Variable load valve device having adjustable bias means to withhold proportional brake control during low level brake requirements

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 proportional control of the brake pressure, there is provided an externally accessible screw adjustment for varying the tension of a bias spring acting on one of the levers, whereby the proportional brake control is withheld until a predetermined level of brake pressure is achieved corresponding to the spring bias.

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. 
SUMMARY OF THE INVENTION 
It is the object of the present invention, therefore, to provide a simple 
arrangement for adjusting a bias force on the components that control the 
variable load valve proportioning action without requiring disassembly of 
the device or replacement of parts. 
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. A compression spring is caged between this adjusting nut 
and the casing to urge the one lever and accordingly the compensating 
piston, which bears against the one lever, in the opening direction of the 
valve element. Thus by varying the adjustment of this spring, the 
influence of the lever system in affecting the proportional control of 
brake pressure is withheld until a desired brake pressure level is 
obtained. In that this adjustment can be accomplished externally of the 
variable load valve device, no disassembly or parts replacement is 
necessary to adapt the degree of proportional load brake control to 
vehicles having different conditions of utilization.

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 bear 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 brakingforce 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 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 washer 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. By reason of this bias force, a 
predetermined brake pressure must be established, as reflected in chamber 
20, before the force on piston 12 is effective through the lever system. 
Consequently, load-dependent proportioning of brake pressure is withheld 
until a predetermined brake pressure is developed. This predetermined 
brake pressure corresponds to the bias force provided by spring 35 acting 
on piston 12 via lever 18, and may be adjusted, as desired, from an easily 
accessible location that is external of the valve body.