Patent Application - MOWER WITH FOUR WHEEL STEERING AND HYDRAULIC FOUR WHEEL DRIVE > Description
This invention relates to a riding mower for cutting grass, the mower having a hydraulic drive system for propulsion in forward and reverse.
Riding mowers are well known for use in cutting grass in relatively wide swaths over a large area. Hydraulic drive systems powered by an internal combustion engine or other power source carried on the mower are also well known for propelling the mower in forward or reverse. Such drive systems often comprise individual hydraulic motors that engage each of the ground engaging wheels of the mower such that every wheel on the motor is a drive wheel. For example, mowers are known having four ground engaging wheels powered by four hydraulic motors to provide four wheel drive (4WD).
Different hydraulic fluid supply circuits can be used to power a 4WD system. One common system is an all parallel circuit in which the hydraulic motors are all connected in parallel to the hydraulic fluid source, typically a single hydraulic pump. Other systems comprise various combinations of series-parallel circuits in which some motors are connected in parallel and others are connected in series.
Each of these systems has its own advantages and disadvantages. Parallel connections provide an inherent differential effect since the motors driving the wheels on the outside of a turn will receive more flow and can rotate faster than the motors driving the wheels on the inside of the turn. Series connections can enhance traction in certain circumstances, but lose the inherent differential effect provided by a parallel connection. Series connections for mowers must be provided with some way to vary the flow passing through the motors on the insides and outsides of a turn, or alternatively to allow the wheels to overrun the motors, to avoid scrubbing of the wheels during a turn.
Various ways have been proposed to allow such flow variation in hydraulic circuits that incorporate series connections between the motors. Actively controlled valves can be used to port more flow to the motors for the wheels on the outsides of the turn as compared to motors for the wheels on the insides of the turn. However, this requires such valves to be present along with a control system for operating the valves as the mower turns. Overrunning clutches and or overrunning hydraulic check valves can be used to allow the wheels to overrun the speed of rotation of the motor shafts when necessary.
Most of the solutions known in the art for providing flow variation in circuits having series connections have been developed for use when only a single pair of the wheels on the mower, typically the front wheels, are steered. The wheels in the non-steered pair of wheels, typically the rear wheels, do not follow dramatically different paths about the center of the turn, and so the flow variation needed for the motors driving the non-steered wheels is not that extreme. However, in a mower with a pair of steerable front wheels and a pair of non-steered rear wheels, the turning radius of the mower is relatively large. Thus, it can be difficult to maneuver the mower in tight spaces without having the deck hit an obstruction, such as a wall or a tree, which the operator is attempting to mow around. Typically, the operator can get close to the obstruction, but may need to leave an uncut strip or swath of grass next to the obstruction in order to avoid hitting it.
It would be an advance in the art to provide a riding mower with four wheel steering (4WS) to shorten the turning radius and have the center of the turn be located at approximately the outer lateral edge of the cutting deck that is adjacent the obstruction. This would permit close cutting up to an obstruction without hitting the obstruction and without leaving any uncut strips of grass. However, in such a 4WS mower with a hydraulic 4WD system, the Applicants have discovered that the demand for different amounts of fluid in the motors driving the various wheels is greatly exacerbated in tight turns. In other words, the Applicants have found that the variation in the amounts of fluid needed by the various motors driving the various wheels greatly increases over the variations normally present in a two wheel steer mower. This invention is directed to providing such a mower, namely a riding mower having 4WS and a hydraulic 4WD system that incorporates at least some series connections between the motors, which can accommodate the necessary flow variations to the motors in a simple and cost-effective manner.
One aspect of this invention relates to a mower with four wheel drive. The mower comprises a frame supported for rolling over the ground by four ground engaging drive wheels comprising a pair of laterally spaced front drive wheels and a pair of laterally spaced rear drive wheels. A grass cutting device is carried on the frame. A hydraulic drive system propels the drive wheels. The drive system comprises a plurality of individual hydraulic drive motors for rotating the drive wheels with a single drive motor being operatively connected to each of the drive wheels such that a pair of front drive motors and a pair of rear drive motors are provided in the drive system. The drive system further comprises a hydraulic fluid supply circuit that includes a pair of identical serial flow circuits. Each serial flow circuit comprises a hydraulic pump that serves as a source of pressurized hydraulic fluid, the pump having an outlet that is directed in a serial manner first through a rear drive motor and then through a front drive motor for a pair of rear and front drive wheels on opposite sides of the frame, the front drive motor having an outlet that is serially connected by a return line back to an inlet of the pump.
