T
here are four basic HST config-urations; two open-circuit and two closed-circuit configurations.Both refer to how the hydraulic lines in the system are connected.
In an open circuit, fluid is drawn into the pump through a reservoir, is routed to the motor, then re-en-ters the reservoir after passing through the hydraulic motor. In a closed circuit, the flow path is un-interrupted — fluid flows in a con-tinuous path from the pump dis-charge port to the fluid motor inlet port, out the motor discharge port and back into the pump inlet.
The two types of open and closed circuit systems are open loop and closed loop, which refer to the con-trol arrangement of the HST. An open-loop system has no means of feedback for speed, pressure, flow, or torque regulation. Any variable control settings are accomplished manually by the operator. Closed-loop control, however, incorporates feedback devices that provide com-munication between the pump and motor, so the HST automatically ad-justs to variations in operating con-ditions of the load, engine, or both.
Fig. 2. Packaged HST en-closes pump, motor, con-trols, conducting system, and all auxiliary components into a single housing. The unit shown accepts input power from a V-belt drive and transmits power to the load through its output shaft. Packaged HSTs are available in a variety of con-figurations, many of which bolt directly to an engine.
fixed-displacement motor, Figure 3A. Although this transmission is in-expensive, its applications are lim-ited, primarily because alternative f o r m s o f p o w e r t r a n s m i s s i o n a r e much more energy efficient. Because p u m p d i s p l a c e m e n t i s f i x e d , t h e pump must be sized to drive the mo-tor at a fixed speed under full load.
When full speed is not required, fluid from the pump outlet passes over the relief valve. This wastes energy in the form of heat.
U s i n g a v a r i a b l e - d i s p l a c e m e n t pump instead of one with a fixed dis-placement creates a constant torque transmission, Figure 3B. Torque out-put is constant at any speed because torque depends only on fluid pressure and motor displacement. Increasing or decreasing pump displacement in-creases or dein-creases motor speed,
re-Torque is constant because system pressure builds to the relief valve set-ting immediately after the control valve shifts. Power lost over the relief valve is the difference between the constant power delivered by the pump and the variable power delivered by the motor.
The area under this curve represents the power wasted when the transmission starts or stops. It also shows the low
ef-ficiency for any operating speed below maximum. A fixed-displacement trans-mission is not recommended for appli-cations requiring frequent starts and stops or when less than full load torque occurs frequently.
Torque/speed ratio
Theoretically, the maximum power a hydrostatic transmission can
trans-mit is a function of flow and pressure.
However, in constant-power trans-missions with variable output speeds, t h e o r e t i c a l p o w e r d i v i d e d b y t h e torque/speed ratio determines actual power output. The greatest constant power that can be transmitted is de-termined by the lowest output speed at which this constant power must be transmitted.
H Y D R O S T A T I C T R A N S M I S S I O N S
NI, TI
P1, Q1 P4, Q4
NO, TO
PR, QR
Torque Power
0 Max.
0 Max.
Motor torque at relief valve setting
Pump power
Motor power
Power lost over relief valve
Motor speed Max.
NO, TO
Torque, power, and flow
0 Max.
Range 1 Range 2
Speed Power
Flow
Torque
NI, TI NO TO
Efficiency - % and torque - lb-ft 3 10 20 40 60 80 100
Speed-rpm
400 800 1200 1600
Power - hp
0 100 200 300
0
Output torque
Overall effieiency
Output power
NI, TI NO, TO
Efficiency - % and torque - lb-ft 310 0 20 40 60 80 100
Speed - rpm
400 800 1200 1600
Power - hp
0 50 100 150 200 250
Output
torque Overall efficiency
Output power
B D
A
C
Fig. 3. Functional hydrostatic transmissions summarized according to types of pumps and motors involved: Fig. A shows HST with fixed-displacement pump and motor; Fig. B has fixed motor and variable-fixed-displacement pump; Fig C has fixed pump and variable-displace-ment motor, and Fig. D has a variable-displacevariable-displace-ment pump and motor.
F o r e x a m p l e , i f t h e m i n i m u m speed represented by point A on the power curve in Figure 4 is half the maximum speed, the torque-to-speed ratio is 2:1. The maximum power that can be transmitted is half the theoret-ical maximum. At point B, corre-s p o n d i n g t o o u t p u t corre-s p e e d A , t h e torque curve decreases as speed in-creases. At maximum output speed, it has dropped to point C.
At output speeds less than half the maximum, torque remains constant at its maximum value, but power de-creases in proportion to speed. The speed at point A is the critical speed and is determined by the dynamics of the HST’s components. Below criti-cal speed, power decreases linearly (with constant torque) to zero at zero
rpm. Above critical speed, torque de-creases as speed inde-creases, which provides constant power.
