BENDIX MC-12 MODULATOR CONTROLLER ASSY Guida alla Risoluzione dei Problemi Scaricare il pdf (Pagina 56)

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We saw earlier that normal atmospheric air is really at

a pressure of 14.7 pounds per square inch. In Figure

11, the reservoir on the right has a volume of six cubic

feet. When another six cubic feet of air is added into

it, the gauge pressure of the air in the reservoir, which

originally read zero, will rise to 14.7 pounds. It follows

that each time a quantity of free air equal to the volume

of the reservoir is forced into it, the gauge pressure will

rise another 14.7 pounds per square inch.

In Figure 12, we see a piston with an air-tight chamber

behind it. When compressed air enters the chamber

it will cause the piston to move until it encounters

a resistance equal to the force developed by the

compressed air. Because the air pressure is based on

pounds per square inch, it follows that the compressed

air will develop a force in pounds on the movable object

equal to the product of the air pressure multiplied by

the effective area of the movable object. If a piston or

a ﬂ exible diaphragm in a brake chamber has an area

of ten square inches and air at ﬁ ve pounds per square

inch pressure is acting on the piston or diaphragm, the

developed force will be 50 pounds. Similarly, if air at a

pressure of ten pounds per square inch is acting upon

it, a force of 100 pounds will be developed.

One key point is that the quantity of air acting on

the piston or diaphragm does not affect the force

developed. The only factors involved are the air

pressure and the area of the piston or diaphragm on

which the air pressure is acting. This means that we

can control the force applied by the braking system

by controlling the air pressure.

The pressure exerted by compressed air is not only

developed in all directions, but it is also equal in all

directions. The compressed air in a reservoir exerts

pressure equally in all directions against the entire

inside surface of the reservoir (the pressure of the

compressed air being overcome by the mechanical

strength of the reservoir walls). Similarly, the force

developed by the air pressure acting on one side

of a piston or a diaphragm may be overcome by an

opposing force acting on the opposite side, and the

opposing force may be compressed air or it may

be mechanical. If the opposing forces are equal,

a balanced condition is reached and there is no

movement of the piston or diaphragm. If the opposing

forces are not equal, the piston or diaphragm will

move, if possible, to assume a position where the

opposing forces are equal. See Figure 13.

This law of balanced pressures and forces is the basic

principle governing the design and operation of the

control and actuating devices in an air brake system.

FIGURE 12 - FUNDAMENTALS OF COMPRESSED AIR

FIGURE 13 - FUNDAMENTALS OF COMPRESSED AIR

Properties of Compressed Air (continued)

Where Supply B

pressure is greater

than Supply A

pressure

Where Supply A

pressure is the same

as Supply B pressure

1 2 ... 51 52 53 54 55 56 57 58 59 60 61 ... 69 70

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BENDIX MC-12 MODULATOR CONTROLLER ASSY Guida alla Risoluzione dei Problemi Pagina 56

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