VEHICLE WITH LOAD CONSCIOUS BRAKE PRESSURE REDUCING VALVE

Abstract: A vehicle comprising sprung and unsprung parts is provided with a brake pressure reducing valve (10) control­ led by a position sensing spring which applies to the control piston (12) a force (S) determined by the relative position of the sprung and unsprung parts. The sensing spring force (S) tends to move the control piston (12) in the direction to open the valve, and is opposed by an internal bias spring (18) which tends to close the valve. By suitable choice of piston areas A₁ and A₂ and spring forces increase (above cut-in pressure) in input pressure (P₁) will always produce an increase in output pressure (P₂) despite the reduction in sensing spring force (S) which will occur due to weight transfer.

Description: This invention relates to a brake pressure control system for a vehicle, the system having the features of the pre-characterising portion of the main claim. It is well known to provide a vehicle with one or more brake pressure reducing valves which operate, under certain conditions, to provide a brake pressure to the rear brakes which is lower than the pressure applied to the front brakes. Such pressure reducing valves may be made load conscious whereby the characteristics of the valve arevaried in accordance with both static and dynamic loading of the vehicle. Vehicle loading may be sensed by incorporating the valve as a load bearing component in a suspension assembly which interconnects the sprung and unsprung parts of the vehicle, whereby the whole or a significant part of the load transmitted from the sprung to the unsprung parts of the vehicles is applied to the valve. Such an arrangement is shown in GB-A-1557006. In the alternative, the valve may be separate from the suspension and may be provided with a sensing spring which applies to the valve a force determined by the relative position of the sprung and unsprung parts of the vehicle. It is with a control system incorporating this type of valve that the present invention is concerned. Since the relative position of the sprung and unsprung parts is dependent on vehicle loading, such an arrangement does render the valve load conscious. However, no significant part of the vehicle load is transferred from the sprung to the unsprung part of the vehicle by the valve since the sensing spring is light and only serves to sense the position of the unsprung part relative to the sprung part. A prior art arrangement of this general type is shown in GB-A-1252700. Whilst valves of the non-load bearing type have various advantages which will be known to those skilled in the art, prior art valves of this type have suffered from a disadvantage, described in more detail below, whereby under certain operating conditions the pressure applied to the rear brakes is considerably less than optimum. These problems can be resolved if the usual arrangement of position sensing spring, in which during certain normal operating conditions the spring applies no force to the control piston of the control valve, is replaced by an arrangement in which the position sensing spring applies a force to the control piston under all normal operating conditions. This can be achieved if the control valve is provided with an internal bias spring which acts in opposition to the position sensing spring. An arrangement of this type is shown in US-A-3909074. In control valves of the type shown in US-A-3909074 the force of the external spring increases as the static loading of the vehicle increases, thereby providing a higher cut-in pressure when the vehicle is more heavily laden. If the control valve is used in association with the rear axle of the vehicle, and is arranged such that the spring force provided by the external spring increases as the vehicle is more heavily laden, then, for any given vehicle loading, the force applied by the external spring will decrease progressively during progressively heavier braking as a result of weight transfer from the rear axle to the front axle of the vehicle. Prior to the present invention, it had been thought that the decrease in the force applied by the external spring as a result of weight transfer effects was not sufficient to materially affect the operating characteristics of the valve. It has now been found that the reduction in the force provided by the external spring is not insignificant as had previously been thought, but rather the force applied by the external spring reduces significantly under heavy braking conditions due to weight transfer effects. The present invention is characterised in that the ratio of the area of the piston (A₁) which is exposed to the pressure prevailing at the inlet to the area (A₂) of the piston which is exposed to the pressure prevailing at the outlet is such that upon an increase in the brake input pressure (P₁) the product of the input pressure (P₁) and the area of the piston exposed to the inlet pressure (A₁) will always increase by more than the resulting decrease in the external spring force as a result of weight transfer effects. As a result of the invention the characteristics of the braking system under heavily laden conditions are improved as compared with the prior art. The invention will be better understood from the following description of the prior art and of a preferred embodiment of the invention, reference being had to the accompanying drawings, wherein: Figure 1 shows schematically a typical prior art valve; Figure 2 is a graph showing the characteristics of the valve of Figure 