Massey Ferguson 5400 - Issue 1.b
09D01.11
Trailer braking - Open centre
D . Partial and maximum trailer
braking
Partial trailer braking (Fig. 8)
The pressure Pb in the trailer braking line (pressure
acting on the active surface (14) of control spool (2)) is
in balance with pressure Py (coming from the tractor
braking system) which acts upon piston (6) of the pilot
flow housing (5).
The channel from port B (to the trailer brake connector)
is cut off from the return port R. Consequently, the oil
is trapped in the trailer braking unit. When the pres-
sures are balanced, the control spool (2) is in position
d.
The flow control valve (1) then moves to position a, an
open position in which it does not ensure a regulating
function.
As when the trailer brakes are released, the flow Qp -
Qx is directed to port N (continuity with the auxiliary
spool valve and the lift spool valve) and the control flow
Qx is directed to return channel R by the control spool
(2).
Maximum trailer braking (Fig. 9)
Limited braking pressure
The flow control valve (1) and the control spool (2) are
in the same switching positions (a and d) as for partial
braking.
The flow rates Qp and Qx are the same as for partial
braking. The maximum allowable braking pressure is
reached (Pb = 150 bar). An increase in the trailer
braking pressure cannot take place, even if the tractor
braking pressure continues to increase.
The pressure relief valve (4) is then pushed to the left.
The springs (8) (15) pre-loaded with the maximum
admissible braking pressure (Pb = 150 bar) for the
trailer, are compressed.
If, due to external conditions, the trailer braking pres-
sure Pb continues to rise, the control spool (2) momen-
tarily opens the communication f between port B
(supply to the trailer brake connector) and the return
channel R.
In all operating positions of the trailer braking valve, the
auxiliary spool valve and the lift spool valve are used
when required by port N (Fig. 11) without causing
significant effect on the trailer brake system, which
has priority on the high flow rate hydraulic circuit.
Fig. 11
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