Abstract:
A suspension system for utility vehicles of the type in which the chassis is supported at one end on an oscillating axle with a central pivot, has the oscillation of this axle controlled by one or two extensible links each in the form of a hydraulic cylinder unit. The second vehicle axle has limited oscillation used to shuttle the spool of a directional control valve which determines the flow of pressurized fluid for operation of the hydraulic cylinder units(s). The control valve can also be electrically operated responsive to the closing of switches responsive to the oscillations of the second axle.

Description:
TECHNICAL FIELD 
     The present invention relates to suspension systems for utility vehicles of the type in which the chassis is supported at one end on an oscillating axle with a central pivot and is supported at the other end by a substantially fixed axle. 
     BACKGROUND OF THE INVENTION 
     The function of a suspension system with an oscillating axle is to allow for travel over irregular ground surfaces. However, in the case of vehicles such as those with a lift or rotating extensible boom, an oscillating axle has been a disadvantage for maximizing stability when the vehicle is stationary and the lift or boom are in operation. The object of the present invention is to provide an oscillating axle system in which there is a solid connection between the oscillating axle and the chassis when needed to maximize stability, and which also permits oscillation during driving of the associated vehicle without loss of stability. 
     SUMMARY OF THE INVENTION 
     In the practice of the present invention a chassis for carrying upper structures such as lifts and extensible booms is mounted on oscillating axles at both ends. These axles are centrally pivoted. One of the axle&#39;s oscillation is limited by fixed motion limiters while the other axle&#39;s oscillation is controlled by a variable length link or links preferably in the form of a hydraulic cylinder unit. When two hydraulic cylinder links are used each is powered to extend whereas when only one hydraulic link is used it is powered to extend and retract. The limited oscillation of the first axle is used to achieve sensing of which end of this axle has the greater load when an uneven loading condition exists. The tilt angle of the other axle relative to the chassis is then responsively adjusted to maximize stability by varying the length of the hydraulic link or links. The direction of the tilt is preferably indicated by a directional valve mounted between the chassis and the limited oscillation axle. This valve controls the direction of flow of pressurized fluid to and from the hydraulic link or links on the other axle. In an alternative arrangement the directional valve may be solenoid operated with the control signals being generated by normally open sensor switches closed by movements of the limited oscillation axle. 
     For ease of explanation in the detailed description and accompanying claims hereinwith presented, the limited oscillation axle will be spoken of as the rear axle and the oscillating axle with the hydraulic link or links will be spoken of as the front axle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic showing the general arrangement of wheels, axles, chassis, and hydraulic extensible links employed in the preferred embodiment of the present invention together with a preferred hydraulic system for controlling the extensible links; 
     FIG. 2 is a schematic of a second embodiment using a single extensible link; and 
     FIG. 3 is a schematic of a modified hydraulic control system for the first and second embodiments. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings, a vehicle chassis 10 is pivotally mounted at the center of front and rear axles 11, 12 at respective pivot pins 13, 14 extending in the fore and aft direction. The front axle 11 is supported by front right and left steerable wheels 15R, 15L and the rear axle has right and left wheels 16R, 16L. The four wheels may be directly driven by respective hydraulic motors. The steering mechanism and driving motors are not shown. 
     Oscillation of the rear axle 12 is limited to about one degree in either direction by stops 17R, 17L on the chassis. The direction in which the front axle 11 must be oscillated to maintain a balanced ground support condition is indicated by which stop is engaged. For example, if the right stop 17R bottoms out against the rear axle 12, this indicates that the included angle at the front axle oscillating pivot 13 between the right portion of the front axle 11 and the chassis 10 must be increased for a balanced ground support condition. 
     Sensing of the right or left oscillating direction of the rear axle 11 can be achieved in various ways. Two examples will be described hereinafter, one in which the oscillating direction is determined by a hydraulic sensor comprising a directional valve unit located between the chassis and rear axle at a location spaced from the oscillating pivot of the rear axle, and the other in which the oscillating direction is determined by electrical sensor switches positioned between the rear axle and chassis. In the first example the hydraulic cylinder unit comprising the sensor has a neutral partly extended position when the rear axle is in balanced position. Further extension of the unit indicates one oscillating direction and retraction from this neutral position indicates oscillation in the opposite direction. 
