The first windshield wipers were operated manually by
moving a lever inside the car back and forth. Today, most of
us take our electric windshield wipers for granted. The wipers
faithfully keep the window clear, moving back and forth across
the windshield countless times as they sweep the water away.
On their highest speed, they move impressively fast, sometimes
shaking the car from side to side. What kind of a mechanism
can move the wiper arms so effectively and so reliably?
Windshield wipers are found on car windshields, some car
and even on the space
shuttle. In this edition of HowStuffWorks,
we'll take a look inside windshield wipers, learn about the
blades and the controls and then explore a new rain-sensing
wiper control system!
Inside the Wipers The wipers combine two
mechanical technologies to perform their task:
A neat linkage converts the rotational output of the
motor into the back-and-forth motion of the wipers.
Motor and Gear
Reduction It takes a lot of force to
accelerate the wiper blades back and forth across the
windshield so quickly. In order to generate this type of
force, a worm
gear is used on the output of a small electric motor.
The worm gear
reduction can multiply the torque of the
motor by about 50 times, while slowing the output speed of the
electric motor by 50 times as well. The output of the gear
reduction operates a linkage that moves the wipers back and
Inside the motor/gear assembly is an electronic
circuit that senses when the wipers are in their down
position. The circuit maintains power to the wipers until they
are parked at the bottom of the windshield, then cuts the
power to the motor. This circuit also parks the wipers between
wipes when they are on their intermittent setting.
Linkage A short cam is
attached to the output shaft of the gear reduction. This cam
spins around as the wiper motor turns. The cam is connected to
a long rod; as the cam spins, it moves the rod back and forth.
The long rod is connected to a short rod that actuates the
wiper blade on the driver's side. Another long rod transmits
the force from the driver-side to the passenger-side wiper
Now let's talk about some details of the wiper blades.
Wiper Blades Wiper blades are like
squeegees. The arms of the wiper drag a thin rubber
strip across the windshield to clear away the water.
systems are designed to wipe 1.5 million times in their
When the blade
is new, the rubber is clean and has no nicks or cracks. It
wipes the water away without leaving streaks. When the wiper
blades age, nicks or cracks form, road grime builds up on the
edge and it doesn't make as tight a seal against the window,
so it leaves streaks. Sometimes you can get a little extra
life out of your wiper blade by wiping the edge with a cloth
soaked in window cleaner until no more dirt comes off the
This wiper blade is supported in six places
for an even pressure distribution against the
Another key to streak-free operation is even
pressure over the length of the rubber blades. Wiper
blades are designed to attach in a single point in the middle,
but a series of arms branch out from the middle like a tree,
so the blade is actually connected in six to eight places. If
ice or snow forms on these arms, it can make the distribution
of pressure uneven, causing steaks under part of the blade.
Some wiper manufacturers make a special winter blade with a
rubber boot covering the arm assembly to keep snow and ice
Pivot Points Most cars
have pretty much the same wiper design: Two blades move
together to clean the windshield. One of the blades pivots
from a point close to the driver's side of the car, and the
other blade pivots from near the middle of the windshield.
This is the Tandem System in the figure below. This
design clears most of the windshield that is in the driver's
field of view.
Some of the different wiper blade schemes
used by various
There are a couple of other designs on some cars. Mercedes
uses a single wiper arm that extends and retracts as it sweeps
across the window -- Single Arm (Controlled) in the
figure above. This design also provides good coverage, but is
more complicated than the standard dual-wiper systems. Some
cars use wiper blades that are mounted on opposite sides of
the windshield and move in the opposite direction, and some
vehicles have a single wiper mounted in the middle. These
systems don't provide as much coverage for the driver as the
standard two-blade system.
Wiper Controls Most wipers have a low and a
high speed, as well as an intermittent setting. When the
wipers are on low and high speed, the motor runs continuously.
But in the intermittent setting, the wipers stop momentarily
between each wipe. There are many different kinds of
switches for wipers. Some cars have just one
intermittent speed, others have 10 discrete settings and still
others have a sliding scale that can be set for almost any
A typical wiper control
Whichever kind of controls your car has, setting them just
right can be tricky -- too fast and the windshield gets dry
and the wipers squeak; too slow and your visibility is blocked
by raindrops. Compounding this is the fact that the amount of
water hitting the windshield changes as your car speeds up and
slows down. It can require almost constant attention to keep
the wipers operating properly. Carmakers may finally have
conquered this problem with the holy grail of wiper technology
-- the rain-sensing wiper.
Rain-sensing Wipers In the past, automakers
have tried to either eliminate the wipers or to control their
speed automatically. Some of the schemes involved detecting
the vibrations caused by individual raindrops hitting the
windshield, applying special coatings that did not allow drops
to form, or even ultrasonically vibrating the windshield to
break up the droplets so they don't need to be wiped at all.
But these systems were plagued by problems and either never
made it to production or were quickly axed because they
annoyed more drivers than they pleased.
However, a new type of wiper system is starting to appear
on cars that actually does a good job of detecting the amount
of water on the windshield and controlling the wipers. One
such system made by TRW
Inc. uses optical sensors to detect the moisture.
The sensor is mounted in contact with the inside of the
windshield, near the rearview mirror.
The sensor projects infrared light into the
windshield at a 45-degree angle. If the glass is dry, most of
this light is
reflected back into the sensor by the front of the windshield.
If water droplets are on the glass, they reflect the light in
different directions -- the wetter the glass, the less light
makes it back into the sensor.
The electronics and software in the sensor turn on the
wipers when the amount of light reflected onto the sensor
decreases to a preset level. The software sets the speed of
the wipers based on how fast the moisture builds up between
wipes. It can operate the wipers at any speed. The system
adjusts the speed as often as necessary to match with the rate
of moisture accumulation.
The TRW system, which is found on many General Motors cars,
including all Cadillac models, can also be overridden or
turned off so the car can be washed.
For more information on windshield wipers and related
topics, check out the links on the next page!