Hydraulic drivetrain for a utility vehicle

A vehicle and a hydraulic propulsion system for the vehicle are provided with first and second motors diagonally arranged relative to one another on the vehicle, and third and fourth motors diagonally arranged relative to one another on the vehicle. First, second and third flow divider combiner assemblies are provided and are arranged in a closed fluid loop with the motors. A first port of each of the assemblies are fluidly connected to one another. The first assembly has a second port fluidly coupled to the first and second motors, and a third port fluidly coupled to the third and fourth motors. The second assembly has second and third ports fluidly coupled to the first and third motors, respectively. The third assembly has second and third ports fluidly coupled to the second and fourth motors, respectively. A method of controlling the hydraulic system is also provided.

TECHNICAL FIELD

Various embodiments relate to hydraulic systems for a utility vehicle with a hydraulic traction circuit to control traction through a vehicle turn.

BACKGROUND

Utility vehicles used in a commercial or industrial environment may include lift equipment, including portable material lifts, telehandlers, scissor lifts, telescopic and articulating booms. These vehicles typically have a hydraulic system that acts to propel the vehicle and operate the work function, such as a lift platform, of the vehicle. The hydraulic system drives the vehicle by controlling the vehicle propulsion, vehicle braking, and vehicle steering. One or more pumps of the hydraulic system are typically driven by an internal combustion engine or other prime mover in the vehicle.

These vehicles often are provided with four-wheel drive, with hydraulic motors providing torque to drive the wheels to provide traction for the vehicle and to propel the vehicle. In a conventional system, the hydraulic traction system or traction circuit divides the fluid flow from the pump and sends one portion to the hydraulic motors associated with the front wheels and front axle and another portion to the hydraulic motors associated with the rear wheels and rear axle. This provides generally equal flow to the hydraulic motors with the motors and wheels turning at the same speed, regardless of the steer angle of the vehicle. During a turn, the vehicle wheels need to turn at different speeds based on their location on the vehicle, and the conventional front/rear split in the hydraulic traction circuit leads to inefficiencies in the hydraulic circuit and in the vehicle as one or more wheels may lose traction and slip or skid in a turn.

SUMMARY

In an embodiment, a vehicle is provided with a chassis having first and second opposite sides extending between third and fourth opposite sides, first and second wheels cooperating to provide a first pair of wheels positioned along the first side of the chassis, and third and fourth wheels cooperating to provide a second pair of wheels positioned along the second side of the chassis. The third wheel is arranged diagonally relative to the first wheel on the chassis, and the fourth wheel is arranged diagonally relative to the second wheel on the chassis. The vehicle has a closed loop hydraulic system with a pump, and first, second, third, and fourth hydraulic motors drivingly connected to the first, second, third, and fourth wheels, respectively, to rotate the associated wheel. The hydraulic system has a first flow divider-combiner assembly with a first port fluidly connected to the pump, a second port fluidly connected to the first and third hydraulic motors, and a third port fluidly connected to the second and fourth hydraulic motors. A second flow divider-combiner assembly in the hydraulic system has a first port fluidly connected to the pump, a second port fluidly connected to the first hydraulic motor, and a third port fluidly connected to the second hydraulic motor. A third flow divider-combiner assembly in the hydraulic system has a first port fluidly connected to the pump, a second port fluidly connected to the third hydraulic motor, and a third port fluidly connected to the fourth hydraulic motor.

In another embodiment, a hydraulic propulsion system for a vehicle is provided with first, second, third, and fourth hydraulic motors. The first and second motors are configured to be diagonally arranged relative to one another on the vehicle, and the third and fourth motors are configured to be diagonally arranged relative to one another on the vehicle. A first flow divider-combiner assembly has a first port, a second port configured to be fluidly coupled to the first and second hydraulic motors, and a third port configured to be fluidly coupled to the third and fourth hydraulic motors. A second flow divider-combiner assembly has a first port, a second port configured to be fluidly coupled to the first motor and a third port configured to be fluidly coupled to the third motor. A third flow divider-combiner assembly has a first port, a second port configured to be fluidly coupled to the second motor and a third port configured to be fluidly coupled to the fourth motor. The first port of the first assembly, the first port of the second assembly, and the first port of the third assembly are configured to be fluidly connected to one another. The first assembly, the second assembly, the third assembly, and the first, second, third, and fourth hydraulic motors are configured to be arranged in a closed fluid loop.

