Cushioning and Decelerating Methods for Cylinders
FLUID POWER - Design Data Sheet 7
Figure 1 - Preferred Method of Stopping a
High Inertia Load in Mid Stroke of a Hydraulic
This circuit is designed to absorb high pressure surges caused
when a hydraulic cylinder is stopped in mid stroke by centering the
4-way directional valve, especially where a very heavy (high
inertia) load is being moved at high speed. On this type of
application the momentum energy of the load may generate a high
pressure surge which may damage circuit components. The load may
also be damaged if stopped too abruptly.
A relief valve, shown in dotted lines, is sometimes used to
dissipate the high pressure surge, and is connected to discharge to
the opposite cylinder port. It should be adjusted to at least 500
PSI higher than maximum system pressure in order that it will not
reduce cylinder tonnage on either the forward or return stroke.
Thus, soft cushioning of the load is usually impractical
A better way is to discharge the relief valve into the system
pressure line, which during deceleration is connected to tank
through the center of the 4-way valve. With the relief valve
connected in this manner it may be adjusted either to a high
pressure for abrupt cushioning, or to a low pressure for soft
cushioning and the cylinder tonnage will not be affected in either
The relief is any adjustable hydraulic relief
valve. The anticavitation check valve should have a low cracking
pressure, 3 to 5 PSI. Its inlet port should be teed into the main
tank return line near the 4-way valve. Normal back pressure in the
tank return line will assist the flow of oil across the check valve
at the time it is needed.
Cushioning is shown for the forward stroke only.
If needed on the return stroke also, one more relief valve and
check valve must be added.
Figure 2 - By-pass Method of Reducing Cylinder Speed
Near the End of its Forward Stroke.
This is a by-pass method of deceleration designed to reduce the
forward speed of a cylinder piston when the cam valve position is
reached. A part of the incoming oil supply is routed through an
optional needle valve to the rod end port of the cylinder and to
tank through the 4-way valve. The cam valve must be experimentally
positioned to a suitable distance from the forward end of the
stroke to produce the desired action.
This circuit works best on loads which have low
inertia. It is a form of meter-in speed control, and like all
meter-in speed control circuits is not very effective in holding
back high inertia loads.
A more gradual reduction in speed will result if
the moving cam is contoured to gradually close the cam valve. The
needle valve can be included for variable adjustment of the rate at
which speed is reduced. If used, it must be a non-compensated type;
a pressure compensated type will not give satisfactory
The cam valve may be smaller in flow capacity
than the 4-way valve, since it will not be required to pass all the
flow to reduce cylinder speed. Its location in the circuit allows
full system pressure to be applied to the rod end of the cylinder
when the return stroke is started.
Figure 3 - Series Method of Decelerating
Cylinder Using Non-Electrical Components.
This circuit is designed for non-electrical
applications to reduce piston speed of an air or hydraulic cylinder
to reduce destructive impact as the piston rod reaches a positive
Normal speed during the main part of the stroke
before the cam valve position is reached, can be adjusted with
As the cam valve is actuated, flow through Valve
4 is cut off. Exhaust flow from the cylinder is metered through
Valve 4 can
be a pressure compensated or non-compensated valve according to the
application. Valve 3 should be a non-compensated type for best
metering at low flows. Check Valve 5 is necessary for normal
start-up and full speed in the return direction of cylinder piston
Figure 4 - Series Method of Decelerating
Cylinder Using Electrical Components and Solenoid
This circuit is designed for solenoid valve
operation, and uses the same flow control valve as in Figure 3.
This circuit is designed to be electrically fail-safe by using a
normally closed (N.C.) Valve 2 for deceleration. If the coil on the
valve should burn out, the cylinder would immediately be placed in
the slow travel mode.
As in Figure 3, Valve 4 sets the maximum speed
during the main part of the stroke. Solenoid Valve 2 is wired
through the COM and N.C. contacts of the limit switch. When the
switch is actuated, Valve 2 closes, and discharge oil from the
cylinder is metered at a slower rate through flow control Valve
In the electrical circuit for the machine, Valve 2 is tied in
with the solenoids on the 4-way valve so if either solenoid on the
4-way valve is energized, Valve 2 also becomes energized, placing
the cylinder in the fast-travel mode.
Figure 5 - Cushioned
Internal Cushions. Cylinders can be ordered with
cushions on one or both ends of the stroke. On cylinders larger
than 2" these cushions are externally adjustable.
Cushions work well on hydraulic cylinders but are
of limited effectiveness on air cylinders and are not generally
Cushions cannot be added in the field.
Figure 6 - Novelty Circuit for Special
This is an experimental circuit suggested by
Airmatic Valve Co. for air cylinders of large bore and long stroke
which are moving loads having high inertia, where other methods of
cushioning may have been ineffective.
As the cylinder moves in either direction,
discharge air, instead of being vented to atmosphere, is discharged
into pressure tanks. Thus, as the cylinder moves, back pressure
builds up, preventing acceleration to unreasonable speeds. The
orifice at the bottom of each tank bleeds off the back pressure,
eliminating a tendency for the cylinder to creep.
The tanks are sized by starting with an oversize
tank and partly filling it with oil or water. The cylinder is
operated, and if it stops before making a full stroke, tank volume
is increased by draining off some of the liquid. When operation is
satisfactory, permanent tanks are built with this capacity.
Download a PDF of Fluid
Power Design Data Sheet 7 - Cushioning and Decelerating Methods for
© 1988 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 information.