Patent Document

FIELD OF THE INVENTION 
   The present invention relates to a braking system of an agricultural vehicle and is applicable in particular to vehicles, such as combine and forage harvesters, having ancillary equipment that may be raised and lowered relative to the ground with consequential raising and lowering of the centre of gravity of the vehicle. 
   BACKGROUND OF THE INVENTION 
   On combines and forage harvesters, as well as on other agricultural vehicles, brakes are used to turn the vehicle within a smaller turning circle than would be achievable by the use of the steering wheels. The brakes are also the preferred method of steering in difficult field conditions. 
   In order to achieve this, two separate braking circuits are provided which have separate brake pedals for braking the left and right sides of the vehicle. The brakes are designed to be very powerful so that steering using the brakes can be achieved with minimal effort. 
   Furthermore, in combine and forage harvesters the hydrostatic drive system is often used for braking. During field operation, the hydrostatic drive system serves as the primary means for stopping the vehicle while the friction brakes acting on the wheels are used primarily for steering. 
   Of course, the same braking systems must be capable of being used when the vehicle is being driven on roads. Under such driving conditions, the two brake pedals are physically connected to one another, so that they cannot be depressed separately, and symmetrical braking is achieved by hydraulically interconnecting the two braking systems so that the same braking pressure is applied to the slave cylinders on both sides of the vehicle. 
   As a result, for driving on normal roads, more braking capacity is available to the driver than is needed and in some countries there is a legal requirement for simultaneous braking using the hydrostatic drive system which increases the maximum braking force still further. 
   The availability of an excessively high braking force presents a particular problem in the case of harvesters that are being driven on a road in that they risk toppling forwards. This problem is aggravated by the fact that, when driven on a road, the header of a harvester, that is to say the attachment on the front of the vehicle which is operable to cut and collect the crop, has to be raised and therefore changes the position of the centre of gravity of the whole vehicle. 
   OBJECT OF THE INVENTION 
   The present invention therefore seeks to avoid the risk of an agricultural vehicle toppling forwards when driven on a road without reducing its braking capacity during field operation. 
   SUMMARY OF THE INVENTION 
   In accordance with a first aspect of the present invention, there is provided an agricultural vehicle having ancillary equipment that may be raised and lowered relative to the ground with consequential changing of the position of the centre of gravity of the vehicle and having a braking system of variable capacity to enable the maximum braking force to be reduced when the position of the centre of gravity of the vehicle is changed in order to prevent the vehicle from toppling forwards during maximum braking. 
   The braking capacity can be reduced in a variety of ways. A first possibility is to provide two slave cylinders on each wheel, both being activated under normal conditions and one being disabled when the braking capacity is to be reduced. 
   In this second aspect of the invention, there is provided an agricultural vehicle having ancillary equipment that may be raised and lowered relative to the ground with consequential changing of the position of the centre of gravity of the vehicle and having a hydraulic braking system comprising a master cylinder and a slave cylinder, wherein the braking system further includes a second slave cylinder associated with the same wheel as the first slave cylinder and a valve for isolating the second slave cylinder from the master cylinder when the position of the centre of gravity of the vehicle is changed, thereby ensuring that the braking force does not exceed a safe limit below which the vehicle does not risk toppling forwards when braking. 
   Another possibility for achieving a variable capacity braking system is to provide a switchable hydraulic amplification stage which operates during field use but not during road use. 
   In this third aspect of the invention, there is provided an agricultural vehicle having ancillary equipment that may be raised and lowered relative to the ground with consequential changing of the position of the centre of gravity of the vehicle and having a hydraulic braking system comprising a master cylinder and a slave cylinder, wherein the braking system additionally comprises a pressure amplification stage and a valve having a first position in which the master cylinder is connected to the slave cylinder by way of the pressure amplification stage and a second position in which the master cylinder is directly connected to the slave cylinder, the pressure amplification stage being bypassed in the second position of the valve to ensure that the braking force does not exceed a safe limit below which the vehicle does not risk toppling forwards when braking. 
