FIELD OF THE INVENTION

[0001] The present invention relates to a brake valve for braking a vehicle, for example an agricultural tractor, in lightly loaded conditions.

BACKGROUND OF THE INVENTION

[0002] Agricultural tractors have traditionally included braking only on the rear axle, which for many years met relevant braking requirements. In time, agricultural tractors have increased in size and operated at higher speeds. Regulatory requirements have also been updated in view of these changes with an objective of maintaining operational safety. As a result, brake systems for modem tractors are typically designed with enough braking power to meet stopping requirements at the highest laden mass and at the tractor’s maximum rated speed.

[0003] When the brake pedal is applied, there is a short amount of pedal travel before pressure begins to build in the front and rear brakes. Brake pressure is directly proportional to pedal force and builds in a linear fashion on both front and rear axles. The brake systems of some agricultural tractors have a different ratio of brake pressure from the front to the rear axles. This is done primarily because the brakes on the front and rear axles may require different pressure to reach the required braking force on each axle.

[0004] An unintended effect of having braking on both axles is aggressive braking when speeds are low and the tractor is lightly laden. In these circumstances, the operator experiences discomfort and is unable to effectively and smoothly control the speed of tlie tractor. Accordingly, there remains a continued need for an improved system for controlling the braking of modern agricultural tractors when lightly loaded. In particular, there remains a continued need for minimizing excessive braking forces when brake pressure is applied.

SUMMARY OF THE INVENTION

[0005] An improved hydraulic brake system for an agricultural tractor is provided. The hydraulic brake system responds to forward movement of a brake pedal by increasing brake fluid pressure at rear wheel brakes and, after a predetermined amount of pedal travel, increasing the brake fluid pressure at the front wheel brakes. Brake fluid pressure at the front wheel brakes increases at a higher rate than at the rear wheel brakes to simultaneously achieve a maximum braking force at both front and rear axles in response to full depression of the brake pedal.

[0006] In one embodiment, the hydraulic brake system comprises a dual, tandem system having left and right pressure modulating valves. Each pressure modulating valve includes a primary brake valve and a secondary brake valve assembled in tandem within a cylinder housing. The primary brake valve is linked by a push rod to a brake pedal, through a modulating spring, and the primary brake valve and the secondary' brake valve each include a movable spool. A first return spring is disposed between the primary spool and the secondary spool, and a second return spring is disposed between the secondary spool and an end wall of the cylinder housing. The second return spring provides a spring force, greater than the spring force for the first return spring, such that the secondary spool does not begin modulating pressure until a predetermined pressure on the rear wheel brakes has been met.

[0007] In these and other embodiments, the hydraulic brake system provides offset braking at die front axle by a predetermined amount. At the start of the offset braking on the front wheel brakes, brake pressure at the front wheel brakes increases proportionally with additional pedal force to a point where brake fluid pressure at. the rear wheel brakes and front wheel brakes are at their respective maximum. As a result, light braking uses only the rear wheel brakes, thereby providing a controlled deceleration under unloaded and lightly loaded conditions while maintaining maximum braking performance at higher speeds and while heavily laden. In addition, while the hydraulic brake system is described in connection with an agricultural tractor, the hydraulic brake system can be used in connection with other vehicles as desired.

[0008] These and other features and advantages of the present invention will become apparent from the following description of the invention, when viewed in accordance with the accompanying drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 is a schematic diagram of a hydraulic brake system in accordance with one embodiment.

[0010] Figure 2 is the schematic diagram of Figure 1 with the brake pedal partially deflected for braking at the rear axle only.

[0011] Figure 3 is the schematic diagram of Figure 1 with the brake pedal being further deflected for braking at the front and rear axles.

[0012] Figure 4 is a graph illustrating front and rear brake pressure as function of brake pedal rotation.

[0013] Figure 5 is a graph illustrating front and rear brake pressure as function of brake pedal rotation including a front axle offset. DETAILED DESCRIPTION OF THE CURRENT EMBODIMENT

[0014] Referring to Figure 1, a hydraulic brake system in accordance with one embodiment is illustrated and generally designated 10. The hydraulic brake system 10 responds to a forward movement of a brake pedal 100 by increasing brake fluid pressure at the rear wheel brakes 102L, 102R and, after a predetermined amount of pedal travel, increasing the brake fluid pressure at the front wheel brakes 104L, 104R. The hydraulic brake system 10 is described below in connection with an agricultural tractor, but can be used with other vehicles as desired.

