Abstract:
A mechanism that interconnects the service brakes of a hydrostatically driven tractor with the hydrostatic pump to more effectively slow or stop the tractor, for example when towing a trailer. The service brakes are connected to a control servo on the hydrostatic pump such that actuation of the service brakes will destroke the pump, eliminating or reducing the torque of the pump as a factor to be overcome in slowing or stopping movement of the tractor. Both mechanical and electronic versions of the mechanism are effective in slowing and stopping the tractor.

Description:
BACKGROUND OF THE INVENTION 
   The invention claimed herein relates generally to hydrostatically driven agricultural vehicles, and more specifically to a braking system for such vehicles that interconnects the normal brakes with the hydrostatic pump to more effectively slow or stop the vehicle. 
   Hydrostatically driven tractors, such as that generally shown in U.S. Pat. No. 6,230,830, have grown in popularity over the last several years, and can be found in use throughout the world. Machines of this type are quite reliable and easy to operate; however, there are several scenarios in which they present operational difficulties or problems that have not heretofore been fully addressed. 
   For example, while traveling at road speeds and pulling a trailer (such as, for example, a trailer or a beet wagon) without trailer brakes, the tractor must, according to industry standards and general safety requirements, be able to stop within a certain distance as a result of a single action or activity of the operator. On a hydrostatically driven tractor the vehicle can be stopped by either using the forward/neutral/reverse lever (hydrostats) or the service brake. To achieve acceptable roading speeds with a hydrostatically driven tractor the variable displacement hydrostatic motor must have a small minimum displacement that, unfortunately, does not provide enough dynamic braking to stop the tractor when the forward/neutral/reverse lever is returned to neutral. Actuating the service brake only, while the tractor is roading, provides insufficient dynamic braking, as the service brakes have to overcome the inertia of the tractor/trailer as well as stall the hydrostatic drive. 
   Another scenario arises while the tractor is traveling at roading speeds, pulling a trailer equipped with trailer brakes. The tractor must be able to be stopped within a certain distance by the operator using a single input and with the trailer brakes actuating before the hydrostats destroke (move to neutral). Returning the hydrostatic forward/neutral/reverse lever to neutral will not apply the trailer brakes, which are coupled hydraulically to the service brakes, and may cause the vehicle and trailer to jackknife. Actuating the service brakes will engage the trailer brakes, however, the stopping distance will be unacceptable as the brakes, as mentioned above, must overcome the torque of the hydrostatic transmission. 
   The above-described scenarios occur under circumstances that provide additional difficulties for the operator. While in a draft application, the tractor must be decelerated while the operator is actuating hydraulic levers and steering at headlands. Actuating the service brakes alone will not overcome the torque of the hydrostatic transmission and the operator will have to move the forward/neutral/reverse lever, which cannot be done concurrently with the other operations—at least not conveniently or easily. The instant invention allows the operator to actuate the service brakes to concurrently destroke the hydrostatic pump and slow the vehicle. When the service brakes are disengaged the tractor accelerates to its original speed. 
   It would be desirable and beneficial to provide a hydrostatically driven tractor that would overcome the above-noted disadvantages of known hydrostatic tractors. 
   SUMMARY OF THE INVENTION 
   It is an object of the instant invention to provide an improved hydrostatically driven agricultural tractor. 
   It is another object of the instant invention to provide an improved hydrostatically driven tractor that is more cost-effective, durable, cheap to produce, easier to use, and reliable than those known in the prior art. 
   It is yet another object of the instant invention to provide an improved braking system for a hydrostatically driven agricultural tractor. 
   These and other objects are obtained by providing a mechanism that interconnects the service brakes of a hydrostatically driven tractor with the hydrostatic pump to more effectively slow or stop the tractor, for example when towing a trailer. The service brakes are connected to a control servo on the hydrostatic pump such that actuation of the service brakes will destroke the pump, eliminating or reducing the torque of the pump as a factor to be overcome in slowing or stopping movement of the tractor. Both mechanical and electronic versions of the mechanism are-effective in slowing and stopping the tractor. 
   The foregoing and other objects, features and advantages of the invention will appear more fully hereinafter from a consideration of the detailed description which follows, in conjunction with the accompanying drawings wherein one primary embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for illustrative purposes and are not to be construed as defining the limits of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages of this invention will become apparent upon consideration of the following detailed description of the invention, especially when taken in conjunction with the accompanying drawings wherein: 
       FIG. 1  is a perspective view of a hydrostatically driven agricultural tractor of the general type to which the instant invention applies; 
       FIG. 2  is a general schematic view of the preferred mechanical version of the braking system of the instant invention; and 
       FIG. 3  is a general schematic view of the preferred electronic version of the braking system of the instant invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings, and particularly  FIG. 1 , an exemplary four-wheel drive tractor incorporating the principles of the instant invention is shown. The tractor  10  includes a forward end  11  supported above the ground G by a front axle assembly  12  and carrying an engine, not shown. The rearward cab end  13  of the tractor  10  is supported above the ground by a rear axle assembly  15  and has an operator&#39;s station  18  mounted thereon. The operator&#39;s station includes a steering wheel  20 , a seat  22 , a service brake pedal  23 , and other control components such as, for example, a forward/neutral/reverse lever to actuate the hydrostatic transmission. Each of the front and rear axles assemblies  12  and  15  is provided with a pair of opposing wheels  19  for mobile movement of the tractor  10  over the surface of the ground G. Though the instant invention works equally well on articulated and non-articulated tractors, the tractor shown is of the articulated type. 