     It is preferred to have two front extension links, one on each side of the pivot axis of the front oscillating axle. When one of the links is extended the other link is retracted. However, as will be later described the system of the present invention can operate with only one front extension link. 
     The invention will first be described with the use of a directional control valve unit 30 between the rear axle 12 and the chassis 10 as indicated in FIG. 1, and with the use of two extension links between the chassis and front oscillating axle taking the form of hydraulic cylinder units 31R, 31L as indicated in FIG. 1. These two units are arranged with their cylinders pivotally connected to the chassis 10 and their piston rods pivotally connected to the front axle 11. The directional control valve 30 may be a 3-position, 4-way valve with an open center configuration. It has its casing pivotally connected at one end to the chassis 10, and has a spool with an extension link 30a pivotally connected to the rear axle 12 at a position offset from the center pivot 14. In FIG. 1 the spool is shown in its center no-flow position. 
     The directional control valve 30 controls flow between a pump/reservoir circuit 29 and a lock valve/check valve circuit 32 connected to the cylinder units 31R, 31L. The latter circuit includes a pair of lock valves 33R, 33L which are spring-loaded to a normal flow-blocking position as shown in FIG. 1, and are in parallel with respective check valves 34R, 34L located in by-pass lines 35R, 35L. The lock valves 33R, 33L are pilot operated and shifted to a flow-through position in opposition to their springs by pressurization of respective cross-over pilot lines 36R, 36L. Bleed lines 37R, 37L are provided at the spring end of the lock valves to bleed off any oil leakage in the lock valves. Flow connection between the lock valves 33R, 33L and the direction control valve 30 is made by lines 38R, 38L. These lines 38R, 38L are also connected by lines 45L-45R, respectively, with the lower piston rod ends of the cylinder units 31L, 31R. The upper piston ends of the cylinder units 31R, 31L are connected to the lock valves 33R, 33L by lines 46R, 46L. In case of undue pressure build-up in the system because of unusual thermal conditions, thermal release lines 47R, 47L connect lines 46R, 46L to the stop valves 33R, 33L to crack open the latter sufficiently to relieve excess pressure in the cylinders 31R, 31L. 
     The pump/reservoir circuit 29 has a pump 39, a reservoir 40, and a pressure relief valve 41. The latter is in parallel with the pump 39 to return pumped oil to the reservoir 40 from a pressure line 42 through a by-pass 43. The pressure line 42 leads to the directional valve 30, and a return line 44 leads from the directional valve 30 to the reservoir 40. Filters are preferably provided in the lines leading to and from the reservoir. 
     The directional control valve 30 functions, when not in its centered blocking position, to connect the pressurized supply line 42 in the pump/reservoir circuit 29 with line 38R or 38L in the lock valve/check valve circuit 32, and to simultaneously connect the reservoir line 44 in the pump/reservoir circuit 29 with whichever of the lines 38R or 38L is not connected to the supply line 42. When line 38R is pressurized, the check valve 34R is unseated and pressurized fluid from the pump 39 flows through supply line 42, directional control valve 30, line 38R, by-pass line 35R and line 46R to the upper end of the cylinder unit 31R. Simultaneously, the lower end of the cylinder 31R dumps to the reservoir 40 via line 45R, line 38L, valve 30, and line 44, and flow from line 38R moves through pilot line 36L and causes lock valve 37L to open. When this occurs the fluid in the upper end of cylinder unit 31L dumps via line 46L, line 38L, valve 30, and line 44 to the reservoir 40, and the lower end of cylinder unit 31L is charged from line 38R via line 45L, thereby retracting cylinder unit 31L as cylinder unit 31R is extended. 
     When the directional control valve 30 is shifted in the opposite direction, so that the line 38R is connected by the valve 30 with the dump line 44 rather than with the supply line 42, and so that the line 38L is connected by the valve 32 with the supply line 42 rather than with the dump line 44, the result is that check valve 34L is unseated and lock valve 33R is opened. Consequently, the cylinder unit 31L is extended and the cylinder unit 31R is correspondingly retracted. 
