Patent Application
20120134848
The present disclosure relates generally to a hydraulic fan circuit, and more particularly, to a hydraulic fan circuit having energy recovery.
Engine-driven machines such as, for example, dozers, loaders, excavators, motor graders, and other types of heavy equipment typically include a cooling system that cools the associated engine and other machine components below a threshold that provides for longevity of the machines. The cooling system consists of one or more air-to-air and/or liquid-to-air heat exchangers that chill coolant circulated throughout the engine and combustion air directed into the engine. Heat from the coolant or combustion air is passed to air from a fan that is speed controlled based on a temperature of the engine and based on a temperature of an associated hydraulic system.
The cooling system fan is generally hydraulically powered. That is, a pump driven by the engine draws in low-pressure fluid and discharges the fluid at elevated pressures to a motor that is connected to the fan. When a temperature of the engine is higher than desired, the pump and motor work together to increase the speed of the fan. When the temperature of the engine is low, the pump and motor work together to decrease the speed of the fan and, in some situations, even stop the fan altogether. Under some conditions, fan rotation can even by reversed such that airflow through the heat exchanger is also reversed to help dislodge any debris that has collected in the heat exchanger.
Although effective at cooling the engine, it has been found that the hydraulic circuit driving the cooling fan described above and/or other hydraulic circuits of the same machine may have excess capacity at times that is not utilized or even wasted. With increasing focus on the environment, particularly on machine fuel consumption, it has become increasingly important to fully utilize all resources.
One attempt to improve hydraulic circuit efficiency is described in U.S. Pat. No. 7,444,809 that issued to Smith et al. on Nov. 4, 2008 (“the '809 patent”). Specifically, the '809 patent describes a hystat system having an engine-driven pump coupled to a motor in a closed circuit configuration. During periods of excess pump capacity, pressurized fluid from the pump is stored in an accumulator for later use. The store of pressurized fluid can then be used to drive the pump and/or motor, thereby reducing a load on the engine. Pressurized fluid from other hydraulic circuits of the same machine, for example from tool actuator circuits, can also be stored in the accumulator and selectively used to drive the pump and motor and thereby further reduce fuel consumption of the engine.
Although the system of the '809 patent may have improved efficiency, it may also have limited applicability. That is, the system provides no isolation of the motor during energy recovery operations. In some applications, such as cooling fan drives, a fan motor driven with accumulated fluid could be caused to operate at undesired speeds as the pressure within the accumulator varies. In addition, driving the fan motor when cooling is unnecessary can waste energy and possibly over-cool the engine.
The disclosed hydraulic fan circuit is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.
In one aspect, the present disclosure is directed to a hydraulic fan circuit. The hydraulic fan circuit may include a primary pump, a high-pressure passage fluidly connected to the primary pump, and a low-pressure passage fluidly connected to the primary pump. The hydraulic fan circuit may also include at least one accumulator in selective fluid communication with at least one of the high- and low-pressure passages, a motor, and a fan connected to the motor. The hydraulic fan circuit may further include a fan isolation valve fluidly connected to the high- and low-pressure passages. The fan isolation valve may be movable between a flow-passing position at which the motor is fluidly connected to the primary pump via the high- and low-pressure passages, and a flow-blocking position at which the motor is substantially isolated from the primary pump.
In another aspect, the present disclosure is directed to another hydraulic fan circuit. This hydraulic fan circuit may include a primary pump, a motor, a fan connected to the motor, and a closed circuit fluidly connecting the primary pump to the motor. The hydraulic fan circuit may also include a high-pressure accumulator in selective fluid communication with the closed circuit, a low-pressure accumulator in fluid communication with the closed circuit, an accumulator discharge valve in fluid communication with the high- and low-pressure accumulators, and a fan isolation valve fluidly connected to the closed circuit and to the motor. The hydraulic fan circuit may further include a controller in communication with the accumulator discharge valve and the fan isolation valve. The controller may be configured to regulate the accumulator discharge valve to selectively pass fluid from the primary pump to the high-pressure accumulator and from the high-pressure accumulator to the primary pump, and to selectively pass fluid from the motor to the low-pressure accumulator and from the low-pressure accumulator to the primary pump. The controller may be further configured to regulate the fan isolation valve to substantially isolate the motor from the primary pump during discharge of the high-pressure accumulator.
