Braking System - Treadle Valve
Section 250-0070
SM 1939 4-00
2
When the brake application force is reduced primary
piston (13) will rise. Inlet/exhaust valve (10) remains
closed but exhaust valve seat (12) will uncover
exhaust passage (6) until the No. 1 delivery pressure
falls sufficiently to restore the balance of forces on
primary piston (13). Primary piston (13) and exhaust
valve seat (12) will then descend to close exhaust
passage (6) again. As the No. 1 circuit delivery
pressure falls relay piston (5) will rise. Inlet valve (2)
remains closed but exhaust valve seat (4) will
uncover exhaust passage (6) until the No. 2 delivery
pressure falls sufficiently to restore the balance of
forces on relay piston (5). Relay piston (5) and
exhaust valve seat (4) will then descend to close
exhaust passage (6) again.
If the brake application force is increased, inlet/
exhaust valve (10) will open and the No. 1 circuit
delivery pressure will rise until a balance of forces
again exists across primary piston (13). The
increased delivery pressure will be transmitted via
transfer holes (9) to relay piston (5). Relay piston (5)
will descend to open inlet valve (2). The No. 2
delivery pressure then rises until a balance of forces
again exists across relay valve (5).
The No. 1 and No. 2 circuit delivery pressures are
therefore maintained at levels in proportion to the
degree of mechanical force exerted by the operator.
This self-lapping action will take place whenever the
brake application force is increaded or decreased.
Removal of the brake application force allows
primary piston (13) and valve stem (18) to make a
full upstroke. The No. 1 delivery pressure is then
swiftly and completely released through exhaust
passage (6). As primary piston (13) rises, the head of
valve stem (18) takes up the limited free movement
between itself and relay piston (5). Relay piston (5) is
then pulled up against spring (7) to ensure a swift and
complete release of the No. 2 circuit delivery
pressure through exhaust passage (6).
Should air pressure be lost from the No. 2 circuit, the
No. 1 circuit will continue to function as described
above. The seals (21) will effectively separate the
two brake circuits.
If air pressure is lost from the No. 1 circuit, the No. 2
circuit will continue to function as follows:
The brake application force will be transmitted to the
No. 2 circuit by primary piston (13) acting directly on
relay piston (5) after taking up the limited movement
between relay piston (5) and valve stem (18).
OPERATION
Numbers in parentheses refer to Fig. 1.
When treadle is pressed down, force is exerted on
spring seat (16), rubber graduating spring (17) and
primary piston (13). Primary piston (13) moves down,
its exhaust valve seat (12) closes exhaust passage
(6) in inlet/exhaust valve (10). The continued
movement of primary piston (13) then pushes inlet/
exhaust valve (10) off inlet valve seat (11). The No. 1
circuit supply air now passes to the delivery port.
As the air flows to the No. 1 delivery port it also
passes through transfer holes (9) to the relay piston
(5). Owing to the downwards force of spring (7) on
relay piston (5), the exhaust valve seat (4) has
already closed the exhaust passage in inlet valve (2).
The air pressure through transfer holes (9) now move
relay piston (5) further down to push inlet valve (2)
off the inlet valve seat (3). The No. 2 circuit supply
air now passes to the delivery port.
Because only a small volume of air needs to pass
through transfer holes (9) to move relay piston (5),
the action of the No. 2 circuit is almost simultaneous
with that of the No. 1 circuit. The No. 1 circuit leads
with a maximum differential of 0.2 bar (2.9 lbf/in²) at a
delivery pressure of 1 bar (14.5 lbf/in²). The
differential diminishes until perfect balance is
achieved at about 5 bar (72 lbf/in²).
The No. 1 delivery air flow continues until the
increasing air pressure under primary piston (13)
together with the upward forces of return spring (14)
and the spring in inlet/exhaust valve (10) overcomes
the brake application force on primary piston (13).
Primary piston (13) then lifts sufficiently to allow
inlet/exhaust valve (10) to close on inlet valve seat
(11) and cut off the No. 1 supply air without letting
exhaust valve seat (12) uncover exhaust passage
(6). The No. 1 circuit is now 'lapped' with both inlet/
exhaust valve (10) and exhaust passage (6) closed.
Similarly, the No. 2 delivery air flow continues until
the increasing air pressure under relay piston (5) and
the upward force of the spring in inlet valve (2)
overcome the downward forces of spring (7) and the
air pressure above relay piston (5). Relay piston (5)
then lifts sufficiently to allow inlet valve (2) to close
on inlet valve seat (3) and cut off the No. 2 supply
air without letting exhaust valve seat (4) uncover
exhaust passage (6). The No. 2 circuit is now 'lapped'
with both theinlet valve (2) and the exhaust passage
(6) closed.
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