In yet another embodiment, a method of operating a hydraulic propulsion system for a vehicle is provided. Fluid flow from at least one pump is divided such that a first portion of the fluid flow is directed from the at least one pump to first and second hydraulic motors and a second portion of the fluid flow is directed from the at least one pump to third and fourth hydraulic motors. First and second wheels connected to the first and second hydraulic motors, respectively, are rotated using the first portion of the fluid flow. Third and fourth wheels connected to the third and fourth hydraulic motors, respectively, are rotated using the second portion of the fluid flow. Fluid flows from the first and third hydraulic motors are combined to the at least one pump. Fluid flows from the second and fourth hydraulic motors are combined to the at least one pump.

DETAILED DESCRIPTION

In various embodiments, a hydraulic system and hydraulic traction circuit are provided for a utility vehicle. The hydraulic traction circuit distributes hydraulic fluid flow to hydraulic motors associated with four wheels of the vehicle to create drive torque to move the vehicle over terrain. The vehicle is therefore four-wheel drive. The fluid flow from a pump is divided to the four hydraulic motors to provide controlled flow to each of the motors such that the associated wheels are rotated at a desired, controlled speed that varies with vehicle steer angle and wheel slip and skid is prevented or reduced. The hydraulic circuit also uses flow divider-combiner valves to divide flows to different pairs of hydraulic motors or individual motors, and also recombine flows from different hydraulic motors or pairs of motors. The flow divider-combiner valves maintain traction for the grounded wheels when one or more wheels lifts off the ground or loses traction by preventing overspeed or runaway of the lifted or slipping wheel. The flow divider-combiner valves, as described later below with reference toFIGS. 4 and 5are typically configured as 50/50 valves where the fluid flow is divided into a 50/50 split, or recombined into a flow with 50% coming from each flow input into the valve. In other examples, the valves may be provided and configured for other percentage splits.

FIGS. 1-3illustrate a utility vehicle10according to an embodiment.FIG. 1is a schematic of a side view of the vehicle10.FIG. 2is a powertrain schematic for the vehicle.FIG. 3illustrates a top schematic view of the vehicle while turning based on a steering input. The vehicle10may be a utility vehicle such as an aerial work platform, a rough terrain telescopic load handler, portable material lift, telehandler, scissor lift, telescopic and articulating boom, and the like. The vehicle10is configured for lifting a load12, such as a person, tools, cargo, and the like, with respect to a support surface14, such as paved or unpaved ground, a road, an apron such as a sidewalk or parking lot, an interior or exterior floor of a structure, or other surfaces. The vehicle10may be any other vehicle that is a conventional vehicle with a hydraulic powertrain.

The vehicle10includes a vehicle lift component16such as a platform, a chassis18, and a support assembly20that couples the platform16and the chassis18. The chassis18is supported on the support surface14by traction devices22, such as wheels. The traction devices22may include tires and/or tracks. The vehicle10has a first axle24with two wheels22and a second axle26with another two wheels22. Axle24may be a front axle, and axle26may be a rear axle. In other embodiments, the vehicle10may have more than two axles. In other embodiments, traction devices22may be aligned with one another along a lateral axis of the vehicle, but not have axles24,26extending between them.

The chassis18has first and second opposite sides or ends30,32, and third and fourth opposite sides or ends34,36that extend between the first and second sides30,32. In one example, the first and second sides30,32correspond to the front and the rear ends of the chassis and vehicle, respectively. The third and the fourth sides34,36correspond to the right- and left-hand sides of the chassis and vehicle, respectively. The vehicle10is configured to move in both a forward and a reverse direction, e.g. in either direction along a vehicle longitudinal axis38when the wheels22are unsteered.

The traction devices22are each hydraulically driven using a hydraulic system40. The hydraulic system40or traction circuit40may be a closed loop system as described herein and shown in greater detail with reference toFIG. 5. A pump42provides pressurized fluid to hydraulic motors44or other features, and the fluid then returns directly to the pump inlet while remaining pressurized. In other examples, the hydraulic system40may be provided as an open loop system with fluid returning to a reservoir or tank via an open return path. The pump42may be a load sensing pump, variable displacement hydraulic pump or other suitable pump.