In yet another aspect, the present disclosure is directed to a method of recovering energy from a hydraulic fan circuit. The method may include pressurizing fluid with a pump, directing the pressurized fluid to drive a fan motor, and accumulating excess pressurized fluid. The method may further include selectively discharging accumulated fluid to drive the pump, and substantially isolating the fan motor from the pump during the discharging.
Machine 200 may be equipped with multiple systems that facilitate the operation of machine 200 at worksite 210, for example a tool system 220, a drive system 230, and an engine system 240 that provides power to tool system 220 and drive system 230. During the performance of most tasks, power from engine system 240 may be disproportionately split between tool system 220 and drive system 230. That is, machine 200 may generally be either traveling between excavation sites and primarily supplying power to drive system 230, or parked at an excavation site and actively moving material by primarily supplying power to tool system 220. Machine 200 generally will not be traveling at high speeds and actively moving large loads of material with tool system 220 at the same time. Accordingly, engine system 240 may be sized to provide enough power to satisfy a maximum demand of either tool system 220 or of drive system 230, but not both at the same time. Although sufficient for most situations, there may be times when the total power demand from machine systems (e.g., from tool system 220 and/or drive system 230) exceeds a power supply capacity of engine system 240. Engine system 240 may be configured to recover stored energy during these times to temporarily increase its supply capacity. This additional supply capacity may also or alternatively be used to reduce a fuel consumption of engine system 240 by allowing for selective reductions in the power production of engine system 240, if desired.
As illustrated in
Primary pump 14 may be an over-center, variable-displacement or variable-delivery pump driven by engine 12 to pressurize fluid. For example, primary pump 14 may embody a rotary or piston-driven pump having a crankshaft (not shown) connected to engine 12 via mechanical output 16 such that an output rotation of engine 12 results in a corresponding pumping motion of primary pump 14. The pumping motion of primary pump 14 may function to draw in low-pressure fluid expelled from motor 18 via a low-pressure passage 24, and discharge the fluid at an elevated pressure to motor 18 via a high-pressure passage 26. Low- and high-pressure passages 24, 26 together may form closed circuit 22. Primary pump 14 may be dedicated to supplying pressurized fluid to only motor 18 via high-pressure passage 26 or, alternatively, may also supply pressurized fluid to other hydraulic circuits associated with machine 200 (e.g., to hydraulic circuits associated with tool system 220, drive system 230, etc.), if desired. Similarly, primary pump 14 may be dedicated to drawing low-pressure fluid from only motor 18 via low-pressure passage 24 or, alternatively, may also draw in low-pressure fluid from other hydraulic circuits of machine 200, if desired. It should be noted that, in some situations, primary pump 14 and motor 18 may be operated in reverse direction and, in these situations, the pressures within low- and high-pressure passages 24, 26 may be reversed.
Motor 18 may include a fixed displacement, rotary- or piston-type hydraulic motor movable by an imbalance of pressure acting on a driven element (not shown), for example an impeller or a piston. Fluid pressurized by primary pump 14 may be directed into motor 18 via high-pressure passage 26 and returned from motor 18 via low-pressure passage 24. The direction of pressurized fluid to one side of the driven element and the draining of fluid from an opposing side of the driven element may create a pressure differential across the driven element (not shown) that causes the driven element to move or rotate. The direction and rate of fluid flow through motor 18 may determine the rotational direction and speed of motor 18 and fan 20, while the pressure imbalance of motor 18 may determine the torque output.
Fan 20 may be disposed proximate a liquid-to-air or air-to-air heat exchanger (not shown) and configured to produce a flow of air directed through channels of the exchanger for heat transfer with coolant or combustion air therein. Fan 20 may include a plurality of blades connected to motor 18 and be driven by motor 18 at a speed corresponding to a desired flow rate of air and/or a desired engine coolant temperature. In one embodiment, a flywheel 28 may be connected to one of fan 20 and motor 18 to rotate therewith. Flywheel 28 may embody a fixed inertia flywheel, a variable inertia flywheel, or another type of flywheel known in the art having one or more rotating masses that move in accordance with a rotation of motor 18 and fan 20. The inertia of flywheel 28 may be selected to increase a free-spinning time of fan 20 after primary pump 14 has stopped driving motor 18. Under most conditions, a typical closed circuit fan may stop spinning after about 3 seconds or less, when no longer driven by a pump. Flywheel 28, however, may have an inertia great enough to cause fan 20 to spin for at least 4 seconds after primary pump 14 has stopped driving motor 18. In another embodiment, flywheel 28 may be incorporated into fan 20 (i.e., fan 20 may be oversized to include the inertia of flywheel 28 that causes it to spin for the at least 4 seconds).
