Rotary Actuators - University of Florida. 5.pdfmotor is e om = Actual output power Input power Input power is hydraulic power measured at motor inlet port and the output power is mechanical

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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.

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INTRODUCTION

Motor performance is a function of pressure.

As Pressure increases, leakage increases speed decreases quantity of mechanical energy delivered to the load decreases.

INTRODUCTIONMotor 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.

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INTRODUCTION

Simple example :Suppose a motor has a displacement of 3.9 in3/rev. Measured flow is 10 GPM. The theoretical output speed is

Nmth = 231Q Vmth

where Nmth = theoretical motor speed(rpm)Q = flow (GPM)Vmth = motor displacement (in3/rev)

INTRODUCTION

Substituting,

Nmth = 231(10) = 592 rpm3.9

Assuming the measured speed is 536 rpm , the motor volumetric efficiency is

evm = 536 x 100 = 90.5%592

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INTRODUCTION

The overall efficiency of a hydraulic motor is

eom = Actual output powerInput 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 (Vmth=3.9in3/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 = ∆PQ1714

Where Pin = input hydraulic power (hp)∆P = pressure drop (psi)Q = flow (GPM)

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INTRODUCTIONSubstituting,

Pin = 2000(10) = 11.67 hp1714

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)

INTRODUCTIONSubstituting,

Pout = 1080(536)63025

= 9.18 hpMotor overall efficiency is

eom = (Pout / Pin) x 100= (9.18 / 11.67) X 100= 78.7 %

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INTRODUCTION

Torque efficiency describes hydraulic motor performance.

etm = Actual output torqueTheoretical torque

Theoretical output torque is Tmth= ∆PVmth / 2π

where ∆P= pressure drop across motorVmth= displacement (in3/rev)

INTRODUCTION

Substituting,

Tmth= 2000(3.9) = 1241 lbf-in2π

Αctual output torque is 1080 lbf-in, torque efficiency is

etm = (1080/1241) x 100 = 87%

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INTRODUCTION

Overall efficiency is product of volumetric and torque efficiencies.

Volumetric efficiency isevm = N/ Nmth

Where N = actual output speed (rpm)Nmth = theoretical output speed (rpm)

INTRODUCTIONSubstitution of

Nmth = 231 (Q/Vmth)

Into previous equation givesevm = NVmth/ 231Q

Torque Efficiency defined by etm= T/ Tmth

where T = measured output torque (lbf-in)Tmth = theoretical output torque (lbf-in)

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INTRODUCTION

Substitution of Tmth= ∆PVmth / 2π

into previous equation, we getetm= 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

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STALL TORQUE EFFICIENCY

Stall Torque Efficiency isesm = (Tms/Tmth) 100

Whereesm = 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 EFFICIENCYMotor 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.

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STALL TORQUE EFFICIENCYStall 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

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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.


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.

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Torque Loss is defined by Tl = Tmth – T

where Tmth = theoretical torque (lbf-in)

T = measured torque (lbf-in)


Torque loss varies with load pressures.Torque loss for this motor is approx. linear with pressure, therefore the torque efficiency is approx. constant.

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COMPARISON OF MOTOR PERFORMANCE CHARACTERISTICS

Motors can be classified into two main groups

High Speed MotorsLow Speed Motors High Torque Motors (LSHT)

Here we compare the three main designs of high speed motors

GearVane Piston


Hydreco Model 1919 Gear motor has a theoretical displacement of 4.53 in3/rev, at 2500 psi max. pressure, and 3000 RPM max. speed

Efficiencies for gear motor with 36 GPM input flow.

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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.


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.

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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, high-torque (two rotors) to the high-speed, low torque (one rotor) configuration.


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. Displacement is 4.57 in3/rev.

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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.


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.)

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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 LOW-SPEED,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.

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One manufacturer uses a disc valve to distribute fluid to the geroler pockets providing improved performanceat low speeds.


Car-Lynn 10,000 series Geroler motor with 40.6 in3/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.

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Volumetric efficiency for this motor was higher than the gear motor.


Vickers Model MHT50 vane motor with 38 in3/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.

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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)


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.

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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 ,equivalent to .

Trial No.1The 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)

%5±rpm15±

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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.

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Trial No.2Performance data for the Model CR-08 motor (Vmth= 7.7 in3/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%


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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 CHARACTERISTICSLoad 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.

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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 isevp = DP + E + (A +BP +CP2)1/2

evp = pump volumetric efficiency (%)P = pressure (psi) ; A,B,C,D,E = constants


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The equation for motor volumetric efficiency is

evm = DP + E + (A +BP +CP2)1/2

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/ 100Q = flow delivered by pump (in3/min); Np = pump speed (rev/min),Vpth = pump displacement(in3/rev); evp = pump volumetric efficiency


Corresponding motor speed is

Nm=(Q/Vmth)(evm)/100

Nm =motor speed (rev/min)

Q =flow to motor(in3/min)

Vmth =motor displacementevm = motor volumetric efficiency(%)

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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)


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.

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Bent Axis MotorsThese 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 bent-axis.Pistons in the cylinders have a spherical end that fits in a plate attached to output shaft.

Bent Axis MotorsSprings 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.

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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 MotorsRefer Fig 5.17 – Variable Displacement Design

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Bent Axis MotorsDesign 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 MotorsMinimum 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 SpeedThe bent axis motor, particularly designs with the lighter pistons, can be operated at a little higher maximum operating speed tan the in-line design.

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Bent Axis MotorsStall 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)

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Radial Piston MotorsPistons 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 MotorsAdvantages:

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.

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Motor-Gearbox CombinationsMany 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 CombinationsStarting 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.

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Oscillating Actuator

Applications that do not need continuous rotation (>360 degrees) are

Industrial mechanisms performing pick-and-place operations

Heavy-duty, large-payload robots.

Oscillating ActuatorVane motors with one or two vanes are used for limited-rotation applications.

The single-vane unit can rotate 2800 and double vane 150 to 1600

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.

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Oscillating ActuatorRefer Fig 5.22

Motors with a helical spline are available with 90, 180, 270 and 3600

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 ActuatorRefer 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.

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END OF CHAPTER 5

THANK YOU

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Rotary Actuators - University of Florida. 5.pdfmotor is e om = Actual output power Input power Input power is hydraulic power measured at motor inlet port and the output power is mechanical