The vehicle10has an engine46or other prime mover to operate the hydraulic system40. The engine46may be a diesel engine, or another internal combustion engine. In other examples, an electric machine or motor may replace or augment the engine46in the system. The output shaft of the engine46may be directly and permanently fixed to the drive shaft of the pump42to rotate and power the pump42, and the pump42may include a swash plate or other mechanism to control the flow output and flow direction from the pump. In other embodiments, a clutch, gearing, or other transmission mechanisms may be provided between the engine46output shaft and the drive shaft of the pump42to control the pump speed relative to the engine speed.

Each hydraulic drive motor44uses pressurized fluid to produce torque and rotate a respective traction device22. For the vehicle10shown with four wheels22, the vehicle has four hydraulic motors44with each motor44associated with a respective traction device22such that the vehicle is four-wheel drive. In one example, the hydraulic motors44have first and second displacements, or dual displacements, to provide different vehicle traction modes. For example, each motor may have a drive mode or high-speed mode with low torque and higher fluid pressures, and a low mode with higher torque and lower fluid pressures. The motors44may be shiftable, for example, using a swash plate with a shiftable angle, and be controlled using a pilot line and shift piston or the like. In other examples, the hydraulic motors44may each be single displacement or fixed displacement.

The vehicle10may also include a hydraulic system to provide pressurized fluid to a function circuit and manifold48. In one example, the function manifold48is driven by a pump49that is rotatably connected or piggy-backed to the pump42. In another example, the pump49is separately driven by the engine or other prime mover in the vehicle. The function manifold48may be provided as a closed loop or open loop system. The function manifold48operates the lift or work function(s) of the vehicle10and other hydraulic functions that the vehicle is equipped with such as hydraulic steering, hydraulic braking, and the like.

The systems40,48and engine46are controlled by a controller50that is in communication with the various components of the systems and system sensors, such as a pump pressure sensor. The controller50may provide or be a part of a vehicle systems controller (VSC), and may include any number of controllers, and may be integrated into a single controller, or have various modules. Some or all of the controllers may be connected by a controller area network (CAN) or other system. The controller50may also be connected to random access memory or another data storage system.

InFIG. 2, the vehicle has wheels60,62,64,66to provide the traction devices22. Two of the wheels60,62are aligned along a first lateral axis of the vehicle70, which may correspond with axle24, to provide a front pair of wheels. The other two wheels64,66are aligned along a second lateral axis of the vehicle, which may correspond with axle26, to provide a rear pair of wheels. The vehicle has two wheels60,62providing a pair of wheels positioned along the front side of the chassis, and two wheels64,66providing a pair of wheels positioned along the rear side of the chassis. The vehicle also has two wheels60,64providing a pair of wheels positioned along the left side of the chassis, and two wheels62,66providing a pair of wheels positioned along the right side of the chassis.

The function manifold48provides hydraulic fluid to control actuators to steer the traction devices22. In one example, wheels60,62are connected to one or more hydraulic actuators74to control the angle of the wheels relative to the chassis and steer the vehicle while wheels64,66are unsteerable such that the vehicle is a two-wheel steering vehicle. In another example of a two-wheel steering vehicle, wheels64,66may be connected to the hydraulic actuator(s) for steering while the wheels60,62are unsteerable. In a further example, all four wheels60,62,64,66are connected to one or more hydraulic actuators74to control the angle of the wheels relative to the chassis and steer the vehicle10such that the vehicle is a four-wheel steering vehicle.

FIG. 3illustrates a two-wheel steering, four-wheel drive vehicle such as vehicle10. The vehicle10is turning or steering, the steer angles of the steerable wheels60,62are such that the axes of rotation for each of the four wheels60,62,64,66coincide or intersect at a single point80defining the center of turn radius. This steering configuration and concept is also known as Ackerman steering. A four-wheel steering, four-wheel drive vehicle may also be configured similarly for an Ackerman steering scenario. In Ackerman steering situations, the radius from the center of turn80to the center of each wheel60,62,64,66is directly proportional to the speed of each wheel60,62,64,66. Therefore, the wheels that are farther away from the center of the turn80need to turn faster than wheels that are closer to the center of the turn80, as the distance that they need to travel is greater. The projected path of each wheel is illustrated inFIG. 3for reference.

