engine at different rpm's, nearly all normal driving
200 pounds a distance of 165 feet in 1 minute. The
conditions can be simulated. The tests and checks can
amount of work involved here is 33,000 foot-pounds
be made without the interference of body noises, such
(165 times 200). If 100 pounds were lifted 330 feet, or if
as you would have when checking the vehicle on the
330 pounds were lifted 100 feet, the amount of work
flight line or deck.
would be the same, 33,000 foot-pounds. When this
amount of work is done in 1 minute, 1 horsepower is
TORQUE WRENCH. Torque is twisting, or
required. If it takes 2 minutes to do this amount of work,
turning, effort. When the lid on a jar is loosened, a
then 16,500 foot-pounds per minute, or 1/2 hp, would
twisting force, or torque, is applied to it. You apply
be required. Applying the formula for horsepower:
torque to a wrench when you tighten or loosen a nut.
ft - lb. per min
LW
Torque is measured in pound-feet (not to be confused
hp =
=
33,000 t
with work, which is measured in foot-pounds). For
33,000
instance, suppose you use a wrench to tighten a nut on a
L = length, in feet, through which W is moved
stud. If the handle of the wrench is 1-foot long and you
apply a 10-pound force to its end, 10 pound-foot of
W = force, in pounds, that is exerted through distance L
torque is applied to the nut. An illustration of a torque
t = time, in minutes, required to move W through L
wrench in use is shown in figure 3-11.
There are a number of devices that you may use to
Do not confuse torque with work or with power.
measure the horsepower of an engine. The most
Both work and power indicate motion, but torque does
common device used for this purpose is the
not. It is merely a turning effort the engine applies to the
dynamometer.
wheels through gears and shafts.
ENGINE DYNAMOMETER. An engine
FRICTION. Friction is the resistance to motion
dynamometer may be used to test an engine that has
between two objects in contact with each other. The
been removed from the vehicle it drives. If the engine
reason a sled will not slide on bare earth is because of
does not develop the manufacturer's recommended
friction. It slides on snow because snow offers little
horsepower and torque at a specific rpm, you know
resistance while the bare earth offers much resistance.
there is need for further adjustments and/or repairs to
Friction is both desirable and undesirable in an
the engine.
automobile or any other vehicle. Friction in the engine
CHASSIS DYNAMOMETER. The chassis
is undesirable because it decreases the power output; in
dynamometer is used for automotive service, since it
other words, it uses up some of the energy of the engine.
can give a very quick report on engine conditions by
This is overcome by using oil so the moving
measuring output at various speeds and loads. This type
components in the engine will slide or roll over each
of machine is very useful in shop testing and adjusting
other smoothly. Friction is desirable in clutches and
automatic transmissions.
brakes, because friction is exactly what is needed for
them to perform their function properly.
On the chassis dynamometer (fig. 3-10), the driving
wheels of the vehicle are placed on rollers. The engine
drives the wheels and the wheels drive the rollers. By
loading the rollers varying amounts and by running the
ASf03011
Figure 3-11.--Torque wrench in use, tightening main bearing
ASf03010
Figure 3-10.--Chassis dynamometer.
stud of an engine.
3-10
