Pressure Compensated Hydraulic Speed Control - Part 1
FLUID POWER - Design Data Sheet 39
Pressure Compensated Flow Control Valves. These
are adjustable speed control valves for hydraulic cylinders and
motors. In addition to the adjustable metering orifice, there is a
"compensating" spool which can modulate the flow during operation
to keep it constant at the value set on the metering orifice by an
operator. The adjustable orifice and the compensating spool work
together, and are built into a common body.
An orifice, by itself, cannot hold the flow constant if the load
on the cylinder or motor should change. Therefore, a pressure
compensated flow control valve is called for on applications where
constant speed must be maintained even when the load changes.
Obviously these valves are not suitable for compressed air, and
work most efficiently on hydraulics at the higher pressures. Below
2000 PSI, their efficiency becomes increasingly poor.
In this issue we intend to show how a speed control system can
be designed by using any kind of adjustable metering orifice, and
adding a separate compensating spool valve to work with it for
Pressure compensated flow control.
Figure 2. Meter-Out Speed Control. The three
components shown - orifice, check, and 2-way spool valve are all
contained within the housing of the flow control valve shown in
Figure 1. However, individual components may be
selected and plumbed separately to achieve the same results. This
approach may be used to advantage when working with a special kind
of adjustable orifice which for one reason or another is not
available with built-in pressure compensation as a completely
self-contained flow control valve. Compensation can be added to any
kind of needle, gate, globe, ball, or diaphragm valve. We will show
there are other advantages to using individual components in place
of a self-contained pressure compensated flow control valve.
The compensator spool is a 2-way, pilot-operated, normally open
spool valve with spring return. If such a valve is hard to come by,
one can be made by modifying a pilot-operated pressure reducing
valve as shown on the backside of this sheet.
Adjustable orifice, compensating 2-way valve, and check
connected to provide pressure compensated meter-out speed
Figure 3. By-Pass Meter-Out Speed Control.This
type of action can only be achieved by assembling individual
components into a speed control system. There is no self-contained
pressure compensated flow control on the market which will give
Figure 3 has the advantage of meter-out control for maximum
cylinder stability, plus the advantage of bleed-off action in the
oil supply to the blind end of the cylinder. The excess oil which
is forced to by-pass to tank can by-pass at load pressure instead
of relief valve pressure, thus saving power and reducing heat
generation during periods when the cylinder is idling or working
The compensator spool in this figure is a 2-way, pilot-operated,
normally closed spool valve. A standard by-pass valve will serve
the purpose if it is a direct-acting rather than a pilot-operated
In Figure 3, assuming the spring in the
compensator spool has a cracking pressure of 75 PSI, when flow
through the adjustable orifice increases to produce a pressure drop
across the orifice of greater than 75 PSI, pressure on the upstream
side of the orifice serves as pilot pressure to cause the
compensator spool to open just enough to by-pass all oil in excess
of that amount which produces 75 PSI drop across the orifice, thus
keeping a constant flow through the orifice.
The by-pass, meter-out circuit of Figure 3 is
the best speed control for cylinder or motor, with best stability
and minimum oil heating, but cannot be used in systems with more
than one branch circuit operating from a pump.
Figure 3. Using
individual adjustable orifice and 2-way, normally closed
by-pass valve, a bleed-off meter-out speed control system may
Figure 4. Hydraulic Motor Speed
Control.Although hydraulic motors can be controlled with
the same speed control circuits used for cylinders, this circuit
gives the best speed regulation, and therefore the widest range of
adjustable speed with acceptable performance.
As background for this circuit, remember that hydraulic motor
performance is best when operating at maximum speed, and becomes
progressively poorer as speed is reduced. Gear and vane-type motors
sometimes will not cover more than a 3 to 1 or 4 to 1 range with
good performance. The reason for this is the internal slippage
which causes the speed regulation to become progressively worse at
Figure 4. Best
speed control for a hydraulic motor.
Widest Range of Variable Speed. To be able to
operate a hydraulic motor over a wide speed range - 10 to 1 or more
- it is necessary to place the adjustable orifice in the motor
outlet. It is also necessary to use a piston-type motor because
this type has a case drain which will carry out the internal
slippage without letting it pass through the adjustable orifice to
worsen the speed regulation.
The compensator is a standard 2-way, pilot-operated spring
returned, spool-type valve, normally open, or, a pilot-operated
pressure reducing valve can be modified for this service as shown
Figure 4 is series metering in the motor inlet.
Alternatively, a 2-way normally closed valve could be used to
bypass excessive oil at the motor inlet. This would be a circuit
comparable to Figure 3. It would give the same
excellent performance and would minimize heating of the oil.
Figure 5. If a suitable 2-way, normally open
spool valve is not available for the circuits of Figures
2 and 4, a pilot-operated pressure
reducing valve may be modified for this service as shown in the
notes on the drawing.
If this valve is used in the motor inlet as in Figure
4, an external pilot connection must be brought out and
the internal passage plugged. But if the valve is used in the motor
outlet as in Figure 2, the spool is piloted from
its own outlet and the external pilot port is not needed. In either
case, however, the pilot poppet and spring must be removed and the
drain connection must be run directly to tank.
Modification of a pressure reducing valve to make a
normally open 2-way valve for use in Figures 2 and 4.
Download a PDF of
Fluid Power Design Data Sheet 39 - Pressure Compensated Hydraulic
Speed Control - Part 1.
© 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