[0015] More specifically, the hydraulic brake system 10 includes a control valve assembly 12, a hydraulic supply 14, and a brake fluid reservoir 16. The hydraulic supply maybe a single supply to the control valve 12 or a separate supply for front and rear brakes. The control valve assembly 12 comprises a split-brake system having two pressure modulating valves: a left pressure modulating valve 20 and a right pressure modulating valve 22. Each pressure modulating valve 20, 22 includes a primary brake valve 24 and a secondary brake valve 26. The primary brake valve 24 is linked through a modulating spring 101 by a push rod 106 to the brake pedal 100. The left pressure modulating valve 20 and the right pressure modulating valve 22 are identical, and for simplicity the left pressure modulating valve 20 will now be described.

[0016] The left pressure modulating valve 20 includes a primary brake valve 24 and a secondary brake valve 26 that are assembled in series within a common housing, with the secondary brake valve 26 being used to supply pressurized brake fluid to the front wheel brakes 104L, 104R only after a sufficient amount of travel of the brake pedal 100. The primaty brake valve 24 is a five-port, three-position directional control valve, and the secondary brake valve 26 is a three-port, three-position directional control valve, The primary brake valve 24 and the secondary brake valve 26 include a movable spool (not shown in the figures), in particular a primary' spool and a secondary spool, respectively. The spool belonging to the primary brake valve 24 is linked through the modulating spring 101 to the push rod 106 and to the pedal 100. A first return spring 28 is disposed between the primary spool and the secondary spool, and a second return spring 30 is disposed between the secondary spool and an end wall of the common housing.

[0017] The primary' brake valve 24 includes a neutral position, a modulating position, and a fill position. The input side of the primary brake valve 24 includes three input ports. The first input port is coupled to the output side of the secondary brake valve 26 of primaiy brake valve 22 on the right side of the valve 12, the second input port is coupled to the hydraulic supply 14, and the third input port is coupled to the brake fluid reservoir 16. The output side of the primary brake valve 24 has two output ports. The first output is linked by a conduit to a hydraulic resolver (or a select-high valve) 31. The second output is linked by a conduit to rear wheel brake 102L. The resolver 31 directs the high pressure from the left and right secondary brake valves to both front wheel brakes 104L, 104R of the tractor vehicle. In another embodiment, there is no resolver 31 and the output of each secondary valve is directed to a front left brake port or a front right brake port. When the primary brake valve 24 is in the fill position, the output of the secondary brake valve 26 is open from the output of the secondary brake valve 26 of valve 22 on the right side of valve 12 to the front wheel brakes 104L, 104R thro ugh the resolver 31, and the hydraulic supply 14 is open to a rear wheel brake 102L, and communication to tiie brake fluid reservoir 16 is blocked. When the primary brake valve 24 is in the modulating position, communication from the output of the secondary brake valve 26 of brake valve 22 on the right side of valve 12 is open to the front wheel brakes 104L and 104R through the resolver 31, all other communication ports are blocked, and no brake fluid is allowed to flow. When the primary' brake valve 24 is in the pressure neutral position, the front and rear wheel brakes are in fluid communication with the brake fluid reservoir 16 and communication to the hydraulic supply 14 is blocked, and communication to the secondary brake valve 26 of brake valve 22 on the right side of the valve 12 is blocked.

[0018] The secondary brake valve 26 likewise includes a fill position, a modulating position, and a neutral position. When the left secondary' brake valve 26 is in the fill position, the hydraulic supply 14 is open to the first input port of the right primary' brake valve 24, and communication to the brake fluid reservoir 16 is blocked. Similarly, when the right secondary brake valve 26 is in the fill position, the hydraulic supply 14 is open to the first input port of the left primary brake valve 24, and communication to the brake fluid reservoir 16 is blocked. In this respect, the hydraulic circuit shown in Figure 1 includes a diagonal split connection between the left and right pressure modulating valves 20, 22. When the secondary brake valve 26 is in the modulating position, all ports are blocked, and no brake fluid is allow'ed to flow. When the secondary brake valve 26 is in the neutral position, it’s output to the primary brake valve 24 on the right side of the valve 12 is connected to the brake fluid reservoir 16 and the hydraulic supply 14 is blocked.

[0019] To reiterate, each primary' pressure modulating valve 24 is linked by a conduit to the corresponding rear wheel brake of the tractor vehicle and via another conduit to the front brakes 104L and 104R through the resolver 31. Within each pressure modulating valve 20, 22 the primary brake valve 24 and the secondary brake valve 26 are assembled in tandem, with the primary brake valve 24 being used to supply hydraulic pressure to the front brakes only after a sufficient amount, of brake pressure has been sent to the corresponding rear brake 102L or 102R. Each pressure modulating valve 20, 22 is coupled to first and hydraulic second supply lines 34. 36 that link the hydraulic supply 14 to the primary and secondary' brake valves 24, 26, respectively. Return conduits 38, 40 link each of the first and second brake valves 24, 26 to the brake fluid reservoir 16.

