3.3. Hydraulic motor 2012 Hydraulic and pneumatic control lecture notes by Siraj K. Page 1 3.3. Hydraulic motors 3.3.1. Introduction A fluid power motor is a device that converts fluid power energy to rotary motion and force. The function of a motor is opposite that of a pump. However, the design and operation of fluid power motors are very similar to pumps. Motors have many uses in fluid power systems. In hydraulic power drives, pumps and motors are combined with suitable lines and valves to form hydraulic transmissions. The pump, commonly referred to as the A-end, is driven by some outside source, such as an electric motor. The pump delivers fluid to the motor. The motor, referred to as the B-end, is actuated by this flow, and through mechanical linkage conveys rotary motion and force to the work. This type of power drive is used to operate (train and elevate) many of the Navy’s guns and rocket launchers. Hydraulic motors are commonly used to operate the wing flaps, radomes, and radar equipment in aircraft. Air motors are used to drive pneumatic tools. Air motors are also used in missiles to convert the kinetic energy of compressed gas into electrical power, or to drive the pump of a hydraulic system. Fluid motors may be either fixed or variable displacement. Fixed-displacement motors provide constant torque and variable speed. The speed is varied by controlling the amount of input flow. Variable-displacement motors are constructed so that the working relationship of the internal parts can be varied to change displacement. The majority of the motors used in fluid power systems are the fixed-displacement type. Although most fluid power motors are capable of providing rotary motion in either direction, some applications require rotation in only one direction. In these applications, one port of the motor is connected to the system pressure line and the other port to the return line or exhausted to the atmosphere. The flow of fluid to the motor is controlled by a flow control valve, a two-way directional control valve, or by starting and stopping the power supply. The speed of the motor may be controlled by varying the rate of fluid flow to it. In most fluid power systems, the motor is required to provide actuation power in either direction. In these applications the ports are referred to as working ports, alternating as inlet and outlet
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3.3. Hydraulic motor 2012
Hydraulic and pneumatic control lecture notes by Siraj K. Page 1
3.3. Hydraulic motors
3.3.1. Introduction
A fluid power motor is a device that converts fluid power energy to rotary motion and force. The
function of a motor is opposite that of a pump. However, the design and operation of fluid power
motors are very similar to pumps. Motors have many uses in fluid power systems. In hydraulic
power drives, pumps and motors are combined with suitable lines and valves to form hydraulic
transmissions. The pump, commonly referred to as the A-end, is driven by some outside source,
such as an electric motor.
The pump delivers fluid to the motor. The motor, referred to as the B-end, is actuated by this
flow, and through mechanical linkage conveys rotary motion and force to the work. This type of
power drive is used to operate (train and elevate) many of the Navy’s guns and rocket launchers.
Hydraulic motors are commonly used to operate the wing flaps, radomes, and radar equipment in
aircraft. Air motors are used to drive pneumatic tools. Air motors are also used in missiles to
convert the kinetic energy of compressed gas into electrical power, or to drive the pump of a
hydraulic system.
Fluid motors may be either fixed or variable displacement. Fixed-displacement motors provide
constant torque and variable speed. The speed is varied by controlling the amount of input flow.
Variable-displacement motors are constructed so that the working relationship of the internal
parts can be varied to change displacement. The majority of the motors used in fluid power
systems are the fixed-displacement type. Although most fluid power motors are capable of
providing rotary motion in either direction, some applications require rotation in only one
direction. In these applications, one port of the motor is connected to the system pressure line
and the other port to the return line or exhausted to the atmosphere. The flow of fluid to the
motor is controlled by a flow control valve, a two-way directional control valve, or by starting
and stopping the power supply. The speed of the motor may be controlled by varying the rate of
fluid flow to it.
In most fluid power systems, the motor is required to provide actuation power in either direction.
In these applications the ports are referred to as working ports, alternating as inlet and outlet
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ports. The flow to the motor is usually Fluid motors are usually classified according to the type
of internal element, which is directly actuated by the flow.
Figure 3.3.1. Actuators classifications
3.3.2. Limited rotation hydraulic motors
Limited rotation actuators, called torque motors, have a wide variety of applications where a
limited specified degree of rotation at the output shaft is required. Rotation is usually limited to
720°. They are used extensively in industry for actuating clamping devices, material handling,
rotating cams for braking mechanisms, tumbling and dumping, positioning and turning, and
many other situations where an economical application of fluid power for limited rotation is
desirable. (Fig. 3.3.2) Vane-type limited actuators apply fluid force to the cross section area of
single or multiple vanes. Rack and pinion type actuators apply fluid force to the cylindrical
chambers which move the rack to drive the pinion gear.
