CHAPTER 5
Rotary Actuators
Fluid Power Circuits and Controls,
John S.Cundiff, 2001
INTRODUCTION
† Concepts developed for pumps are
applicable to hydraulic motors.
† Motors convert fluid energy back into
mechanical energy and thus are the mirror
image of pumps
† Typical motor designs are gear, vane and
piston.
1
INTRODUCTION
† Motor performance is a function of
pressure.
† As Pressure increases,
„ leakage increases
„ speed decreases
„ quantity of mechanical energy delivered
to the load decreases.
INTRODUCTION
† Motor volumetric efficiency is
evm =
Actual motor speed
Theoretical motor speed
† Pump volumetric efficiency is
evp =
Actual flow
Theoretical flow
† Purpose of the pump is to produce flow. Load sets the
pressure.
† Purpose of the motor is to receive this flow and
reproduce rotary motion.
2
INTRODUCTION
† Simple example :
„ Suppose a motor has a displacement of 3.9
3
in /rev. Measured flow is 10 GPM. The
theoretical output speed is
Nmth = 231Q
Vmth
where Nmth = theoretical motor speed(rpm)
Q
= flow (GPM)
3
Vmth = motor displacement (in /rev)
INTRODUCTION
„ Substituting,
Nmth = 231(10) = 592 rpm
3.9
Assuming the measured speed is 536
rpm , the motor volumetric efficiency is
evm = 536 x 100 = 90.5%
592
3
INTRODUCTION
† The overall efficiency of a hydraulic
motor is
eom = Actual output power
Input power
† Input power is hydraulic power
measured at motor inlet port and the
output power is mechanical power
delivered by the motor output shaft.
INTRODUCTION
† The motor in previous example
3
(Vmth=3.9in /rev) operates at a 2000 psi
pressure drop across the ports.
Measured flow to the motor is 10 GPM,
then hydraulic power input is
Pin = ∆PQ
1714
Where Pin = input hydraulic power (hp)
∆P = pressure drop (psi)
Q = flow (GPM)
4
INTRODUCTION
† Substituting,
Pin = 2000(10) = 11.67 hp
1714
Measured torque is 1080 lbf-in at 536 rpm.
† Output mechanical power is
Pout = TN / 63025
Where Pout = mechanical power (hp)
T = measured torque (lbf-in)
N = measured speed (rpm)
INTRODUCTION
„ Substituting,
Pout = 1080(536)
63025
= 9.18 hp
Motor overall efficiency is
eom = (Pout / Pin) x 100
= (9.18 / 11.67) X 100
= 78.7 %
5
INTRODUCTION
† Torque efficiency describes hydraulic
motor performance.
etm = Actual output torque
Theoretical torque
Theoretical output torque is
Tmth= ∆PVmth / 2π
where ∆P= pressure drop across motor
3
Vmth= displacement (in /rev)
INTRODUCTION
† Substituting,
Tmth= 2000(3.9) = 1241 lbf-in
2π
Αctual output torque is 1080 lbf-in,
torque efficiency is
etm = (1080/1241) x 100 = 87%
6
INTRODUCTION
† Overall efficiency is product of
volumetric and torque efficiencies.
Volumetric efficiency is
evm = N/ Nmth
Where
N
= actual output speed (rpm)
Nmth = theoretical output speed (rpm)
INTRODUCTION
Substitution of
Nmth = 231 (Q/Vmth)
Into previous equation gives
evm = NVmth/ 231Q
† Torque Efficiency defined by
etm= T/ Tmth
where
T = measured output torque (lbf-in)
Tmth = theoretical output torque (lbf-in)
7
INTRODUCTION
† Substitution of
Tmth= ∆PVmth / 2π
into previous equation, we get
etm= 2πT/∆PVmth
Multiplying these two together,
etm evm = (NVmth/231Q) x 2πT
∆PVmth
= 2πTN/231∆PQ = TN/36.76∆PQ
INTRODUCTION
† By definition, Overall Efficiency
eom = Pout / Pin
= 2πTN/231∆PQ
= TN/36.76∆PQ
Then
eom = evm etm
Inserting data from previous example ,
eom = 0.905(0.87) = 0.787
8
STALL TORQUE EFFICIENCY
† Stall Torque Efficiency is
esm = (Tms/Tmth) 100
Where
esm = stall torque efficiency(%)
Tms = measured torque developed at
stall (lbf-in)
Tmth= theoretical torque (lbf-in)
Stall is defined at output speeds less
than 1 rpm .
