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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,

  • and we'll start with the hanging scale.

  • 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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B1 中級 美國腔

机械秤如何工作(How does a Mechanical Scale work? (Spring Scale))

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    joey joey 發佈於 2021 年 04 月 26 日
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