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Hydraulic and pneumatic control lecture notes by Siraj K. Page 3
Figure 3.3.2 (a) Rack and pinion limited rotation actuator (courtesy of Flo-Tork, Inc. ); (b) Vane
type limited rotation actuator (courtesy of Bird-Johnson ); (c) Typical applications of limited
rotation actuators (courtesy of Bird-Johnson).
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Analysis of torque capacity
The following nomenclature and analysis are applicable to a limited rotation hydraulic motor
containing a single rotating vane:
=RR Outer radius of rotor (in, mm)
=VR Outer radius of vane (in, mm)
=L Width of vane (in, mm)
=p Hydraulic pressure (psi, Pa)
=F Hydraulic force acting on vane (lb, N)
=A Surface area of vane in contact with oil (in2, m2)
=T Torque capacity (in. lb, N.m)
The force on the vane equals the pressure times the vane surface area:
( )LRRppAF RV −==
The torque equals the vane force times the mean radius of the vane:
( ) ( )2
RVRV
RRLRRpT
+∗−=
On arrangement we have
( )22
2 RV RRpL
T −= 3.3.1
A second equation for torque can be developed by noting the following relationship for
volumetric displacement VD:
( )LRRT RV22 −= π 3.3.2
Combining equations (3.3.1) and (3.3.2) yields
π2DpV
T = 3.3.3
3.3.2. Types of hydraulic motors
Continuous rotation actuators, called hydraulic motors, provide sustained rotation in either
direction. Some hydraulic motors are also convertible to serve as hydraulic pumps if a
mechanical drive is applied to the output shaft, but this is not usually recommended without
special provision because of port timing and other internal part arrangements. Vane motors, for
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example, have spring loaded vanes, whereas vane pumps usually do not. This is not the case with
axial piston motors, however , which are widely advertised as combination pump-motors.
Hydraulic motors differ from pumps in other respects. Because the case is pressurized from an
outside source, case drains are provided to protect shaft seals (Fig .3.3.3). These may be piped
directly to the low pressure reservoir, or through a crossover check valve arrangement to the
exhaust port of the motor. External drain lines or crossover check valve arrangements are needed
only for series circuits or meter out circuits. This is necessary during reversing, braking, and
other operating conditions which would otherwise subject the case drain to system pressure.
Maximum pressure at the case drain is usually 100-250 psi. Port timing is an additional factor
that may be different between pumps and motors.
Figure 3.3.3. Hydraulic case drain arrangements
Hydraulic motors are available as fixed or variable displacement unit s so that speed variation
with rotation in either direction is possible.
Gear motors
External gear positive displacement motors operate in the reverse manner of their pump
counterparts (Fig. 3.3.4). They are available in sizes to 20 in3 per revolution. Fluid supplied to
the inlet port circulates around the out side of the gear teeth driving both gears, although only
one gear is connected to the motor output shaft. The gear teeth seal where they mesh and
between their ends and the motor housing. Fluid is trapped in cavities formed by the gear teeth
and the motor housing and transported around the out side diameter of the gears to the low
pressure port side of the motor.
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Industrial gear motors are the least expensive to manufacture, have overall efficiencies to 90%,
and operate in the speed range of 1000 to 2500 rpm. Recent developments in gear tooth materials
and technology have extended the speed range of gear motors in excess of 20,000 rpm for motor
size s in the 0.063-0.093 in3 per revolution displacement range. Their small size, high speed, and
high power (4-5.5 hp) make these motors ideal for spindle drives in the machine tool industry.
Figure 3.3.4. External gear positive displacement motor
Vane motors
Vane motors operate similarly to vane pumps with the exception that unlike their pump
counterpart s, spring loading is used to insure positive contact between the vanes and the
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eccentric cam ring (Fig 3.3.5). Seals, high speed bearings, and case drains also are given special
attention.
Vane motor showing spring loaded vanes
Figure 3.3.5. Vane-type motor.