STALL TORQUE EFFICIENCY
† Motor RPM is the
independent
variable. We would
not expect speed to
affect output torque
because
Tmth= ∆P (Vmth/2π)
† N does not appear in
the equation.
† However, there is a
drop in torque,
below 200 RPM.
9
STALL TORQUE EFFICIENCY
† Stall torque is important in mobile applications where high
torque is required to start a stationary vehicle.
† Hydraulic motors must be designed for stall torque rather
than operating torque characteristics.
† A characteristic of high speed motors that creates problems
at low output speed is called cogging, where speed is jerky.
† Low speed, high torque motors were developed to address
these low speed problems.
TYPICAL PERFORMANCE DATA FOR
A GEAR MOTOR.
† Manufacturers data for
gear motor.
† Sloping horizontal
curves are ∆P across
motor.
† Sloping vertical curves
are flows to motor.
† X-axis is output speed.
† Y-axis is output
torque.
† Efficiency shown in
table on next slide
10
TYPICAL PERFORMANCE DATA FOR
A GEAR MOTOR.
† The 10 GPM curve was used for all three operating
points, 1000, 2000, 3000 psi.
† Volumetric and overall efficiencies follow the same
trend as gear pump.
TYPICAL PERFORMANCE DATA FOR
A GEAR MOTOR.
†
Leakage increases as pressure increases; consequently
efficiency decreases.
†
A secondary effect is the deformation of the components.
†
Clearance between parts increases with pressure, thus
effective area of leakage pathway increases.
11
TYPICAL PERFORMANCE DATA FOR
A GEAR MOTOR.
† Torque Loss is defined by
Tl = Tmth – T
where Tmth = theoretical torque (lbf-in)
T
= measured torque (lbf-in)
TYPICAL PERFORMANCE DATA FOR
A GEAR MOTOR.
† Torque loss varies with load pressures.
† Torque loss for this motor is approx. linear
with pressure, therefore the torque
efficiency is approx. constant.
12
COMPARISON OF MOTOR
PERFORMANCE CHARACTERISTICS
† Motors can be classified into two main
groups
„ High Speed Motors
„ Low Speed Motors
„ High Torque Motors (LSHT)
† Here we compare the three main designs of
high speed motors
„ Gear
„ Vane
„ Piston
COMPARISON OF MOTOR
PERFORMANCE CHARACTERISTICS
† Hydreco Model 1919 Gear motor has
a theoretical displacement of 4.53
3
in /rev, at 2500 psi max. pressure,
and 3000 RPM max. speed
† Efficiencies for gear motor with 36
GPM input flow.
13
COMPARISON OF MOTOR
PERFORMANCE CHARACTERISTICS
† Volumetric efficiency
decreases linearly as
pressure increases.
† Torque efficiency is
almost constant above
1500 psi.
† Overall efficiency is a
maximum at 1500 and
decreases to 76% at
2500 psi.
COMPARISON OF MOTOR
PERFORMANCE CHARACTERISTICS
† A Vickers Model 25M
(65) vane motor is rated
for 3000 rpm at 2500
psi maximum pressure.
† Torque efficiency for this
design is higher than for
gear motor and is
relatively constant from
500 to 2500 psi.
14
COMPARISON OF MOTOR
PERFORMANCE CHARACTERISTICS
† Vane motors have two unique displacement
settings by using two rotors.
† Flow directed to only one of the rotors will
produce twice the speed but only half the
torque.
† Operator adjusts a valve on the outside of
the motor to switch from low-speed, hightorque (two rotors) to the high-speed, low
torque (one rotor) configuration.
COMPARISON OF MOTOR
PERFORMANCE CHARACTERISTICS
† Fixed displacement axial piston motor
(Sauer-Danfoss Model 90-075 MF)
has a maximum speed of 3950 rpm
and a rated pressure of 6000 psi.
