has probably washed your clothes hundreds of times, but have
you ever wondered what's inside that trusty washing machine?
How does it spin the clothes so fast without leaking water?
Why is it so heavy? How does the agitator switch
directions? In this edition of HowStuffWorks,
we'll venture inside a washing machine to answer all of these
questions and more.
We'll start by explaining how the washing machine cleans
clothes, then we'll take a look at how the machine is put
together. We'll look at the plumbing, the drive mechanism and
Operating a washing machine
is pretty simple:
- There are a few things to decide before you start your
load of clothes, such as how big the load is (small, medium,
large, extra large), what temperature the water will be for
the wash and rinse cycles (cold/cold, warm/cold, warm/warm,
hot/cold), how the machine should agitate (delicate, knit,
permanent press, heavy), and how long the cycles should last
(number of minutes, based on how soiled your clothes are).
- After you fill the tub with clothes, the machine fills
the tub with water, and then stirs the clothes around using
- After some time agitating, the washer drains the water
and then spins the clothes to remove most of the water.
Then, it refills, and agitates the clothes some more to
rinse out the soap. Then it drains and spins again.
Inside a Washing Machine
If we take a look
under the washing machine, you'll see what makes it so heavy.
Motor and counterweight
Yes, that is in fact a block of concrete in the
picture above. The concrete is there to balance the equally
motor, which drives a very heavy gearbox that is attached
to the steel inner tub. There are lots of heavy components in
a washing machine.
The washing machine has two steel tubs. The inner
tub is the one that holds the clothes. It has an
agitator in the middle of it, and the sides are
perforated with holes so that when the tub spins, the
water can leave.
The outer tub, which seals in all the water, is
bolted to the body of the washer. Because the inner tub
vibrates and shakes during the wash cycle, it has to be
mounted in a way that lets it move around without banging into
other parts of the machine.
The inner tub is attached to the gearbox, which is
attached to the black metal frame you see in the
picture above. This frame holds the motor, gearbox and the
Cable-and-pulley support system
The picture above shows just the black metal frame, without
the tub or gearbox. The cable that you see on the left
side of the picture is the other end of the same cable that
you see on the right side. There are a total of three
pulleys, so that if one side of the frame moves up, the
other side moves down. This system supports the weight of the
heavy components, letting them move in such a way as not to
shake the entire machine.
But, if all of these parts are just hanging by cables, why
don't they swing around all the time?
A laundry machine has a damping system that uses friction to
absorb some of the force from the vibrations.
In each of the four corners of the machine is a mechanism
that works a little like a disc
brake. The part attached to the washer frame is a
spring. It squeezes two pads against the
metal plate that is attached to the black frame. You
can see where the pads have polished the plate from movement
The plumbing on the washing
machine has several jobs:
The washing machine has hookups for two water
lines on the back, one for hot water and one for cold.
These lines are hooked up to the body of a solenoid
- It fills the washing machine with the correct
temperature of water.
- It recirculates the wash water from the bottom of the
wash tub back to the top (during the wash cycle).
- It pumps water out the drain (during the spin cycle).
Water control valves
The image above shows the back and front of the solenoid
valve. You can see that there are two valves, but they
feed into a single hose. So depending on the
temperature selected, either the hot valve, the cold
valve or both valves will open.
Before the hose releases water into the wash tub, it sends
it through an anti-siphon device.
This device prevents wash water from being sucked back into
the water supply lines, possibly contaminating the water for
your house or even your neighborhood. You can see that the
white, plastic device has a big opening that allows air
in. The water from the hose shoots into the device and
turns downward, exiting through the tube on the other end. But
while it is inside the device, it is open to the atmosphere.
This means that if there were suction on the water supply
line, it could not possibly suck any water in from the washing
machine; it would get only air.
Water inlet and overflow port
The picture above shows the inlet through which
water enters the washing machine. The nozzle to the right is
an overflow port, which is connected to a pipe that
dumps water out the bottom of the washing machine (onto your
floor), instead of letting it overflow the tub and possibly
get the motor wet.
The rest of the plumbing system, the
part that recirculates the water and the part that drains it,
involves the pump.