Fluid entering the motor under high pressure acts against the rectangular surfaces of the vanes
while the chamber volume increases, and then is exhausted as it decreases (Fig. 3.3.6). Balanced
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vane motors admit fluid from two ports located 180° apart to reduce side thrusts on the
supporting shaft and bearings, and discharge the fluid 90° later through two similarly located
discharge ports. One piece motor body and cartridge element construction permit interchange to
adapt the unit to a wide variety of flow capacities and ease repair with minimum disassembly and
number of spare parts. Fluid viscosities between 55 and 275 SSU are recommended at normal
operating temperatures of 120°F with a maximum of 180°F. Most vane motors are not
recommended for use with water-based emulsion fire-resistant fluids .
Figure 3.3.6. Balanced vane motor operation (Courtesy of Sperry Vickers)
Piston type motors
Piston motors are the most like their pump counterparts and incorporate only minor changes to
effect the conversion. They are available in both axial and radial designs. In-line-type motors are
similar to pumps and make use of both the fixed displacement swash plate and variable
displacement adjustable yoke. Bent axis motors are available with fixed displacement angles as
well as variable displacement angles. Radial piston motors are used widely in low speed, high
torque applications. Piston motors also operate at the highest efficiencies, speeds, and pressures
available.
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Fixed displacement in-line swash plate motors commonly use nine pistons with the swash plate
at 15° (Fig. 3.3.7). They are available in single or double shaft versions. Reversal of the motor is
accomplished only by reversing the direction of fluid flowing through the motor.
Figure 3.3.7. Fixed displacement in-line piston motor (courtesy of Sperry Vickers).
When used as a motor, hydraulic pressure on the piston create s tangential force s on the angled
swash plate (Fig. 3.3.8). This gives the required turning moment to the shaft. As fluid is directed
to the rotor through the inlet and kidney-shaped port in the valve plate, it imparts a force (F) on
the pistons in line with their axis.
Figure 3.3.8. Vector forces in in-line piston motor (courtesy of Sperry Vickers).
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The axial force is resolved into its component angle force s (FI) and (Fz). While the force (F1) is
directed perpendicular to the surface of the stationary swash plate and is not available to
accomplish work, the force (Fz) is in a direction parallel to the surface of the swash plate and so
is free to move down the angled surface imparting motion to the piston group, cylinder block,
and output shaft. Fluid under pres sure is directed to the piston s through the kidney-shaped inlet
port while the piston s are descending the swash plate angle , and directed from the pistons to the
exhaust port as they ascend the swash plate angle.
Variable displacement in-line piston motors differ from fixed swash plate motors in that the
angle of the swash plate is changeable (Fig. 3.3.9). Angles of + 15° and -15° from the center
position are common. Reversal of the motor is accomplished by tilting the yoke over center by
the action of a servo yoke actuating piston or manual control. The torque from variable
displacement motors varies with the change in volume displacement caused by tilting of the
yoke. If the motor receives fluid at constant flow rate from the pump, a decrease in torque is
accompanied by an increase in speed. The fluid horsepower of the motor is constant since the
product of pressure (p) and flow rate (Q) input are constant. Neglecting losses, the horsepower
output is also constant.
252,5
NTBhp
∗=
Figure 3.3.9. Variable displacement in-line piston motor (courtesy of Sperry Vickers).
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Increases or decreases in torque resulting from increases or decreases in motor displacement will
be accompanied by inverse decreases or increases in motor output speed. As the angle of the
actuating yoke is reduced to approach 0°, the speed of the variable displacement motor increases.
Further decrease in the angle of the yoke will cause the motor to stall and cause excessive
pressure in the system which must be released to redirect the constant flow provided by the
pump.
3.3.3. Hydraulic motor theoretical torque, power and flow rate
Motors convert fluid energy back into mechanical energy and thus are the mirror image of
pumps. It is not surprising that the same mechanisms are used for both. The typical motor
designs are gear, vane, and piston. Motor performance is a function of pressure. As pressure
increases, leakage increases, speed decreases, and thus the quantity of mechanical energy
delivered to the load decreases. Due to this a hydraulic motor delivers less torque than it should
theoretically. The theoretical torque (which is the torque that a frictionless hydraulic motor
would deliver) can be determined by the following equation developed for limited rotation
hydraulic actuators:
( ) ( ) ( )π2
/3 psiprevinVlbinT D
T
∗=∗ 3.3.4
Using metric units we have
( ) ( ) ( )π2
/3 PaprevmVmNT D
T
∗=∗
Thus, the theoretical torque is proportional not only to the pressure but also to the volumetric
displacement.