3
Displacement is 4.57 in /rev.
15
COMPARISON OF MOTOR
PERFORMANCE CHARACTERISTICS
† Torque efficiency
increases to a
maximum at 3000
psi, remaining
constant at higher
pressures.
† Volumetric efficiency
decreases from 99%
at 1000 psi to 90.5%
at 6000 psi.
COMPARISON OF MOTOR
PERFORMANCE CHARACTERISTICS
† At 2500 psi, the overall efficiency of the piston
motor is 92.5% and a piston pump is 93.3%.
† If pump and motor are used as hydrostatic
transmission, the overall efficiency neglecting
losses is
0.933 x 0.925 = 0.86
† Gear pump and motor combination has overall
efficiency of 62% (same operating pressure.)
16
COMPARISON OF MOTOR
PERFORMANCE CHARACTERISTICS
† Direct comparison of overall
efficiency for the 3 designs.
† At p<1000 psi, the vane
motor has the highest
overall efficiency.
† At higher pressures, the
piston motor has a higher
overall efficiency.
PERFORMANCE OF LOWSPEED,HIGH TORQUE MOTORS
† The “geroler” motor is similar to a gerotor motor.
† Instead of a gear running inside another gear, the gear
operates inside a housing with rollers in place of the outer
gear teeth. (Refer Fig5.7 and 4.2)
† Some motors run effectively at speeds as low as 1 rpm.
† Geroler’s are reversible by changing the fluid flow direction
into the motor, the direction of shaft rotation changes.
17
PERFORMANCE OF LOWSPEED,HIGH TORQUE MOTORS
†One manufacturer uses a disc valve to distribute fluid to
the geroler pockets providing improved performance
at low speeds.
PERFORMANCE OF LOWSPEED,HIGH TORQUE MOTORS
† Car-Lynn
10,000 series Geroler motor with
3
40.6 in /rev displacement has a max. speed of
254 rpm and Maximum pressure is 3000 psi.
† Overall efficiency increases from 79.5% at
500 psi to 86% at 1000 psi and remains
nearly constant for the higher pressure.
18
PERFORMANCE OF LOWSPEED,HIGH TORQUE MOTORS
† Volumetric efficiency for this motor was higher than
the gear motor.
PERFORMANCE OF LOWSPEED,HIGH TORQUE MOTORS
† Vickers Model MHT50 vane motor
3
with 38 in /rev max. displacement is
rated for a max. pressure of 4000 psi.
† Max. continuous speed at 3000 psi. is
200 rpm and max. speed at 2000 psi
is 350 rpm.
19
PERFORMANCE OF LOWSPEED,HIGH TORQUE MOTORS
† Volumetric efficiency
declined linearly as
pressure increased
from 1000 to 3000
psi.
† Torque efficiency
increased as pressure
increased from 1000
to 2000 psi.
(remained const. at
higher pressures)
PERFORMANCE OF LOWSPEED,HIGH TORQUE MOTORS
† Decrease in volumetric efficiency was
offset by the increase in torque
efficiency in the 1000-2000 psi range
† Overall efficiency decrease was
moderated.
† Overall efficiency decreased from
2000 to 3000 psi.
20
PERFORMANCE OF LOWSPEED,HIGH TORQUE MOTORS
† Comparison of low-speed,
high-torque geroler, and
vane motor
† These two designs have
approximately equal
performance in 1000-2000
psi range.
† Other factors would be
considered in making a
choice between the designs.
† For higher pressure the
vane motor is ideal.
DESIGN EXAMPLE FOR GEAR
MOTOR APPLICATION
† Gear motors can be an optimum selection for
a given application.
† Exercise:
„ A motor load is expected to average 1000 lbf-in,
with peaks as high as 1500 lbf-in. The desired
speed is 300 rpm, and quality control requires that
this speed not fluctuate more than ± 5% ,equivalent
to ± 15rpm .
† Trial No.1
„ The Model CR-04 motor has been discussed earlier in
Fig 5.2.