In Ackerman steering and in the present disclosure, the geometry is such that a sum of the turn radii of left rear and right front wheels64,62is very similar to the sum of the turn radii of the right rear and left front wheels66,60, the difference not exceeding 5% in a typical application and vehicle configuration. In other words, sum of the turn radii of each pair of diagonally opposed wheels, e.g. wheels60,66and wheels62,64, is substantially similar and typically within 10% or even 5% of each other.

In a conventional vehicle with a hydraulic traction circuit, and as shown inFIG. 4, the hydraulic fluid from the pump100is divided equally using a flow divider-combiner valve102and sent separately to hydraulic motors106associated with the front pair of wheels108and hydraulic motors110associated with the rear pair of wheels112. The flow from the front wheel hydraulic motors106is recombined equally using a flow divider-combiner valve114and returned and sent to the pump100, and the flow from the rear wheel hydraulic motors110is recombined equally using a flow divider-combiner valve118and returned to the pump100. As can be seen from the illustration inFIG. 3, a sum of the radii for the front wheels108is larger than a sum of the radii for the rear wheels112in the turn, and therefore the existing division of flow provides too much or too little flow to hydraulic motors106,110in the system. Furthermore, and as shown inFIG. 3, for flow re-combinations, the inner wheels and outer wheels in a front or rear pair have vastly different radii and speeds, e.g. the left rear and right rear wheels may differ by 100% in a turn, and recombining these flows may also lead to inefficiencies in the system. In other words, while steering, the front wheels108have to turn faster than the rear wheels112, and the wheels at the outside of the turn have to turn faster than the wheels on the inside of the turn. When driving straight, the flow divider-combiner valves102,114,118allows for wheels108,112with traction to apply torque while wheels108,112with poor traction do not slip excessively. However, when the vehicle is turning in a tight radius, forcing all wheels108,112to go the same speed based on the hydraulic circuit arrangement of the flow divider-combiner valves102,114,118causes the wheels at different speeds to fight each other and waste power. In a conventional vehicle, a certain amount of unequal flow to the wheels is allowed to account for speed differentials between motors by providing a bypass flow. The bypass flow has an orifice or flow control valve104,116,120to allow for a short cut parallel for the valves102,114,118that permits an amount of flow to bypass the flow divider-combiner valve, and allow divided flow from one side to be redirected to divided flow on the other side of the valve based on pressure differentials. However, through a turn as the speed differential and bypass flow increases, the hydraulic fluid is heated. The orifices104,116,120, or another flow control valve, may be used to control bypass flow and to allow pressure to be maintained on wheels with traction; however, this bypass can only provide a limited amount of flow equalization and also leads to inefficiencies in the system.

FIG. 5illustrates a hydraulic traction circuit150for use with the vehicle10and hydraulic system40ofFIGS. 1-3according to an embodiment of the present disclosure. Reference numbers inFIG. 5are the same reference numbers inFIGS. 1-3for elements that are the same or similar. The hydraulic circuit150divides flow to wheel pairs other than the front pair and the rear pair, and divides and recombines flow from different pairs of wheels on the vehicle. In one example, the hydraulic circuit divides hydraulic fluid to pairs of diagonally arranged wheels using a flow divider-combiner valve. In this arrangement, substantially half of the pump flow to the hydraulic circuit goes to one diagonal pair of motors while the remaining half goes to the other diagonal pair of motors. The flow is then recombined to the pump with the flow from the left-hand pair being recombined and the flow from the right-hand pair being recombined.

InFIG. 5, the pump42is rotated by the engine46in the vehicle. The pump42may be provided with a swash plate to control the direction of flow output from the pump42, e.g. a first direction in the circuit150to rotate the hydraulic motors44in a first direction, for example a forward vehicle direction, or in a second direction in the circuit150to rotate the hydraulic motors44in a second direction, for example a reverse vehicle direction. In other examples, the pump may be provided with associated flow direction control valves to control the direction of flow in the fluid circuit.

