Load based suspension motion limiting

[0001] This document claims priority based on U.S. provisional; application Ser. No. 60/631,558, filed Nov. 29, 2004, and entitled LOAD BASED SUSPENSION MOTION LIMITING, under 35 U.S.C. 119(e).

[0002] This applies to an articulated crawler dozer with an independent suspension system.

[0003] conventional construction vehicles (dozers, loaders, backhoes, skid steers, graders, etc) do not usually have cushioning suspension systems but are, at most, equipped with pneumatic tires. The consequence is that the machine ride can be very harsh dependant upon the operating conditions of the machine. Presented herein is an articulated dozer with an independent suspension system that reduces the harshness of the ride.

[0004] Agricultural tractors are increasingly incorporating suspension systems to reduce the shock to the vehicle and increase the operational speed of the vehicle.

[0005] Traditionally, blade equipped machines such as crawlers or graders are structurally rigid. Such rigidity is desirable to prevent vertical motion in response to changing soil conditions. The cutting edge is typically angled back at the top so that it will shave off material when elevated material is contacted. A consequence of this characteristic is that a vertical force is generated on the blade cutting edge when hard soil conditions are encountered. If the machine is not sufficiently rigid, the blade will lower and dig into the ground under these conditions. When soft soil is encountered and the vertical force reduced, the blade will tend to rise to a higher elevation. An analogy can be made to a plane that is used in woodworking. The tool shaves off high regions without gouging, and moves over low regions without any affect to the material. Thus, the addition of suspension to a machine such as this creates a situation that is counter to the desired conditions stated above. The suspension may allow the blade to move up or down based on the load encountered from the contours of the ground.

[0006] An exemplary articulated vehicle using the invention includes a front lower A-frame and a rear lower A-frame as well as an articulated chassis having a front portion and a rear portion. The front and rear A-frames are pivotally attached to the articulated chassis via ball joints, the point of attachment for the front lower frame being forward of the chassis articulation joint and the point of attachment for the rear lower frame being rearward of the chassis articulation joint. Relative lateral movement between the lower A-frames and the portions of the articulated chassis to which they are attached are constrained due to pan hard rod connections between the A-frames and the articulated chassis at each end of the articulated chassis. Toward each end of the chassis two front suspension cylinders and two rear suspension cylinders situated between the front chassis potion and the front A-frame and between the rear chassis portion and the rear A-frame, respectively, support the articulated chassis above the A-frames allowing relative vertical movements between the A-frames and the chassis. In order to control vehicle roll, the head end of the right front suspension cylinder is hydraulically connected to the rod end of the left front suspension cylinder and the head end of the left front suspension cylinder is hydraulically connected to the rod end of the right front suspension cylinder. This arrangement reduces the effective cylinder area to the rod area of the cylinder and creates a higher pressure in the system which is desirous for improved suspension control.

[0007] Due to the structural requirements of the invention, the A-frames and track systems may comprise a significant portion of the vehicle weight. When an upward force is applied, the suspension system can allow the chassis to move upward and away from the track system until the suspension system reaches equilibrium or the end of travel for the suspension cylinders. This travel produces a looseness in the suspension and does not allow a full blade load to be generated when desired.

[0008] Naturally, such a suspension system will tend to allow variations in the height of the blade above the ground as the blade encounters greater and lesser resistance resulting in varying vertical loads on the blade and the suspended chassis of the vehicle. The suspension height of the chassis will tend to vary with the vertical load generated causing undesirable results with respect to blade control.

[0009] The head end pressure for each of the hydraulic suspension cylinders is determined by the load on that cylinder divided by the rod area of that cylinder in the system described. This pressure is also exerted on at least one of a control valve and an accumulator.

[0010] When a force on the cylinder is removed, the pressure drops to approximately zero. If the lifting force continues to exist, the cylinder can continue to extend by allowing the oil that is in the rod of one cylinder to flow to the head of the other cylinder. This is an undesirable condition.