1; Figure 3 shows schematically a valve for use in an embodiment of the invention; and Figure 4 is a graph showing characteristics of the valve of Figure 3. Referring firstly to Figure 1 there is shown schematically a typical prior art brake pressure reducing valve 1 of the type employing an axle position sensing spring separate from the vehicle suspension. In use the valve is, for example, mounted on the vehicle body and is connected to the rear axle by a position sensing spring which applies to the control piston 2 a force S dependent on the relative position of the rear axle and body. As the rear axle and body move towards each other with increased vehicle loading the force S increases. The sensing spring is set such that when the vehicle is stationary and carrying only the driver the force S is small and is substantially zero at the cut-in pressure of the valve 1 (the pressure at which the valve 1 will begin to provide a lower pressure at the outlet 3 than the pressure at the inlet 4). Thus the cut-in pressure under driver only conditions is determined by the equation: P c A 3 = K where: P c = cut-in pressure A₃ = area of the rod of piston 2 K = the force of spring 5 For driver only conditions the general equilibrium equation of the valve 1 at pressure above cut-in is: P₁A₁ + K = P₂A₂ where: P₁ = pressure at inlet 4 P₂ = pressure at outlet 3 A₂ = area of the piston head A₁ = A₂ - A₃ Thus, for driver only the cut-in pressure is determined solely by the spring 5 and the rise in outlet pressure at pressures above cut-in is determined mainly by the area ratio weight transfer having no effect on the rise in outlet pressure. The overall driver only characteristics of the valve are given by the line OAB of Figure 2 and gives a reasonable approximation to the computed ideal driver only characteristic curve X of Figure 2. When the vehicle is laden the position sensing spring will apply a force S to the control piston and the cut-in pressure is given by: P c A 3 = K + S Accordingly cut-in pressure is dependent on vehicle loading, as required. At pressures above cut-in: P₁A₁ + S + K = P₂A₂ Where: S is the force of the sensing spring which reduces with weight transfer. It has previously been thought that, for any given vehicle loading, the rate of reduction in (S + K) due to weight transfer is substantially less than the rate of increase in P₁A₁ and above cut-in the rise in outlet pressure will again be determined mainly by the ratio Thus, the predicted characteristics of the valve of Figure 1 when the vehicle is fully laden are given by the line OCD plotted on Figure 2, i.e. it was thought that effect of loading the vehicle was to increase the cut-in pressure (from A to C), but to leave substantially unaltered the rate of rise of outlet pressure at pressures above cut-in (line CD parallel to line AB). The line OCD gives a good fit to the computed ideal fully laden characteristic line Y. However, it has now been found upon more detailed analysis of vehicle dynamic characteristics that the reduction in factor (S + K) is not insignificant as had previously been thought, but rather the factor (S + K) reduces significantly during braking due to weight transfer to the front of the vehicle. This has an adverse effect on the ratio of input to output pressure and results, under extreme conditions, in the phenomenon that a rise in P₁ produces a smaller rise in the factor P₁A₁ than the resultant decrease in the factor S. Thus, a rise in P₁ produces a fall in P₂. This characteristic is represented by the line OCE of Figure 2 and compares unfavourably with the computed ideal fully laden characteristic Y. The practical effect of this is severe under utilisation of the rear brakes resulting in un-necessarily long stopping distance on good road surfaces. Referring now to Figure 3 the valve 10 comprises a body 11 housing a control piston 12 carried by a piston rod 13 which is acted upon by a position sensing spring (not shown). The piston 12 houses a valve 14 which is normally held open by a pin 15 secured to the valve body 11 but which closes as the piston 12 moves to the right and thereby interrupts communication between the inlet 16 and outlet 17. An internal bias spring 18 applies a substantially constant force K' to the piston tending to close the valve 14. The bias spring 18 acts in opposition to the position sensing spring, but the position sensing spring is designed and set so that it always applies to the control piston a force S which is greater than the substantially constant force K' of the bias spring 18. Cut-in pressure P c is given by P c A 3 = S - K' The position sensing spring is set such that under driver only conditions the correct cut-in pressure is obtained. The position sensing spring characteristics are selected so that the desired progressive increase in cut-in pressure is obtained with progressive increase in vehicle loading. Above cut-in pressure, the valve of Figure 3 satisfies the following equation for both driver only and laden conditions: P₂A₂ = P₁A₁ + S - K' Since it is appreciated that the factor (S - K') will not be constant but will decrease with increasing brake pressure due to weight transfer effects, the area ratio is selected such that, upon an increase in brake input pressure, the factor P₁A₁ will always increase more than S - K' will decrease. Thus, an increase in P₁ will always produce an increase in P₂, despite weight transfer effects. The driver only characteristics OA'B' and fully laden characteristics OC'D' of the valve of Figure 3 are plotted in Figure 