Low- and high-pressure passages 24, 26 may be interconnected via multiple different crossover passages. In the exemplary embodiment, two different crossover passages interconnect low- and high-pressure passages 24, 26, including a makeup/relief passage 30 and a pressure-limiting passage 32. Makeup/relief passage 30 may provide makeup fluid to low- and/or high-pressure passages 24, 26 to help ensure that hydraulic fan circuit 10 remains full of fluid, and also provide a leak path for high-pressure fluid within low- and/or high-pressure passages 24, 26 such that damage to the components of hydraulic fan circuit 10 may be avoided. Pressure-limiting passage 32 may provide for pilot pressure control associated with a displacement of primary pump 14.
One or more makeup valves 34, for example check valves, may be located within makeup/relief passage 30 to selectively connect the output from a charge pump 36 with low- and/or high-pressure passages 24, 26 based on pressures of fluid in the different passages. That is, when a pressure within low- and/or high-pressure passage 24, 26 falls below a pressure of fluid discharged by charge pump 36, makeup valve(s) 34 may open and allow fluid to pass from charge pump 36 into the respective passage(s). Charge pump 36 may be driven by engine 12 to rotate with primary pump 14 and draw in fluid from a low-pressure sump 38 via a tank passage 40, and discharge the fluid into makeup/relief passage 30 via a valve passage 42.
One or more relief valves 44 may also be located within makeup/relief passage 30. Relief valves 44 may be spring-biased and movable in response to a pressure of low- and/or high-pressure passages 24, 26 to selectively connect the respective passages with a low-pressure passage 46, thereby relieving excessive fluid pressures within low- and high-pressure passages 24, 26. An additional spring-biased pressure relief valve 48 may be located within low-pressure passage 46 and selectively moved by a pressure within low-pressure passage 46 between flow-passing and flow-blocking (shown in
A resolver 50 may be disposed within pressure-limiting passage 32 and associated with a pilot pressure limiter 52. Resolver 50 may be configured to connect fluid from the one of low- and high-pressure passages 24, 26 having the greater pressure with pilot pressure limiter 52. In most instances, resolver 50 connects the pressure from high-pressure passage 26 with pilot pressure limiter 52 (shown in
Pilot pressure limiter 52 may be in fluid communication with a pilot passage 54 that extends between charge pump 36 and a displacement actuator 56 of primary pump 14. Specifically, pilot pressure limiter 52 may be connected to pilot passage 54 via a passage 58. When pilot pressure limiter 52 moves toward the flow-passing position described above, pilot fluid from within pilot passage 54 may be allowed to drain to low-pressure sump 38. The draining of pilot fluid from pilot passage 54 may reduce a pressure of fluid within pilot passage 54.
The pilot fluid in passage 54 may be selectively communicated with displacement actuator 56 to affect a displacement change of primary pump 14. Displacement actuator 56 may embody a double-acting, spring-biased cylinder connected to move a swashplate, a spill valve, or another displacement-adjusting mechanism of primary pump 14. When pilot fluid of a sufficient pressure is introduced into one end of displacement actuator 56, displacement actuator 56 may move the displacement-adjusting mechanism of primary pump 14 by an amount corresponding to the pressure of the fluid. Pilot pressure limiter 52 may limit the pressure within pilot passage 54 based on a pressure of fluid within low- and high-pressure passages 24, 26 and, accordingly, also limit the displacement of primary pump 14.
