Selectable flow divider drive system

A selectable flow divider drive system includes a hydraulic fluid reservoir and a plurality of drive motors in fluid communication with the hydraulic fluid reservoir. A hydraulic pump is connected between the hydraulic fluid reservoir and the plurality of drive motors and directs hydraulic fluid from the hydraulic fluid reservoir to the plurality of motors. The hydraulic pump is operable in a high flow condition and a low flow condition. A flow divider component is interposed between the hydraulic pump and the plurality of motors. The flow divider component selectively divides hydraulic fluid flow to each of the plurality of drive motors, where a flow divider of the flow divider component is sized for the low flow condition of the hydraulic pump. A bypass valve is disposed upstream of the flow divider that selectively bypasses the flow divider when the hydraulic pump is operated in the high flow condition.

BACKGROUND OF THE INVENTION

The invention relates to aerial work platforms and, more particularly, to a hydraulic drive system for an aerial work platform.

Aerial Work Platforms (AWPs) typically use hydrostatic drives systems. They could be two- or four-wheel drive with a gear reduction and a hydraulic drive motor at each driven wheel. A hydraulic pump is typically driven directly from the engine and includes a network of traction valves or flow dividers to split the flow to each driven wheel. These flow dividers are intended to divide the total flow among each driven wheel even if one wheel would lose traction. The flow dividers typically include provisions for leakage or bypass of perfect flow division to allow for varying wheel speeds, as would be required for steering utilizing the Ackerman principle. Otherwise, wheels could be forced to slide during turns or the engine could stall.

If this bypass is too large, the flow division is compromised as a large portion of the flow could be lost to leakage. This situation is also seen when the flow dividers are sized for high flow conditions and used for the low flow conditions as the resulting leakage rate of the dividers can be a much larger portion of the available flow. This is of particular importance with AWPs since much of the driving is done at low flow conditions and with a migrating center of mass potentially unloading the ground contact load of the off side tires.

AWPs traditionally are designed with these traction valves sized for the high flow rate needed to achieve a max drive speed, which influences the efficiency and power requirements of not only high flow conditions but also low flow performance.

BRIEF SUMMARY OF THE INVENTION

The flow dividers according to preferred embodiments are sized for the low flow condition and are automatically or manually bypassed during high flow conditions. This arrangement allows for a more cost effective system as well as for improved flow division in low flow conditions, which is of particular importance for operation of an AWP.

In an exemplary embodiment, a selectable flow divider drive system includes a hydraulic fluid reservoir and a plurality of drive motors in fluid communication with the hydraulic fluid reservoir. A hydraulic pump is connected between the hydraulic fluid reservoir and the plurality of drive motors and directs hydraulic fluid from the hydraulic fluid reservoir to the plurality of motors. The hydraulic pump is operable in a high flow condition and a low flow condition. A flow divider component is interposed between the hydraulic pump and the plurality of motors. The flow divider component selectively divides hydraulic fluid flow to each of the plurality of drive motors, where a flow divider of the flow divider component is sized for the low flow condition of the hydraulic pump. A bypass valve is disposed upstream of the flow divider that selectively bypasses the flow divider when the hydraulic pump is operated in the high flow condition.

The system may additionally include a bypass solenoid valve in fluid communication with the bypass valve, where the bypass solenoid valve acts to selectively close the bypass valve. Preferably, the bypass valve is biased open. A filter may be interposed between the hydraulic pump and the bypass solenoid valve.

In one arrangement, the system includes four drive motors, where the flow divider component includes a plurality of flow dividers including one first stage flow divider and two second stage flow dividers, in a dividing mode, the first stage flow divider dividing the hydraulic fluid flow for input to the two second stage flow dividers, and the two second stage flow dividers further dividing the hydraulic fluid flow for input to the four drive motors. In a combining mode, exhaust fluid from the motors is directed to the second stage flow dividers, which combine the fluid for input to the first stage flow divider.

The system may also include a check valve associated with each of the drive motors that maintains a preset pressure through the drive system.

