Lesson video

Hi there everybody, my name is Mr. Booth, and welcome to your "Design and technology" lesson for today, it's brilliant that you could join me.

We have got a great lesson today all about movement with mechanisms. We are gonna be looking at levers and linkages and also cams. This lesson is part of the "Prototypes with mechanisms: robotics and automation" unit.

Today's outcome, I want you to be able to explain the different types of motion within simple mechanisms. We have five key words that I want you to keep a lookout for in today's lesson.

Therefore, first is fulcrum, a fixed pivot point, that's how levers rotate around.

We then have effort, the input force that we usually have to put in.

We then have load, this is the output force.

We have linkages.

Now, this is a set of levers joined together to transfer and control motion.

And then finally, cam, a mechanical component that converts rotary motion into linear motion.

Let's have a look at our learning cycles.

Two of those today, the first is all about levers and linkages, and then we'll have a look at cams. So let's go straight into levers and linkages.

A lever is a rigid or stiff bar that turns around a fulcrum.

Levers have three changeable elements that we're gonna look at.

The first is the fulcrum, a fixed pivot point to move around.

And you can see here we're trying to open a tin of paint using a screwdriver.

And the fulcrum is the edge of the tin there, that's what we're wedging the screwdriver against.

We then have the effort.

This is the input force, and this is being provided by me trying to open this tin of paint by pushing down on the screwdriver.

We then have the load, this is the output force.

And this of course comes from the end of the screwdriver lifting up the lid of that tin.

A lever is a very simple way to gain what we call mechanical advantage or MA, and this is of all about trying to make things move a little bit easier.

And the longer that lever, the more mechanical advantage we have.

Quick check for understanding, what does mechanical advantage mean? Is it making something heavier, moving objects faster, making moving or lifting something easier, or increasing the size of an object? Pause the video now, have a go at this, and come back to me when you've got your answer.

It is of course C, making moving or lifting something easier, well done.

We're now gonna look at the three classes of lever.

The first, class 1, the fulcrum is positioned anywhere between the load and the effort.

So you can see there, we have the effort pushing down on the lever.

The fulcrum in this case is right in the middle and the load is on the other side.

And a good example of this is scissors.

First class levers can provide a mechanical advantage by actually amplifying the force.

And like I said previously, the longer the lever, the more that force is amplified.

Quick check for understanding: The scissors user first class lever mechanism.

On the picture below, I would like you to label the fulcrum, the load, and the effort.

Pause the video now come back to me when you've completed it.

So how did you get on? Well, hopefully you've got number one as the load, that's where the shearing force will happen.

We've then got the effort, obviously that's where we put our fingers.

And the fulcrum is that bit in the middle where it's connected.

The class 2 lever.

The load is positioned anywhere between the fulcrum and the effort.

So you can see there's a slight difference in this.

We've got the fulcrum on the left-hand side, the load is in the middle.

And then we are doing that kind of lifting effort, if you like, on the lever.

Now, what I want you to do is can you think of any products that we use quite regularly and possibly in the garden that might use something similar to this.

Pause the video now, have a go at this, and come back to me when you think you've got an answer.

Well, what did you think of? Well, it is of course a wheelbarrow that uses a second class lever.

Second class levers are common in tools and machines designed to reduce the effort needing for heavy lifting.

So if you need to lift something quite heavy or also squeezing.

So once again, here we have our wheelbarrow.

We know it's a second class lever mechanism.

On the picture, I would like you to label the fulcrum, the load, and the effort.

Pause video now, have a go at this, and come back to me when you've got your answer.

So there we go, we've got the load, we've got the fulcrum there, and we've also got the effort, well done.

Now, onto the class 3 lever.

And you can see this looks slightly different.

The effort is positioned anywhere between the fulcrum and the load.

And you can see on this one, we have obviously two of those levers either side.

And we're providing effort from both sides to kind of almost like pinch the load there.

Now if you can ever think of a product that might be similar to this, it is of course, tweezers.

And third class levers are useful for tasks requiring control, accuracy, and also precision such as using tweezers.

Another check for understanding, which diagram is an example of a third class lever? Is it A, B, or C? Pause the video, now have a go at this, and come back to me when you've got your answer.

Well, it is of course B, the one in the middle there, that is your third class lever, well done.

Linkages are a set of levers joined together.

And the reason for this is because they can change the direction of a force.

They can change the magnitude, so that's the size or the amount of a force.

Or they can transfer one type of motion into another motion.

They can also do a kind of mix of those things as well.

And you might have a toy similar to this one at home.