In some situations, it may be desirable to inhibit the pressure limiting provided by pilot pressure limiter 52, for example when an extreme displacement position of primary pump 14 is desired. For this reason, a pressure override valve 59 may be disposed within passage 58, between pilot pressure limiter 52 and pilot passage 54. Pressure override valve 59 may be a spring-biased, solenoid-actuated control valve that is movable based on a command from a controller 62. Pressure override valve 59 may be movable between a flow-passing position (shown in
A directional control valve 60 may be associated with displacement actuator 56 to control what end of displacement actuator 56 receives the pressurized pilot fluid and, accordingly, in which direction (i.e., which of a displacement-increasing and a displacement-decreasing direction) the displacement-adjusting mechanism of primary pump 14 is moved by displacement actuator 56. Directional control valve 60 may be a spring-biased, solenoid-actuated control valve that is movable based on a command from controller 62. Directional control valve 60 may move between a first position at which a first end of displacement actuator 56 receives pressurized pilot fluid, and a second position at which a second opposing end of displacement actuator 56 receives pressurized pilot fluid. When the first end of displacement actuator 56 is receiving pressurized pilot fluid (i.e., when directional control valve 60 is in the first position), the second end of displacement actuator 56 may be simultaneously connected to low-pressure sump 38 via directional control valve 60. Similarly, when the second end of displacement actuator 56 is receiving pressurized pilot fluid (i.e., when directional control valve 60 is in the second position), the first end of displacement actuator 56 may be simultaneously connected to low-pressure sump 38 via directional control valve 60. One or more restrictive orifices 64 may be associated with pilot passage 54 to reduce pressure fluctuations in the pilot fluid entering and exiting the ends of displacement actuator 56 and, thereby, stabilize fluctuations in a speed of pump displacement changes.
A pressure control valve 66 may also be associated with pilot passage 54 and displacement actuator 56 and configured to control movement of displacement actuator 56 by varying a pressure of pilot passage 54. Pressure control valve 66 may be movable from a first position (shown in
At least one accumulator may be associated with closed circuit 22. In the embodiment of
Discharge control valve 76 may be a double-acting, spring-biased, solenoid-controlled valve that is movable between three distinct positions based on a command from controller 62. In the first position (shown in
Low- and high-pressure accumulators 68, 70 may be in fluid communication with pilot passage 54. Specifically, a fill passage 81 may fluidly connect each of low- and high-pressure discharge passages 72, 74 to pilot passage 54. A check valve 83 may be disposed within fill passage 81 between pilot passage 54 and each of low- and high-pressure accumulators 68, 70 to help ensure a unidirectional flow of fluid from charge pump 36 into low- and high-pressure accumulators 68, 70.
High-pressure accumulator 70 may also be in fluid communication with another hydraulic circuit 100 that forms a portion of, for example, tool system 220, drive system 230, or another system of machine 200. In particular, an auxiliary supply passage 102 may fluidly connect hydraulic circuit 100 to high-pressure accumulator 70 to fill high-pressure accumulator 70 with waste or excess fluid having an elevated pressure. A check valve 104 and a restrictive orifice 106 may be disposed within auxiliary supply passage 102 to help provide for a unidirectional flow of fluid with damped oscillations from hydraulic circuit 100 into high-pressure accumulator 70. A sensor 108, for example a pressure sensor, temperature sensor, viscosity sensor, etc., may be associated with auxiliary supply passage 102 to provide a signal to controller 62 indicative of a fluid parameter of auxiliary supply passage 102 and/or high-pressure accumulator 70. Hydraulic circuit 100 may include a tool actuation circuit, a transmission circuit, a brake circuit, a steering circuit, or any other machine circuit known in the art.
During accumulator discharge, as will be described in greater detail below, it may be beneficial to substantially isolate motor 18 from low- and high-pressure passages 24, 26 (i.e., to substantially block direct fluid flow to motor 18 via low- and high-pressure passages 24, 26). For this reason, a fan isolation valve 84 may be fluidly connected to low- and high-pressure passages 24, 26, between motor 18 and low- and high-pressure accumulators 68, 70. Fan isolation valve 84 may be a spring-biased, solenoid-controlled valve that is movable between two distinct positions based on a command from controller 62. In the first position (shown in
When motor 18 is isolated by fan isolation valve 84 (i.e., when fan isolation valve 84 is in the second position), fluid may still circulate through motor 18, and fan 20 may still be spinning. To help control fluid temperatures during this time, hydraulic fan circuit 10 may include a motor flushing valve 86 and a pair of check valves 88 in fluid communication with a motor makeup valve 90. Motor flushing valve 86 may be in fluid communication with isolated portions of low- and high-pressure passages 24, 26, and configured to move between three positions based on the pressures of fluid within these passages. In the first position (shown in
Motor makeup valve 90 may be disposed between pressure-limiting passage 32 and branching passage 92, and movable based on a pressure of fluid within pressure-limiting passage 32 to selective allow fluid into branching passage 92. In particular, fluid in a low-pressure makeup passage 94 connected to pressure-limiting passage 32 at a low-pressure side of resolver 50 may push on one end of motor makeup valve 90, while fluid in a high-pressure makeup passage 96 connected to pressure-limiting passage 32 at a high-pressure side of resolver 50 may push on an opposing end of motor makeup valve 90. The one of low- and high-pressure makeup passages 94, 96 having the higher pressure at a given point in time may urge motor makeup valve 90 to a position at which fluid from the lower pressure passage flows into branching passage 92. Motor makeup valve 90 may be spring biased toward a position at which fluid from both the low- and high-pressure makeup passages 94, 96 passes through to branching passage 92.