Pump and plumbing
In the picture above, you can see how the pump is hooked
up. This pump is actually two separate pumps in one:
The bottom half of the pump is hooked up to the drain line,
while the top half recirculates the wash water. So how does
the pump decide whether to pump the water out the drain line
or back into the wash tub?
This is where one of the neat tricks of the washing machine
comes in: The motor that drives the pump can reverse
direction. It spins one way when the washer is running a
wash cycle and recirculating the water; and it spins the other
way when the washer is doing a spin cycle and draining the
Let's take a closer look at the pump:
If you look carefully, you can see the vanes of the
bottom layer of the pump. When water enters the pump's
inlet, these vanes, or fins, push the water around and
force it back out of the pump by way of the outlet.
This type of pump can operate in both directions -- which port
is the inlet and which is the outlet depends on which
direction the pump is spinning in.
Take another look at the pump. If the pump spins clockwise,
the bottom pump sucks water from the bottom of the wash tub
and forces it out the drain hose, and the top pump tries to
suck air from the top of the wash tub and force it back up
through the bottom, so that no water recirculation takes
If the pump spins counter-clockwise, the top pump sucks
water from the bottom of the tub and pumps it back up to the
top, and the bottom pump tries to pump water from the drain
hose back into the bottom of the tub. There is actually a
little bit of water in the drain hose, but the pump doesn't
have the power to force much of it back into the tub.
Take another look at the drain hose in the picture
above -- notice how it loops all the way to the top of the
machine before heading back down to the drain. Because one end
of the hose is hooked up to the bottom of the tub and the
other is open to the atmosphere, the level of water inside the
drain hose will be the same as the level inside the tub. If
the drain hose didn't go all the way up to the top of the
machine, then the tub could never fill all the way. As soon as
the water reaches the bend in the hose, it goes out the drain.
There are also times when the pump does not spin at all.
The washer just churns the water that is in the tub without
recirculating it. For this situation, the pump is hooked up to
the motor by way of a clutch.
Clutch and flexible coupling
In this picture, you see the flexible coupling that
hooks the clutch up to the pump. The coupling is
needed because the motor and clutch are mounted to the frame,
which can move freely with the inner tub, whereas the pump is
mounted to the stationary outer tub.
On the bottom of the clutch is a set of four teeth.
When the electromagnet
engages, it raises an arm up into these teeth, which stops
them from rotating. Once the teeth are stopped, the clutch
starts to engage. After a couple of revolutions, it locks up
to the motor shaft and the pump starts to turn with the motor.
The drive mechanism
on a washing machine has two jobs:
- To agitate the clothes, moving them back and forth
inside the wash tub.
- To spin the entire wash tub, forcing the water out.
There is a really cool gearbox that handles these
two jobs, and it uses the same trick as the pump does. If the
motor spins in one direction, the gearbox agitates; if it
spins the other way, the gearbox goes into spin cycle.
First, let's see how everything is hooked up:
Gearbox, pump, tubs and rubber
In this picture, the frame has been removed. You can see
the pump mounted to the outer tub, and the gearbox, which
holds the inner tub. A piece of rubber seals the outer
tub to the gearbox. The inner tub is mounted to the gearbox on
the other side of the seal.
The inner tub has been removed from the outer tub in the
picture above. It is resting on the gearbox, and the
plastic agitator is visible in the center of the tub.
Here you can see the top side of the gearbox with the seal
cut and the inner tub removed. The inner tub bolts to the
three holes in the flange of the gearbox. You can see
from the buildup of crud on top of the gearbox that it has
been exposed to wash water for many years. A hollow
tube extends from the center of the gearbox. Inside this
tube is a splined shaft -- the spline on top of the
shaft hooks into the plastic agitator.
Inside the Gearbox
The gearbox is one
of the coolest parts of the washing machine. If you spin the
pulley on the gearbox one way, the inner shaft turns slowly
back and forth, reversing direction about every
half-revolution. If you spin the pulley the other way, the
flange spins at high speed, spinning the whole tub with it.