The description as it relates toFIG. 5below described the fluid flow in a first direction through the circuit150; however, one of ordinary skill in the art understands that the flow may be reversed through the circuit to reverse the direction of the hydraulic motors44and vehicle. The flow combiner-divider valves or assemblies152,154,156therefore each operate as a divider valve with fluid flow in one direction, and as combiner valves with fluid flow through the valve in the opposite direction. Flow divider-combiner valves as used herein may include any form of a device or assembly that combines and divides fluid flow in a manner similar to that described with respect to the flow divider-combiner valves152,154,156. For example, elements152,154and156may each be provided by a spool-valve or spool-type; gear pump, rotary, or motor-type; or another flow divider-combiner device or flow divider-combiner assembly.

For a flow divider-combiner valve152,154,156, as a wheel loses traction and begins to slip and the motor44begins to freewheel, there is a pressure drop across the flow valve instead of across the hydraulic motor associated with the wheel. The valves152,154,156may be self-controlling such that this pressure differential controls a pilot line to the valve and changes the flow split of the valve to restrict flow to the motor associated with the wheel with less traction. The flow combiner valves152,154,156may also control the flow split in the system when one of the wheels is slipping or losing traction.

The pump42outlet is fluidly connected to a first flow divider-combiner valve152. The first valve has three ports. Ports as used herein may include any form of a fluid connection, including an opening or passageway provided in a separate connector device or in the component itself, for example as an aperture or drilled passage in a manifold structure.

The first port160is fluidly connected to the pump42. The second port162is fluidly connected to a fluid line164for the first and third hydraulic motors166,168. Fluid flow in the line164is at a common pressure for the first and third hydraulic motors, neglecting any friction flow losses in the system, etc. The line164includes a fluid junction, such as a T-junction, to fluidly connect the port162with each of the motors166,168at a common fluid pressure.

The third port170is fluidly connected to a fluid line172for the second and fourth hydraulic motors174,176, and fluid flow in the line is at a common pressure for the second and fourth hydraulic motors, neglecting any friction flow losses in the system, etc. The line172includes a fluid junction, such as a T-junction, to fluidly connect the port170with each of the motors166,168at a common fluid pressure.

As shown in the Figure, the first and third motors166,168are not associated with a common axle or along a common lateral axis such that they are diagonally arranged, and the second and fourth motors174,176are not associated with a common axle or along a common lateral axis such that they are diagonally arranged. Therefore, the first and third motors166,168are arranged diagonally relative to one another on the chassis and vehicle as shown, and the second and fourth motors174,176are arranged diagonally relative to one another on the chassis and vehicle.

A second flow divider-combiner valve154is provided in the circuit150. The second valve154has three ports. The first port180is fluidly connected to the pump42. The second port182is fluidly connected to a fluid line for the first hydraulic motor166. The third port184is fluidly connected to a fluid line for the fourth hydraulic motor176. As shown in the Figure, the first and fourth motors may lie along a side of the vehicle, such as the left-hand or right-hand side of the vehicle.

A third flow divider-combiner valve156is provided in the circuit150. The third valve156has three ports. The first port190is fluidly connected to the pump42. The second port192is fluidly connected to a fluid line for the second hydraulic motor174. The third port194is fluidly connected to a fluid line for the third hydraulic motor168. As shown in the Figure, the second and third motors may lie along a side of the vehicle, such as the left-hand or right-hand side of the vehicle.

The pump42, the first valve152, the second valve154, the third valve156, and the first, second, third, and fourth hydraulic motors166,168,174,176are arranged in a closed fluid loop or fluid circuit. The pump42is positioned between and fluidly couples the first valve152to the second and third valves154,156. As shown inFIG. 5, a fluid junction, such as a T-junction, may be used to fluidly connect the ports180,190with the pump42such that the fluid in this section of the circuit is at a common fluid pressure.