Controller 62 may embody a single or multiple microprocessors, field programmable gate arrays (FPGAs), digital signal processors (DSPs), etc. that include a means for controlling an operation of hydraulic fan circuit 10 in response to signals received from sensor 108, one or more engine sensors 110, a pump displacement sensors 112, and a motor speed sensor 113. Numerous commercially available microprocessors can be configured to perform the functions of controller 62. It should be appreciated that controller 62 could readily embody a microprocessor separate from that controlling other machine-related functions, or that controller 62 could be integral with a machine microprocessor and be capable of controlling numerous machine functions and modes of operation. If separate from the general machine microprocessor, controller 62 may communicate with the general machine microprocessor via datalinks or other methods. Various other known circuits may be associated with controller 62, including power supply circuitry, signal-conditioning circuitry, actuator driver circuitry (i.e., circuitry powering solenoids, motors, or piezo actuators), and communication circuitry.
Controller 62 may be in communication with valves 59, 60, 66, 76, and 84 to control operations of hydraulic fan circuit 10 during at least two distinct modes of operation based on input from sensors 108, 110, 112, and 113. The modes of operation may include a normal mode during which primary pump 14 drives motor 18 to cool engine 12, and an energy recovery mode during which motor 18 drives primary pump 14 to recover energy directed back to engine 12. These modes of operation will be described in more detail in the following section to further illustrate the disclosed concepts
The disclosed hydraulic fan circuit may be applicable to any heat engine where cooling and energy recovery is desired. The disclosed hydraulic fan circuit may provide for energy recovery from any machine circuit through the selective use of accumulator storage and discharge. Operation of hydraulic fan circuit 10 will now be described.
During the normal mode of operation, engine 12 may drive primary pump 14 to rotate and pressurize fluid. The pressurized fluid may be discharged from primary pump 14 into high-pressure passage 26 and directed into motor 18. As the pressurized fluid passes through motor 18, hydraulic power in the fluid may be converted to mechanical power used to rotate fan 20 and flywheel 28. As fan 20 rotates, a flow of air may be generated that facilitates cooling of engine 12. Fluid exiting motor 18, having been reduced in pressure, may be directed back to primary pump 14 via low-pressure passage 24 to repeat the cycle.
The fluid discharge direction and displacement of pump 14 during the normal mode of operation may be regulated based on signals from sensors 108, 110, 112, and/or 113, for example based on an engine speed signal, an engine temperature signal, a motor speed signal, a pump displacement signal, an accumulator pressure signal, and/or another similar signal. Controller 62 may receive these signals and reference a corresponding engine speed, engine temperature, pump displacement angle, motor speed, accumulator pressure, or other similar parameter with one or more lookup maps stored in memory to determine a desired discharge direction and displacement setting of primary pump 14 and a corresponding rotation direction and speed of fan 20. Controller 62 may then generate appropriate commands to be sent to directional control valve 60 and pressure control valve 66 to affect corresponding adjustments to the displacement of primary pump 14.
