Advertisement
1 Introduction
Automatic adjustment
The automatic adjuster makes an adjustment to the operation of
the brake pad to compensate for pad wear. Every time the brake
is applied, the system senses whether adjustment is required or
whether the running clearance of the brake pads to the brake
rotor is still within the built-in tolerance and does not need to be
adjusted.
The built in tolerance is determined in the design by the clearance
between the ball-ended drive pin (44) that is rigidly fixed to the
operating shaft and the fork on the end of the adjuster shaft (43).
a) Operation without adjustment
From the rest position, the push rod of the air cylinder moves
forward, rotating the operating shaft (21). Just as the pistons (35)
have moved forward by the inbuilt running clearance, the ball
ended drive pin (44) starts to contact the driving side of the fork
on the end of the adjuster shaft (43). Further movement of the air
actuator push rod rotates the operating shaft (21), now causing
the adjuster shaft (43) to rotate because the inbuilt clearance has
been taken up. The outer drive sleeve (39) is fixed to the adjuster
shaft (43) and rotates the inner drive sleeve (38) via the clutch
pack (37). The inner drive sleeve (38) is linked to the intermediate
gear (42) by a unidirectional friction spring (40) and this tries to
rotate the tappets (34). However, the friction in the threads of the
tappets (34) and pistons (35) has started to increase due to the
clamping force on the pads and this prevents the pistons and
tappets rotating relative to one another. The pistons cannot rotate
in the housing (41) and due to the high torque to turn the tappets
(34) the clutch pack (37) 'slips', preventing adjustment of the
mechanism below the correct running clearance.
12
ArvinMeritor Air Disc Brake ELSA 195
b) Operation with adjustment
As a result of pad or rotor wear, the running clearance is now
greater than the inbuilt tolerance and adjustment of the
mechanism is now required. From the rest position, the push rod
of the air cylinder moves forward, rotating the operating shaft
(21). Just as the pistons (35) have moved forward by the inbuilt
running clearance, the ball-ended drive pin (44) starts to contact
the driving side of the fork on the end of the adjuster shaft (43).
Further movement of the operating shaft, now causes rotation of
the adjuster shaft (43) via the ball-ended drive pin (44).
Driving through the clutch plates (37) and the unidirectional
friction spring (40), the intermediate gear (42) rotates. Due to the
excessive running clearance the tappets now rotate in the pistons.
The pistons (35) cannot rotate and are, therefore, wound out from
their housing (41). When the pads finally contact the rotor, the
clamping force increases the thread friction in the tappets (34)
and pistons (35). The torque to turn the tappets (34) then
increases and the clutch pack (37) driving the intermediate gear
(42) starts to slip, preventing further adjustment. The adjustment
is not wound back during the return of the actuation mechanism.
As the operating shaft (21) now returns to the brakes off position,
the ball-ended drive pin (44) travels back through the clearance in
the fork on the end of the adjuster shaft (43). Once this clearance
is taken up, the adjuster shaft (43) then rotates in the reverse
direction, rotating the inner drive sleeve (38) via the clutch pack
(37). However, in this direction, the unidirectional friction spring
(40) cannot drive the intermediate gear (42), leaving the tappets
(34) and intermediate gear (42) in the adjusted state.
The system is now once again in its starting position.
Advertisement