This is our Hoberman sphere, which you can see, as you raise it, it expands the toy, makes it seem to be bigger.

We have, first of all, the lazy tong linkage.

Interlinked arms allow the product to contract and expand.

So in this case, it is contracted.

And if we squeeze the two end levers together, it will expand, it will stretch out.

And a good example of this will be a folding clothes horse that you might see someone at home drying their clothes on.

And there you have a lovely GIF showing you how that works, and you can see that matches our lazy tong linkage illustration perfectly.

Then we have what's known as a reverse motion linkage.

And just like the name, if the input is pulled, the output is pushed.

In this case, you can see we have a look closer look at the pivots, we have a fixed pivot in the middle.

So you've gotta imagine that that pivot will allow the levers to rotate around it, but it won't move.

So if I pull the input, obviously the top moving pivot will move, but the output will move in the opposite direction, which is why we call it the reverse motion linkage.

Now, reverse motion linkages allow products to open and close, enabling products to be more compact.

And in this case, you can see we have this stroller or this pram, depending on what you want to call it.

Quick check for understanding, what is false about a reverse motion linkage? Is it A, it enables products to become compact? B, the input and output move in opposite directions? C, it transfers rotary to linear motion? Which one of those is false? Pause the video now, have a go at this, and come back to me when you've got your answer.

It is of course C, it transfers rotary to linear motion, it doesn't do that of course.

So we're now looking at parallel motion linkage.

The input and the output motion move in the same direction.

So if I push the input, the output will also move in the same direction.

And why do we call it a parallel motion linkage? Well that's because a lot of the levers in this will move in parallel around those fixed pivots.

And this is really useful for items such as toolboxes that open up to reveal sections.

And you can see on the side there, you can see those parallel motion linkages.

So we now have bell cranked linkage.

The input motion moves the output motion by 90 degrees.

The reason it's called a bell crank linkage is 'cause this is the same linkage that is used when ringing bells.

So if I push the input, it will rotate around the fixed pivot, and it will move my output by 90 degrees as you can see on this, and my output, in this case, goes upwards.

Now this is the same linkage that you'll find on your bike brakes.

Obviously you pull the brake on the handlebar, which is then translated when it gets to the tire through 90 degrees onto the brake pad.

So next time we're on your bike, have a close look at that, and you'll be able to see it.

Quick check for understanding, which linkage enables the input and output motions to move in the same direction.

Is it A, B, or C? Pause the video now, have a go at this, and come back to me when you've got your answer.

It is of course, C, that is the parallel motion linkage, well done.

So we're now onto your first task, task A.

The picture shows, the picture I'm gonna show in a second, the picture shows a pair of kitchen tongs.

kitchen tongs use a lever mechanism.

I would like you to tell me which class lever does the kitchen tongs use and why? And then I would like you to label the picture to show the fulcrum, effort, and load.

And there's your image.

Pause the video now, have a go at this task, and then come back to me when you've completed it.

So how did you get on? Well, first of all, I wanted you to tell me what class of lever it was.

Well, this is third class lever, and that's because the tongs require precision and accuracy.

And of course we then had the different parts of it.

A is the load, B is the fulcrum, and C is the effort.

Next, I would like you to fill in the blanks.

So you have three linkages there, three images of linkages there.

I would like you to name the linkage, and then give me the direction of input/output motion.

Pause the video now, have a go at this, and come back to me when you completed it.

So the first one we have is of course the reverse motion linkage, and the input and output move in opposite directions.

Then we have the parallel motion linkage, input and output moving the same direction.

And then finally, bell crank, output is 90 degrees to the input motion.

Well done with that task, let's move on.

We're now onto cams. There are four different types of motion that we need to consider when talking about cams. The first one is rotary, rotating round in a circle, usually around a central point like an axis.

We then have oscillating, rotating back and forth in a curve.

We have linear, so traveling in a straight line.

And then reciprocating, so in a straight line, but traveling backwards and forwards.

Quick check from understanding, which image represents reciprocating motion? Is it A, B, or C? And here, we have for A, a sewing machine, B, we have a bike wheel, and C, we have a clock pendulum.

Pause the video now have a go at this, and come back to me when you've got your answer.

So the reciprocating motion in this is the sewing machine needle, of course, 'cause it goes up and down.

Now, cams convert rotary motion into reciprocating motion.

Let's have a look at these so we have a follow up there, which is following what the cam does, and that's known as reciprocating motion.

And there we have the cam, which is of course rotary motion.