Building a closed-circuit HST The descriptions of closed-circuit hydrostatic transmissions in Figure 3 concentrate on parametric considera-tions only. Additional funcconsidera-tions must be provided to achieve a practical HST.
Pump-end components — Con-sider, for example, a constant-torque HST — the type used most commonly
— with a servo-controlled, variable-displacement pump driving a fixed-dis-placement motor, Figure 5A. Because this is a closed-circuit HST, slip flow accumulates in the pump and motor cases and is removed through a case drain line, Figure 5B. The combined
1998/99 Fluid Power Handbook & Directory
A/131 H Y D R O S T A T I C T R A N S M I S S I O N S
A
NO, TO
B Charge pump
A B
C
C
To or from motor
To or from motor
D E
D To or
from pump
To or from pump
NO, TO
F
G
E To or
from pump
To or from pump
Fig. 5. Progression of constant-power HST circuits — from a bare pump and motor to an assembly with basic accessories.
Torque
Torque and power
Speed Power
Maximum42 Maximum
0
A B
C
Fig. 4. Critical speed (indicated by point A) in a constant-power HST is the lowest speed at which maximum constant power can be transmitted.
case drains flow to the reservoir through a heat exchanger.
One of the most important features of a closed-circuit HST is a charge pump. The charge pump is usually an integral part of the main pump pack-age but can also be an independent pump gang-mounted with the drive pumps it serves, Figure 6. Whatever the arrangement, the charge pump performs two functions. First, it pre-vents cavitation of the main pump by replenishing the fluid lost by the closed system through pump and mo-tor slip. It also provides pressurized fluid required by the variable-dis-placement control mechanism.
Referring now to Figure 5C, low-pressure relief valve A on the dis-charge side of the dis-charge pump sets control pressure. Although charge pressures vary from one pump manu-facturer to another, they typically r a n g e b e t w e e n 2 5 0 a n d 3 0 0 p s i . Back-to-back replenishing check v a l v e s B a n d C s u p p l y
make-up fluid to the appro-priate low-pressure line.
Motor-end components
— A typical, closed-circuit HST also requires cross-over relief valves D and E, Figure 5D. These usually are integrated into the mot o r p a c k a g e . T w o c r o s s -over relief valves are in-stalled to prevent excessive pressure from developing in either supply line due to s h o c k - l o a d f e e d b a c k through the motor, an over-r u n n i n g l o a d , o over-r s i m i l a over-r conditions. These valves
limit pressure in either pressure sup-ply line by routing high-pressure fluid to the low-pressure line. These relief valves perform the same func-tion as a system relief valve in an open circuit. However, they are lo-cated at the fluid motor end because this is where overpressures originate in closed-circuit HSTs.
In addition to cross-over relief valves, shuttle valve F is included.
The shuttle valve is always shifted by high-pressure fluid, which connects the low-pressure line to low-pressure relief valve G. Valve G routes excess charge pump flow to the motor case,
then through the drain line to the pump case. Fluid then returns to the charge pump reservoir through the heat exchanger.
Cavitation control
The stiffness of an HST depends on the compressibility of the fluid and the compliance of system components, namely, tubing, and hoses. The influ-ence of these components can be com-pared to the effect a spring-loaded ac-cumulator would have if connected to the supply line through a tee fitting.
Under light loads, the effective accu-mulator spring compresses slightly; der heavy loads, the accumulator un-dergoes substantial compression, and there is more fluid in the accumulator.
This additional fluid volume must be supplied by the charge pump.
The critical factor is the rate of pressure rise in the system. If pres-sure rises too rapidly, the rate of vol-ume increase on the supply side
(so-called compressibility flow) may e x c e e d t h e f l o w c a p a c i t y o f t h e charge pump, and the main pump may cavitate. Perhaps circuits pow-ered by variable-displacement pumps with automatic controls pose the most serious threat of danger. When such a system cavitates, pressure drops or disappears altogether. The automatic controls attempt to respond, resulting in an unstable system.
Mathematically, the rate of pressure rise can be expressed as:
dp/dt = BeQcp4V
where: Beis effective bulk modulus of the system, psi
V is volume of fluid on pressure side in in.3, and
Qcpis charge pump output in in.3/sec.
Application example #2
Assume that the HST of Figure 5 is connected with 2 ft of 11/2-in. ID steel tubing. Neglecting the volumes of pump and motor, V is about 30 in.3For oil in steel tubing, Beis 200,000 psi.
Assuming the charge pump delivers 6 gpm (28 in.3/sec), then the rate of pres-sure rise is:
dp/dt = (200,000 328)4 30
= 190,000 psi/sec.