[0011] The invention overcomes this problem by adding a standard pilot operated check valve to the rod end port of each cylinder. The pilot end of each check valve is connected to the head end of each corresponding cylinder. When the suspension is operating in a normal (weight bearing) condition, the pressure on the head end of the cylinder holds the check valve in an open position and the suspension operates normally with no affect from this valve. When the weight on the cylinder is eliminated, the pressure in the head end of the cylinder is reduced to approximately zero. In this condition the check valve closes and the oil is trapped in the rod end of the cylinder. This enables the suspension to carry loads in the opposite direction, and eliminates the free travel mentioned earlier. When the suspension load returns to the normal (weight bearing) condition, the check valve opens and the suspension resumes normal operation.

[0021] The exemplary embodiment of the invention described herein is applied to a crawler dozer with four independent track systems. In this configuration, the track systems are mounted such that they can move in a way that they can follow the contour of the ground. Each of the tracks pivots about a drive wheel.

[0022] FIG. 1 illustrates a vehicle in which the invention may be used. The particular vehicle illustrated in FIG. 1 is a four track articulated dozer 10 having a front portion 20 a rear portion 30; an articulation mechanism 40 between the front portion 20 and the rear portion 30; first and second track systems 50, 60; and third and fourth track systems 70, 80. The front portion 20 includes a blade 22 and a blade mounting frame 23 as well as an operator cab 21.

[0023] A first A-frame 200 is pivotally connected to both the first and second track systems 50, 60, i.e., rocker arms 51, 61 at first and second track frame pivots 51a, 61a. This first A-frame 200 is connected to a front chassis portion 100 primarily at the top of the “A”, i.e., at a narrower portion of the first A-frame 200, with a first spherical ball joint 101. This first spherical ball joint 101 is located in proximity to but forward of the articulation joint 40. Laterally the first A-frame 200 is connected to the vehicle frame with a first linkage (first pan-hard rod) 300 (see FIG. 3) to keep the position of the first A-frame approximately centered under the front chassis portion 101, restricting relative lateral motion. The front chassis portion 100 is vertically connected to the first A-frame 200 by a first suspension cylinder 231 having a first cylinder head end 231a and a first cylinder rod end 231b; and a second suspension cylinder 232 having a second cylinder head end 232a and a second cylinder rod end 232b. Each of the first and second suspension cylinders 213, 232 is, respectively, connected, hydraulically, to hydraulic accumulators 251, 252. A mechanism senses the position of the first A-frame 200 relative to the front chassis portion 100 at each cylinder location, and controls the vehicle height, via hydraulic balancing circuit 240, by adding or removing hydraulic fluid to and from the first and second suspension cylinders 231, 232 on a continuous basis. These cylinders primarily support the vehicle weight.

[0024] It is also desired to control vehicle roll position at this front axle 203. To accomplish this, the first cylinder head end 231a is hydraulically connected to the second cylinder rod end 232b. Conversely the second cylinder head end 232a is hydraulically connected to first cylinder the rod end 231b of the first cylinder 231. This methodology reduces the effective cylinder area to be equal to the rod area of the cylinder. This creates a higher pressure in the system which is desirous for improved suspension control.

[0025] The first and second suspension cylinders 231, 232 are attached to the first A-frame 200 at respective points behind the first and second track frame pivots 51a, 61a so that they operate at an increased pressure level. This helps contribute to the roll stability mentioned above by increasing the pressure proportionally. However, when a force on the cylinders 231, 232 is removed, the pressure drops to approximately zero and if a lifting force continues to exist, the cylinders 231, 232 can continue to extend by allowing the oil in the rod ends 231b, 232b to flow to the head ends 232a, 231a respectively, a condition of instability. This instability is overcome by adding standard pilot operated check valves 231c, 232c at the rod ends 231b, 232b of the suspension cylinders 231, 232 respectively. The pilot lines of check valves 231c, 232c are connected to the head ends 231a, 232a of the suspension cylinders 231, 232 respectively.

