Bendix Commercial Vehicle Systems SD-3 SAFETY ACTUATORS - Manual (Page 2)

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DESCRIPTION
The DD-3 ™ and SD-3 ™ are air operated actuators with a
mechanical push shaft lock. The DD-3 ™ actuator (see Fig.
1) is a double diaphragm device which provides braking for
service, emergency, and parking. It is available in two sizes:
type 24 and type 30.
The SD-3 ™ actuator is a single diaphragm device used
primarily on off-highway vehicles. It provides service braking
and parking. It is available as a diaphragm type in type 30
(see Fig. 2) and in rotochamber configuration in type 36 and
50 (see Fig. 3).
The locking mechanism function is identical in all SD-3 ™
and DD-3 ™ actuators.
In normal operation, the SD-3 ™ or DD-3 ™ safety actuator is
similar to a brake chamber or rotochamber. However, the
addition of a locking roller mechanism locks the push rod
in the extended position unless the rollers are held out of
place by the application of air pressure at the lock port A
(Fig. 3).
In the normal operating position, air pressure is applied to
lock port A and into cavity B. This pressure acting on piston
C moves it forward, contacting rollers D, pushing them up
ramp E. Spring G then holds rollers D between ramp E and
collar ramp F and away from push rod H. The push rod is
then free to move back and forth with the application and
release of service air pressure.
In the park position, air pressure is vented from lock port A
and cavity B, permitting piston C to move back. Spring G
forces rollers D against collar ramp F and push rod H.
During this sequence, a parking brake application has been
made with air pressure. However, if air pressure behind
diaphragm J is lost for any reason, push rod H will attempt
to move back to the released position but the wedge shape
of collar ramp F will force the rollers against the push rod,
thereby preventing any return movement of the push rod.
The brake application is now held mechanically and will be
maintained until air pressure is again applied to the lock
port. The rollers will be released when the push rod is slightly
extended with a service application.
OPERATION - DD-3 ™ ACTUATOR (FIG. 1) -
NORMAL RUNNING
For normal operation full reservoir pressure is applied
continuously at the lock port, keeping the rollers disengaged
and permitting the shaft to extend and retract unrestricted.
Applications may be made by means of applying air
pressure on either the service diaphragm or the auxiliary
diaphragm. The auxiliary diaphragm has about 80% of the
effective area of the service diaphragm.
EMERGENCY
The auxiliary diaphragm of the DD-3 ™ actuator may be
connected to an alternate source of air pressure thus
providing an emergency capability.
PARKING
The DD-3 ™ actuator parking function is activated by releasing
the air pressure from the lock port and making a brake
application. The application may well be made from the same
alternate source as used for the emergency application. It
should preferably be made on the auxiliary diaphragm for
reasons which will be explained under “releasing.”
If the air application is released or leaks off, the shaft will
retract slightly and then be clamped by the wedging action
of the rollers. The brakes may now be considered to be
applied and to remain applied regardless of any exhaustion
of energy or loss of air pressure.
RELEASE OF PARKING APPLICATION
To release a parking application, air pressure in excess of
40 psi must be applied at the lock port and a brake application
slightly greater than the locked-in application must be made.
Thus if the park application is made with the auxiliary
diaphragm, an application of equal air pressure on the larger
area service diaphragm will provide the heavier application
required to free the lock rollers and release the park
application.
OPERATION - SD-3 ™ ACTUATOR (FIG. 3) -
NORMAL RUNNING
For normal operation full reservoir pressure is applied
continuously at the lock port, keeping the rollers disengaged
and permitting the shaft to extend and retract unrestricted.
In this mode, the SD-3
™ actuator functions the same as a
standard brake chamber or rotochamber.
PARKING (SD-3 ™ ACTUATOR)
The SD-3 ™ actuator may be used for parking by releasing
the air pressure from the lock port and making a limited
application at a pressure somewhat less than the cut-in
pressure of the air governor. For instance, if the governor
cut-in pressure is 85 psi, the suggested parking pressure
would be 75 psi. As the application is released, the lock
mechanism will engage the shaft and retain the park
application.
FIGURE 3
J
B C E A H
G
D
F