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• - [Jared] Most scales nowadays are digital,

• they use electricity to determine how much something weighs.

• What I'm interested in is these old style mechanical scales,

• sometimes called spring scales.

• No batteries required here.

• In this video, we'll take a part two of these scales

• to see how they work on the inside.

• (upbeat music)

• The idea of using springs in a scale

• has been around for about 200 years now.

• This one is called a floor scale

• and this one is called a hanging scale.

• So let's spring right into the details here,

• These are commonly used at the grocery store.

• The more weight or the harder that you pull on the bottom,

• the more that the red pointer spins on the inside.

• Take away the weight and the red pointer goes back to zero.

• Let's take it apart and see what's on the inside.

• The first thing you'll notice

• are two large springs on the inside.

• They're hooked onto the top piece,

• which is bolted to the frame of the scale.

• In the center, we have the gear system.

• This piece is called the rack.

• Hidden inside of here is called the pinion gear.

• As the rack goes back and forth, the pinion gear spins too.

• The rack is bolted to the frame that the springs are on.

• So when the springs go down,

• the rack goes down too, which then spins the gear.

• There's another tiny spring here which pulls on the rack.

• This ensures that we have good contact with the pinion gear.

• The center of the gear goes out

• and pokes through the center of the dial

• so that we can attach the red pointer.

• Now, as the scale is pulled down, the spring stretch,

• which moves the rack, turns the gear, turns the red pointer,

• which then shows how much the object weighs.

• Now, may happen that the red pointer starts to get off.

• It's not pointing directly at zero

• when there's nothing on the scale.

• This might happen when you attach a basket to the scale.

• In this case, we need to calibrate the scale.

• That's the job of the calibration knob,

• also known as a zeroing screw.

• You can see that it goes

• all the way through and comes out the other side.

• These two are called the pivot plates.

• They both have a hole in the middle

• which is where they attach.

• As you spin the calibration knob,

• it will push against the metal pivot plates.

• So as you turn the knob,

• the whole spring assembly will go up or down,

• which will just barely move the red pointer.

• Once it's right at zero,

• then you are ready to start weighing again.

• The reason that a scale like this works

• is because of something called Hooke's Law.

• It's the relationship between how much force

• you pull on a spring versus how far that spring stretches.

• So just for an example,

• let's put a one kilogram object on this spring.

• It stretches one centimeter.

• Okay, now let's double the weight.

• Now it stretches two centimeters.

• For each extra kilogram,

• the spring stretches another centimeter.

• If you put it on a graph,

• then it's a straight line,

• or another words, it's a linear relationship.

• This means it's predictable.

• We can now use the spring

• to determine the weight of an object,

• figure out how far the spring stretches,

• which will then tell us the weight of the object.

• For this spring, the numbers are easy.

• One kilogram for every one centimeter.

• But maybe you've got a really stiff spring.

• This one takes five kilograms

• to stretch it only one centimeter.

• Maybe you've got a really flexible spring.

• It takes barely any weight at all

• to stretch it one centimeter.

• When you study Hooke's Law in your physics class,

• you might see an equation like this, F equals kx.

• The K is how stiff the spring is,

• x is how far the spring is stretched,

• which results in a downward force

• or how heavy the object must be.

• Hooke's Law only works up to a certain point.

• Normally, you take the weight off the spring,

• and the spring goes back to where it started.

• However, if you put a very large weight on a small spring,

• you will probably stretch that spring so much

• that it won't go back.

• You've reached the elastic limit,

• and Hooke's Law no longer works.

• This spring is somewhat useless to use in a scale now.

• So keep that in mind,

• it is possible to break these scales.

• With the hanging scale,

• there's two springs in here working together.

• Since we know how stiff they are,

• we can use how far they stretch to turn a gear,

• which then tells us how much an object weighs.

• Now let's take a look at the floor scale.

• Two tiny springs, hold it all together.

• It's easiest to unhook it from the bottom.

• The front cover comes off and the insides become visible.

• The big difference here

• is that instead of a red pointer moving across the dial,

• this time the whole dial actually moves

• while the red pointer is stationary.

• The dial is attached to another pinion gear

• with a rack moving across it.

• This of course is very similar

• to the hanging scale that we saw earlier.

• The end of the rack is attached to a small spring

• which pulls it towards the edge.

• Things get interesting at the other end of the rack.

• We've got a lever that can pivot back and forth.

• It's attached to the rack,

• and remember, the rack is constantly being pulled this way.

• The only thing stopping it

• is another metal plate right beneath it.

• This plate is held up by our main spring.

• There's only one of them this time,

• and it's quite a bit smaller.

• Our spring is still hanging

• but the end of it is attached to a metal plate.

• When the plate goes down, the spring is stretched.

• This also means the lever is allowed to rotate.

• The metal plate is pressed down

• by four metal bars that go across the scale,

• two long ones and two shorter ones

• that hang right beneath it.

• All four of the bars

• also rest on the edges of the scale case.

• The lid to the scale has four supports on the bottom.

• Each of these supports

• rests on one of the four bars in the scale.

• When you stand on the scale,

• your weight is distributed down through the four bars,

• over to the metal plate,

• which moves down and stretches the spring,

• allows the lever to rotate, which moves the rack,

• which then rotates the gear that moves the dial.

• No matter where you stand on the scale,

• your weight gets distributed to the tiny spring

• which uses Hooke's Law to determine your weight.

• Just like with the hanging scale,

• this one may need to be zeroed out if it gets off.

• The calibration dial is down here.

• There's two parts to this, the bottom which can spin,

• and the top, which fits inside of it.

• Notice the screw threads around the side.

• When you spin the bottom, the top goes up or down.

• This moves the resting position of this spring,

• which will then ripple through and affect

• where the dial is when there's no weight on the scale.

• My name's Jared.

• I make 3D animations on how things work.

• If you enjoyed this video,

• hit that subscribe button and the bell

• so you're notified when I make a new video.

• Thanks for watching, and I will see you next time.

• (upbeat music)

- [Jared] Most scales nowadays are digital,

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# How does a Mechanical Scale work? (Spring Scale)

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joey joey posted on 2021/04/26
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