Now consider the effect of plumbing the system with 20 ft of 11/2-in. ID, three-wire braided hose. The hose man-ufacturer would have to provide the volumetric expansion coefficient in in.3/ 1000-psi to calculate the effective bulk modulus. Assume, for this exam-ple, that Beis about 84,000 psi. Then:
dp/dt = (84,000 328) 4 294.5
= 7986 psi/sec
Increasing the output of the charge pump would be the most effective way to prevent the tendency of such a system to cavitate. Alter-nately, if changes in the ex-ternal load are not continu-ous, an accumulator can be added to the charge circuit.
In fact, some HST manufac-turers provide a port for con-necting an accumulator to the charge circuit.
I f t h e s t i f f n e s s o f t h e H S T i s l o w , a n d i t i s equipped with automatic controls, the HST should be started with pump dis-placement at zero. In addition, accel-eration of the displacement mecha-nism should be limited to prevent jerky starts, which, in turn, could generate excessive pressure surges.
Some HST manufacturers provide damping orifices in the stroking cir-cuit for this very purpose.
This discussion demonstrates the multi-faceted role of the charge pump system in a closed-circuit HST. There-fore, system stiffness and control of the rate of pressure rise may be the primary considerations for determining charge pump delivery, rather than simply main pump and motor slip flows.
H Y D R O S T A T I C T R A N S M I S S I O N S
Fig. 6. Gang mounting multiple pumps provides a single, compact assembly that supplies two or more independent circuits from the rear drive pad of a gasoline or diesel engine. In this example, two variable-displacement axial-piston pumps are visible at left; a fixed-displacement vane pump, at right, serves as a charge pump.
I
ntensifiers, or boosters, convert low-pressure fluid power into higher-pressure fluid power. One type of intensifier resembles a cylinder in appearance and operation. Another type is commonly known as a multiple-section, internal-gear flow divider;when it is appropriately connected in a circuit, pressure intensification results.
Cylinder-type intensifiers, Figure 1, have a large and a small piston mounted on a common rod or shaft.
Each piston is housed in bores of ap-propriate diameter. The end of the rod serves as the small piston in many models. A source of low-pressure fluid (usually shop air although oil is not uncommon) is made available to the large piston, while the smaller pis-ton most often receives oil at low pres-sure. When activated, the low-pres-sure fluid powers the large piston to increase or intensify the pressure out-put from the smaller piston chamber.
This fluid is pumped to a work cylin-der for the job at hand.
Area ratios
The ratio of intensification is the same as the ratio of the two piston
ar-intensifier produces high-pressure oil for operation of the entire work cylin-der stroke. These circuits are recom-mended when the work cylinder ap-proach stroke is short compared to its higher-pressure stroke.
The dual-pressure circuit uses an air-oil tank to extend the work cylinder through its lower-pressure approach stroke before the intensifier begins op-eration. This circuit can save up to 90% of the air required for single-pres-sure operation.
Flow dIvider intensification
An internal gear pump with multiple gear sections mounted on a common shaft usually can be a multi-sectioned pump or a rotary flow divider. When used as a flow divider, the amount of energy expended across the divider as it operates remains the same, minus a small efficiency loss.
When output of one or more of the divider sections is returned to tank, the same energy situation remains true: the energy expended remains the same, mi-nus a small efficiency loss.
When output of one or more of the divider sections is returned to tank, the same energy situation remains true: the energy expended remains the same.
This means that the input horsepower of the gear sections returning fluid to tank is applied through the common shaft to the working gear sections to drive them as motors to increase their power output, again minus the small ef-ficiency losses.
When the flow divider is connected in a circuit in this manner, pressure fluid downstream of the working gear sections can be raised perhaps to a level above the pressure capability of the main pump and relief valve setting.
This use also can reduce time that the main system pump must operate at maximum pressure.
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eas. If these areas are in a ratio of 10:1, for example, and 100 psi shop air strokes the larger piston, the resulting pressure can be 1000 psi, depending on the load that is to be moved.
These intensifiers can be single or double acting. The single-acting inten-sifier, often called a one shot, must have some mechanical means to return the pistons once they have operated.
Double-acting intensifiers use external, manually-operated valving to cycle the intensifier. Another variation uses the external valve to control the beginning and end of the intensification cycle, and also has internal valving mechanically operated by the larger piston as it bot-toms, to provide reciprocating motion of the piston and pumping of intensi-fied fluid to the work cylinder. An ad-ditional variation of cylinder intensi-fiers uses a acting, double-rod-end cylinder in which each double-rod-end alternately feeds intensified-pressure fluid to the work actuator, thus provid-ing intensified-pressure in each direc-tion of intensifier stroke.
