Spool-Type Proportional Flow Dividers
FLUID POWER - Design Data Sheet 26
Flow dividers - are devices which split the flow from a pump
into two streams for independently operating two branch circuits.
Advantages of flow divider operation are:
- Only one pump is required. This is a particular advantage in
the saving of space, and reducing cost.
- Each branch circuit is isolated from the other: changing loads
on one branch will not affect the pressure or flow in the other
branch. Flow dividers are only for hydraulic systems; they are not
applicable to most industrial air systems.
Two general methods are used for dividing a flow of oil. One
method uses a sliding spool; the other method uses rotary
construction, and operates similarly to two gear-type pumps with
shafts coupled together. The two gear sections meter proportional
amounts of oil to each outlet.
The action of spool-type and rotary-type dividers is quite
different, and each requires a degree of technical knowledge to
apply them properly and on suitable applications.
Spool-type dividers can be built to deliver proportional flows
from two outlets, or to deliver a priority flow from one outlet
with excess flow coming out the other outlet. Only the spool-type,
proportional divider will be described in this issue; the other
types will be the subject of a later issue.
Spool-type proportional dividers usually have equal division to
the two outlets (1:1 ratio). They are practical with ratios as high
as 4:1. Beyond this, the accuracy is poor.
The two most common types of applications are shown in the
diagrams, Figures 1 and 2.
Figure 1. Divider
splits pump flow equally or unequally
for independent operation of two cylinders which
may be the same or different bore and stroke.
Figure 1. Two Independent Circuits. Each branch
circuit receives its proportional share of the pump oil. It is most
important to have a relief valve on each branch to protect the pump
if the cylinder in one branch should be overloaded or should stall.
This would not only overload the pump, but would cause the other
cylinder to stop because the flow divider spool would cut off its
flow. Usually the two relief valves should be adjusted to the same
The 4-way valves may have any type of center position. Tandem
center valves are shown, and the pump would be unloaded when both
vales are centered. But remember, if only one cylinder is working,
the other side of the flow divider cannot unload through the tandem
center of its 4-way valve. The divider spool will move into a
throttling position. Only when both 4-way valves are centered can
the pump unload to a minimum pressure about 100 PSI (this minimum
PSI is due to pressure drop through the divider).
Design Limitations of Figure 1. A flow divider
instead of two separate pumps is recommended only on those systems
where both cylinders will be working for approximately the same
length of time during the same time intervals, a certain amount of
heat will be generated in the oil, and this must be considered in
the system design. If possible, select bore diameters of cylinders
or displacements of hydraulic motors so both branch circuits will
be able to work at about the same pressure. Remember that one
outlet of a flow divider cannot be unloaded while the other outlet
Figure 2. Flow
dividers keep two unequally loaded
cylinders moving at the same rate of speed,
forward and reverse. Dividers split the flow equally
to two cylinders of identical bore and stroke.
Figure 2. For synchronizing the movement of two
cylinders, the flow divider should have a 1:1 ratio, and the
cylinders should have the same bore and stroke. Accuracy of flow
division is usually about ± 5%, but may vary slightly according to
volume of oil flow, degree of unbalance in load between the
cylinders, and sometimes other factors.
To synchronize the cylinders in both directions of travel, two
flow dividers must be used. Or, one unit may be used if it is
designed to divide flow in one direction and combine equal flows in
the reverse direction. A unit designed only for dividing will not
combine equal flows; reverse flow is uncontrolled as it would be in
passing through a pair of plain orifices. To reduce power losses in
reverse flow, check valves may be placed around the divider.
Clamp and Work Application for a Proportional Flow
In this unusual application, a flow divider valve is used
instead of a sequence valve which normally would be used to program
two hydraulic cylinders in a clamp and work application.
Figure 3. Unusual
clamp and work circuit using a
flow divider instead of a sequence valve. The clamp
is isolated from the work cylinder by the flow divider.
In any clamp and work application, the clamp acts first, and
when it has "bottomed out" against the workpiece, it must come up
to near full clamping force before the work cylinder moves a
cutting tool into the work. A problem in some circuits using a
sequence valve is to limit the hydraulic pressure on the clamp to
less than that on the work cylinder, usually to protect the
workpiece from distortion or damage.
In this circuit, the clamp relief valve can be adjusted to any
desired pressure equal to or less than the setting of the pump
relief valve. Then, the work cylinder can operate at full pump
relief pressure while the clamp cylinder can be limited to any
desired lower pressure.
Both cylinders start to move when the 4-way valve is shifted to
forward position, and the work cylinder must be located a
sufficient distance from the workpiece so the cutting tool will not
engage until the clamp cylinder has reached the workpiece and built
up clamping force. Since the clamp cylinder is usually of small
bore and short stroke, and the work cylinder much larger, the clamp
should move to clamping position quickly, before the work cylinder
can move very far.
At the option of the designer, the flow divider can have a 1:1
division ratio, or can have up to a 4:1 ratio, depending on the
relative size of the two cylinders and the action desired. When the
clamp stalls against the workpiece, its share of the oil goes
across the clamp relief valve and generates heat during this
period, and this fact must be considered in the design.
When the 4-way valve is shifted to retract the cylinders, all
pump oil goes to the work cylinder, and the clamp retracts by
spring force. If the clamp is doubleacting, its rod port may be
teed into Point A, causing it to retract by power. Return oil will
pass in reverse through the divider as through an uncompensated
orifice. If desired, a free return check valve can be connected on
the left side of the flow divider.
On applications where the clamp must remain holding during
retraction of the work cylinder, the circuit must be modified to
retain the oil in the clamp until the work cylinder reaches home.
Pressure build-up at Point A can be used to open a sequence valve
or pilot open a dump valve to release the clamp.
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Design Data Sheet 26 - Spool-Type Proportional
© 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