„ Find the intersection of the 1000 lbf-in line and the 300
rpm line. (Refer Fig 5.2)
21
DESIGN EXAMPLE FOR GEAR
MOTOR APPLICATION
DESIGN EXAMPLE FOR GEAR
MOTOR APPLICATION
„ Input flow of 6 GPM is required.
„ Interpolating between the 1500 and 2000 psi
curves, the pressure drop will be ∆P = 1810
psi.
„ When the torque requirement increases to
1500 lbf-in, projecting the intersection of 1500
lbf-in line and the 6GPM curve, N0= 218 rpm.
This output speed represents a 27% speed
droop.
22
DESIGN EXAMPLE FOR GEAR
MOTOR APPLICATION
† Trial No.2
„ Performance data for the Model CR-08 motor
3
(Vmth= 7.7 in /rev) are given in Fig5.11.
„ Intersection of 300 rpm and 1000 lbf-in lines
show that the flow requirement is 10.8 GPM
and ∆P = 975 psi.
„ If torque requirement increases to 1500 lbf-in,
move along line of constant flow, the output
speed drops to 290 rpm, a speed drop of only
3.3%
DESIGN EXAMPLE FOR GEAR
MOTOR APPLICATION
23
INTERACTION OF PUMP AND
MOTOR CHARACTERISTICS
† From Chapter 4, Pump flow
output decreases as load
pressure increases.
† Flow to the motor does not
stay constant. It decreases
as load pressure increases.
† Refer Fig 5.12 where a
hydraulic motor is used to
drive a time varying load.
INTERACTION OF PUMP AND
MOTOR CHARACTERISTICS
† Load pressure as a function
of time [P(T)] is given in
Fig 5.13.
† Requirement is to plot the
percentage change in the
motor output speed as
pressure varies.
† The prime mover turns at a
constant 1800 rpm
independent of the torque
required to drive the pump.
24
INTERACTION OF PUMP AND
MOTOR CHARACTERISTICS
† The Hydreco Model 1919 gear pump and motor
were chosen for the gear design.
† The axial piston design is represented by the
Sauer-Danfoss Model 90-075 pump and motor
operated at max. displacement
† Equation for pump volumetric efficiency is
2 1/2
evp = DP + E + (A +BP +CP )
evp = pump volumetric efficiency (%)
P = pressure (psi) ; A,B,C,D,E = constants
INTERACTION OF PUMP AND
MOTOR CHARACTERISTICS
25
INTERACTION OF PUMP AND
MOTOR CHARACTERISTICS
† The equation for motor volumetric efficiency is
2 1/2
evm = DP + E + (A +BP +CP )
evm
= motor volumetric efficiency
P
= pressure (psi)
A, B, C, D,E = constants presented in table
† The curvature of the plotted curve is so small that the A,B,C
constants are negligible and thus the equation reduces to the
equation for a straight line.
Pump out flow is given by
Q = Np Vpth evp/ 100
3
Q = flow delivered by pump (in /min); Np = pump speed (rev/min),
3
Vpth = pump displacement(in /rev); evp = pump volumetric efficiency
INTERACTION OF PUMP AND
MOTOR CHARACTERISTICS
† Corresponding motor speed is
Nm=(Q/Vmth)(evm)/100
Nm
=motor speed (rev/min)
Q
=flow to motor(in /min)
3
Vmth =motor displacement
evm = motor volumetric efficiency(%)
26
INTERACTION OF PUMP AND
MOTOR CHARACTERISTICS
† Motor reference speed was chosen as the
speed at 500 psi pressure.
∆Nm=((Nm–Nm0)/Nm0) 100
∆Nm = motor speed change (%)
Nm = motor speed (t=t)
Nm0 = motor speed (t=0)
INTERACTION OF PUMP AND
MOTOR CHARACTERISTICS
† Motor speed changes for
both designs
† Gear-pump combination,
the speed change ranges
from -23 to +4%, a total
change of 27%.
† Piston-pump motor
combination has a max.
speed change of -7% as
pressure varies.
27
Bent Axis Motors
† These were developed to
improve the operating and
stall torque efficiencies of
high speed motors.
† Construction & Working :
„
„
A series of cylinders are
mounted around the
center line of the bentaxis.
