Chapter 8 Machines
249
Transmissions
A transmission provides gear shifting to allow an increase or decrease
in speed without overworking either an engine in a vehicle or a human
pedalling a bicycle. See Figure 8-51. If you are a cyclist, you know that
it is very difficult for you to start in the highest gear. You must start out
in a low gear and gradually change to higher gears so that the speed of
pedalling remains relatively constant. The same is true for a car. Without
a transmission, a car would not have enough power to accelerate from a
standstill. When a transmission is in low gear, the engine has to turn
several times to make the drive shaft and the wheels turn once. As the
transmission moves through the gears, from low to high, the drive shaft
and the engine turn at approximately the same speed. The vehicle speed
increases and the engine speed drops.
An automatic transmission performs the same function as a standard
transmission, except that it shifts gears automatically by using internal
oil pressure. A third type of transmission is a continuously variable
transmission (CVT). This type uses belt systems to provide an infinite
number of gear ratios. It improves engine efficiency by allowing the
engine to match its speed to the load more efficiently. Currently there
are also many other choices that are partly manual and partly automatic
(automated manuals).
Rack
Pinion
gure 8-49. Rack-and-pinion gears
convert rotary motion to straight-line motion.
Cog wheel
gure 8-50. An inclined railroad with a cog
wheel and a drill press are both good examples
of rack-and-pinion gears.