Gearbox agitation mechanism
Here you can see a gear with a link attached
to it. This link is just like the one attached to an old steam
train wheel -- as the gear (along
with the link) turns, it pushes another pie-shaped piece of
gear back and forth. This pie-shaped gear engages a
small gear on the inner shaft, which leads to the
spline. In addition to rotating the inner shaft in
alternating directions, there are other gears within the
system that provide a gear
reduction to slow the rotation. Because the motor spins
only at one speed, spin-cycle speed, a gear reduction is
necessary to facilitate the slower wash cycle.
When the washer goes into spin cycle, the whole mechanism
locks up, causing everything to spin at the same speed as the
input, which is hooked up to the motor. The interesting thing
here is that when the motor spins the gearbox in one
direction, the agitator runs, and when it spins it the other
way, the whole machine locks up. How does it do this?
In the figure above, notice the gear with the angled
teeth. There is also a smaller gear with angled teeth
behind the big one in the foreground. These are the only two
gears with angled teeth. Depending on which way the gears are
spinning, the angle on the teeth will tend to force the inner
gear to slide either to the left or to the right inside the
gearbox. If it slides to the left, it engages a mechanism that
locks up the gearbox.
Gearbox lockup mechanism
You can see a small notch in the outer shaft. This
notch is hollow, and is attached to the shaft with the small
gear. When the small gear moves, it moves this outer shaft
with it, and the small notch engages the single tooth
that is fixed to the lockup mechanism. When the gearbox is
locked up, both the inner shaft, which drives the agitator,
and the outer shaft, which drives the tub, spin at the same
speed as the input pulley.
The controls for this machine were
designed before microcontrollers
were being used in appliances. In fact, there is not a single
resistor or capacitor
in the whole machine. First, let's take a look at the cycle
switch -- you'll be amazed at what is inside.
The cycle switch has the job of determining how long the
different parts of the cycle last.
Inside the cycle switch
Inside the switch is a little motor equipped with a very
reduction that makes the control dial turn very slowly. In
the top half of the switch, there is a set of six
contacts. These are actuated by the small pieces of
metal in the plastic arm on the dial. As the dial spins,
bumps on the dial raise and lower the six metal pieces,
which close and open the contacts in the top half of the
Inside the cycle switch
If you look at the shape of the bumps, you can see why the
dial on the washer spins only one way: The front side of the
bumps has a slope that raises up the metal pieces gradually;
but the back side doesn't, so if you try to turn the knob
backward, the metal pieces wedge against the bumps.
This bumpy plastic disk is really the software program that
runs your washing machine. The length of the bumps determines
how long each part of the cycle lasts, and the length of the
space between bumps determines how long the machine pauses
before moving on to its next task.
The speed and temperature control switches are much
simpler than the cycle control switch.
Speed and temperature control
These switches control the speed of the motor and determine
which of the hot/cold water supply solenoids will open during
the wash and the rinse cycles. If hot is selected, only the
hot water solenoid valve will open when the machine fills; if
warm is selected, both will open; and if cold is selected,
only the cold water solenoid valve will open.
Inside the speed/temperature
The speed/temperature control is pretty simple. Each
plastic rocker engages two sets of contacts,
either opening or closing the circuit connected to those
contacts. For each switch, there is always one closed and one
open set of contacts.
The level sensor uses a pressure switch to
detect the water level in the tub.
Water level control switch
This switch controls how high the tub fills with water.
Water level control switch
The big end of the hose connects to the bottom of the tub,
while the small end connects to the switch. As the water level
in the tub rises, water rises in the hose also; but the air in
the hose is trapped, so as the water rises, the air is
Inside the water level control
Inside the housing of this switch is a little
piston. The pressure in the hose pushes the piston up.
When it is raised far enough, it pops up and closes an
electrical contact. This set point, where the contact is lost,
is adjustable, and in the picture you can see the cam
mechanism that is connected to the adjuster knob on the
control panel of the washer. As the cam turns, it presses a
spring against the cylinder, making it harder for the
cylinder to pop up. This means that the water level will have
to rise some more before the pressure in the hose will be high
enough to trigger the switch.
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