Section 250-0290
Braking System - Air Pressure Protection Valve
2
SM 1938 4-00
'21' and '22', while circuit '23' is fed through an element
which incorporates a non-return valve (Non-Feedback
Element). Should there be a failure in either of circuits
'21' or '22' then the pressures in circuits '23' and '24'
will be unaffected as their supply comes through non-
return valves. Similarly if there is a failure in circuit '24'
and circuit '23' will be unaffected because of its Non-
feedback element (i.e. it is protected to a minimum of
6 bar). However, if there is a failure in circuit '23' then
the pressure in circuit '24' will fall to 4 bar minimum at
which its element will close protecting it from any
further drop. As with circuits '21' and '22', the element
feeding the failed circuit will tend to remain closed
allowing the non-failed circuit to recharge.
Safety Function Operation
Letters in parenthesis refer to Fig. 3 & 4.
Under normal running conditions with all circuits
pressurised there is pressure on piston (A), fed from
circuit '21', which is sufficient to lift piston (A) and
compress spring (D) (refer to Fig. 3). In this condition,
hole (Z) in piston (A) is above 'O' ring (E) thereby
sealing off connection (Y), therefore the operation of
the pressure protection valve is unaffected by the
safety function.
However, if while the vehicle is stationary there is a
leak in the service reservoirs which is sufficient to
deplete the pressure in those reservoirs to below
3.5 bar ± 0.5 bar, then piston (A) will move down
under the force of spring (D) and hole (Z) will move
below 'O' ring (E) (refer to Fig. 4). In this condition,
holes (Y & Z) will be connected allowing air to pass
from circuit '23' to atmosphere through the hole in
spring retainer (B). The rate of air loss through is
16 to 35 Nl/min at 8 bar.
It is this action of the safety function which will drop the
pressure in the spring brake reservoir, thereby
preventing release of the spring brakes until the
pressure in the service reservoirs has built up
sufficiently to move up piston (A) to close off hole (Z).
REMOVAL AND INSTALLATION
Numbers in parentheses refer to Fig. 1.
The pressure protection valve is a non-serviceable
item and should be replaced completely, if damaged,
as follows:
Note: Tighten all fasteners to standard torques listed
in Section 300-0080, STANDARD BOLT AND NUT
TORQUE SPECIFICATIONS.
Fig. 3 - Safety Function - Disengaged
A - Piston
B - Spring Retainer
C - Circlip
D - Spring
E - 'O' ring
M - Valve Body
X - Connection Point
Y - Hole
Z - Hole
SM - 2449
Fig. 4 - Safety Function - Engaged
SM - 2450
A - Piston
B - Spring Retainer
C - Circlip
D - Spring
E - 'O' ring
M - Valve Body
X - Connection Point
Y - Hole
Z - Hole
M
B
C
D
E
Z
A
X
Y
M
B
C
D
E
Z
A
X
Y
Содержание TA30
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