   Other possibilities for a dual capacity or variable capacity braking system will be readily apparent to the person skilled in the art, but the preferred approach is to selectively limit the braking force by limiting the pressure in the hydraulic braking circuits during road use. The advantage of this approach is that it involves minimal alteration to existing braking systems and can therefore if necessary be retrofitted easily to existing vehicles. 
   In the fourth and most preferred aspect of the invention, there is provided an agricultural vehicle having ancillary equipment that may be raised and lowered relative to the ground with consequential changing of the position of the centre of gravity of the vehicle and having a hydraulic braking system comprising a master cylinder and a slave cylinder, wherein the braking system further includes a pressure relief valve for limiting the hydraulic pressure applied to the slave cylinder when the centre of gravity of the vehicle is raised, thereby ensuring that the braking force does not exceed a safe limit below which the vehicle does not risk toppling forwards when braking. 
   Preferably, the agricultural vehicle has two hydraulic braking systems with separate brake pedals each acting on a respective side of the vehicle, so as to enable the vehicle to be steered by the application of a braking force to only one side of the vehicle and the master cylinders of the two braking systems additionally comprise pressure equalisation ports that are connected to one another, each port including a non-return valve that is opened as soon as the associated brake pedal is depressed, so that equal pressures are applied to the slave cylinders on the opposites sides of the vehicle when the two brake pedals are depressed simultaneously. 
   The pressure relief valve is conveniently connected in this case to the hydraulic line interconnecting pressure equalisation ports of the master cylinders. 
   In order to activate and disable the pressure relief valve selectively, a two position valve may suitably be arranged in series with the pressure relief valve, the two position valve serving to isolate the pressure relief valve from the pressure equalisations ports in its first position but not in its second position. 
   The two position valve could simply be a manually operated ON/OFF valve with an interlock that obliges it to be opened before the two brake pedals can be connected to one another for driving on a road. 
   It is preferred, however, to provide a normally closed solenoid valve that is operated automatically in dependence upon the selected drive ratio, the speed of the vehicle, the height of the header or any other parameter indicative of road use, as opposed to field use, of the vehicle. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described further, by way of example, with reference to the accompanying drawing, in which: 
       FIG. 1  is a block diagram of the hydraulic circuit of a braking system of an agricultural vehicle constructed in accordance with the first and fourth aspect of the invention, 
       FIG. 2  is section through a valve of a braking system constructed in accordance with the second aspect of the invention, 
       FIG. 3  is a block diagram of part of the hydraulic circuit of a braking system constructed in accordance with the third aspect of the invention, and 
       FIG. 4  is a block diagram similar to that of  FIG. 3  showing the braking system during field use of the vehicle when the brake pedals are depressed separately. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a braking system comprising two (left and right) master cylinders  10 ,  10 ′ connected to two slave cylinders  12 ,  12 ′ by way of respective hydraulic pressure lines  14 ,  14 ′ to form separate left and right braking circuits. The two braking circuits are identical and only one of them needs to be described. 
   In each of the braking circuits, the master cylinder  10  comprises a housing  10   a  containing a piston  10   b  that is moved by means of an actuating rod  10   c  connected to a respective brake pedal. When the piston  10   b  is moved by depressing a brake pedal, fluid under pressure is supplied through a port  10   d  to the slave cylinder  12 , the piston of which acts on a brake pad on a respective side of the vehicle. 
   Each of the master cylinders  10 ,  10 ′ is further connected through a port  10   e  that incorporates a non-return valve  10   f , to a common reservoir  16  at ambient pressure through a line  18 . The reservoir maintains the circuits filled with hydraulic fluid as their volume increases through wear of the brake pads. 
   As so far described, each hydraulic circuit is entirely conventional and operates in the same manner as the braking system of most road vehicles. 