     When the vehicle is traveling over an uneven terrain the spool in the directional control 30 valve shuttles up and down such that rarely are the cylinder units 31R, 31L fully extended or retracted. The flow paths of the fluid in the lock valve/check valve circuit 29 are such that extension and retraction of the cylinder units is dampened. 
     Although it is preferred to provide a pair of variable length links for the front oscillating axle, one on each side of the oscillation axis, it is possible to use a single variable-length link in the form, for example, of a hydraulic double-acting cylinder with a powered extension and retraction rather than one side of the cylinder being vented to the reservoir as in the case of the double link arrangement previously described. 
     FIG. 2 is a schematic of the single link embodiment in which the link comprises a double-acting cylinder unit 131 pivotally connected to the chassis 10 and front axle 11 at a location spaced to the right (as shown) or left of the oscillation pivot 13 of the axle. A modified stop valve/check valve circuit 132 is provided eliminating the lines 45R, 45L from the circuit 32, and instead of a pair of spring-loaded lock valves being connected to a pair of hydraulic cylinder units as in the circuit 32 in the first embodiment, the circuit 132 in the second embodiment has a pair of normally-closed, spring-loaded lock valves 133E, 133R connected to opposite ends of the single hydraulic unit 131 by lines 144E, 144R. When one of the lock valves is in a flow-through condition the other lock valve is in a no-flow condition. This comes about by cross-over pilot lines 135R, 135E which, when pressurized, cause the respective lock valve 133R, 133E to shift to a flow-through position for flow away from the cylinder unit 131 to the reservoir. Lines 144E, 144R are also connected to by-pass lines 135E, 135R which contain check valves 134E, 134R and connect to lines 138E, 138R leading to the direction control valve 30. When the spool in valve 30 moves such as to connect the pump 39 in circuit 29 to line 138E via line 42 and valve 30, check valve 134E opens and exposes the upper piston end of the cylinder unit 131 to a supply of pressurized fluid. At the same time, stop valve 133R is opened by pressurization of pilot 135R to permit fluid on the lower rod side of the cylinder in unit 131 to dump to the reservoir 40 in circuit 29 via lines 144R and 138R, valve 30, and line 44. Likewise, when the spool in valve 30 moves such as to connect the output of the pump 39 to line 138R, check valve 134R opens and exposes the lower rod end of the cylinder unit 131 to a supply of pressurized fluid. At the same time, stop valve 133E is opened by pressurization of pilot 135E to permit fluid on the upper piston side of the cylinder in unit 131 to dump to reservoir 40 via lines 144E and 138E, valve 30 and line 44. 
     In an alternative embodiment shown in FIG. 3 the flow control valve 32 is replaced by a spring-centered flow control valve 132 which is operated by two solenoids 132R, 132L to control the direction of flow from the pump/reservoir circuit 29 to the hydraulic cylinder units 31R, 31L in the first embodiment or to the hydraulic cylinder unit 131 in the second embodiment. These two solenoids are controlled by respective normally open switches 50R, 50L arranged to be closed, respectively, responsive to engagement of the stops 17R, 17L on the rear axle due to right and left downward rocking of the chassis. Closing of switch 50R, for example, causes energizing of the solenoid 51&#39;R for a relay switch 51R in a power circuit including a lead 52R from battery 49 to the switch 50R, and a lead 53R from switch 50R to the solenoid 51&#39;R for a normally open relay switch 51R. A lead 54R connects between the relay switch 51R and the solenoid 132R so that the solenoid 132R is activated to shift the spool in the direction control valve 132 in a direction resulting in raising of the right front corner of the chassis relative to the right front wheel whenever switch 50R is closed. 
     Likewise, closing of switch 50L responsive to rocking of the chassis toward the left part of the rear axle causes activation of solenoid 132L by a power circuit like that previously described, and namely, one containing a solenoid 51&#39;L for a normally open relay switch 51L, a lead 52L from battery 49 to the switch 50L, a lead 53L from switch 50L to the solenoid 51&#39;L, and a lead 54L between the relay switch 51L and the solenoid 132L. 
     It will be appreciated that when the spool of the solenoid-operated direction control valve 132 is shifted responsive to closure of sensor switch 50R or 50L, the result is the same as when the spool of valve 32 is shifted responsive to downward or upward rocking, respectively, of the chassis relative to the right rear wheel of the vehicle. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.