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Fluid Power Drives

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Fluid Power Drives FLUID POWER DRIVES HYDRAULIC HYDRAULIC ADJUSTABLE-SPEED DRIVES..........A197 • PV-MF (variable ADJUSTABLE-SPEED ROTARY ACTUATORS ..........................................A199 pump, fixed motor) DRIVES FLUID MOTORS....................................................A201 • PF-MV (fixed pump, HYDRAULIC PUMPS.............................................A203 variable motor) ydraulic, or fluid, • PV-MV (variable adjustable speed pump, variable motor) H drives are manufactured to plications calling for close speed or • PF-MF (fixed pump, fixed motor) deliver 1 to 5,000 hp. These smooth power regulation, because of their with flow control valves. The general operating units can be controlled high overall operating efficiencies. characteristics of these units are sum- manually at the unit or by remote Axial piston pumps and motors com- marized in Table 1. control devices. All of the following monly have wobble or swash plates PV-MF — This is the most common drives are designed for installation that determine effective piston design. Output speed is adjusted by between a prime mover, such as an stroke, and hence displacement. varying the pump displacement. The electric motor, and the driven load. Output speed of a hydraulic motor pump can be stroked “across center” Some units are made with the prime is a direct function of its displacement to reverse the direction of oil flow and mover integral with the fluid drive. and the volume of fluid supplied. the direction of motor shaft rotation. The major types of these fluid Hence, varying the displacement of Since motor displacement is constant, drives include hydrostatic hydroki- either pump or motor varies drive output torque is constant if pressure netic, and hydroviscous. speed. is constant, Figure 1. Horsepower is Output torque-horsepower re- variable and a direct function of out- Hydrostatic lationships — Hydraulic adjustable put shaft speed at a given pressure speed drives are frequently classified setting. Applications for this drive in- Hydrostatic drives, typically rated according to their output characteris- clude machine tools, printing and pro- from 10 to 300 hp, incorporate posi- tics. Analysis of the load characteris- cessing machinery, foundry and con- tive displacement pumps and positive tics will usually determine the best tinuous casting equipment, textile displacement motors. Mechanical en- drive type for the job. machinery, elevators, and hoists and ergy is transmitted from the prime Broadly speaking, varying the vol- winches. mover to the pump. The pump im- ume of fluid delivered to a fixed dis- PF-MV — Pump output is constant parts energy through a fluid to a hy- placement motor will produce varying with a constant input speed. Hy- draulic motor. The prime mover can speed, constant torque, and varying draulic motor displacement is varied be an electric motor, a gasoline or horsepower at the output shaft. A to change output speed. Reducing mo- diesel engine, or a take-off from the fixed rate of fluid supplied to a vary- tor displacement increases output main machine drive. ing displacement motor develops speed. However, if this displacement Hydrostatic drives offer several fea- varying speed, variable torque, and is reduced to near zero, the drive tures that make them particularly constant horsepower. When both speed will attempt to reach an infinite adaptable to many adjustable speed pump and motor are variable dis- value, Figure 2. Therefore, output applications: infinitely adjustable placement types, speed, torque, and speeds are usually kept within design stepless change of speed, torque, and horsepower can be varied. limits by limiting displacement ex- horsepower; no damage even if stalled Hydrostatic adjustable speed tremes with mechanical stops. The at full load; operation in both direc- drives fall into four standard drive displacement range of a variable dis- tions of rotation at controlled speeds; categories based on the types of placement motor is commonly 4:1. set speeds held accurately against pumps and motors (fixed or variable Output torque is inversely propor- driving or braking loads; small size displacement): tional to output speed, maintaining and low weight per horsepower output. Output speed can be adjusted if ei- Table 1 — Performance characteristics of hydrostatic drives ther the pump or motor has variable based on constant pump speed and torque measured at displacement. If both are fixed dis- constant pressure. (C denotes constant; V denotes variable.) placement units, valves control fluid flow to the hydraulic motor. Pump Motor Torque Output speed Power Variable displacement pumps and motors are usually piston or vane de- Variable Fixed C V V signs. (Gear pumps and motors are Fixed Variable V V C generally not adaptable to variable Variable Variable V V V displacement.) Piston units have Fixed Fixed C C C proven to be the most capable in ap- 1998 PT Design A197 relatively constant power. This pattern is particularly useful in