The pump42and circuit150are configured to provide fluid flow in a first direction such that fluid flows sequentially from the pump42to the first valve152to the hydraulic motors44to the second and third valves154,156and to the pump42such that each of the hydraulic motors44operate at a controlled speed to rotate the associated wheels. The fluid circuit is arranged such that the speed of each motor is controlled to approach a speed defined as a function of the vehicle speed and the steer angle of the wheel. Wheel torque is a function of fluid pressure at the motor, road load, requested vehicle speed and acceleration, and the like, and the wheels speeds are controlled to approach equal torque output at each wheel.

The pump42and circuit150are configured to provide fluid flow in a second direction such that fluid flows sequentially from the pump42to the second and third valves154,156to the hydraulic motors44to the first valve152and to the pump42such that each of the hydraulic motors44at a controlled speed to rotate the associated wheels.

Each of the first, second, and third valves152,154,156are provided as 50/50 valves for dividing or recombining fluid flows to or from the second and third ports of each valve, where the fluid flow is divided into a 50/50 split, or recombined into a flow with 50% coming from each flow input into the valve or device. In other examples, the valves152,154,156may be provided with other predetermined percentages for flow division or recombination, for example, based on different wheel geometries for the vehicle. In further examples, the valves152,154,156may be controllable to vary the percentages for flow division and recombination, for example, to further reduce or eliminate bypass flow in the associated orifices200,202,204.

For example, the circuit and vehicle may be configured to non-equal gear ratios or different gear ratios between the front and rear axles, or between the front wheels60,62and rear wheels64,66. In one example, the front axle wheel60,62drives may use a higher gear ratio than the rear wheel64,66drives, because the vehicle weight distribution permits more tractive force to be applied on the higher loaded front axle associated with wheels60,62. In this case, the front motors166,174would need to spin faster than the rear motors168,176such that the four wheels22turn at the same speed as one another. For the case where all four motors44have the same displacement, the front motors166,174would need more fluid flow than the rear motors168,176. As the diagonal pairs of motors are each hydraulically connected by a respective open fluid junction164,172, e.g. a T-junction, the flow division occurs automatically. The flow divider-combiner devices154,156are used to constrain the flow to the desired predetermined percentages for flow division or recombination from the front and rear pairs of motors44, and the devices154,156are constrained to divide and combine at a non-equal ratio, e.g. other than 50/50. If the vehicle and gearing is configured such that the front axle provide 70% of the torque and the rear axle provides 30% of the torque, the flow divider-combiner devices154,156are set to use that ratio for flow division or control while the flow divider-combiner device152to the diagonal pairs of motors may remain as a 50/50 valve. For example, the valve154may be set to combine a fluid flow with 70% of the flow through port182and 30% of the flow through port184, and the valve156may be set to combine a fluid flow with 70% of the flow through port192and 30% of the flow through port194.

The exact same effect can be achieved by keeping the gear ratios the same for each wheel, and using different displacement motors on the front and rear axles. Again, the divider/combiner ratio for the front/rear would need to match the ratio of displacements.

The second and third valves154,156are arranged for parallel fluid flow to or from the pump42based on the flow direction in the circuit150. Each of the first, second, and third valves152,154,156has an associated bypass device200,202,204fluidly coupling the second and third ports to allow flow from the second port to flow to the third port, or vice versa, based on any pressure imbalances between the second and third ports and associated fluid lines. Each bypass device200,202,204may be a valve or an orifice. As the fluid flow in the present disclosure is controlled to reduce wheel speed differentials as the vehicle is steered, less fluid flows through the bypass devices and more tractive effort is available at the wheels to propel the vehicle.

During operation, fluid flow from the pump42is divided by the first valve152such that a first portion of the fluid flow is directed from the pump42to a first pair of hydraulic motors166,168and a second portion of the fluid flow is directed from the pump42to a second pair of hydraulic motors174,176. The fluid flow may be divided using the first valve152positioned downstream of the pump. The two wheels60,66connected to the first pair of hydraulic motors166,168, respectively, are rotated using the first portion of the fluid flow. The two other wheels62,64connected to the second pair of hydraulic motors174,176, respectively, are rotated using the second portion of the fluid flow. Flow from one of the motors in each of the first and second pairs are combined and returned to the pump, and these motors166,176may be positioned and recombined as pairs on one side of the vehicle such as the left-hand side. Flow from the other one of the motors in each of the first and second pairs are combined and returned to the pump, and these motors174,168may be positioned and recombined as pairs on one side of the vehicle such as the right-hand side. Motors in each pair of motors may be positioned diagonally relative to one another on the vehicle.