Low- and/or high-pressure accumulators 68, 70 may be charged during the normal mode of operation in a least three different ways. For example, when primary pump 14 is driven to pressurize fluid, any excess fluid not consumed by motor 18 may fill high-pressure accumulator 70 via discharge control valve 76, when discharge control valve 76 is in the second position. Similarly, fluid exiting motor 18 may fill low-pressure accumulator 68. Low- or high-pressure accumulators 68, 70 may only be filled while discharge control valve 76 is in the second position and pressures within low- or high-pressure passages 24, 26 are greater than pressures within low- or high-pressure accumulators 68, 70, respectively. Otherwise, low- or high-pressure accumulators 68, 70 may discharge fluid into low- or high-pressure passages 24, 26 when discharge control valve 76 is moved to the second position. The movement of discharge control valve 76 may be closely regulated based at least in part on the signal provided by pressure sensor 108, such that low- and high-pressure accumulators 68, 70 may be charged and discharged at the appropriate times. It should be noted that only one of low- and high-pressure accumulators 68, 70 may be filled at a time, while the other of low- and high-pressure accumulators 68, 70 will be discharging, and vice versa.
Alternatively or additionally, low- or high-pressure accumulators 68, 70 may be continuously charged via charge pump 36. Specifically, at any time during normal operation, when a pressure of fluid from charge pump 36 is greater than pressures within low- or high-pressure accumulators 68, 70, fluid may be passed from charge pump 36, through fill passage 81, and past check valves 83 into the respective low- or high-pressure accumulator 68, 70. Pressure relief valve 78 may help ensure that low-pressure accumulator 68 does not over-pressurize during charging by charge pump 36. Again, it should be noted that only one of low- and high-pressure accumulators 68, 70 may be fill or discharge at a time.
High-pressure accumulator 70 may also be charged by hydraulic circuit 100. That is, at any time during normal operations, when a pressure of fluid from hydraulic circuit 100 is greater than a pressure within high-pressure accumulator 70, fluid may be passed from circuit 100, through auxiliary supply passage 102, and past check valve 104 into high-pressure accumulator 70.
When the signal from engine sensor 110 indicates that sufficient cooling has been obtained (i.e., when the demand for cooling air flow has been reduced) and fan 20 may be slowed or even stopped, controller 62 may implement the energy recovery mode of operation. During the energy recovery mode of operation, controller 62 may command fan isolation valve 84 to isolate motor 18 from primary pump 14, and then command discharge control valve 76 to move to one of the second and third positions depending on the desired flow direction of primary pump 14. At about this same time, controller 62 may command override valve 59 to move to the flow-blocking position and also command pressure control valve 66 to begin destroking primary pump 14. When the appropriate valve commands have been issued, fluid from within one of low- or high-pressure accumulators 68, 70 may discharge into low- or high-pressure passages 24, 26, respectively, via passages 72, 74, discharge control valve 76, and passages 80, 82, thereby driving primary pump 14 as a motor. By driving primary pump 14, hydraulic power from the accumulated fluid may be converted to mechanical power directed into engine 12 via mechanical output 16. This power assist may help to increase a power supply capacity and/or decrease a fuel consumption of engine 12 during the energy recovery mode of operation.
During discharge of one of low- or high-pressure accumulators 68, 70, while motor 18 is isolated from primary pump 14, fan 20 may continue to spin. As described above, fan 20, if equipped with flywheel 28 or oversized to integrate the mass of flywheel 28, may spin for an extended period of time without motor 18 being driven. In one example, the extended period of time may be at least 4 seconds. In this manner, a significant amount of engine cooling may still be possible during discharge of low- or high-pressure accumulators 68, 70, and the speed of motor 18 may be substantially unaffected by the changing fluid pressures within the accumulators. In addition, energy from the accumulated fluid may not be wasted on unnecessarily driving motor 18.
It is contemplated that accumulator discharge could alternatively occur without complete motor isolation, if desired. Specifically, fan isolation valve 84 could be controlled to move to any position between the first and second positions described above such that a desired amount of pressurized fluid from high-pressure accumulator 70 passes through and drives motor 114 (referring to the embodiment of
The disclosed hydraulic fan circuit may be relatively inexpensive and provide multiple levels of energy recovery. In particular, because the hydraulic fan circuit largely utilizes existing components to recover otherwise wasted energy, the cost of the system may remain low. Further, because low- and high-pressure accumulators 68, 70 can fill with fluid from different sources and discharge in different ways, an amount of energy recovery may be increased.
It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed hydraulic fan circuit. For example, although the disclosed pumps and motors are described as being variable and fixed displacement or variable and variable displacement type devices, respectively, it is contemplated that the disclosed pumps and motors may alternatively both be fixed displacement type devices, if desired. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed hydraulic fan circuit. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.
