Hydro-pneumatic actuator with automatic slack adjuster

A hydro-pneumatic actuator for providing a two-stage buildup of the hydraulic output pressure. A primary hydraulic piston, to which a pneumatic input piston actuator is connected, acts in a larger bore of a hydraulic cylinder to transfer fluid via a central passage in a spring-loaded, differential area, secondary hydraulic piston, that is coaxially disposed in the hydraulic cylinder in spaced-apart relationship with, and downstream of, the primary piston. A larger face of the secondary piston is in the larger bore and the smaller face is in a reduced diameter bore of the hydraulic cylinder, so that a pressure buildup in the hydraulic cylinder effects actuation of the secondary piston to close a check valve and thereby inter-fluid communication between the larger and smaller bores of the hydraulic cylinder via the central passage. The smaller face of the secondary piston is therefore effective to transmit the hydraulic force with a high gain multiplication factor following attainment of an initial predetermined force in low gain. The arrangement includes a displacement cylinder to measure the pneumatic piston stroke and an expansion chamber into which fluid is drawn from the reservoir when piston stroke is excessive for subsequent transfer to the hydraulic cylinder.

BACKGROUND OF THE INVENTION 
The present invention is related to hydro-pneumatic type brake actuators 
and more particularly to brake actuators of the above type, which are 
suitable for use in freight type railway brake systems. 
At the present time, railroads typically employ traditional automatic air 
brake systems. Each car in a train is normally equipped with auxiliary and 
emergency reservoirs which are charged from a brake pipe extending through 
the train, and a control valve which responds to changes in the brake pipe 
pressure to control the flow of air to and from the car brake cylinders. 
Since the railroad industry has standardized on relatively low braking 
pressures, and practical considerations limit the diameter of the car 
brake cylinders, it has become necessary to employ force-multiplying 
linkages between the brake cylinder and brake shoes in order to obtain the 
high braking forces required at the brake shoes. Such a brake rigging 
arrangement inherently increases the stroke of the brake cylinder piston 
required to move the brake shoes enough to take up the clearance space 
between the brake shoes and wheel treads. Accordingly, the brake cylinder 
clearance volume, or in other words the piston stroke required to bring 
the shoes into wheel contact, must be relatively large and thus requires a 
considerable amount of air. This in turn requires relatively large air 
reservoirs, which are space consuming and thus impose a further 
restriction on the area needed for the force-multiplying brake rigging. 
The gradual acceptance of hydraulic brake systems in the rapid transit 
segment of the railway industry suggests the possibility of using 
hydraulics as a means of transmitting brake forces to the brake shoes in 
freight type service. Such an approach would be advantageous in 
eliminating the need for the cumbersome, mechanical brake rigging 
presently employed on freight cars to transmit the brake cylinder forces 
to the brake shoes. Ideally, such an arrangement would require only a 
single hydro-pneumatic actuator device on each car corresponding to the 
brake cylinder in a conventional mechanical brake rigging system. 
Mechanical advantage sufficient to obtain the desired high brake shoe 
forces would be obtained by a large ratio piston of the hydraulic 
actuator. Because of this high ratio piston and the brake shoe clearance 
to be taken up, the stroke of the actuator piston in the hydraulic 
actuator would be necessarily large, and thus require a considerable 
amount of air simply to bring the brake shoes into braking engagement with 
the wheel treads. This would unduly enlarge the size of the hydraulic 
actuator, as well as require relatively large air reservoirs. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a compact, low-cost, yet 
highly reliable hydro-pneumatic brake actuator device having a high ratio 
input to output hydraulic pressure developed in two-stages, in order to 
reduce the piston stroke required to take up the clearance between the 
brake shoes and wheel treads of a railway vehicle. 
Another object of the invention is to provide a hydro-pneumatic actuator of 
the above type, which automatically compensates for variations in the 
clearance between the brake shoes and wheel treads due to brake shoe wear 
or replacement, for example, in order to maintain the stroke of the 
pneumatic piston and thus the air demand constant. 
Briefly, the hydro-pneumatic actuator according to the present invention 
converts an input force, such as the pneumatic output of an air brake 
control valve device, into a proportionally higher hydraulic pressure. 
During the first stage of operation, a pneumatic piston drives a primary 
hydraulic piston having a relatively large pressure area in order to 
obtain a large volumetric displacement of hydraulic fluid for a given 
displacement of the pneumatic piston. The pneumatic piston displacement is 
such that when brake shoe/wheel engagement occurs, the primary hydraulic 
piston will simultaneously develop hydraulic force to actuate a secondary 
hydraulic piston having a smaller pressure area than the primary piston 
pressure area. This results in a higher force multiplication being 
obtained after the brake shoe clearance has been taken up. The initial 
multiplication ratio in effect during movement of the brake shoes into 
braking engagement permits a shorter piston stroke and therefore less air 
consumption. 
