevent pressure build-up on the pilot (from valve
spool leakage) which would cause the check valve to open and allow
the cylinder to drift.
Note that neither a pilot-operated check valve nor any other
external means will prevent cylinder drift if the drift is caused
by leaking piston seals.
If the reactionary load (while the
cylinder is stopped) is pulling instead of pushing on the piston
rod, the pilot-operated check should be located in the rod end
line, and the opposite cylinder port on the 4-way valve should be
vented to tank in neutral. Since pressure can intensify in the rod
end of the cylinder when the rod port is blocked (as by a
pilot-operated check valve), the application must be evaluated to
be sure intensification will not cause damage.
Figure 2. A
pilot-operated check valve eliminates valve leakage.
Figure 3. If two or more cylinder branch circuits
are operated from one pump, the upstream cylinder (Cylinder 1 in
Figure 3) may drift when the downstream cylinder
is operating at high pressure.
Leakage across Valve 1 spool, entering the blind end will cause
pressure intensification in the rod end of the cylinder. This
intensified pressure, being higher than pump pressure, may leak
across the valve spool into the pump line, causing forward drift of
the cylinder, unless the reaction or gravity load is sufficient to
prevent it. To calculate drifting force, multiply rod area times
inlet pressure to Valve 1.
Figure 3. On an
open center system, the upstream cylinder may drift
due to valve leakage when the downstream cylinder is
Closed Center System
Figure 4. A closed center system is one which
includes two or more branch cylinder circuits operating in parallel
from one pump. The pressure inlet port on all directional valves,
whether individual valves or one bank valve is used, is closed to
oil flow when the valve spool is in center neutral position. Pump
pressure builds up to relief valve or compensator level and is
maintained during the time all valve spools are centered.
To avoid power waste and heat build-up in the oil, a variable
displacement pump with a pressure compensator is usually employed
in a closed center system.· When all valve spools are centered and
system pressure holds at its maximum level, the leakage across the
4-way valve spools is at a maximum and frequently will cause one or
more of the cylinders to drift, especially those with large
diameter rods and those with little reactionary load against
In Figure 4 the possible leakage paths across
the spool of Valve 1 are shown in dotted lines. Leakage into the
blind end of Cylinder 1 will cause pressure intensification in the
rod end. The intensified pressure, being higher than pump pressure,
may leak across the valve spool and into the pressure line, thus
causing the cylinder to drift forward. However, if there is a dead
load or reactionary load against the piston rod greater than the
drifting force of rod square inch area times pump pressure, the
cylinder cannot drift.
A suggested solution for closed center systems is shown for
Cylinder 2 in Figure 4. A lock valve may be placed
in the lines to the cylinder. A lock valve is a double section
pilot-operated check valve. In addition to the lock valve, the
spool in Valve 2 must be a float center type, venting both cylinder
ports to tank when the spool is centered. The lock valve will
prevent cylinder drift in either direction. The vented ports on the
4-way valve will prevent spool leakage from building up on the
pilots of the check valve and holding them open. Spool leakage will
simply go to tank.
Closed center system with two branch circuits.
Leakage in the Piston Seals
If the piston seals are not leaktight, this may also cause the
cylinder to drift either under the influence of valve spool leakage
or reactionary load against the piston rod. This type of drift can
sometimes be minimized under certain conditions of operation:
Figure 5. If the reactionary or gravity load is
tending to pull the rod out as in this figure, there is no way to
arrange the external circuit to guarantee there will be no drift in
either a closed or open center system. Apparently the only remedy
is to select a cylinder with leaktight piston seals. Even this may
not eliminate drift due to leakage across the valve spool. If drift
develops after a period of usage, the piston seals should be
Figure 5. Load
pulling against rod.
Figure 6. However, if the gravity or
reactionary load is pushing against the piston rod, there is a
chance of minimizing drift by using a 4-way valve with blocked
cylinder ports in neutral If leakage across the valve spool could
be eliminated, it would be impossible for the cylinder to drift
inward regardless of how great the piston seal leakage might be.
For each 1 cubic inch of leakage from the blind end, for example,
there is less than l cubic inch of volume on the rod end for it to
To further minimize drift due to internal leakage through the
piston seals, the 4-way valve itself should be a high quality model
rated for low internal leakage. Note that a pilot-operated check
valve or lock valve would not help in piston seal problems. Since
there is no pressure intensification in the rod end of the cylinder
of Figure 6, pilot pressure obtained from the blind end of the
cylinder would not allow a check or lock valve to close, holding it
open for free flow in both directions.
Piston seal leakage can be a very serious problem in closed
center systems, and sometimes it may be virtually impossible to
eliminate cylinder drift. Cylinders and valves for these systems
should have a very low leakage rating.
Figure 6. Load
pushing against rod.
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Power Design Data Sheet 54 - Cylinder Drift Problems and Their
© 1990 by Womack Machine Supply Co. This
company assumes no liability for errors in data nor in safe and/or
satisfactory operation of equipment designed from this