Minimizing Shock in Hydraulic Systems
FLUID POWER - Design Data Sheet 23
Momentary high pressure surges passing through the fluid in a
hydraulic system generate noise and may cause hoses to jump and
tubing to vibrate. Needless to say, these pressure spikes are a
strain on a system and should be eliminated in so far as possible.
They cause accelerated wear in pumps and motors by internal bending
of shafts and overloading of bearings. A catastrophic failure may
eventually occur when a line ruptures or the housing of a component
Pressure spikes are generated principally during the shifting of
valves and during sudden decompression of oil under high pressure.
Valve problems will be discussed in this issue: decompression shock
will be the subject of a later issue.
Hydraulic Shock From 4-Way Valve Operation
Figure 1. A
tandem center spool generates pressure spikes
while spool is traveling between positions.
Figure 1. Tandem center 4-way valves offer a
convenient and economical way to control a hydraulic cylinder and
to unload the pump when the valve spool is centered. But these
valves are apt to generate a pressure spike in the pump line while
the spool is moving between positions.
Expanded diagram of tandem center spool.
Closed porting in the crossover positions is
Figure 2. This is an expanded diagram of the
standard tandem center spool used in Figure 1. At
intermediate points between the end positions and center, the spool
travels through crossover positions in which the pump flow is
momentarily blocked. A pressure spike is produced because of the
momentarily increase in energy level due to the pressure and
velocity heads in the pump line.
Figure 3. Open
porting in the crossover positions reduces
pressure spikes but spool leakage rate is higher.
Figure 3. Tandem center valves are available
from most manufacturers which have special spools with open porting
in the crossover positions. These do reduce the shifting shock but
have more spool leakage because of reduced sealing length on the
spool. On manual valves, the load can be dropped if the operator
hesitates while shifting the spool.
When designing industrial hydraulic systems using solenoid 4-way
valves, we suggest limiting the use of tandem center valves to low
power systems, say under 25 HP. On high power systems, closed
center valves should be used with the unloading arrangement of
Figure 4. This
circuit eliminates spool shifting shock in solenoid valve circuits.
Turn page over for description.
Figure 4. Vent-type pump unloading is preferred
over tandem center valve unloading on industrial hydraulic systems
using solenoid 4-way valves for two good reasons:
- Pressure spikes generated when the valve spool shifts are
virtually non-existent. By the time the spool reaches crossover
position, high flow and high pressure have already been dumped to
tank through the vented relief valve.
- Several branch circuits can be operated from one pump in a
parallel circuit with full pressure available to all branches
simultaneously. Tandem center valves operating several branch
circuits must be connected in series, and full pressure is not
available to all branch circuits at the same time.
The relief valve in Figure 4 functions not only
as an adjustable maximum pressure limiter but also, when vented, as
a pump unloader. It must be a 2-stage valve, also called a
pilot-operated relief, and must have an external vent port
(sometimes called an RC, remote control port).
All branch circuits should have 4-way valves with blocked
pressure port so they can be connected in parallel. Closed center,
float center, or 2-position (no center neutral) types may be used.
The pump is unloaded by venting the pilotoperated relief valve when
all branch circuit solenoids are de-energized. When Solenoid C is
de-energized, the relief valve loses its pressure holding ability,
opens up, and allows the pump oil to flow to tank with very little
resistance. When Solenoid C is energized, the vent line is blocked
and the relief resumes its normal functioning as a maximum pressure
limiter at the pressure set on its adjusting knob.
When designing the electrical control circuit, each time any one
of the valve solenoids, A, B, etc. is energized, Solenoid C must
also be energized through a separate set of contacts in order to
load up the pump to obtain pressure. When all 4-way valves are
centered, Solenoid C must also be de-energized, to unload the
Figure 5. Cushion
Relief Valves for Hydraulic Motor
Figure 5. Cushion Relief Valves. Most of us are
familiar with cushion valves across the ports of hydraulic motors
to decelerate the motor safely to a stop when the spool of the
4-way valve is centered or when it is passing through a closed port
crossover position. These valves should be non-adjustable, and have
a cracking pressure about 500 PSI higher than the maximum system
pressure. They give very abrupt stopping, sometimes too abrupt.
Most hydraulic motors should be protected with cushion valves,
the exceptions being low speed operation (below 500 RPM) or when
the load is mainly friction or pure torque, rather than rotation of
a large mass.
Infinitely Adjustable Motor Cushioning
Figure 6. Adjustable Cushioning. This is a much
better cushion circuit where a softer stop is needed. The two
cushion relief valves, one for each direction of rotation, may be
set for any degree of deceleration from very abrupt to very soft.
Instead of discharging to the opposite motor port as in
Figure 4, they are connected to the pump pressure
line. Pressure in this line blocks them while the motor is running,
and though they may be set very low they cannot limit the system
pressure. But when the 4-way valve spool is centered, this pump
line goes to tank and cushioning becomes effective.
To prevent cavitation of the motor inlet when oil discharges
across the cushion relief valves, a pair of check valves is added.
These should have low cracking pressure, 3 PSI, and should be
plumbed to the tank port of the 4-way valve where they can take
advantage of back pressure in the tank return line to help the
check valves open.
Relief valves for cushioning should fie poppet-type (not
spool-type), direct-acting (not pilot-operated). This type is
considered to have the fastest response to pressure spikes.
Figure 7. Relief
Valve With Pressure Compensated Pump
Figure 7. Pressure Compensated Pumps. Circuits
for these pumps ordinarily use a closed center 4-way directional
valve. The question is whether a relief valve should be used to
absorb pressure spikes generated when the 4-way valve spool is
centered and before the compensator has time to act. Each
manufacturer will furnish data on compensator response and
whether a relief valve should be used. However, even with a very
fast acting compensator of 50 milliseconds, if the connecting line
between pump and 4-way valve is very short, the enclosed volume of
oil may not be sufficient to contain the rise of pressure spikes,
and a miniature relief valve is advised. As a general rule, a
relief valve is always recommended if the response is slower than
Hose Connections. Valve shifting and
decompression shocks can be minimized although not completely
eliminated by using hose connections, instead of rigid plumbing
between a hydraulic cylinder and its 4-way control valve.
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Design Data Sheet 23 - Minimizing Shock in
© 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