Generally, cylinder intensifier cir-cuits are either single- or dual-pressure circuits. In a single-pressure circuit, the
Intensifiers
Fig. 1. Double-acting intensifier uses rod end as small piston area, discharges intensified fluid out port at left end.
Low-pressure ports Seals
High-pressure inlet passage
High-pressure pistons
Low-pressure piston
,,,,,,,,,,,,,,,,, ,,,,,,,,,,,,,,,,, ,,,,,,,,,,,,,,,,, ,,,,,,,,,,,,,,,,, ,,,,,,,,,,,,,,,,, ,,,,,,,,,,,,,,,,, ,,,,,,,,,,,,,,,,, ,,,,,,,,,,,,,,,,, ,,,,,,,,,,,,,,,,, ,,,,,,,,,,,,,,,,, ,,,,,,,,,,,,,,,,, ,,,,,,,,,,,,,,,,, ,,,,,,,,,,,,,,,,, ,,,,,,,,,,,,,,,,, ,,,,,,,,,,,,,,,,, ,,,,,,,,,,,,,,,,, ,,,,,,,,,,,,,,,,,
Ring
A
B B
B
B A
A
A
B
B
A1
A2
A1 B
A2
B
B
B
A1
A2
A1 B
A2
B
Vane
(a) Basic
Rotor
A-Inlet B-Outlet
Reduced
rise Normal
rise
(b) Reduced Rise
Split Rise
(c) Even Split (d) Even Split
B
asic low-speed/high-torque (LSHT) motor designs include:internal gear, vane, radial pis-ton, axial pispis-ton, axial ball pispis-ton, rolling vane, and radial-piston/con-stant-acceleration cam.
Many factors influence the operat-ing performance of LSHT motors;
consequently, direct comparison is vir-tually impossible. Here are some gen-eral points to consider:
Gerotor motors are more economi-cal, but their leakage rates tend to lower volumetric efficiency, making them better-suited for low pressure op-eration. Their mechanical efficiency is reasonable.
Vane motors have a large number of leakage paths and tend to have lower volumetric efficiency at low speeds. These motors are radially-bal-anced, which improves their mechani-cal efficiency and extends their operat-ing life. Vane motor operation tends to improve at lower pressures.
Rolling-vane motors require pre-cisely-controlled tolerances and tend to cost more. However, their volumet-ric efficiency is nearly constant at all speeds. These motors are also radially balanced.
Radial-piston motors exhibit good leakage characteristics and conse-quently, good volumetric efficiency
ball pistons are small, so volumetric efficiency can be good. Torque effi-ciencies are about 80%.
Construction
There are many variations among basic LSHT motor designs. The fol-lowing are representative:
Gear motors usually are of Gerotor design and consist of a Gerotor set, a splined drive coupling, and a commu-tator valve. The Gerotor set has a sta-tionary outer ring which is part of the motor housing, and a rotor. The outer ring has integral gear teeth which mesh with mating teeth on the rotor. The ro-tor has one less tooth than the outer ring. A 6-lobe/7-lobe gear set has a 6:1 mechanical advantage. Pressure fluid forces the rotor to revolve inside the outer ring while orbiting around the center of the outer ring. A coupling transmits the motion of the rotor to the output shaft. Each tooth of the rotor is in sliding contact with the outer ring at all times.
The commutator valve, connected to and rotating with the output shaft, ports pressure fluid to the spaces be-tween the gear teeth. Pressure and re-turn passages in the commutator valve are connected to the motor ports through the housing. As a valve ro-tates, the fluid passages which keep the throughout their speed range. Starting
torques are good; starting torque effi-ciencies for motors with an eccentric crankshaft are about 85%. Motors with constant-acceleration cams have start-ing torque efficiencies to 95%.
Radial piston motors using eccen-tric crankshafts or ecceneccen-tric, circular cam rings may exhibit some torque and speed variation caused by har-monic piston motion. Effects which may occur at very high speeds include intense whine and flow pulsation. At very low speeds there may be torque or speed flutter, or cogging of the out-put shaft. Close attention to the manu-facturer’s operating recommendations for maximum and minimum speed limits is essential.
Constant-acceleration cam radial piston motors eliminate these har-monic difficulties because at any mo-ment, the sum of piston velocities is al-ways zero. However, these motors are more expensive than eccentric crankshaft types.
Axial-piston motors have good volumetric efficiencies, particularly at lower pressures, and usually have good starting torque characteristics.
Axial-ball piston motors with mul-tiple wave cams are pressure-balanced to operate without pulsation or vibra-tion. Operating clearances around the