As shown inFIG. 5, first and second wheels60,62on a first axle are connected to first and second hydraulic motors166,174, respectively. Third and fourth wheels64,66on a second axle are connected to third and fourth motors168,176, respectively. The pump42is fluidly connected to the first and third motors166,168via a flow divider valve152to direct a first portion of fluid flow received by the valve from the pump to the first and third motors. The pump42is also fluidly connected to the second and fourth motors174,176via the flow divider valve152to direct a second portion of fluid flow received by the valve from the pump to the second and fourth motors.

In various examples, the hydraulic circuit divides hydraulic fluid to motors associated with pairs of wheels arranged on the left-hand side or the right-hand side of the vehicle, and recombines hydraulic fluid from motors associated with pairs of wheels arranged diagonally.

For flow re-combinations and as shown inFIG. 3, the inner wheels have a similar, short turn radius and the outer wheels have a similar, long turn radius. Therefore, the fluid flows from the hydraulic motors on the left side are recombined with one flow combiner-divider valve, and flows from the hydraulic motors on the right side are recombined with another flow combiner-divider valve. This results in much smaller flow differences between input flows into each of these flow divider-combiner valves than if re-combining flows from the rear wheel motor pair and re-combining flows from the front wheel motor pair as is conventionally done.

Additionally, the divided flows and the recombined flows are associated with different pairs of hydraulic motors which handles additional degrees of freedom and provides an improved result compared to dividing and combining flows to and from the same hydraulic motors, as shown inFIG. 4.

The hydraulic traction circuit150inFIG. 5provides a similar level of performance when the vehicle is driving straight ahead, with or without effective traction, as the system shown inFIG. 4. However, the hydraulic traction circuit150inFIG. 5offers a substantial improvement over the conventional circuit when the vehicle is turning, for example, when steered to a tight turn radius, as the pressure required to maintain a given tractive force is much lower. Improvements are provided in both two- and four-wheel steer vehicles using the traction circuit ofFIG. 5compared to a conventional circuit; however, greater improvements may be generally seen in two-wheel steer vehicles as the speed differentials between the wheels is greater in turns.

In the proposed solution, a bypass device, such as a bypass orifice or a bidirectional flow control valve, is retained and provided in parallel with the flow divider, but flows across the shortcut are much smaller. The bypass device remains to correct inaccuracies and error in the flow valves, and other factors such as tire wear and inflation levels and asymmetric vehicle loading.

For example, testing results indicate that the hydraulic circuit pressures between a conventional hydraulic traction circuit and the circuit150according to the present disclosure and as shown inFIG. 5are generally equivalent when the vehicle is in a straight-ahead configuration, e.g. no steering input or turning. Test data indicates that when a vehicle with the circuit150according to the present disclosure is in a maximum steering angle configuration, it has a drive pressure of only approximately 25% greater than when it is in a straight-ahead configuration. Conversely, when a vehicle with a conventional circuit accordingFIG. 4is in a maximum steering angle configuration, it has a drive pressure of approximately 125% greater than when it is in a straight-ahead configuration. Furthermore, the conventional circuit has nearly double the drive pressure of the disclosed circuit150ofFIG. 5when both are in a maximum steer angle configuration. Therefore, the disclosed circuit ofFIG. 5allows for a more uniform drive pressure through varying steer angles to pairs of wheels to allow for controlled speeds, and a more equal torque distribution at the wheels.

In another example, modelling results indicate that bypass flow required during a maximum steer angle configuration for the vehicle are significantly reduced in the circuit150according to the present disclosure compared to a conventional circuit when both are in a maximum steer angle configuration. The flow divider-combiner valves of the conventional circuit had an average bypass flow for the circuit of over two and a half gallons per minute. The flow divider-combiner valves of the circuit150ofFIG. 5had an average bypass flow for the circuit of less than one gallon per minute, thereby showing a dramatic improvement in the efficiency in the hydraulic traction system.

