BENDIX MC-12 MODULATOR CONTROLLER ASSY Průvodce řešením problémů stáhnout pdf (Strana 27)

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Spring Bra ke Valves

The SR-1

™

spring brake valve is used in dual circuit

brake systems and serves two functions; during normal

operation, it limits hold-off pressure to the spring

brakes to 90 or 95 psi. If a loss of pressure occurs in

the rear brake service supply, the valve will provide

a modulated spring brake application, proportional to

the driver’s service braking pressure delivered to the

front axle.

The Bendix

SR-7

spring brake modulating valve

is used in dual air brake systems with spring brake

actuators. It provides a rapid application of the spring

brake actuator when parking, modulates the spring

brake actuator application (using the dual brake valve

should a primary failure occur in the service brake

system) and prevents compounding of service and

spring forces.

Simple Spring Brake Anti-compound

System

General Operation

Brake compounding can occur in a spring brake

parking system due to the direct in-line arrangement

of both the spring chamber and brake chamber. It

occurs in unprotected systems when parking AND

service brake applications are made at the same time.

An example of this situation occurs when a vehicle is

parked on a steep incline; the driver holds the service

brakes applied (preventing the vehicle from rolling

backward), then actuates the park control which

“sets” or applies the spring brakes. For a brief time,

the air applied service brakes and the mechanical

spring brakes both exert a braking force on the slack

adjusters and foundation brakes. The forces of the

spring and air applications are additive and can cause

damage to the foundation brake components (cam

shaft splines, shoes, drum, etc.) and/or slack adjuster.

An anti-compounding system is especially important in

protecting the adjusting mechanism of automatic slack

adjusters from damage caused by over-torque that

occurs during a compounded application of the brakes.

Anti-compounding prevents the simultaneous

application of both the air and spring brakes by

directing application air to the spring brakes when both

are applied at once. In the simple anti-compounding

schematic shown here, the double check valve allows

service application air to apply the service brakes AND

move into the spring cavity if they are also applied (no

air pressure and springs are also applying brakes).

Park Control

Valve

Spring Brake

Parking Quick

Release Valve

Service

Brake

Valve

Service Quick

Release

Valve

Double

Check

Valve

SR-7

Spring

Brake Valve

SR-1

™

Spring

Brake Valve

The anti-compounding function of the double check

valve is built into several air brake devices such as

the SR-7

, R-7

™

, R-8

™

, R-14

and QR-1C

valves.

When these devices are used in the system, a

separate double check valve for anti-compounding

is not needed.

It is recommended that the service connection to the

anti-compounding device (double check valve) come

from a point between the service brake chamber and

the ﬁ rst “upstream” service device (in this case a quick

release valve).

Spring Brake Valves

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