[0026] A second A-frame structure 210 is pivotally connected to both the third and fourth track systems 70, 80, via rocker arms 71,81, through third and fourth track frame pivots 71a, 81a. As illustrated in FIG. 2, the second A-frame 210 is connected to a rear chassis portion 210 primarily at the top of the “A”, i.e., at a narrower portion of the second A-frame 210, via a spherical ball joint 211. This connection point is located in proximity to but rearward of the articulation joint 40. Laterally the second A-frame 210 is connected to the rear chassis portion 110 with a linkage (pan-hard rod) 310 to the second A-frame 210 approximately centered under the rear chassis portion 110. The rear chassis portion 110 is vertically connected to the second A-frame 210 by a third suspension cylinder 233 having a third cylinder head end 233a and a third cylinder rod end 233b; and a fourth suspension cylinder 234 having a fourth cylinder head end 234a and a fourth cylinder rod end 234b. These suspension cylinders 233, 234 are hydraulically connected together and are, respectively, flow connected to hydraulic accumulators 253, 254. A mechanism senses the position of the second A-frame 210 relative to the second chassis portion 110 at a point midway between the third and fourth suspension cylinders 233, 234 indicating the average location, and controls the vehicle height, via hydraulic balancing circuit 241, by adding or removing hydraulic fluid from the third and fourth suspension cylinders 233, 234 on a continuous basis.

[0027] It is desired to have the rear axle oscillate to ensure all 4 tracks maintain ground contact at all times. This is done by connecting the third and fourth head ends 233a, 234a of the third and fourth cylinders 233, 234 together to allow oil to flow from one to the other as needed. The third and fourth rod ends 233b, 234b of the third and fourth cylinders 233, 234 are also connected together likewise. Thus, the third and fourth cylinder head ends 233a, 234a are hydraulically connected and the third and fourth cylinder rod ends 233b, 234b are hydraulically connected (see FIG. 7).

[0028] First and second balancing circuits 240,241 are hydraulic circuits that maintain the nominal distances between the front chassis portion 100 and the front A-frame 200 and the rear chassis portion 110 and the rear A-frame 210. A load sense line directs a hydraulic pump to provide fluid at an appropriate pressure to maintain the distances.

[0029] The third and fourth cylinders 233,234 are attached to the second A-frame 210 at a point behind the track frames, i.e., rocker arm pivots 71, 81 so that they operate at a reduced pressure level. This lowers the pressure of the system for a smoother ride. In the exemplary embodiment illustrated in FIGS. 2 and 6, the third hydraulic suspension cylinder 233 is connected to the rear chassis 110 at pivot 112a and to the second A-frame 210 at pivot 212a. Similarly, the fourth hydraulic suspension cylinder 234 is connected to the rear chassis 110 and the second A-frame at pivots 112b and 212b respectively. This lowers the pressure of the system for a smoother ride.

[0030] The blade mounting structure, referred to as the C-frame 23, is operatively attached to the first A-frame 200. This ensures the blade level (right to left with respect to the operator) will be consistent with the tracks and not affected by vehicle chassis motion enabled by the suspension system motion.

[0031] The front suspension operates, in part, as follows. When the suspension is operating in a normal (weight bearing) condition, the pressures on the head ends 231a, 232a of the front suspension cylinders 231, 232 holds the check valves 231c, 232c in open positions allowing fluid to flow between the rod ends 231b, 232b and the head ends 232a and 231a respectively. When forces on the cylinders 231, 232 are removed, the pressures at the head ends 231a, 232a of the suspension cylinders 231, 232 reduce to approximately zero psi, the pressure communicated to the pilot operated check valves 231c, 232c. In this condition the check valves 231c, 232c close and hydraulic fluid is trapped in the rod end of the cylinder, enabling the suspension to carry loads in the opposite direction and eliminating free travel.

[0032] Having described the illustrated embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.