rewind drives that require con- stant web speeds. PV-MV — A vari- able-pump, variable- motor de- sign offers the widest operating Figure 1 — Torque and horsepower characteristics of PV-MF drive. range of any hydrostatic drive. Output speed can be varied by adjusting Figure 4 — Typical integrated hydrostatic the displacement of the transmission. pump, the motor, or both. Output torque and through the speed range. power are infinitely ad- PF-MF (with valving) — Pump justable across the com- output volume and motor torque are plete speed range in both constant, and output speed and horse- directions of rotation. power are variable. The flow-control Increasing pump dis- valve limits the amount of fluid avail- placement and main- able to the motor and thus controls taining the hydraulic output speed. Efficiency of this sys- motor at maximum dis- tem is relatively low compared with placement, delivers es- other hydrostatic drives, because of sentially constant torque the throttling action of the valve. Low output, at a constant in cost, this drive is usually applied pressure, while increas- where only small speed changes are ing output speed and required. horsepower, Figure 3. Integrated hydrostatic trans- With pump displace- missions — Frequently termed Figure 2 — Torque and horsepower characteristic of PF-MV drive. ment and system pressure held con- IHSTs, these units consist of a pump stant, the drive delivers essentially and motor contained in the same constant horsepower. housing and share a common valving If motor displacement surface. These compact units offer ex- increases, speed tremely short paths for oil flow, and drops, and torque in- eliminate high-pressure oil leaks creases proportion- either to the reservoir or to the out- ally, and vice versa. side, Figure 4. In some installations, pump and motor dis- Hydrokinetic placement are con- trolled simultane- Usually applied in applications ously to provide rated 1 to 5,000 hp, hydrokinetic ad- constant horsepower justable speed drives vary the output speed and torque by varying the amount of fluid in the vortex that cou- ples the input to the output, Figure 5. The fluid vortex size can be varied during operation by controlling the Figure 3 — Torque and angle of the scoop tube that removes horsepower fluid from the vortex. The tube angle characteristic of PV-MV can be controlled manually or by elec- drive. tric, hydraulic, or pneumatic means. A204 1997 Power Transmission Design Since the output is only “connected” are splined to the in- to the input by the fluid, and without put shaft. The plain any direct mechanical connection, steel discs are keyed there is a 2 to 4% slip. This slip re- to the output shaft. duces efficiency, but offers good shock Both sets of discs are protection to the driving and driven free to slide axially. A equipment. A heat exchanger re- pump provides cool- moves the heat generated by this slip. ing oil to the inner di- Hydrokinetic drives offer an operat- ameter of the compo- ing speed range of about 8:1. In most sition-faced discs, cases, the minimum speed is specified which have a pattern to allow the oil to Figure 7 — Rack-and-pinion. because operating at too low a speed flow through the disc pack. This oil will cause drive instability. also flows between the discs and trans- In addition to controlling the shaft mits the torque from the input to the Rack-and-pinion — One side of output speed and torque, these units of- output plates. the piston contains a gear rack that fer a smooth or soft-start characteristic. The output shaft contains a cylin- engages a pinion to turn the output der and an actuating shaft. Output torque can be doubled piston that produces an in versions built with two parallel pis- axial force on the disc ton-rack units. High tolerance for side stack. This controls the and end loading makes these actua- oil-film thickness be- tors suitable for heavy-duty applica- tween the rotating tions, Figure 7. plates, and adjusts the Piston-chain — Two pistons, a output torque. For max- chain, and a sprocket convert fluid imum output, the out- pressure into torque. The larger pis- put plates are forced ton pulls the chain in response to pres- firmly against the input surized fluid, while the smaller piston plates with no slip. seals the return side of the chain A heat exchanger dis- against leakage. Torque remains con- sipates the heat gener- stant throughout the stroke. Design Figure 5 — Hydrokinetic adjustable speed ated by the slipping action that occurs constraints include the strength of the drive. during reduced speed operation. chain and sprocket as well as the bulk of the unit itself, Figure 8. Hydroviscous ROTARY ACTUATORS Sometimes termed oil shear drives, these units are adjustable-speed, con- Devices known as stant-torque, variable-slip drives. rotary actuators They are similar to other variable-slip produce limited re- drives in efficiency and application. ciprocating rotary Capable of operating at zero slip, hy- force and motion by droviscous drives are rated from 25 to rotating an output over 300 hp.
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