BENDIX MC-12 MODULATOR CONTROLLER ASSY Guide de dépannage télécharger pdf (Page 52)

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Introducing both weight and speed into the comparison

again, a 10,000 pound vehicle traveling 60 miles per

hour has 18 times as much energy of motion as a

5,000 pound vehicle traveling at 20 miles per hour.

If the same stopping power is used, a 5,000 pound

vehicle which stops from 20 miles per hour in 30 feet,

when loaded to 10,000 pounds and is traveling at 60

miles per hour will require 18 times (540 feet), or more,

to stop. Note: Many other factors, including the road

surface, brake friction material and tire condition, etc.

would also affect the stopping distance.

Leverage

Having reviewed the forces involved in braking a

vehicle, consideration must also be given to how

these forces are developed and directed to do the

braking work. Almost all braking systems make use

of one of the oldest mechanical devices governing the

transmission and modiﬁ cation of force and motion,

the lever.

A lever is deﬁ ned as an inﬂ exible rod or beam capable

of motion about a ﬁ xed point called a fulcrum, and it is

used to transmit and modify force and motion.

Figure 5 illustrates three simple types of levers; the

only difference in them being the location of the fulcrum

in relation to the applied force and the delivered force.

All shapes and sizes of levers used in a typical brake

system are one of these three types.

The simple law of levers is that the applied force

multiplied by the perpendicular distance between

the line of force and the fulcrum always equals

the delivered force multiplied by the perpendicular

distance between the fulcrum and the line of force.

Thus, with a leverage arrangement as shown in view

5(a), an applied force of 100 pounds two feet from

the fulcrum will give a delivered force of 200 pounds

at a point one foot from the fulcrum. With a leverage

arrangement as shown in Figure 5(b), an applied force

of 100 pounds three feet from the fulcrum will lift 300

pounds at a point one foot from the fulcrum.

Note that in both cases the delivered force exceeds the

applied force because the applied force is farther from

the fulcrum than the delivered force. With a leverage

arrangement as shown in Figure 5(c), the delivered

force is the farthest from the fulcrum; therefore, it is

less than the applied force. If the applied force in

this case is 300 pounds at a point two feet from the

fulcrum, the delivered force at a point three feet from

the fulcrum will be 200 pounds.

The delivered force of any lever is determined by

multiplying the applied force by the distance it is

from the fulcrum and then dividing this answer by the

distance the delivered force is from the fulcrum.

In determining the distance at which any force is

acting on a lever, the true length of the lever arm is the

perpendicular distance from the force to the fulcrum,

regardless of the shape of the lever. The lever arm

is always measured at right angles to the direction of

the force.

The product of the force acting on a lever, multiplied

by the distance the force is from the fulcrum, is called

the turning moment, and when this relates to a shaft,

it is called torque. The turning moment or torque

is usually expressed in inch-pounds, foot-pounds,

foot-tons, etc., depending upon whether the force is

measured in pounds or tons and whether the distance

is measured in inches or feet. As an example – a

force of 100 pounds acting on a lever arm ﬁ ve inches

long would result in a turning moment or torque of 500

inch pounds.

The most easily recognized lever in an air brake

system is the slack adjuster. The length of the lever

arm of a slack adjuster is always the perpendicular

distance between the center line of the brake camshaft

opening and the center line of the clevis pin.

Another form of lever – not always recognized – is the

brake cam. All brake cams are levers and are used to

transmit and modify the torque and turning motion of

the brake camshaft in such a way that the brake shoes

are spread and forced against the brake drum, not only

in the proper direction but also with the proper force.

Spreading the shoes in the proper direction, of course,

FIGURE 5 - LEVERAGE

5(a)

5(b)

5(c)

Leverage

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