BENDIX DD-3-SD-3 SAFETY ACTUATORS Manuel - Page 2

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DESCRIPTION

The DD-3

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and SD-3

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mechanical push shaft lock. The DD-3

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.

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FIGURE 3

In normal operation, the SD-3

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.

are air operated actuators with a

actuator (see Fig.

™

C E

A H

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or DD-3

™

safety actuator is

OPERATION - DD-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.

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

connected to an alternate source of air pressure thus

™

providing an emergency capability.

PARKING

The DD-3

actuator parking function is activated by releasing

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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

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

standard brake chamber or rotochamber.

PARKING (SD-3

ACTUATOR)

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The SD-3

actuator may be used for parking by releasing

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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.

ACTUATOR (FIG. 1) -

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actuator may be

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ACTUATOR (FIG. 3) -

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actuator functions the same as a

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