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 TM 9-8000
positive In performance for they do not prevent the action
place. Therefore, in some conditions, the vehicle may
of the differential assembly from taking
move while the brake is applied.
Section V. MECHANICAL BRAKE SYSTEM
b. Two Cross-Shafts. In some mechanical brake
34-21. Means of Actuation. The energy supplied by the
arrangements, two cross-shafts (fig. 34-18) are
operator's foot in pushing down on the brake pedal is
provided, so that the hand brake is applied to the shoes
transferred to the brake mechanism on the wheels by
In the rear wheels only. The front and rear cross-shafts
various means. A mechanical hookup has been used
are connected together by an interconnecting rod Joined
since the earliest motor vehicles, but hydraulic pressure
to a lever on each shaft. An overrunning slot must be
is used most extensively at the present time.
provided on the interconnecting rod (fig. 34-18) and the
Mechanically operated braking systems are obsolete on
hand brake pull rod if the rear wheel brakes only are to
today's automobiles, but these systems may be used for
be applied by the brake hand lever. Some braking
a portion of the braking systems in many vehicles.
systems Incorporate additional shoes on the rear wheels
for the hand brake, in which case the hand brake has its
34-22. Hookup.
own cross-shaft and hookup, and is In no way connected
with the foot or service brakes.
a. Cross-Shaft. In a mechanical braking system, the
force applied to the pedal is transmitted to the brakes by
c. Front Wheel Control. The hookup of the brake rods
means of rods and cables. The mechanical linkage may
to the front wheels must be designed to allow the wheels
be arranged in various ways, but its operation Is
to turn without locking In any position. If the front wheels
essentially the same. In order to have all the wheel
lock, steering is destroyed. One means of controlling the
brakes applied uniformly, a cross-shaft is provided near
front wheels is the use of the brake rod attached to a
the center of the vehicle frame, on which the levers
camshaft, with a small universal joint above the steering
connected to the rods and cables leading to the wheel
knuckle pivot. A more popular and satisfactory method
brakes are mounted. The brake pedal also is connected
employs a flexible steel cable connecting the end of the
to a lever on the cross-shaft (fig. 34-18). When the pedal
brake rod to the brake camshaft lever (fig. 34-18).
Is depressed, the cross-shaft (fig. 34-18) is turned so
Because of the flexibility of the cable and its sheath,
that levers on the top of the shaft are turned backward
motion of the wheel does not affect tension on the cable.
and those on the bottom of the shaft forward. By
Flexible steel cables also may be used advantageously
connecting the front brake rods to the levers on the top
on the rear wheels because of the vibration of the wheels
of the shaft, and the rear brake rods to the levers on the
when traveling over the road. On some braking systems,
bottom of the shaft, all the brake rods are pulled together
the pressure on the wheel brakes is distributed evenly by
and the four wheel brakes are applied at the same time.
various types of equalizers. Equalizers are designed to
The usual practice In mechanical systems has been to
take up all the slack In the hookup to each brake so that
link the hand brake to the foot brake so that both operate
all brakes will be applied at the same time. This prevents
the same braking units. With such an arrangement, the
the possibility of too much pressure being applied to any
hand brake linkage Incorporates an overrunning slot in
one brake, which would lock that wheel and probably
the hand brake pull rod where It is connected to a lever
make the vehicle skid. One of the main reasons why the
on the cross-shaft. This allows the pedal linkage to
mechanical braking system has been supplanted Is the
operate without Interference from the hand lever. Both
difficulty of maintaining equal pressure on all brakes.
controls may be operated Independently by also
Incorporating an overrunning slot In the foot brake
linkage.
34-21
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