Overtravel of the pneumatic piston is compensated for by drawing hydraulic 
fluid from the reservoir into a fixed volume displacement chamber during 
the application stroke. In the event overtravel occurs due to brake shoe 
or wheel wear, the hydraulic fluid drawn from the reservoir is forced into 
the hydraulic system to make up for the additional slack created by the 
shoe/wheel wear. In the event of undertravel, as occurs following brake 
shoe changeout, the excess hydraulic fluid in the hydraulic system is 
forced back into the reservoir through the pressure relief check valve 
during the application stroke.

DESCRIPTION AND OPERATION 
The hydro-pneumatic brake actuator 1 comprises a pneumatic cylinder 2 and a 
hydraulic cylinder 3. Reciprocally disposed within a bore 4 of pneumatic 
cylinder 2 is a pneumatic piston 5 having a return spring 6 that urges the 
pneumatic piston toward its release position, as shown. An end cover 7 
cooperates with cylinder 2 and piston 5 to form an actuator chamber 8 to 
which air may be connected via a pipe fitting 9 in end cover 7. For 
example, a standard AB, ABD, or ABDW type air brake control valve device 
10 may be employed as the source of control air supplied to fitting 9. 
A hollow push rod 11 connects piston 5 to a primary hydraulic piston 12 
that operates in a bore 13 of hydraulic cylinder 3. The hollow of push rod 
11 forms a displacement cylinder 14 having a displacement piston 15. A 
passageway 16 in piston 5 connects chamber 8 to the face of piston 15. As 
viewed in the drawing, the left side of hydraulic piston 12 cooperates 
with cylinder 3 and an end wall 17, through which hollow push rod 11 
passes, to form an expansion chamber 18. A passageway 19 in the forward 
end of displacement chamber 14 is connected to expansion chamber 18. 
Another passageway 20 connects expansion chamber 18 to a reservoir 21 in 
which hydraulic fluid is stored. A one-way check valve 22 permits fluid to 
flow from reservoir 21 to expansion chamber 18 and prevents fluid flow in 
the opposite direction. Another one-way check valve 23 in a branch passage 
24 of passage 19 connects hydraulic fluid from displacement cylinder 14 to 
the hydraulic cylinder 3 and prevents fluid flow in the opposite 
direction. 
In addition to bore 13, hydraulic cylinder 3 includes a reduced diameter 
bore 25. A secondary hydraulic piston 26 having a through passage 26a 
operates in hydraulic cylinder 3, with one end constituting a piston 
operating in bore 13 and the opposite end constituting a reduced diameter 
piston operating in bore 25. A stop 27 in bore 25 locates secondary piston 
26 in spaced-apart relationship with primary piston 12 under the influence 
of a bias spring 28. A second stage check valve 29 is biased by a spring 
30 aganst a stop 31 in bore 25, so as to be normally spaced-apart from the 
reduced diameter piston face of secondary hydraulic piston 26 by a slight 
amount when piston 26 is against stop 27. An outlet port 32 is provided in 
hydraulic cylinder 3 for connection to the hydraulic lines leading to the 
wheel brake cylinders in a railway vehicle brake system, for example. 
Also, a passage 33 opens into bore 13 behind the larger piston face of 
secondary piston 26 to connect hydraulic fluid from reservoir 21 to a 
chamber 34 containing spring 28. Another passage 35 opens into bore 13 
between the hydraulic pistons 12 and 26 to connect hydraulic fluid from 
reservoir 21 to hydraulic cylinder 3 via a one-way check valve 36. 
In an air brake controlled, hydraulic brake system for freight cars, the 
hydro-pneumatic actuator device 1 of the present invention responds to the 
air pressure from the air brake control valve device 10. This air pressure 
is supplied to chamber 8 and actuates pneumatic piston 5 in a rightward 
direction against the force of return spring 6. During the first stage of 
operation, the primary hydraulic piston 12, which is directly driven by 
pneumatic piston 5, displaces a relatively large volume of hydraulic fluid 
from cylinder 3 via passage 26a, unseated check valve 29 and outlet port 
32. The volumetric displacement of fluid is determined by the distance 
through which the brake shoes must move to engage the wheel treads for 
braking and by the size of the pistons in the vehicle brake cylinders that 
are operated by means of the hydraulic fluid pressure provided by actuator 
device 1. Passage 35 is located so that piston 12 normally passes just 
beyond the passage prior to brake shoe/wheel engagement, thus preventing 
fluid in cylinder 3 from being forced back into reservoir 21 via check 
valve 36. The bias spring of check valve 36 must be stronger than the 
resistance offered by the brake rigging to prevent the hydraulic fluid in 
cylinder 3 from backdumping into reservoir 21 prior to piston 12 passing 
over passage 35. The distance hydraulic piston 12 moves during this first 
stage of operation is such that brake shoe/wheel engagement is intended to 
occur prior to primary piston 12 engaging secondary piston 26. When 
shoe/wheel contact is made, any further displacement of primary piston 12 
rapidly builds up sufficient hydraulic pressure in cylinder 3 to force 
secondary hydraulic piston 26 rightwardly against the force of its spring 
28 until the smaller face of piston 26 engages second stage check valve 
29. This movement of secondary piston 26 is due to the differential 
pressure area between the larger and smaller piston faces on which the 
pressurized hydraulic fluid in cylinder 3 acts. 