The positioning of the divider-combiner valves in relation to the T-junctions also provides for control over the flow to each of the motors through varying steer angles and under other conditions where one or more wheels may lose traction and slip or skid. The divider-combiner valves control the flow, e.g. by creating a set or forced pressure drop in the system across the valve or by creating or retaining a back pressure, and the flow may be divided into equal flows or be combined equally based on the flow direction through the valve. In contrast, the T-junctions fluidly connect hydraulic motors at the same fluid pressures such that it is an open connection, and the motors are able to use the fluid flow as needed to rotate, which may be at different relative speeds. The T-junctions and divider-combiner valves are also arranged such that one or two wheels of the vehicle may lose traction and be in a “free-wheel” state without runaway, and the motors for the remaining wheels continue to receive pressurized fluid flow to propel the vehicle.

The circuit150ofFIG. 5therefore allows for a significant reduction of drive pressure in a turn compared to a conventional system, and the engine is less likely to stall in a turn, particularly when the vehicle is traversing a grade. Also, as fluid flow is distributed in a controlled manner between the pairs of hydraulic motors and wheels, the wheels are not fighting each other and so are less likely to begin slipping when traction is limited. Additionally, it may be possible to use a smaller engine with the vehicle with circuit150, thereby providing cost and weight improvements as well as fuel economy. As the motor speeds are controlled, the resulting torque at the wheels is more balanced with the system150according to the present disclosure, the vehicle has a reduced tendency to disturb or tear up the ground surface in a turn. Additionally, as the fluid flow to pair of wheels is more balanced and the speeds of the motors is controlled, and the required drive pressures are lower, the hydraulic fluid undergoes less heating in the circuit150ofFIG. 5compared to a conventional circuit, thereby increasing the duty cycle of the vehicle, and also allowing for downsizing or removal of a hydraulic fluid cooler in the vehicle.

FIG. 6illustrates a variation of the hydraulic traction circuit150for use with the vehicle10and hydraulic system40ofFIGS. 1-3according to another embodiment of the present disclosure. Reference numbers inFIG. 6are the same reference numbers inFIGS. 1-4for elements that are the same or similar. The hydraulic traction circuit150ofFIG. 6provides for similar flow division and recombination as that described above with reference toFIG. 5via the use of alternative components.

InFIG. 6, two closed loop drive pumps replace the single drive pump42and the flow divider-combiner valve152in the circuit150ofFIG. 5. In one example, a first pump212and a second pump214are provided as illustrated to pump the fluid in the circuit, and pumps212,214additionally provide the function of a flow divider-combiner assembly210. The pumps212,214may be mechanically linked to one another such that they rotate at the same speed. The pumps212,214share a common port216or fluid line on one side and have separate ports218,220on the other side. For example, pump212is fluidly connected to ports216and218, and pump214is fluidly connected to ports216and220. The assembly210and pumps212,214are operable to provide fluid flow in either direction, e.g. into or out of port216such that fluid flows in a first direction, for example a forward vehicle direction, or in a second direction in the circuit150to rotate the hydraulic motors44in a second direction, for example a reverse vehicle direction.

The ports218,220, or fluid lines, may be connected by a bypass device222such as a valve or an orifice, to allow for bypass flow between ports218,220based on any pressure imbalances between the ports218,220and associated fluid lines.

In one examples, the assembly210may be provided without a drive link to an external load or other device such that the assembly210operates as a standalone component in the circuit150and vehicle. In other examples, the pumps212,214may be provided using two variable displacement pumps coupled to a single engine, or two fixed displacement pumps coupled to a single variable speed motor.

In other examples, other discrete hydraulic flow control components may be provided and arranged in the hydraulic circuit150in order to control the flow to each of the hydraulic motors, and for flow division and recombination, or to function similarly as the assembly210as described herein.

With reference to the fluid flow in the circuit being in the first direction, the assembly210is used to pump fluid through port218to one pair of diagonally arranged hydraulic motors174,176, and through port220to the other pair of diagonally arranged hydraulic motors166,168. The return flow from the pair of motors166,176is re-combined in one flow divider-combiner device154, and the return flow from the pair of motors168,174is re-combined in another flow divider/combiner device156. The return flow from both flow divider-combiner devices154,156combines at the dual pump inlet in the common fluid line or port216at a common fluid pressure.