4. It will be appreciated that the line OC'D' of Figure 4 provides a much better approximation to the ideal curve Y than does the line OCE of Figure 2, and that this is achieved without sacrificing the driver only characteristics. Whilst the above described valve is of general utility, it is particularly useful in an X split system (a system in which each front brake is linked to the diametrically opposed rear brake) which incorporates anti-lock controls on the front brakes only. In such a system, an impending front wheel lock will result in automatic relief of brake pressure in the front wheel in question and in the diametrically opposed rear wheel. When the brake pressure is reapplied, the cut-in point of the reduction valve associated with the rear wheel in question will be lower than upon initial application because of weight transfer caused by the other brakes. If the valve of Figure 1 is used, there will be little or no increase in output pressure after cut-in, and indeed there may be a decrease. If the valve of Figure 3 is used, however, the output pressure will rise rapidly after cut-in due to the high ratio of coupled with the relatively small additional weight transfer which will occur under these conditions. It will be appreciated that in the above described arrangement according to the invention the valve provides a variable output in accordance with weight transfer effects even in the case of driver only loading. In addition to enabling better rear brake utilization in the case of straight line braking, the invention offers significantly better braking on cornering in the driver only case when two valves are employed as part of an X-split system. This is because the two valves associated with the X-split system will be located adjacent the respective rear wheels and will thus detect the transverse weight transfer which occurs during cornering. the effect will be to prevent locking of the more lightly laden (inner) rear wheel and to provide better utilization of the more heavily laden (outer) rear wheel, even in the driver only case.

Claims: A brake pressure control system in a vehicle having a sprung part supported on a plurality of unsprung parts by way of a plurality of suspension assemblies, the system comprising: at least one brake pressure reducing valve (10) having an inlet (16) connected to a source of hydraulic brake fluid, an outlet (17) connected to a rear brake operating cylinder, and a control piston (12) which has a first area (A₁) exposed to the pressure prevailing at the inlet (16) and a second area (A₂) exposed to the pressure prevailing at the outlet and which is moveable in a first direction to interrupt communication between the inlet and the outlet; a position sensing spring which applies to the control piston (12) at all times when the vehicle is in use a force (S) which is determined by the relative position of the sprung and unsprung parts of the vehicle; and a bias spring (18) which applies to the control piston (12) a substantially constant force (K) which tends to move the control piston in the first direction and which opposes the force (S) applied to the control piston by the position sensing spring, characterised in that the area ratio A₁:A₂ is such that upon an increase in brake input pressure (P₁) the factor P₁A₁ will always increase by more than the resulting decrease in the difference (S-K') between the position sensing spring force (S) and the force (K') of the bias spring (18). A brake pressure control system according to claim 1 characterised in that the closure member (14) of the control valve is housed within the control piston (12) and the flow path from the inlet (16) to the outlet (17) is via a passage formed in the control piston (12). A brake pressure control system according to claim 2 characterised in that the closure member (14) is spring biased towards its closed position by a closure member spring and is normally held open by a pin (15) which extends through the passage formed in the control piston (12) to engage the body (11) of the brake pressure reducing valve (10). A brake pressure control system according to claim 3 characterised in that the cut-in pressure of the brake pressure reducing valve (10) is given by the formula: P c A 3 = S - K Where: P c = cut-in pressure A₃ = area of the piston rod of the control piston K = force applied to the control piston by the bias spring S = force applied to the control piston by the position sensing spring. A brake pressure control system according to claim 4 characterised in that at pressures above cut-in pressure (P c ), the outlet pressure is given by the formula: P₂A₂ = P₁A₁ + S - K Where: P₂ = outlet pressure P₁ = inlet pressure A₁ = area of control piston exposed to inlet pressure A₂ = area of control piston exposed to outlet pressure. A brake pressure control system according to any preceding claim characterised in that the bias spring (18) is located within the body (11) of the brake pressure reducing valve (10), and acts directly on the valve body (11) and the control piston (12). A brake pressure control system according to any preceding claim wherein the vehicle is equipped with an X-split braking system and a separate brake pressure reducing valve is associated with each circuit of the X-split system, each brake pressure reducing valve being located adjacent a respective rear wheel of the vehicle.

Inventors: FARR, GLYN PHILLIP REGINALD
Applicants: LUCAS INDUSTRIES PUBLIC LIMITED COMPANY
Application year: 1984
Main classification: B60T8
IPC codes: B60T8