In another exemplary embodiment, an aerial work platform machine includes a vehicle chassis supporting a liftable work platform, a plurality of wheels secured to the vehicle chassis, and the selectable flow divider drive system for driving the plurality of wheels.

The aerial work platform machine may additionally include operator controls coupled with operating components for lifting and lowering the work platform and for driving the aerial work platform machine, and a control system receiving input from the operator controls and controlling operation of the operating components according to the input. The control system communicates with the selectable flow divider drive system and controls the selectable flow divider drive system according to operating parameters of the aerial work platform machine. The control system may control output of the hydraulic pump between the high flow condition and the low flow condition based on the operating parameters of the aerial work platform machine. The control system may be configured to energize the bypass solenoid valve when the output of the hydraulic pump is changed from the high flow condition to the low flow condition.

An operator preferably controls output of the hydraulic pump between the high flow condition and the low flow condition via the operator controls.

In yet another exemplary embodiment, a method of driving a plurality of hydraulic motors using a selectable flow divider drive system includes the steps of (a) interposing a flow divider component between a hydraulic pump and the plurality of hydraulic motors, the flow divider component selectively dividing hydraulic fluid flow to each of the plurality of drive motors; (b) selectively operating a hydraulic pump in a high flow condition and a low flow condition, the hydraulic pump directing hydraulic fluid from the hydraulic fluid reservoir to the plurality of hydraulic motors; and (c) bypassing a flow divider of the flow divider component when the hydraulic pump is operated in the high flow condition, wherein the flow divider is sized for the low flow condition of the hydraulic pump.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1shows an exemplary aerial work platform10for which the drive system of the invention is suitable. The machine10includes a chassis12supporting drive wheels14. In the exemplary AWP shown, a turntable16is supported on the chassis12. The turntable16supports a boom configuration18, and a work platform20is supported at a distal end of the boom configuration18. Operator controls22on the work platform20enable the operator to position the boom and drive the vehicle from the work platform20. The operator controls22communicate with a control system24(shown schematically inFIG. 1) that controls operating parameters of the vehicle10based on input from the operator controls22as well as external sensors and operating conditions.

FIG. 2is a schematic drawing of the selectable flow divider drive system for controlling drive functionality of the vehicle10. A hydraulic motor30is associated with each wheel14for driving the wheel14in forward and reverse directions. The hydraulic motors30are driven with hydraulic fluid from a hydraulic fluid reservoir32via a pump34. The pump34operates under the control of the controller24and circulates hydraulic fluid from the reservoir32to the hydraulic motors30via a flow divider component36. The pump34is operable in a high flow condition (for max speed) and a low flow condition (for max torque/traction).

With continued reference toFIG. 2, the flow divider component36includes a plurality of traction valves or flow dividers38,40interposed between the hydraulic pump34and the plurality of hydraulic motors30. The flow dividers38,40selectively divide the hydraulic fluid flow to each of the plurality of drive motors30. In a preferred arrangement, one first stage flow divider38first divides the hydraulic fluid flow for input to two second stage flow dividers40, which subsequently divide the hydraulic fluid flow for input to the four drive motors30. In this flow direction, the flow dividers38,40are in a dividing mode. When the flow direction is reversed, where the exhaust oil from the motors30enters the outlet ports of the second stage flow dividers40, the second stage flow dividers40combine the fluid and send the combined flow to the outlet ports of the first stage flow divider39. This flow direction is called a combining mode. In this manner, the vehicle can maintain traction control in both the forward and reverse directions. The flow dividers are also known as flow divider/combiners. As would be appreciated by those of ordinary skill in the art, the flow divider component36could be operable with a single flow divider providing hydraulic drive flow for only two wheels.

The flow divider component36also includes a bypass valve42disposed upstream of the flow dividers38,40. The bypass valve42is preferably biased open and serves to selectively bypass the flow dividers38,40when the hydraulic pump34is operated in the high flow condition. A bypass solenoid valve44is provided in fluid communication with the bypass valves42. When energized, the bypass solenoid valve44acts to selectively close the bypass valves42.

