- [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 in other 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,
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Thanks for watching, and I will see you next time.
(upbeat music)