   Tractor  10  is provided with a drive system that is operatively connected to the engine to provide operative power for the front and real axle assemblies  12  and  15 . The drive system may include a splitter gearbox as generally shown in U.S. Pat. No. 6,227,326 mounted on the front of the rear axle assembly  15 . The drive system also includes the input drive components, including a hydrostatic pump for powering the traction drive of the tractor  10 , a hydraulic pump for pressuring the hydraulic system of the tractor  10 , a power takeoff (PTO) mechanism; and the output drive components, including a variable displacement hydrostatic motor to provide operative power to both the front and rear axle assemblies  12  and  15  through front and rear output drive shafts. At the rear of the tractor is a drawbar, a three-point hitch, and auxiliary hydraulic couplings for providing power to operate and brake towed implements, trailers and the like 
   Very broadly, the instant invention consists of a mechanism, mechanical and/or electronic, that destrokes the hydrostatic pump when the service brake of the tractor is applied. The term “destroke” as used herein refers to the act of moving the hydrostatic pump from a first position of engagement, either for forward or reverse movement of the tractor, to a second neutral position. 
   In prior tractor braking mechanisms, actuation of the service brake pedal  23  alone will not overcome the torque of the hydrostatic transmission, and the operator will have to move the forward/neutral/reverse lever, which cannot be effectively done concurrently with the other normal operational activities of the operator. The instant invention allows the operator to actuate the service brakes to concurrently destroke the hydrostatic pump and slow the tractor. When the service brakes are disengaged, the tractor accelerates to its original speed, i.e., the forward/neutral/reverse lever moves to its position prior to engagement of the service brakes. 
   Referring now to  FIG. 2 , the mechanical version of the invention comprises a mechanically actuated cartridge valve  40 , integral with the hydrostatic pump mechanical servo  42 . As will be described in further detail below, servo  42  is connected via a cable/spring mechanism to the service brake master cylinder  44  such that the two work simultaneously to slow and stop tractor  10 . The cartridge valve  40  shunts the servo pistons within the hydrostatic pump  42 , causing pump  42  to destroke to neutral, assisting the service brakes in slowing or stopping the tractor. Once the service brakes are released, the servo pistons within the hydrostatic pump return to their original setting and the tractor will accelerate to the previous speed. The movement of the master cylinder bell crank  46  is greater than the travel of the shunt cartridge valve, and, therefore, the spring  48 , which has an initial tension greater than the opening force of valve  40 , ensures that the cartridge valve  40  opens while allowing the bell crank  46  to move through its full range of travel. 
   Though it is obvious from the drawings, it is worthwhile to note that the tension spring  48  is connected to servo  42  by cable  50 . Cable  50  may be an enclosed or shielded cable, but in any event, requires mounting brackets  52  and  54  to hold the cable in position and ensure reliable operation. The bell crank  46  is pivotably mounted, about pivot point  60 , to the tractor by a bracket  62 . The pivot points of spring  48 , service brake pedal  23  and master cylinder  44 ,  64 ,  66  and  68 , respectively, are above the pivot point  60  so that the spring  48  may operate to return the bell crank, master cylinder and servo to a neutral position when actuation of the service brakes is stopped. 
   The electronic version of the invention is shown in  FIG. 3 , and comprises an electronically controlled hydrostatic pump  70 , a programmable microprocessor  72 , and a potentiometer  74  attached to the service brake bell crank  75 . Actuation of the service brake pedal  23  provides an input from potentiometer  74  to the microcontroller or computer, via wires  76 , and the programming (i.e., software) then sends a signal to servo  80 , via wires  82 , destrokes hydrostatic pump  70 , assisting the brake in slowing and stopping the tractor. Tension spring  78  works in similar fashion to spring  48  in  FIG. 3  to return the system to neutral. 
   It will be understood that changes in the details, materials, steps and arrangements of parts which have been described and illustrated to explain the nature of the invention will occur to and may be made by those skilled in the art upon a reading of this disclosure within the principles and scope of the invention. The foregoing description illustrates the preferred embodiment of the invention; however, concepts, as based upon the description, may be employed in other embodiments without departing from the scope of the inventions. Accordingly, the following claims are intended to protect the invention broadly as well as in the specific form shown. 
   In the embodiments shown, the brake pedal, bell crank and master cylinder (or the brake pedal, bell crank and potentiometer) may be referred to as a “sensing device”. The cartridge valve integral with hydraulic pump control servo (or the programmable microprocessor) may be referred to as a “control device”. The cable mechanism (or wires) may be referred to as a “signal transfer device”. and the tension spring may be referred to as a “return device”.