Automatic load-dependent brake control device having wide range of pressure adjustment

In a variable load valve device of the type employing a walking beam type lever arrangement via which the control and compensating pistons are interconnected to obtain load-dependent control of the brake pressure, there is provided a screw adjustment that acts through an auxiliary piston to vary the tension of a bias spring acting on one of the levers of the lever arrangement to bias the compensating piston in order to withhold the load-dependent brake control until a predetermined level of brake pressure is developed. The auxiliary piston is subject to brake control pressure supplied to the variable load valve inlet and in one embodiment of the invention, the auxiliary piston is formed to urge the piston in a direction to counteract the spring force and in another embodiment, to urge the piston in a direction to supplement the spring force.

BACKGROUND OF THE INVENTION 
The present invention is related to load responsive brake apparatus and 
more particularly to a variable load valve device of the type in which the 
load proportioning aspect is provided by a lever arrangement for 
interconnecting the pistons controlling the valving between the variable 
load valve inlet and outlet to obtain mechanical advantage in effecting 
closure of the valving when the brake pressure is at a level commensurate 
with the vehicle load. 
Vehicles employing load dependent, brake force regulation on only one axle, 
for example the rear axle, are subject to wheel lock-up on the front axle 
under certain braking conditions. During brake requirements, a partially 
or fully loaded vehicle has its rear axle brake pressure modulated in 
accordance with the rear axle load, while full brake pressure is directed 
to the front axle. Due to the reduced rear axle brake force, the operator 
may find it necessary to increase the brake pressure to properly control 
the vehicle, thus causing the front axle wheels to lock up. In addition, 
brake shoe lining wear on the front axle wheels is accelerated. 
In order to counteract these disadvantages, load dependent or variable load 
valves of the above type have been developed to withhold brake pressure 
regulation during the initial phase of a relatively heavy brake 
application, or throughout the brake process during a relatively light 
brake application. These developments rely upon preadjustment of 
components at the factory and thus have the disadvantage of not being able 
to be adjusted in the field for adaptation to different conditions of 
vehicle utilization except by disassembly of the device. 
In copending application, Ser. No. 139,616, assigned to the assignee of the 
present invention, there is provided an arrangement by which an external 
adjustment can be made to set the desired level of normal brake pressure 
development prior to initiation of load-dependent brake pressure 
regulation. Such an arrangement permits field adjustment to accommodate 
different conditions of use of the vehicle, but introduces the possibility 
of the normal brake pressure development being adjusted such as to 
adversely affect the range of load-dependent pressure regulation. For 
example, the adjustment might be such that the initial brake pressure 
development continues until a relatively high pressure is reached before 
the load-adjustment pressure phase begins. In such a case, only a 
relatively short range of load-dependent brake pressure regulation is 
possible, and depending upon the condition of use of the vehicle, may be 
undesirable. 
SUMMARY OF THE INVENTION 
Accordingly, the object of the present invention is to modify the external 
adjustment arrangement mentioned above in such a way that the final 
load-dependent brake pressure can be kept within a predetermined range, 
while at the same time permitting the initial brake pressure development 
to be varied. 
In accomplishing this objective, a control piston and a compensating piston 
of the variable load valve device are coaxially arranged and 
interconnected by an arrangement of two oppositely pivotable levers 
separated by a movable fulcrum member. One of the levers is engageable 
with the compensating piston and the other lever is connected to the 
control piston. A normally seated valve element that is engageable with 
the compensating piston is unseated in response to actuation of the 
control piston by the supply of brake pressure to the control piston 
operating chamber. The unseated valve conducts brake pressure via the 
unseated valve element to the brake cylinders and to a pressure chamber 
containing the compensating piston. The force of this pressure acting on 
the compensating piston is amplified through the lever system to 
counteract the opposing force of the control piston. When the control 
piston force is counterbalanced, the compensating piston allows the valve 
element to be seated, terminating further buildup of brake pressure. 
At the end of the one lever opposite its pivot end is a pin that projects 
through an opening in the casing and is formed with threads to receive an 
adjusting nut. Between the adjusting nut and a compression spring is a 
piston via which the spring compression may be adjusted by the threaded 
pin and nut. This piston is subject to the delivery fluid pressure acting 
on the compensating piston such as to either counteract or supplement the 
spring force, thus providing for full adjustment of the normal brake 
pressure development prior to load-dependent control, without limiting the 
available control range desired for a specific condition of use of the 
vehicle.