Closure of check valve 29 terminates the first stage of operation, during 
which a relatively large volume of hydraulic fluid is displaced in 
hydraulic cylinder 3 for a given unit of travel of pneumatic piston 5, and 
initiates a second stage of operation, during which a higher force 
multiplication is achieved than during the first stage of operation. When 
check valve 29 closes, the hydraulic pressures are generated via the 
smaller face of secondary piston 26, thus providing a higher input to 
output multiplication ratio than during the first stage of operation. 
Since no further hydraulic expansion occurs during this second stage of 
operation, it will be appreciated that the pneumatic piston stroke is 
complete and thus the higher force multiplication in effect at this point 
is of no consequence insofar as affecting the pneumatic piston stroke and 
thus the air requirement. The two-stage operation thus affords the 
necessary force multiplication to produce the desired brake shoe forces 
without the accompanying adverse effect (high air consumption) of a long 
piston stroke and large piston volume found in single stage actuator 
devices that are required to produce high ratio input/output forces. 
In order to maintain proper brake shoe clearance and piston stroke, an 
automatic double-acting, hydraulic slack adjusting arrangement is 
provided, which compensates for over-travel of pneumatic piston 5 due to 
brake shoe/wheel wear and undertravel due to the replacement of worn brake 
shoes with new brake shoes. The combined effect of the pneumatic piston 
actuating air supplied to one side of displacement piston 15 via passage 
16 and the reduction of pressure on the other side, brought about by the 
volumetric increase of expansion chamber 18, as the hydraulic piston 12 
advances in bore 13 of cylinder 3, causes displacement piston 15 to force 
the hydraulic fluid in displacement cylinder 14 into the voided volume of 
expansion chamber 18 during the stroke of pneumatic piston 5, until 
displacement piston 15 bottoms out at the end of the displacement 
cylinder. The volume of fluid in displacement cylinder 15 is exactly equal 
to the voided volume of expansion chamber 18 for the desired stroke of 
pneumatic piston 5 required to move the brake shoes into brake engagement 
with the wheel treads. 
If the stroke of pneumatic piston 5 exceeds the desired distance, due to 
brake shoe/wheel wear having increased the brake shoe clearance, for 
example, an additional voided volume of expansion chamber 18 will occur as 
hydraulic piston 12 advances further into hydraulic cylinder 3 than 
normal. This additional voided volume is supplied with hydraulic fluid 
from reservoir 21 via check valve 22 and passage 20 until brake shoe/wheel 
engagement occurs. 
During the following release stroke of piston 5 under the influence of 
return spring 6, in response to air pressure being released from actuating 
chamber 8 by control valve device 10, hydraulic fluid in expansion chamber 
18 is forced back into displacement cylinder 14, as hydraulic piston 12 is 
retracted in bore 13 of hydraulic cylinder 3, until displacement piston 15 
is reset in its leftward-most position. This fluid returned to 
displacement cylinder 14 is attributed to the desired normal piston 
travel. Any excess fluid in expansion chamber 18, due to overtravel, is 
subsequently forced through passages 19 and 24, and check valve 23 into 
hydraulic cylinder 3 by the continued retraction of hydraulic piston 12. 
In this way, additional hydraulic fluid consistent with the accumulated 
overtravel of piston 5, due to brake shoe/wheel wear, is supplied to the 
hydraulic system to take up the excess clearance space between the brake 
shoes and wheel treads in order to maintain both the desired shoe 
clearance and the desired pneumatic piston travel. 
When worn brake shoes require replacement, the maintained brake shoe 
clearance will be reduced by the difference between the new and worn shoe 
thickness. Consequently, the piston travel will be reduced on the initial 
brake application following brake shoe change-out. Consequently, brake 
shoe/wheel engagement will occur prior to hydraulic piston 12 passing over 
passage 35. Hydraulic fluid force is developed in cylinder 13 sufficient 
to overcome the bias spring of check valve 36 and thereby accommodate 
fluid flow into reservoir 21 via passage 35 and check valve 36 to allow 
continued movement of pneumatic piston 5 and hydraulic piston 12 until the 
latter crosses passage 35. This reduces the amount of active hydraulic 
fluid in the hydraulic system to allow increased retraction of the brake 
shoe during a subsequent brake release to thereby reestablish the desired 
brake shoe clearance. 
From the foregoing description and operation, it will be seen that any 
leakage of hydraulic fluid past any of the three high-pressure seals in 
primary piston 12 and the differential pistons of secondary piston 26 will 
bleed directly or indirectly back into the storage reservoir 21, thus 
minimizing hydraulic fluid loss.