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How Washing Machines Work
by Karim Nice

It 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 the controls.

Cleaning Clothes
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 an agitator.

  • 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 heavy electric 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 concrete weight.

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.

Vibration-damping system

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 during vibration.

The plumbing on the washing machine has several jobs:

  • 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).
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 valve.

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.

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 water.

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 place.

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.

Drive Mechanism
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 seal

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.

Inner tub

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.

Gear reduction

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 helical 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.

Cycle switch

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 large gear 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 switch.

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 switches

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 control

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 plumbing

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 compressed.

Inside the water level control switch

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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