DESCRIPTION AND OPERATION 
In the variable load valve body 1, there is situated a control piston 2, 
the upper chamber 3 of which is connected with the atmosphere. Below the 
control piston 2, there is a chamber 4 that is connected via an inlet 
connection 5 to an operator's brake valve (not shown). A valve element 6 
forms, in conjunction with a valve seat 7, an inlet valve 6, 7 opening 
into a chamber 8, and in conjunction with a valve seat 9, an outlet valve 
6, 9 venting to the atmosphere. An outlet connection 10 leading to brake 
cylinders (not shown) also opens into chamber 8. Control piston 2 is 
connected to a piston rod 11 that passes through valve element 6 and 
projects into a compensating piston 12, that is coaxial with control 
piston 2. 
At the lower end of piston rod 11, there is attached a fork member 13, to 
the lower end of which is hinged a lever 15 by means of a pin 14, which 
lever is also pivotable on a pin 16 fastened to the body 1. In conjunction 
with a fulcrum member 17 configured as a roller, and a lever 18 pivotable 
on a pin 19 fastened in the body 1, the lever 15 forms a mechanical 
amplification device for transmitting the forces arising on control piston 
2 and compensating piston 12. Piston 12 is in contact at its lower end 
with the upper side of lever 18, whereas its upper end serves as the 
outlet-valve seat 9, which bears on valve element 6. An annular chamber 20 
above compensating piston 12 is connected with chamber 8 via a bore 21. A 
chamber 22 below piston 12 is continuously connected with the atmosphere 
via an exhaust port and filter 23. A compression spring 24 connects piston 
12 with piston rod 11 of control piston 2. 
Fulcrum member 17 is supported in a mounting 25 with a guide extension 26. 
Connected to mounting 25 is a piston rod 27 of a piston 28, in a chamber 
29 that is subject to a pressure medium provided by a load-dependent 
pressure from the pneumatic-cushion bellows (not shown) and introduced via 
the connection 30. 
The slide extension 26 projects movably into a bore in the body. Carried in 
a recess of the slide extension 26, between the mounting 25 and an 
adjustable stop, there is situated a pretensioned adjustment spring 32. 
The operation of the automatic load-dependent braking-force regulator is as 
follows: 
The compressed air directed into chamber 4 by the brake valve during a 
brake application passes into chamber 8 via the open inlet valve 6, 7 and 
then into the brake cylinder via outlet connection 10. Simultaneously, 
this pressure in chamber 4 lifts the piston 2, which in turn transmits its 
movement to fulcrum member 17 and lever 18 via piston rod 11, with the 
fork 13 attached to it and the lever 15 hinged to it by means of the pin 
14. In addition, the compressed air passes via bore 21 into annular 
chamber 20 above piston 12 and forces the latter against the lever 18. 
The force of piston 12 sufficient to overcome the opposing force of the 
lever 15 controlled by the piston 2 now depends on the position of the 
support 17, which is determined by the movement of control piston 28 
controlled by the air-cushion pressures via the connection 30. As soon as 
this occurs, piston 12 moves downward, allowing valve element 6 to close 
the inlet valve 6, 7. 
On partial release of the braking, control chamber 4 is partially vented. 
This disturbs the force equilibrium on piston 12. The pressure in chamber 
20 prevails and moves piston 12 downward, thereby disengaging valve 
element 6 to open outlet valve 6, 9 until a new state of equilibrium is 
reached. On further decrease in pressure in chamber 4, the release of 
brake pressure is effected in the same manner until the pressure in both 
chambers 4 and 20 is depleted. A relief valve (not shown) may be employed 
to assure that no residual pressure remains in outlet connection 10, when 
the pressure at inlet connection 5 has been depleted. 
If there is no longer any brake pressure present, then the piston 2 is 
released. Its force working against the downward pressure of piston 12 is 
lost, whence the piston 12 is moved downward further by the pressure still 
present in chamber 20. By this action, the outlet valve 6, 9 opens and the 
brake cylinders are vented through a bore in piston 12 and via a vent 23. 
Since the chamber 20 also becomes free of pressure, the spring-loaded 
piston 12 again moves upward, lifting the valve element 6, thus opening 
inlet valve 6, 7, to restore the variable load valve device to its 
original release position. 
In accordance with the invention, there is attached to the one lever 18 a 
pin 33 that projects through an opening formed by casing section 34 in 
body 1. The projecting end of pin 33 is threaded to receive a nut 37. A 
spring 35 encircles pin 33 and rests between the body 1 and a piston 36 
that bears against nut 37. By adjusting nut 37, the effective length of 
spring 35 and thus its compressive force may be varied to provide a 
desired bias on piston 12 in the opening direction of valve element 6. 
In FIG. 1, one form of piston 36 is provided with an annular pressure area 
38 in a chamber 39 formed by the casing section 34 and piston 36. This 
chamber 39 is connected with chamber 4 via a passage 40, such that the 
brake pressure P.sub.1 in chamber 4 acts on piston 36 to counteract the 
compressive force of spring 35 exerted on lever 18 via pin 33. 
As shown in FIG. 3, the resulting pressure P.sub.2 delivered to the vehicle 
brakes follows the characteristic curve (b), which has a more shallow 
slope than that of curve (a), representing the relationship of pressures 
P.sub.1 versus P.sub.2 prior to the present invention; i.e., the pressure 
curve produced by the arrangement of the invention in copending 
application, Ser. No. 139,616. 
In FIG. 2 is shown a piston 36a, which corresponds to piston 36 in FIG. 1, 
but is arranged with an annular pressure area 38a in a chamber 39a formed 
by the casing section 34a and piston 36a, so that the brake pressure 
P.sub.1 acts on piston 36a to supplement the compressive force of spring 
35a, instead of counteracting it, as in FIG. 1. 
The resultant pressure P.sub.2 delivered to the vehicle brakes in this 
arrangement is represented by characteristic curve (c) in FIG. 3, which 
has a steeper slope than that of curve (a). 
In accordance with the present invention, the piston arrangement of FIG. 1 
is employed, when it is desired to increase the initial pressure 
development prior to load-dependent brake control, as by adjusting pin 33 
to increase the compression of spring 35. The effect of this is to raise 
the break point of curve (b) which, however, produces a final 
load-dependent brake pressure P.sub.2 corresponding substantially to brake 
pressure curve (c), due to the fact that the effect of piston 36, in 
counteracting the spring force, produces a more shallow pressure curve 
(b). 
Conversely, the piston arrangement of FIG. 2 is employed, when it is 
desired to decrease the initial pressure development prior to 
load-dependent brake control, as by adjusting pin 33 to decrease the 
compression of spring 35. The effect of this is to lower the break point 
of curve (c), which, however, produces a final load-dependent brake 
pressure P.sub.2 corresponding substantially to brake pressure curve (c), 
due to the fact that the effect of piston 36a, in counteracting the spring 
force, is to produce a steeper pressure curve (c).