Speed Regulation of Hydraulic Motors - Part 2
FLUID POWER - Design Data Sheet 52
In this issue we will continue our study of how speed regulation
of a hydraulic motor circuit is a direct measure of the range over
which its speed can be varied with acceptable performance. Internal
slippage of the motor itself is a very important factor, and was
the topic for the preceding issue. The circuit used for speed
control is also quite important, and is the topic for this
Remember, that to get as wide an adjustable range of speed as
possible for a hydraulic motor, the motor itself must have low
internal slippage, the speed control valve (if a pressure
compensated type is used), must have low internal slippage, and the
best speed control circuit should be selected which is compatible
with the components.
Before going into this study, be sure you understand the
foundation laid in the previous issue, Design
Data Sheet 51, on the subject of speed regulation in
BY-PASS SPEED CONTROL
Figure 1. The flow control valve is placed to
divert the unwanted oil to tank. While this is usually the simplest
method of speed control, and tends to produce less heating in the
oil than the other methods described, the speed regulation of the
total circuit is very poor because slippage in both the pump and
the motor contribute to poor regulation. This method should be used
only on applications where a very limited range of speed is needed,
no greater than a 2:1 range.
A further disadvantage is that only one branch motor circuit can
be used on the pump line.
By-pass method of motor speed control.
SERIES METER-IN METHOD
Figure 2. When the flow control valve is placed
in series between pump and motor, the effect of slippage in the
pump is eliminated, leaving motor slippage as the main factor which
limits the range of adjustable speed. Overall speed regulation is
approximately equal to the regulation of the motor itself. Twice
the speed range can be covered successfully as with the by-pass
method of Figure 1, or roughly about a 4:1 speed
range for most applications.
Although oil heating tends to be somewhat greater than in
Figure 1, several branch motor circuits can be
operated from one pump.
Series meter-in method of motor speed control.
THREE-PORT FLOW CONTROL
Figure 3. This circuit uses a special "3-port"
flow control valve, and is a combination of the by-pass and series
Speed regulation is about equal to the series circuit of
Figure 2, and the oil heating is no more than the
by-pass circuit of Figure 1, since surplus oil,
not needed by the motor, is allowed to discharge to tank through
the 3rd port of the flow control valve at a pressure only slightly
greater than actual load pressure, instead of at relief valve
Like the by-pass method of Figure 1, only one
branch motor circuit can be operated from the pump.
Three-port flow control method.
SERIES METER-OUT METHOD
Figure 4. This is the only method which permits a
full range of adjustment of motor speed from near zero RPM up to
maximum rated motor speed, and which holds speed constant at the
selected RPM from zero load up to full load. But this circuit can
only be used with selected components.
The motor must be one of the piston types because this is the
only type in which the internal slippage can be drained out without
coming out of the motor outlet port. Motor speed is in direct
proportion to the oil coming from the outlet (not the flow entering
the inlet). If, then, the flow control valve is placed in the motor
outlet, speed will not vary from no load to full load. Inlet flow
and slippage will increase with loading, but outlet flow will
remain constant and can be accurately metered regardless of
The pump can be any type but the best results, with minimum oil
heating are obtained with a variable displacement piston pump with
pressure compensator. It must produce more than enough flow to take
care of working oil plus slippage oil in the motor under all
conditions of loading and speed.
Series meter-out speed control gives best speed
RECOMMENDATIONS FOR INCREASING SPEED RANGE BY IMPROVING
- Select a motor which has a maximum continuous speed rating no
higher than the top speed in the range you wish to cover. Do not
use a motor which has extra displacement, more than needed to get
your top speed. The use of an over-capacity motor will cause speed
regulation and performance to be poor at the low end of your speed
- After estimating the lowest motor speed at which motor
performance is acceptable, if the speed range is not wide enough
for the application, it can be extended on the upper end, by going
into the intermittent speed range as shown in Figure
Figure 5. Speeds between Point A (zero RPM) and
Point B are unusable because motor performance is unacceptable. So
the speed range cannot be extended downward. In the speed range
between Points B and C motor performance is acceptable. For a wider
speed range, the motor can be operated up in the intermittent speed
range, up to Point D. However, prolonged operation in this area
will reduce life expectancy. Efficiency will be poor because of
increased porting losses, and these losses may overheat the
Figure 5. The
adjustable speed range of a hydraulic motor can be
extended by operating it into its rated intermittent speed
- Since internal slippage is greater when operating at high
pressure, better speed regulation and a wider range of speed can
sometimes be achieved by selecting a different motor model one
which will produce the same horsepower but at a lower pressure with
a higher oil flow.
- Place the speed control valve in series between the pump and
the motor, as in Figures 2 or 3,
or in series with the motor outlet as in Figure 4
if a piston motor is used.
- In a given frame size, particularly with gear-type or vane-type
motors, the model with highest displacement will usually cover the
widest speed range. For example: if a certain motor is offered in a
number of displacements from 0.5 to 2.0 C.I.R. (cubic inches per
revolution), it is likely that the 2.0 C.I.R. model will have
better speed regulation and will therefore cover a wider speed
range. Although the larger displacement model will have a larger
slippage volume, the percentage of slippage in relation to total
displacement will be less, and it will produce the same horsepower
while operating at a lower pressure.
This matter of motor speed regulation and speed range is covered
in much greater detail in the book "Industrial Fluid
Power - Volume 3" available from Womack
Download a PDF of Fluid
Power Design Data Sheet 52 - Speed Regulation of Hydraulic Motors -
© 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