Pistons in the cylinders
have a spherical end that
fits in a plate attached to
output shaft.
Bent Axis Motors
„ Springs hold the piston against the plate.
„ Fluid enters the motor and flows into the cylinder.
„ Piston extends, pressing against the plate, making it
rotate.
„ This rotation causes the cylinder carrier to rotate and the
next cylinder is aligned with inlet port.
„ Piston extends and produces next rotation.
28
Bent Axis Motors
† Efficiencies for the bent axis motor are similar
to the efficiencies for axial piston motor.
† Bent Axis Motors are available as
„ Fixed displacement units
„ Variable displacement units.
Bent Axis Motors
†
Refer Fig 5.17 – Variable Displacement Design
29
Bent Axis Motors
† Design considerations :
„ Maximum speed of an axial piston motor (in-line
design or bent axis design) is limited by the oil film
between the piston and the wall of the cylinder.
„ Two design features reduce the loss of oil film between
piston and cylinder bore:
† Lighter pistons are used.
† A synchronizing mechanism minimizes the side load
on the pistons.
Bent Axis Motors
„ Minimum displacement
† In- line axial motors can be taken to zero displacement,
but bent axis motors cannot.
† Variable displacement bent axis motors can be set back to
a minimum displacement, but not back to zero. They
cannot be taken out of the circuit like the in-line axial
motors.
„ Maximum Operating Speed
† The bent axis motor, particularly designs with the lighter
pistons, can be operated at a little higher maximum
operating speed tan the in-line design.
30
Bent Axis Motors
„ Stall Torque Efficiency
† The major advantage touted by bent motor
manufacturers is higher stall torque efficiency.
† Bent axis motors have a stall torque efficiency
about 5 % points higher than an in-line axial
motor.
Radial Piston Motors
† Radial Piston Motors produce very high torque
at low speed.
† They are used as wheel motors for large
equipments.
† Working of Radial Piston Motors.
(Refer Fig 5.18)
31
Radial Piston Motors
† Pistons operate in radial bores in a stationary
cylinder block.
† The surrounding housing that rotates has two
cam rings.
† The pistons each have two cam rollers.
† An extending piston forces the rollers against
the two cam rings causing the housing to
rotate.
Radial Piston Motors
Advantages:
† Radial piston motors can operate at pressures up to
5000 psi.
† They tend to be robust.
† Under normal operating conditions, design life is
15,000+ hours.
† Manufacturer states that full torque is available at
any speed.
32
Motor-Gearbox Combinations
† Many hydraulic motors are used for applications in
which desired output is in the 50 to 500 rpm range
rather than 500 to 5000 rpm range.
† High-speed motors typically drive the gearbox that
reduces the speed and increases the torque.
† Testing has been done to compare a low-speed,
high-torque (LSHT) motor with a high speed motor
driving a planetary gearbox.
Motor-Gearbox Combinations
† Starting torque was 93%
for the wheel motor and
74.5% for the
combination.
† Torque efficiency over the
entire operating range was
higher for the wheel
motor.
33
Oscillating Actuator
† Applications that do not need
continuous rotation (>360 degrees)
are
„ Industrial mechanisms performing pick-andplace operations
„ Heavy-duty, large-payload robots.
Oscillating Actuator
† Vane motors with one or
two vanes are used for
limited-rotation
applications.
† The single-vane
unit can
0
rotate 280 and double
0
vane 150 to 160
† Direction of rotation is
determined by a valve that
directs fluid into one
chamber or the other.
† These motors generate
torque up 500,000 lbf-in.
34
Oscillating Actuator
† Refer Fig 5.22
† Motors with a helical
spline are available with
0
90, 180, 270 and 360
of rotation.
† Rotation is set by the
length and pitch of the
helix.
† Units with torque rating
up to 1,000,000 lbf-in
are available.
Oscillating Actuator
† Refer Fig 5.24
† Two cylinders can be
used to power the rack
in a rack-and pinion
actuator.
† This can produce a
torque output in excess
of 50,000,000 lbf-in.
† Rotation is limited only
by cylinder stroke.
35
END OF CHAPTER 5
THANK YOU
36