One aspect of this invention relates to a mower with four wheel drive. The mower comprises a frame supported for rolling over the ground by four ground engaging drive wheels comprising a pair of laterally spaced front drive wheels and a pair of laterally spaced rear drive wheels. A grass cutting device is carried on the frame. A hydraulic drive system propels the drive wheels. The drive system comprises a plurality of individual hydraulic drive motors for rotating the drive wheels with a single drive motor being operatively connected to each of the drive wheels such that a pair of front drive motors and a pair of rear drive motors are provided in the drive system. The drive system further comprises a pair of serial hydraulic flow circuits with each serial flow circuit directing pressurized hydraulic fluid in a serial manner through one front drive motor and one rear drive motor. One drive motor in each serial flow circuit is provided with a return recirculation path protected by a check valve which check valve opens to allow hydraulic fluid leaving the one drive motor through an outlet thereof to flow back to an inlet of the front drive motor to permit the one drive motor to speed up during turns of the mower if the drive wheel to which the one drive motor is coupled is on an outside of the turn. The return recirculation paths in the pair of serial flow circuits are shut off when the mower is being operated in reverse or when the mower is being braked by the operator retarding the speed of rotation of the drive motors such that hydraulic braking is maintained on the drive wheels whose motors have the return recirculation paths.
This invention will be described more completely in the following Detailed Description, when taken in conjunction with the following drawings, in which like reference numerals refer to like elements throughout.
Referring first to
Mower 2 is equipped with a steering wheel 12 adjacent operator's seat 6 to allow the operator to steer mower 2. A four wheel steering (4WS) system is used on mower 2 such that turning or rotating steering wheel 12 causes each wheel 8 on mower 2 to pivot about a substantially vertical axis.
In a full right hand turn as shown in
A hydraulic four wheel drive (4WD) system is provided on frame 4 for powering or driving each wheel 8. In other words, each wheel 8 of mower 2 is not only a support wheel but a drive wheel as well.
In the 4WD system of this invention, each drive wheel 8 is powered by its own individual hydraulic motor 20. Thus, left rear drive wheel 8lr is powered by a left rear hydraulic motor 20lr, left front drive wheel 8lf is powered by a left front hydraulic motor 20lf, right front drive wheel 8rf is powered by a right front hydraulic motor 20rf, and right rear drive wheel 8rr is powered by a right rear hydraulic motor 20rr. Each motor 20 is carried on its corresponding drive wheel 8 to pivot with drive wheel 8 as drive wheel 8 pivots about its vertical steering axis y. Each drive motor 20 has an output shaft 22 that rotates the adjacent drive wheel 8. Note that the representation of
Referring first to
Each pump 24 is connected to a pair of drive wheels 20 in a series circuit 30. Thus, for example, first pump 24a of first series circuit 30a has an output line 32 that serially connects to the inlet of left rear drive motor 20lr, the outlet of left rear drive motor 20lr is then serially connected by a series line 34 in a catty-corner manner to the inlet of right front drive motor 20rf, and then the outlet of right front drive motor 20rf is serially connected by a return line 36 to the inlet of first pump 24a. The flow from first pump 24a through the two drive motors serially supplied by first pump 24a has been shown by the arrows A in
The basic pump and wheel motor plumbing shown in
In addition, even in the unlikely event that both drive wheels 8 in one of the series circuits 30 simultaneously lose traction and slip, the presence of the other series circuit 30, namely the other pump that is also serially powering drive motors 20 for the remaining pair of catty-corner arranged drive wheels 8, provides a redundancy in traction that is likely to keep mower 2 moving. It is extremely unlikely that all four drive wheels 8 will find themselves in conditions in which they all slip at once. At least one drive wheel 8, and more likely more than one drive wheel 8, in at least one of the series circuits 30 is likely to retain traction. Thus, mower 2 of this invention will have very good traction even in adverse conditions.