The theoretical horsepower (which is the horsepower a frictionless hydraulic motor would
develop) can also be mathematically expressed:
( ) ( )000,63
rpmNlbinTHP T
T
∗∗=
( ) ( ) ( )π2
/3 rpmNpsiprevinVD ∗∗= 3.3.5
In metric units, theoretical power (W)
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( ) ( ) ( ) ( ) ( ) ( )π2
///
3 sradNPaprevmVsradNmNTWpowerlTheoretica D
T
∗∗=∗∗=
Also due to the leakage, a hydraulic motor consumes more flow-rate than it should theoretically.
The theoretical flow-rate is the flow-rate a hydraulic motor consume if there where no leakage.
As is the case for pumps, the following equation gives the relationship among speed, volumetric
displacement and theoretical flow-rate.
( ) ( ) ( )231
/3 rpmNrevinVgpmQ D
T
∗=
3.3.6
Or
( ) ( ) ( )srevNrevmVsmQ DT /// 33 ∗= 3.3.7
3.3.4. Hydraulic motor performance
The performance of any hydraulic motor depends on the precision of its manufactures as well as
the maintenance of close tolerances under design operating conditions. As in the case for pumps,
internal leakage (slippage) between the inlet and outlet reduces the volumetric efficiency of a
hydraulic motor. Similarly, friction between mating parts and due to fluid tolerance reduces the
mechanical efficiency of the hydraulic motor.
Gear motors typically have an overall efficiency of 70 to 75% as compared to 75 to 85% for vane
motors and 85 to 95% for piston motor. Some systems require that a hydraulic motor start under
load. Such system should include a stall torque factor when making design calculations. For
example only about 80% of the maximum torque can be expected if the motor is required to start
either under load or operate at the speed below 500 rpm.
Motor efficiencies
Hydraulic motor performance is evaluated on the same three efficiencies (volumetric,
mechanical and overall efficiencies) used for hydraulic pumps. They are defined for motor as
follows:
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1. Volumetric efficiency (ηηηηv)
The volumetric efficiency of a hydraulic motor is the inverse of that for a pump. This is because
a pump does not produce as much flow as it should theoretically, whereas a motor uses more
flow than it should theoretically due to leakage. Thus, we have
A
Tv Q
Q
motorbyconsumedrateflowactual
consumeshouldmotorrateflowltheoretica=
−−
=η 3.3.8
Determination of volumetric efficiency requires the calculation of the theoretical flow-rate,
which is defined for a motor. Substituting the values of calculated theoretical flow-rate and
actual flow-rate (which is measured) in to equation above yields the volumetric efficiency for the
given motor.
2. Mechanical efficiency (ηm)
The mechanical efficiency of a hydraulic motor is the inverse of that for a pump. This is because
due to friction, the pump requires a greater torque than it should theoretical where as a motor
produces less torque than it should theoretically. Thus, we have
T
Am T
T
deliverllytheoreticashouldmotortorque
motorbydeliveredtorqueactual==η
3.3.9
The following equations (3.3. ) and (3.3. ) allow for the calculation of TT and TA, respectively:
( ) ( ) ( )π2
/3 psiprevinVlbinT D
T
∗=∗ 3.3.10
Using metric units we have
( ) ( ) ( )π2
/3 PaprevmVmNT D
T
∗=∗
( ) ( )rpmN
motorbydeliveredHPactuallbinTA
000,63∗=∗ 3.3.11
Using metric units we have
( ) ( )sradN
motorbydeliveredwattageactualmNTA /
000,63∗=∗
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3. Overall efficiency (ηηηηo)
As in the case for pumps, the overall efficiency of a hydraulic motor equals the product of the
volumetric and mechanical efficiencies.
motortodeliveredpoweractual
motorbydeliveredpoweractualmvo =∗= ηηη
3.3.12
In English units, we have
( ) ( )
( ) ( )1714
000,63gpmQpsip
rpmNlbinT
A
A
o ∗
∗∗
=η
3.3.13
In metric units, we have
( ) ( )( ) ( )smQPap
sradNmNT
A
Ao /
/.3∗
∗=η
3.3.14
Note that the actual power delivered to a motor by the fluid is called hydraulic power and the
actual power delivered to the load by a motor via a rotating shaft called brake power.
