Introduction

Recap the mechanisms that were built last week; motors, gears, idler gears. Can you describe the motion created by each and how they are different? 

 

Idler Gear

Thirdly, have a look at Idler gears.

 

 

Build and test their Idler gear model with the same simple algorithm as above, observe the motion created. The gears in this kit are called “Spur” gears and they come in 2 different sizes (big and small).  Can they explain how gears work? 

 

• How many teeth does the BIG gear have?  
• How many teeth does the SMALL gear have? 
• Can they describe the motion of each gear? 
• What do they notice about the direction of the rotation of each gear?

 

Idler gears are an example of a gear train (two or more gears linked together). The idler gear is the smaller gear that sits between the driver and the driven gears, it transmits the rotary motion between these two gears. The idler gear will turn in the opposite direction to the driver and driven gears as adjacent gears must always turn away from each other. This pattern would continue if you added more gears to the train, every second gear would turn in the same direction.

 

image courtesy: https://commons.wikimedia.org/wiki/File:Animated_3_Gear_Row.gif

 

Crown Gear

 

Build the ‘crown gear’ prototype with the pieces shown in the pop-out menu (on the left). What looks different about the gears in this mechanism?

 

Get the model moving with some simple code and consider:

• Why are the teeth bent on the crown gear?
• Can you think of where a crown gear could be used? 

 

Crown gears key concepts

Crown (or bevel) gears change the axis of rotation through 90°. The teeth have been specially cut so the gears will mesh at right-angles to each other, where spur gears must be parallel. 

This toy race car is an example of a bevel gear in action. The motor can sit in the middle of the car with its small driver gear attached, this turns the bevel gear which allows the motor to turn the axle that drives the back wheels (which is at 90° to the motor axle.)

 

A traditional windmill is another example of where a crown gear might be seen (shown at (a) on the diagram below) The wind turned the sails which turned the driver axle. This met the driven axle at a right angle with a crown gear to allow the rotary motion to be transferred down to the mill stone which turned to grind the crops.  

 

Cams

Build the cam prototype with the pieces listed in the WeDo software:

 

 

Test the mechanism with this algorithm:

 

 

• What happens now when you press go?
• How does the sensor start things in this algorithm? What kind of sensor do you think it is?
• What happens to the green wheel when the cam turns?

 

A cam is a shaped piece of metal or plastic fixed to a rotating shaft. 

A cam mechanism has three parts: cam, slide and follower. 

 

 

The camshaft is connected to the motor and rotates continually, turning the cam. The follower is a rod that rests on the edge of the turning cam. 

The follower can dwell (not move) and is also free to move up and down, but is prevented from moving from side to side by a slide or guide. Lego WeDo Cams are Pear shaped. The pear shape of this cam means that for half the cycle, the follower will dwell. Then, as the pointed part of the cam approaches, the follower is pushed up (rises), then, as the point passes, falls and dwells - and the cycle starts again.

 

Cams key concepts

• A cam mechanism has three parts: cam, slide and follower.
• The camshaft rotates continually, turning the cam. The follower is free to move up and down, but is prevented from moving from side to side by a slide or guide. 
• The follower can only do three things:

1. 1. Rise (move up)
   2. Fall (move down) or
   3. Dwell (remain stationary)

• Cams change rotary motion into Reciprocating motion (moving backwards and forwards in a straight line)

 

Cams can be different shapes, the shape of the cam will change the motion of the follower:

 

 

 

Cams in an engine

A internal combustion engine is a great example of where cams are used in every day life. The cams rotate to open and close engine valves. This lets the combustion gases into into the combustion chamber where a spark plug creates a spark which ignites the gases causing combustion. This small explosion creates force which drives the engine.

 

A camshaft:

 

The cams can be seen rotating at the top of this diagram of a 4-stroke engine. The numbered stages are:

• 1 = Induction
• 2 = Compression
• 3 = Power
• 4 = Exhaust

Animation courtesy of https://en.wikipedia.org/wiki/Internal_combustion_engine#mediaviewer/File:4StrokeEngine_Ortho_3D_Small.gif