A Study of Rotary-Type Flow Dividers
FLUID POWER - Design Data Sheet 31
A flow divider accepts a flow of oil under pressure, usually
from a hydraulic pump, and divides it into two streams of equal (or
unequal) flow volume for operation of two independent branch
circuits from one pump, or for synchronization of two identical
cylinders. Spool-type flow dividers were described in Design Data
Sheet 26. This issue covers rotary type flow dividers.
These operate quite differently, and require different circuit
A rotary flow divider is like two gear-type pumps or motors with
shafts joined, except that the two sections are built in one
housing and use a common shaft. Being mechanically joined, they are
forced to rotate at the same speed, and to meter equal flows to
their outlet ports.
Most rotary flow dividers are limited to two sections of equal
displacement, and this discussion will be limited to this type.
When a greater number of sections, or unequal displacement between
sections is used, problems of extreme pressure intensification may
Rotary dividers are more efficient than spool-type dividers
because the assigned power of an unused outlet port can be
transferred mechanically to the other section instead of being
converted into heat as is the case with spool-type dividers. Not
only does this reduce heat in the oil, it conserves input power to
the system, it permits the pump to operate at a lower average
pressure, and makes possible the efficient operation of a system
with unequally loaded branch circuits (an advantage over spool-type
Rotary dividers are probably most efficient when the combined
displacement of both sections is approximately equal to pump
displacement. They are definitely limited to proportional dividing;
priority dividing (as in a power steering system) is not possible
A rotary flow divider has a unique ability to transmit torque from
one section to the other through their common shaft. To take
advantage of this, the 4-way valve must have open flow to tank when
in center position. A tandem or open center spool should be used; a
valve with closed pressure port in neutral would cause the unused
power to be converted into heat across a relief valve instead of
being transferred through the flow divider and used in the other
Even though relief valves may be used elsewhere in the system, it
is important to place a relief valve on each outlet port of the
flow divider, and to set each valve to the maximum pressure
required in that branch.
A relief valve directly on the pump - pressure line is optional,
and may not be necessary. If desired, one may be used as an extra
safeguard, to back up the ones on the flow divider outlets, in the
event the flow divider should lock up. A pump relief valve, by
itself, would not give complete protection because of the pressure
intensification which may develop (see opposite side of this
sheet). If used, it should be adjusted to the same pressure as the
relief valve on the side of the flow divider which is most heavily
Figure 1 shows a 2-branch circuit with a
different load on each branch cylinder. Both 4-way valves are in
neutral. Gauge readings on the cylinders are taken as the pressure
which is required to move each load.
Figure 1. Basic
circuit for operating two branch circuits
from one pump. Branch circuits are carrying different
The reaction of pump pressure to branch circuit
when the branches are working, is shown in Figures 2 and
Figures 2 and 3 on this sheet show pump pressure required when
both branches are working and when only one branch is working.
Diagrams are in block form to illustrate flow divider action.
They are not complete working circuits. A pressure relief valve
(not shown) should be connected across each outlet of the flow
divider. All 4-way valves should have tandem center spools.
Operation of Two Independent Branches
Figure 2. Block diagram of Figure 1. Two branch
circuits, carrying unequal loads, are to be operated from one pump.
Notice the pressure level at which the pump operates in relation to
the pressure appearing in each branch.
Figure 2. Block
diagram of the circuit of Figure 1.
Both branch circuits are working. Compare the pump
pressure with that on each of the two cylinder ports.
Cylinder 2 requires 500 PSI for its load. This pressure is
supplied entirely by the pump through Section 2 of the flow
Cylinder 1 requires 1,500 PSI for its load. A part of this
pressure, approximately half of it, is furnished directly by the
pump working through Section 1 of the flow divider. The remainder
is furnished by torque developed in Section 2 as that section acts
like a hydraulic motor, furnishing power to drive Section 1 which
is acting like a hydraulic pump. In other words, any excess of
power not needed in the branch with lighter load (Section 2 in this
case), is transferred mechanically to Section 1 and is used there
instead of being dissipated in heat as in a spool-type divider. If
the flow divider has a 1:1 ratio, the pump should be working at a
pressure which splits the difference between pressure requirements
in the two branches.
Figure 3. This is also a block diagram of the
circuit of Figure 1, but with one branch working and the other
branch stopped. The 4-way valve to Cylinder 1 is shifted to a
working position and Cylinder 1 is extending. The 4-way valve to
Cylinder 2 is in neutral, and this part of the pump oil can flow to
tank. Remember, these 4-way valves should be tandem center type to
allow free oil flow to tank without passing across a relief valve
when the 4-way valve is centered.
Figure 3. Block
diagram of the circuit of Figure 1.
Cylinder on the right is stopped and its share of
power is transferred to the working cylinder on the
Half of the 1,500 PSI needed by Cylinder 1 is supplied directly
from the pump through Section 1 of the divider. The other 750 PSI
is furnished by power transfer through the divider from Section 2,
where it ) is not needed, to section 1. If the divider has a 1:1
ratio, the pump only has to put up half the pressure needed if one
side is not working and if that unused section of the divider is
vented to tank.
Synchronizing Two Cylinders
Another type of application for rotary-type flow dividers is to
keep two cylinders (attached to the same mechanism) in step, moving
at the same rate of speed. The cylinders should be identical in
bore and stroke, and the flow divider should have a 1:1 ratio. The
4-way valves may have any type of spool center.
One limitation of a rotary divider is that it cannot be used in
reverse as a flow combiner. One section, connected to the lagging
cylinder, would cavitate. Two possible circuit arrangements for
keeping two cylinders in step are shown in Figures 4 and 5.
Figure 4. A 2-section divider may be used in
the pump line. The two outlets than feed a pair of identical 4-way
valves, usually solenoid type, which are always energized and
de-energized at the same time.
Figure 4. Block
diagram of synchronizing circuit
for two cylinders using one flow divider and two
4-way valves which must be shifted simultaneously.
Figure 5. Where only one 4-way valve is to be
used, and where synchronization is to be in both directions of
travel, two flow dividers are required, one for each direction of
travel. Four relief valves are required, one connected to each
outlet port of each flow divider.
Figure 5. Block
diagram of synchronizing circuit
for two cylinders using one 4-way valve and two
dividers, one for each direction of cylinder travel.
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Power Design Data Sheet 31 - A Study of Rotary-Type Flow
© 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