   Because two separate braking circuits are provided for the left and right side of the vehicle, during field operation the left and right sides of the vehicle can be braked separately by operating only one or other of the two brake pedals and this allows the vehicle to be steered by means of its brakes. 
   Such braking is inappropriate, for obvious reasons, when the vehicle is to be driven on a road. Thus, during road use, the brake pedals are mechanically connected to one another so that they cannot be operated separately. One cannot rely on the mechanical coupling of the brake pedals to ensure equal braking on both sides of the vehicle and this is instead accomplished, in a known manner, by interconnecting the hydraulic circuits. To this end, each of the master cylinders  10  and  10 ′ additionally comprises a pressure equalisation port  10   g  that incorporates a non-return valve  10   h . The piston  10   b  has a shoulder which acts on the closure member of the non-return valve  10   h  as soon as the brake pedal is depressed to connect the working chamber of the cylinder to a pressure equalisation line  20 . If only one of the brake pedals is operated during field use, only one of the non-return valves  10   h  will be open and high pressure will not be able to flow from the actuated braking circuit to the other. However if both pedals are depressed, even by unequal amounts, the two circuits will be able to communicate with one another through the pressure equalisation line  20  to ensure symmetrical braking when the vehicle is in road use. 
   As so far described, the braking system is known in the context of brake steered agricultural vehicles. Master cylinders having an additional pressure equalisation which also incorporate a non-return valve in the pressure equalisation port that is opened as soon as the piston is moved are currently available and their internal construction need not therefore be described in greater detail herein. 
   The illustrated embodiment of the present invention comprises a solenoid operated two position valve  24  and a pressure relief valve  26  arranged in series with one another in a line  22  that leads from the pressure equalisation line  20  to the line  18  connected to the fluid reservoir  16 . 
   The solenoid valve  24  is shown in its normal position for field use wherein it maintains the pressure equalisation line  20  isolated and does not therefore interfere with pressure supplied to the slave cylinders  12 ,  12 ′. 
   However, when a parameter is sensed that indicates that the vehicle risks toppling because it is being driven on a road with the header raised, the solenoid valve  24  is moved into its other position in which it connects the pressure equalisation line  20  to the pressure relief valve  26 . 
   The pressure relief valve  26  is a spool valve that is acted upon in one direction by a spring and in the opposite direction by a pilot pressure derived from its intake port. As soon as the pressure applied to the relief valve  26  exceeds a threshold, which as represented by an arrow in the drawing may be adjustable to suit the vehicle, the valve opens and connects the pressure equalisation line  20  to the ambient pressure in the reservoir  16 . In this way, the pressure delivered to the slave cylinders  12 ,  12 ′ is limited to the value set by the pressure relief valve and the risk of the vehicle toppling is avoided. 
   The signal for operating the solenoid valve  24  may be derived from any suitable source, for example from a sensor that responds to the selected drive ratio, the speed of the vehicle or the height of the header. 
   It will be seen from the drawing that the additional components required to eliminate the risk of the vehicle toppling during road use are the two contained within the box drawn in dotted lines, namely the two position solenoid valve  24  and the pressure relief valve  26 . These two components can be formed as a sub-assembly that may be connected using only two hydraulic connections to an existing braking system, thus making it possible to modify existing vehicles with relative ease. 
   Though the embodiment of  FIG. 1  is preferred because of the ease of retrofitting, the capacity of the braking system can be altered in other ways as will now be described with reference to two further embodiments of the invention, shown in  FIG. 2  and in  FIGS. 3 and 4 , respectively. In the case of both these further embodiments, all the components shown in  FIG. 1 , other than the two valves  24  and  26 , are present. These are the components that are conventionally to be found in an agricultural vehicle that can be steered by asymmetrical braking. 
   In the case of the alternative embodiment, a valve body  50  as shown in  FIG. 2  is provided, in addition to which a second brake calliper (not shown), which incorporates an additional slave cylinder, is provided on each wheel. Lines  60  and  62  lead to the separate slave cylinders on the left side of the vehicle and lines  60 ′ and  62 ′ lead to separate slave cylinders on the right side of the vehicle. Once again, primed reference numerals will be used to avoid unnecessary repetition of description. 