When mower 2 is operating in straight ahead mode with all four drive wheels 8 having traction, the hydraulic flow through the two series circuits 30a and 30b will be equal. Pumps 24 are preferably sized identically to one another. Each pump 24 will output the same flow and this flow will pass serially and equally through each drive motor 8 in the pair of drive motors fed by the pump. The representation of
Referring now to
In order to accommodate these substantially different flow requirements, the hydraulic circuit includes a pump output crossover path 40 placed between the output lines 32 of the first and second pumps with a first orifice 42 being placed in pump output crossover path 40. First orifice 42 has a fixed diameter that is sized to allow a portion, but not all, of the flow from second pump 24b to crossover and join the flow from first pump 24a to supply the additional flow requirements of left rear drive motor 20lr. This additional flow in pump output crossover path 40 is represented by the arrows B in
Now, considering the two front drive motors, the flow output from second pump 24b, which is headed towards left front drive motor 20lf, has been reduced by the amount of the pump crossover flow B described in the previous paragraph. However, left front drive motor 20lf, to which this reduced flow is being directed, actually needs the most flow of all drive motors 20. Accordingly, there is a rather large deficit in flow that must be made up or compensated for in some fashion.
The flow deficit for left front drive motor 20lf is made up from various sources. The first increment of deficit flow makeup arises from the realization by the Applicants that the amount of flow exiting left rear drive motor 20lr, which is receiving 2.40× due to the pump output crossover path 40, is substantially more than the amount of flow, i.e. 1.56×, required by right front drive motor 20rf. This abundance or excess of flow is siphoned off or removed in a series crossover path, represented generally as 44, that bridges across the series flow lines 34 connecting the front and rear drive motors in each series circuit 30.
Series crossover path 44 includes two parts: 1.) a first crossover flow sub-path 46 through a second orifice 48 of fixed diameter; and 2) a second crossover flow sub-path 50 through a bidirectional pressure relief valve 52 (or a pair of oppositely opening pressure relief valves 52). In a maximum right hand turn, enough flow can crossover between the series circuits through the first and second crossover flow sub-paths 46 and 50 such that substantially the entire excess of flow exiting the outlet of left rear drive motor 20lr is diverted into the other series circuit 30b, namely into the fluid supply line going to left front drive motor 20lf. Referring to
However, due to the extremely different amounts of flow required by the various drive motors, the series crossover flow C, while substantial and representing about 50% of the capacity of the pump 24, is still not enough to provide the flow requirements for left front drive motor 20lf. Accordingly, a return recirculation path 60 is provided between the outlet and the inlet of left front drive motor 20lf, effectively allowing enough of the return flow passing away from left front drive motor 20lf to be recirculated back to the inlet of left front drive motor 20lf. The flow in return recirculation path 60, as represented by the arrows D in
In many slight or moderate right hand turns of mower 2, the return recirculation flow D described above will not occur. In such slight or moderate turns, the amount of flow from the series crossover flow C when added to the flow exiting right rear drive motor 20rr will be enough to supply left front drive motor 20lf. Only in severe right hand turns or in a maximum right hand turn will there be a need for check valve 62 in return recirculation path 60 to open to add the additional return recirculation flow D to the crossover flow C.
In the situation shown in
Each of the return recirculation paths 60 for series circuits 30a and 30b can be placed on or off by a piloted on-off valve 70. One side of on-off valve 70 receives a pilot pressure through a fourth orifice 72 from the output side of one pump 24 as indicated by the dotted pilot pressure supply line 74 in
However, when mower 2 operates in reverse or when mower 2 is being braked by the operator by reducing the depression on accelerator pedal 28, the two on-off valves 70 will close as the combination of the pressure along line 76 and the bias of spring 78 will now be sufficient to move valves 70 to their closed positions. In such closed positions, return recirculation paths 60 are now shut off and cannot be used to bridge between the lines 34 and 36 around the respective right front drive motor 20rf and left front drive motor 20lf. This ensures that hydraulic braking will be maintained on all four drive wheels, both the front drive wheels and the rear drive wheels, when operating in reverse or when the operator consciously wishes to slow down in forward by letting up on accelerator pedal 28. Hydraulic braking will be maintained on the front drive wheels since the fluid recirculation path 60 around each front drive wheel, which otherwise would permit a freewheeling of each front drive wheel, is closed and thus cannot supply the additional recirculation fluid needed to accomplish this freewheeling. The front drive wheels 8 are compelled to rotate at whatever speed is being commanded by the operator and hydraulic braking is thus maintained on the front drive wheels in reverse or when braking.
Various modifications of this invention will be apparent to those skilled in the art. Thus, the scope of this invention will be limited only by the appended claims.