When the follower moves up, we describe the motion as rising.

When it comes back down again, we describe it as falling.

Quick check for understanding, rotary motion in cams is changed into which motion in followers? Is it A: linear, B: reciprocating, or C: oscillating? Pause the video now, have a go at this and come back to me when you've got your answer.

It is of course reciprocating, and in that previous case, it was rising and then falling.

The shape of the cam determines the speed and the magnitude, which is the height of the motion.

Here we have a pear cam.

We also have a snail cam, and you can see the difference in what the follower is doing, and also a eccentric cam as well.

Now, what I want you to do is very closely look at these animations and have a look at what the follower is doing.

Quick check for understanding, which of the following is an eccentric cam? Is it A, B, or C? Pause the video now, have a go at this, and come back to me when you've got your answer.

It is of course C, which is eccentric.

So we now need to look at friction.

Friction is resistance, which is encountered when two things rub together.

When the edge of the cam moves against the follower, friction occurs, as I'm sure you can see from this animation.

This can affect a mechanism's performance.

The shape of the follower can be changed to counter this.

So we can actually adjust the shape of the follower to try and reduce that friction.

And we could do that in a number of ways.

Quick check for understanding, which shape cam is this mechanical toy using? Is it A: eccentric, B: snail, or C: pear? Pause the video now, have a go at this, and come back to me when you've got your answer.

It is of course, snail.

You can see that it is rising slowly, and then suddenly falling very quickly.

Below are three types of followers that we can use with cams. We have what is known as the flat, large surface area, so lots of friction and where will occur.

We then have the roller.

Now, this reduces frictions, so it wears well and spins smoothly.

So you've gotta imagine that little wheel on the end of the follower is actually rotates and that follows the rotation of the cam.

We then have knife-edge.

Friction is concentrated in only one point, so it wears quickly, but it also means it's very accurate as well.

Now, the shapes differ depending on the application because of the amount of friction produced.

Quick check for understanding, which follower will encounter the least friction from the cam's edge? Is it A: the flat follower, B: the roller follower, or C: the knife-edge follower? Pause the video now, have a go at this, and come back to me when you've got your answer.

It is of course, the roller, that's gonna have the least friction.

Let's have a closer look at each of our cams. So we have the eccentric cam, a circle with its center off to the side.

You can see it is not centralized on the axis.

It makes the follow up move up and down in a smooth and regular way.

Have a look at that follower.

You can see very predictably it's going up and down in a regular way.

Now, lots of uses for this, there are simple toys with a smooth movement.

We then have the pear-shaped cam, shaped just like a pear, that's why we call it a pear-shaped cam.

The followers stay still for parts of the turn, and then quickly rises and falls.

So this is quite good for timed movement.

And the uses, mechanical toys that pause and move.

And then we have the snail cam, and it looks like a snail shell, which is why we call it that.

The follower slowly rises and then suddenly drops.

It's used when a quick drop is needed, and a use for this often packaging machines use these to release items quickly.

So when I onto Task B.

First of all, I would like you to sketch the shape of the following cams: pear, eccentric, and snail.

Number two, a mechanical toy has been designed that requires a cartoon character to rise up gradually and then suddenly drop.

Suggest a suitable cam shape, and then explain how the cam and follower work together to produce this motion using sketches and annotations.

Pause the video now, have a go at this task, and come back to me when you've completed it.

So how did you get on? Well, let's see the sketched cams. Well first of all we have a pear.

It looks like a pear shape, there you go.

And then we have eccentric, which is that one there with the off-centered circle.

And then finally, the snail cam.

And then we wanted you to suggest a suitable cam shape for this toy, and then also explain how the cam and follower work together to produce this motion using sketches and annotations.

"A suitable cam shape could be a snail-shaped cam." "As the cam rotates, its shape moves the follower slowly upwards, which moves the character up slowly.

When the follower gets to the highest position, the cam then drops it suddenly to the lowest position." That is what your answer could have been.

So that brings us to the end of today's lesson.

Let's have a quick summary.

A lever is a very simple way to gain mechanical advantage to make moving or lifting something easier.

Levers have three changeable elements on a bar, the fulcrum, effort, and load.

Linkages are mechanisms that can change the magnitude and directions of a force.

They can transform one type of motion into another.

Cams convert rotary motion into reciprocating motion.

The shape of a cam determines the speed and magnitude of the motion.

The shape of the bottom of the follower changes the accuracy of the motion.

However, this affects wear and friction.

You've been absolutely brilliant today, I'll look forward to seeing you all next time, goodbye.