   The valve body  50  shown in  FIG. 2  has two intake ports connected to the lines  14 ,  14 ′ from the master cylinders  10 ,  10 ′ and four output ports connected to the lines  60 ,  62 ,  60 ′ and  62 ′. A valve spool  52  is mounted in a bore in the valve body  50  and at each end the spool is acted upon by a spring  55 ,  55 ′ and the pressure in a control chamber  54 ,  54 ′ that communicates with one of the intake ports. When equal pressures are applied to the two control chambers  54 ,  54 ′, the spool  52  adopts its illustrated position. Here the lines  14  and  14 ′ are connected to the lines  60 , and  60 ′ through passages  66  and  66 ′ in the body  50  while the two further passages  68  and  68 ′ that lead to the lines  62  and  62 ′ are closed by the valve spool  52 . Thus, when both brake pedals are depressed during road use of the vehicle, only one set of brake pads acts on each side of the vehicle. 
   When only the left side is braked, the pressure in the control chamber  54  will move the valve spool  52  to the right as viewed, allowing fluid to flow through the passage  68  to the line  62  leading to the slave cylinder of the second calliper and two sets of brakes will be activated on the left side of the vehicle but none on the right, causing the vehicle to be steered to the left. To allow hydraulic fluid to be drained from the second slave cylinder when the brake pedal is released and the passage  68  is again closed by the valve spool  52 , a non-return valve  64  is provided which allows fluid to flow from the line  62  to the line  14  only in the direction that reduces the braking force on the wheels. 
   By operation of the valve body  50 , it is ensured that maximum braking force can still be applied when braking only one wheel, by means of the first and second slave cylinders. The braking force will however be divided by two when braking both wheels simultaneously because only the first slave cylinder on the left and right wheel is used due to an equal pressure being applied to both sides of the valve spool  52 . 
   The embodiment of  FIGS. 3 and 4  comprises a valve  30  connected to the lines  14 ,  14 ′ of  FIG. 1  and two pressure amplifiers  38  and  38 ′ each connected to a respective one of the slave cylinders  12  and  12 ′ in  FIG. 1 . 
   The valve  30  is generally similar to the valve  50  in  FIG. 2  and has control chambers  34  and  34 ′ connected to the lines  14  and  14 ′ from the master cylinders. As in the case of the valve spool  52  in  FIG. 2 , the spool  32  of the valve  30  adopts a central position, shown in  FIG. 3 , when both brake pedals are depressed during road operation and an end position, shown in  FIG. 4 , when only one brake pedal is depressed during field operation. 
   During road operation, the master cylinder lines  14  and  14 ′ are connected to the slave cylinders  12  and  12 ′ through lines  42  and  42 ′ that bypass the pressure amplification stages  38  and  38 ′ so that the latter have no effect. On the other hand, when the spool  32  moves to its end position shown in  FIG. 4  because only the left brake pedal is depressed, the passage  42  is blocked by a land of the spool  32  and instead the pressure from the master cylinder line  14  is applied through the passage  40  to the amplification stage  38  which increases the pressure applied to the slave cylinder  12  by a factor equal to the ratio of the surface areas of the opposite ends of its piston. Similarly, on depressing the right brake pedal, the master cylinder line  14 ′ is blocked by another land of the spool  32 , preventing pressure to be transmitted to the right slave cylinder  12 ′. 
   Accordingly, when driving on the road and depressing both brake pedals simultaneously, the spool  32  is forced to stay in the middle of the valve  30 , because of the equal pressure in the master cylinder lines  14  and  14 ′. 
   However, because of the pressure amplification stages  38  and  38 ′, the system is able to create a higher braking force when the left or right brake is used alone. The system is dimensioned such that the braking force is within safe braking limits when driving on the road.

Technology Category: 4