Parking
Brake
The parking brake holds one or more
brakes continuously in the applied position. The parking brake employs
the regular drum brakes on the rear wheel. Instead of hydraulic
pressure, however, a simple mechanical linkage is used to engage the
brake shoes. When the parking-brake pedal is depressed (or, in some
cars, a hand lever is raised), a steel cable pulls taut a tension lever;
other cables draw the brake shoes firmly against the drums. The release
knob slackens the cables and disengages the brake shoes. The parking
brake is self adjusting. An automatic adjuster in the piston moves on
the thrust screw to compensate for lining wear.
Piston,
Rings, and Wrist Pin
The piston converts the potential energy
of the fuel into the kinetic energy that turns the crankshaft. The
piston is a cylindrical shaped hollow part that moves up and down inside
the engine's cylinder. It has grooves around its perimeter near the top
where rings are placed. The piston fits snugly in the cylinder. The
piston rings are used to ensure a snug "air tight" fit. The
wrist pin connects the piston to the connecting rod. Pistons are made of
aluminum, because it is light and a good heat conductor. The piston head
or "crown" is the top surface against which the explosive
force is exerted. It may be flat, concave, convex or any one of a great
variety of shapes to promote turbulence or help control combustion. In
some, a narrow groove is cut into the piston above the top ring to serve
as a "heat dam" to reduce the amount of heat reaching the top
ring.
Power
Brakes
Power brake are of four general types:
vacuum suspended; air suspended; hydraulic booster, and
electro-hydraulic booster. Most power brakes use vacuum suspended units,
which contain a large vacuum-powered booster device to provide the added
thrust to the typical power-brake. Pressure on the brake pedal pushes
forward a rod connected to the pistons of the two master cylinders. The
pistons begin forcing fluid into the front and rear brake lines. At the
same time, the brake-pedal pushrod positions the vacuum-control valve so
that it closes the vacuum port and seals off the forward half of the
booster unit. The engine vacuum line then draws off the air, creating a
low-pressure vacuum chamber. Atmospheric pressure in the control chamber
then pushes against the diaphragm, dividing the two chambers. The
pressure on the diaphragm, which is locked to the pushrod, forces it
forward, supplying even more pressure on the pistons. The safe driver is
always ready to apply the total force needed to stop their vehicle, even
if the engine quits (removing the power assist).
Push Rod
A push rod is a connecting link in an
operating mechanism. Two examples are the rod between the valve lifter
and rocker arm on an overhead valve engine, and push rods at the piston
ends which apply pressure to the brake shoes.
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