[0020] With brakes completely released (no input force on the brake pedals 100), each pressure modulating valve 20, 22 is at reservoir 16 pressure. Within each cylinder 20, 22, the primary' brake valve 24 and the secondary' brake valve 26 are in the position shown in Figure 1. When a braking operation is commanded by the driver by pressing on the brake pedals 100, the primary spool contained within the primary brake valves 24 moves from the neutral position, shown in Figure 1, to the fill position, shown in Figure 2. That is, initial brake actuation forces the primary’ brake valves 24 to move to the position shown in Figure 2. This brake pedal movement causes an increase in fluid pressure, transmitted to the rear wheel brakes 102L. 102R. Releasing die force on the brake pedals 100 allows the fluid to flow back to the primary brake valves 24 in the position shown in Figure 1, and ultimately the reservoir 16. However, continued brake pedals movement causes the secondary brake valves 26 to move from the neutral position to the fill position, shown in Figure 3, and the primary' brake valves 24 is able to increase hydraulic pressure at the front wheel brakes 104L, 104R.

[0021] As noted above, the secondary brake valves 26 interrupts the connection between the hydraulic supply 14 and the front wheel brakes 104L, 104R until a predetermined minimum amount of rear brake pressure. Therefore, in a state where the brake pedals 100 are not depressed by a predetermined amount, die brake fluid pressure in the rear wheel brakes 102L, 102R increases while the brake fluid pressure in the front wheel brakes 104L, 104R is unchanged. In addition, the reaction area of the spools for braking on the front brakes 104L and 104R may be different than the reaction area of the spools for braking on the rear brakes 102L and 102R. This allows the brake pressure on the front brakes 104L and 104R to rise at a different rate than on the rear brakes 102L and 102R so as to meet at a preset maximum simultaneously in response to the full application of the brake pedals 100.

[0022] As shown in Figure 4 for example, a graph depicting the front/rear brake pressure without an offset is illustrated. As the pedal angle reaches a first percentage of travel, the hydraulic pressure at the front and rear wheel brakes increases simultaneously. By contrast, Figure 5 depicts the embodiment of Figures 1-3, which provides offset braking at the front brakes by a predetermined amount, In particular, as the pedal angle reaches the first percentage of travel, the hydraulic pressure at the rear wheel brakes 102L and 102R increase linearly and the hydraulic pressure at the front wheel brakes 104L and 104R remain at zero. As the pedal angle passes through a second percentage of travel (greater than the first percentage), the hydraulic pressure at the front wheel brakes increases linearly, but at a faster rate when compared to the rear wheel brakes. Consequently, maximum braking at the front and rear axles is simultaneously reached at 100% of pedal travel in the illustrated embodiment. In other embodiments, the amount of offset and the rate of pressure rise at the front brakes 104L and 104R may be different than illustrated in Figure 5 as needed to meet the specific braking requirements of the tractor.

[0023] The description above describes the operation of the brake valve 12 as if both brake pedals 100 are depressed at the same time. This is the normal operating mode for the brake valve 12. A mechanical latch may be used to ensure that both pedals 100 are applied at the same time. If one only one brake pedal 100 is applied, there will be no brake pressure communicated to the front brakes 104L and 104R due to lack of communication through the primary brake valve 24 on the opposite side of the brake valve 12. In another embodiment of a single version of the brake valve 12, it comprises just one tandem valve 20.

[0024] The above description is that of current embodiments of the invention. Various alterations and changes can be made without departing from the spirit and broader aspects of the invention as defined in the appended claims, which are to be interpreted in accordance with the principles of patent law including the doctrine of equivalents. This disclosure is presented for illustrative purposes and should not be interpreted as an exhaustive description of all embodiments of the invention or to limit the scope of the claims to the specific elements illustrated or described in connection with these embodiments. For example, and without limitation, any individual element(s) of the described invention may be replaced by alternative elements that provide substantially similar functionality or otherwise provide adequate operation. This includes, for example, presently known alternative elements, such as those that might be currently known to one skilled in the art, and alternative elements that may be developed in die future, such as those that one skilled in the art might, upon development, recognize as an alternative. Further, the disclosed embodiments include a plurality of features that are described in concert and that might cooperatively provide a collection of benefits. The present invention is not limited to only those embodiments that include all of these features or that provide all of the stated benefits, except to the extent otherwise expressly set forth in the issued claims. Any reference to claim elements by ordinal terms, for example “first,” “second,” and “third,” are used for clarity, and are not to be construed as limiting the order in which the claim elements appear. Any reference to claim elements in the singular, for example, using the articles “a,” “an,” “the” or “said,” is not to be construed as limiting the element to the singular.