3.3.5. Hydrostatic transmissions
Hydrostatic drive
Introduction
The hydrostatic drive is a fluid drive which uses fluid under pressure to transmit engine power to
the drive wheels of the machine. Mechanical power from the engine is converted to hydraulic
power by a pump-motor team. This power is then converted back to mechanical power for the
drive wheels. The hydrostatic drive can function as both a clutch and transmission. The final
gear train can then be simplified, with the hydrostatic unit supplying infinite speed and torque
ranges as well as reverse speeds. There two basic types of hydraulic transmissions:
• Hydrodynamic
• Hydrostatic
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Figure 3.3. 10 Hydrostatic drives
Figure 3.3.11 Basic type of hydraulic transmissions
Figure 3.3.12 shows one piston for the pump and one for the motor. To provide a pumping action
for the pistons, a plate called a swashplate is located in both in the pump and motor. The piston
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against the swashplates. The angle of the swashplates (figure 3.3.12) can be varied so that the
volume and pressure of oil pumped by the pistons can be changed or the direction of oil flow
reversed.
Figure 3.3.12 Two connected cylinders with swashplates
A pump or motor with a movable swashplate is called a variable-displacement unit. A pump or
motor with a fixed swashplate is called a fixed displacement unit. Figure 3.3.13 shows a variable
displacement pump driving a fixed displacement motor. As the pump pistons rotate, they move
across the sloping face of the swashplate, sliding in and out of their cylinder bores to pump oil
out. The more the pump swashplate is tilted, the more oil it pumps with each piston stroke and
faster it drives the motor.
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Figure 3.3.13 Variable displacement pump driving fixed displacement motor
Pressure results whenever the flow of a fluid is resisted. The resistance may come from
acceleration of the machine or normal load. The motor swashplate is at a fixed angle so that the
strokes of its pistons are always the same. Thus its speed of rotation can not be changed except
as it is driven faster or slower by the pump oil. The point to remember now is that a given
volume of oil forced out of the pump will cause the motor to turn at a given speed. More oil will
speed up the motor; less oil will slow it down. The pump is driven by the machine’s engine and
so is linked to the speed set by the operator. It pumps a constant stream of high-pressure oil to
the motor.
Since the motor is linked to the drive wheels of the machine, it gives the machine its travel
speed. Only three factors control the operation of a hydrostatic drive:
• Rate of oil flow-gives the speed
• Direction of oil flow-gives the direction
• Pressure of oil-gives the power
Control of these three factors is infinite, giving endless selections of speed and torque in a
hydrostatic drive. The pump-motor team is the heart of the hydrostatic drive, although the
complete hydraulic system (figure 3.3.14) also includes a reservoir to supply the oil, a filter to
remove dirt, and a cooler to remove excess heat from the oil.
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Figure 3.3.14 Complete system for a hydrostatic drive (closed hydraulic loop)
Basically however, the pump and motor are joined in a closed hydraulic loop; the return line
from the motor is joined directly to the intake of the pump, rather than to the reservoir. The
charge pump simply supplies the oil, drawing it from the resorvoir.
Types of hydrostatic drives
Displacement is the quantity of fluid which a pump can move (or motor can use) during each
revolution. It is directly related to the horsepower output of the drive. (Horsepower is a
combination of torque x speed.) As you have already learned, pumps and motors can have a
fixed dispacement or variable displacement. Four pump-motor combinations are possible:
1. Fixed displacement pump driving a fixed displacement motor.
2. Variable displacement pump driving a fixed displacement motor.
3. Fixed displacement pump driving a variable displacement motor.
4. Variable displacement pump driving a variable displacement motor.
The point tor remember is that for each combination the output power must equal the input
power minus negligible power losses. Let’s look at each combination (figure 3.3.15).
No. 1 circuit is like a gear drive; it transmits power without altering the speed or horsepower
between the engine and the load. A constant input speed and torque gives a constant output
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horsepower. If either the speed or torque is increased holding the other constant the output
horsepower will increase.
Since the pump flow is variable in No. 2 circuit, the output speed is variable and torque output is
constant for any given pressure. This circuit gives variable speed and constant torque.
Output speed in No. 3 circuit is varied by changing the motor displacement. For constant power
input if the motor displacement decreases, output speed increases but output torque drops.
No. 4 circuit is the most flexible and expensive of the circuits but also the most difficult to
control. It is capable of operating like any of the above combinations.
Figure 3.3.15 Pump-motor combinations for hydrostatic drives
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