Other components of the hydraulic circuit include a filter46interposed between the hydraulic pump34and the bypass solenoid valve44. The filter46forms part of a circuit of hydraulic fluid circulating the hydraulic pump34. A leakage reservoir48collects output leakage from the bypass valves42, which occurs when the bypass valve solenoid44is energized, sending pressurized fluid to the bypass valve42, which shifts the bypass valve42causing leakage in the spring chamber to vent to the reservoir48. Additionally, a check valve50is associated with each of the drive motors30that serves to maintain a preset pressure through the drive system. The check valves50are configured such that upon the occurrence of a pressure loss in the system, and the pressure drops below a preset value, the check valves50are opened to maintain pressure and prevent cavitation. An exemplary preset pressure is 377 psi.

Leakage drains52are also provided for the motors30.

As noted, the flow dividers38,40are sized for the low flow condition of the hydraulic pump34. As such, during high flow conditions, the flow dividers38,40are manually (by the operator) or automatically (by the control system24) bypassed. Thus, when driving at max speed, i.e., high flow condition (in an exemplary construction, about 3.1 mph), the flow dividers38,40are bypassed by the bypass valves42. This reduces pressure losses in the system, which in turn reduces the engine horsepower required, allowing a smaller engine to be selected for the application or more power available for increased performance. This also reduces heat in the hydraulic system, which in turn reduces leakage between components, thereby increasing the efficiency of the drive pump, drive motors, and related components.

When the operator manually selects the max torque position of the drive select switch on the operator control panel22, the drive pump34changes its displacement from high flow condition (e.g., 2.75 cu in.) to low flow condition (e.g., 1.55 cu in.). This reduction in flow changes the speed of the machine from MAX to LOW (e.g., 3.1 to 1.5 mph), thereby reducing the engine horsepower requirement. This happens within a two second ramp down time.

At the same time, the bypass solenoid valve44is energized, which sends pilot pressure to the traction bypass valves42. The pilot pressure shifts these valves42to their closed position, which blocks the flow path around the flow divider valves38,40. Fluid is now forced through the flow divider valves38,40to divide the flow. There is a two second delay in this transition, which allows the pump flow rate to be reduced accordingly, then the bypass valves42are closed.

As noted, the flow divider valves38,40are sized for the reduced flow rate. Traditional systems are not as efficient at lower flow rates because they are sized for higher flows. Smaller traction valves reduce manifold size and cost.

If the machine is “out of transport” (either the boom is raised or extended, or the turntable is swung substantially off center), the control system24automatically changes from max speed to max torque drive mode. This also activates the traction control system automatically as described in the manual operation above.

If the machine is driven on a slope or grade of 6° or more, the control system24automatically changes from max speed to max torque drive mode. This also activates the traction control system automatically. This allows the pump displacement to be reduced on grades which in-turn reduces the engine horsepower required.

Although not required, the system is further enhanced when combined with an oscillating axle system. When driving in the max speed condition, the flow dividers38,40are bypassed. If a wheel looses contact with the ground, the flow will not be distributed to the other wheels, and loss of performance will result. An oscillating axle increases the likelihood of maintaining contact of all four wheels with the ground.

FIG. 3shows exemplary conditions when the flow dividers38,40are bypassed. As can be seen, the conditions where bypass results are limited, and when combined with an oscillating axle, this bypassing would rarely hinder the operation of the machine. In this system, the axle is free to float when the machine is “in transport.”

The selectable flow divider drive system results in increased high speed efficiency. The ability to use a smaller engine results in reduced cost and reduced fuel consumption, while reduced heat generation results in extended oil life, a smaller hydraulic tank size, and improved high ambient temperature performance. Still further, the system results in improved low speed performance as less undesired flow divider leakage and thereby better wheel speed control. The use of smaller traction valves further reduces manufacturing costs. Additionally, changing the pump displacement to regulate the speed of the vehicle allows for the use of fixed motors, which is a further cost reduction. No extra valving is required to change motor displacement when using fixed motors, which also reduces costs. The hydraulic hosing in the drive system is sized for the max flow rate. In the max torque drive mode, the flow rate is reduced, so the pressure drop throughout the hydraulic hosing is reduced, thereby reducing heat and leakage in the system.