Bendix Commercial Vehicle Systems SD-3 User Manual

Bendix® DD-3™ & SD-3™ Safety Actuators

SD-02-4600

SERVICE

DIAPHRAGM

AUXILIARY

DIAPHRAGM

RETURN

SPRING

REAR

SEAL

FIGURE 1 - DD-3

SEPARATOR

™

SAFETY ACTUATOR

PUSH PLATE

& SHAFT

ASSY.

EXHAUST

DIAPHRAGM

LOCKING

PISTON O-RING

GASKET

LOCKING

PISTON

CAP O-RING

ROLLER SPRING

FRONT SEAL

ROLLERS (8)

SPLASH PLATE

EMERGENCY

PORT

LOCKPORT

DRAIN SLOT

SERVICE PORT

FIGURE 2 - SD-3

™

SAFETY ACTUATOR

DRAIN SLOT

SERVICE PORT

LOCKING PORT

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.

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.

EAH

FIGURE 3

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.

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.

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