Matching a Hydraulic Motor to the Load
FLUID POWER  Design Data Sheet 42
On the design of a new machine which is to be run with a
hydraulic motor, a determination of required speed and horsepower
must be made so a model with suitable ratings can be selected. This
issue describes several methods of making such a determination.
Hydraulic vs. Electric Motor
Characteristics
Designers who are experienced only in selecting electric motor
drives need to be careful in designing hydraulic drives because of
important differences between these two motors.
Normally, an electric motor is selected on the basis of
horsepower. It is selected to match an existing power source which
provides constant voltage and frequency. If it happens to be a
little oversize for the job, no harm is done, although it may cost
a little too much.
This is not true of a hydraulic motor. It is selected on the
basis of torque and speed rather than horsepower. It cannot be
selected without also considering the pressure and flow of the pump
which will supply it with fluid. If it is inadvertently selected
oversize for the job, its performance will not be as good as if it
were correctly sized. Hydraulic pump and motor must work as a team.
For optimum performance, the pump and motor must be correctly
matched.
An electric motor is relatively easy to select because, at any
given horsepower, there are only a few fixed speeds from which to
choose, and it does not have to be matched to the alternator which
supplies the power. On an induction type electric motor, starting
torque is always greater than running torque, so this type motor
can start any load which it can run. It can start under a full
torque load, or even an overload, although if started too
frequently, there will be an excessive heat buildup in the
windings.
The reverse is true of a hydraulic motor. Starting torque is
uncertain, but nearly always is less than running torque, so this
type motor cannot necessarily start under full load but it can run
any load it can start. Usually, it cannot start a full torque load
unless it has been oversized to gain starting torque. If purposely
selected oversize, then it will likely have poor speed regulation
with the result that it cannot cover as wide a speed range with
acceptable performance.
Torque Determination
Torque, not horsepower, is the basis for proper selection of a
hydraulic motor. In nearly every application the desired speed is
already known, leaving only the torque to be determined.
Torque is always the most significant factor in any hydraulic
motor system  much more significant than either speed or
horsepower. Unless the motor has sufficient torque to start the
load or to run it at maximum speed, the system can never work
properly no matter how much horsepower is available.
After a hydraulic motor has been tentatively selected, the
horsepower can be computed from speed and torque. If this turns out
to be more than is available from the pump, then some of the speed
must be sacrificed, but never the torque
Load torque is the twisting or turning effort which must be
exerted on the load shaft to start the load and/or to run it at
maximum rated speed. This torque must be supplied by the hydraulic
drive motor. The amount of torque which will be required from the
motor can be estimated in one of several ways:
Torque Estimation by Measurement
It is essential that characteristics of the load be considered
when attempting to estimate hydraulic motor torque for starting
and/or running. Some loads require more torque to break them away
from a standstill than to run them after they have started. Other
loads require only a small amount of breakaway torque but torque
requirements increase in proportion to speed.
Breakaway Torque. Figure 1. Breakaway, or
starting torque can be measured with a torque wrench on small
machines, or by using a long lever loaded with weights until the
load shaft breaks away and starts to rotate. Torque is calculated
by multiplying lever arm length, in feet, times the weight
necessary to break the shaft away. Torque will be in footlbs. On
this type load, if the hydraulic motor can supply breakaway torque,
it can also run the load at any speed.
Figure 1.
Breakaway torque of a load can be measured
with a lever and weight, a spring scale, or a torque
wrench.
Electric Motor Substitution. The running torque
can be determined by temporarily coupling an electric motor to the
load shaft and measuring the motor line current while it is
running. By referring to a torque/current performance curve for the
motor, the running torque can be determined with a fair degree of
accuracy. Then, a hydraulic motor can be selected and the system
PSI can be established which will give the required running torque.
Of course the electric motor speed should approximate the maximum
running speed of the load.
Torque Estimation by Comparison
This is the easiest method where applicable. The proposed new
machine can be compared with an existing machine which does
essentially the same job, and on which the torque and speed, or
horsepower, are known.
But perhaps the new machine is to produce twice as many parts
per hour as the existing machine. Then basically the new machine
will require twice the horsepower. Perhaps the new machine will
manufacture parts of the same size but twice as fast; hence twice
the horsepower. Or perhaps larger parts are to be manufactured
which will require twice the torque but at the same production
rate. Again, twice the horsepower will be required. Suppose the new
machine is to manufacture the larger parts and at twice the rate of
the existing machine. Then four times the HP is needed.
Estimation is by comparing horsepower; then the HP can be
divided by speed to find the hydraulic motor torque.
Caution! If the existing machine is powered
with an electric motor and the new machine with a hydraulic motor,
and if high starting torque is required, the hydraulic motor must
be selected by comparing its starting torque with the starting
torque of the electric motor on the existing machine. An induction
type electric motor can produce about 3 times the starting torque
of most hydraulic motors of the same horsepower rating.
Torque Estimation by Calculation
On machines of new design where there is no way to measure input
torque, nor to compare performance with an existing machine, torque
and HP must be calculated.
Since the final output of the machine will be mechanical, the
starting point for calculations is to compute the mechanical output
HP.
If the machine output will be linear motion, use this formula to
compute HP output:
HP = (F × D) ÷ [t ×
33,000], in which:
F is the linear
force output, in lbs.;
D is distance, in feet, through which the force
is exerted;
t is time, in minutes, that the machine exerts
force, F, through distance, D.
If the machine output will be rotary, use this formula to
compute the mechanical HP output;
HP = T × RPM ÷
5,252, in which:
T is the torque
produced, in ft. lbs.;
RPM is the speed of the rotary output.
Torque must be calculated on
most new designs.
No calculations are possible unless
the force, and travel distance during a specified time interval are
known or assumed.
After HP output is determined, work
backwards toward the input shaft of the machine, one step at a
time, adding sufficient HP at each step to make up for friction
losses in gear boxes, bearings, etc. This will give the estimated
HP required from a hydraulic motor coupled to the input shaft.
In a similar manner start with the
output speed and work back toward the input, adjusting for speed
changes through chain drives, gear boxes, etc., finally arriving at
the speed of the input shaft to which the hydraulic motor will be
coupled.
From these estimations of input HP and
speed, the hydraulic motor torque can be computed:
T (torque in ftlbs) = HP ×
5252 ÷ RPM
Winch Example: A
winch, having a 20" diameter drum, must raise a 15,000 load at a
speed of 3 ft/second. Mechanical HP is calculated as follows:
HP = (15,000 × 180] + [1 ×
33,000] = 81.8 HP
15,000 lbs. is force to move load; 180
ft. is distance traveled in one minute (computed from 3 ft/sec); 1
represents a time of 1 minute.
Drum RPM is computed from Formula (D)
in box; input speed for the hydraulic motor is computed as
described above in this column.
Drilling Machine. On
a machine for drilling, the drill manufacturer will usually furnish
HP and recommended speed specifications. These figures can be
adjusted back to the input shaft of the drilling machine, making
allowances for frictional losses and speed changes.
Vehicle Wheel Motor.
Calculate the total drawbar pull (horizontal thrust) to overcome
all road and operating conditions. Using the wheel radius,
calculate wheel torque with Formula (C) in the box below. Convert
travel speed into wheel RPM. Select hydraulic wheel motor to meet
these conditions. Information for estimating drawbar pull and
making calculations is given in "Industrial Fluid Power
 Volume 3" published by Womack Machine
Supply Co.
Download a PDF of Fluid Power
Design Data Sheet 42  Matching a Hydraulic Motor to the
Load.
© 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
information.
