BENDIX MC-12 MODULATOR CONTROLLER ASSY Guide de dépannage télécharger pdf (Page 60)

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II. Mechanical Systems

General

Vehicle manufacturers must also comply with

Government regulations for braking performance

(e.g. U.S. FMVSS 121, Canadian CMVSS 121).

As with the air or pneumatic side of the system,

the vehicle manufacturer must carefully design the

brake geometry, size and power of the foundation

brake components to achieve the desired vehicle

performance. Maintenance personnel must likewise

strive to maintain this performance.

Actuators

Brake chambers convert air pressure into mechanical

force. Always maintain the chambers to their original

performance condition. If chamber return springs

need replacement, the springs should be replaced with

springs of the proper spring load. When replacing,

remember the chamber return spring affects the net

force delivered by the chamber, and is especially

important in low service brake applications. For

this reason always replace the return spring on both

chambers on an axle.

Chamber diaphragm life will vary according to the type

of service and the diaphragm environment. Experience

will generally dictate replacement frequency, and it is

good practice to replace all diaphragms on the vehicle

at the same time.

Also, in the case of S-Cam brakes, make sure

the chamber push rods are in line with the slack

adjusters. If misaligned, the chamber rod can rub

on the chamber’s non-pressure plate and cause a

dragging brake.

Another factor that inﬂ uences the brake chamber

output force is the chamber push rod length. Ideally,

the push rod length should be adjusted so that when

the chamber reaches half its maximum stroke a 90

degree angle (approximately) is formed between the

slack adjuster and chamber push rod.

Foundation Brakes

Braking torque is established by the vehicle

manufacturer and is determined by the designed

axle weight. Brake size (diameter), brake block or

pad characteristics and the foundation brake design

(S-Cam, air disc, etc.) inﬂ uence brake torque. In

the case of S-Cam brakes, the torque is carefully

evaluated in relation to drum capacity, drum area, and

lining area and in the case of air disc brakes, rotor/pad

Air Brake System Balance: Mechanical Systems

speciﬁ cations. It is recommended that you consult the

vehicle manufacturer before attempting any changes

that would affect brake torque.

Mechanical friction within the foundation brake can

affect brake output torque. The components of the

foundation brake are subject to high forces and should

be periodically checked and lubricated as necessary.

Friction material (e.g. brake block and lining segments,

etc.) affect vehicle stopping performance and are a

factor in wear balance from front to rear. Replacement

material should be chosen for its stopping performance

as well as wear characteristics. At a minimum, always

replace friction material on both ends of an axle

whenever one side is being serviced. For rear axles,

it is recommended that friction material on all axles be

replaced at one time.

With air disc brakes, the rotor-friction couple is carefully

designed and validated for optimal performance.

When replacing air disc brake pads, always select

replacement pads that have been validated to perform

with the rotor used, in order to prevent premature or

uneven pad wear or damage to critical disc brake

components (e.g. cracked rotors) which can adversely

affect braking performance.

Brake Adjustment

One of the most important factors in obtaining

maximum mechanical output of the chamber is proper

brake adjustment. All chamber strokes should be

adjusted to approximately the same stroke, and the

adjustment should be made so that the chamber stroke

is as short as possible without the brakes dragging.

Improperly adjusted brakes waste air, reduce leverage

and contribute to poor brake performance. In many

cases, complaints of poor or insufﬁ cient braking power

can be taken care of by proper brake adjustment.

Brake System Maintenance

Because no two vehicles operate under identical

conditions, maintenance and maintenance intervals

will vary. Experience is a valuable guide in determining

the best maintenance interval for any one particular

vehicle.

In Section 9, Air Brake System Troubleshooting, you

will ﬁ nd some suggested tests for determining the

overall condition of the air brake system and to help

quickly locate problem areas. The tests check both

leakage and device function. Performing these tests

on a quarterly basis and recording the results of the

tests in the vehicle maintenance records would provide

a valuable basis for performance comparisons/trends

and assist in deciding maintenance intervals.

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BENDIX MC-12 MODULATOR CONTROLLER ASSY Guide de dépannage Page 60

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