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Rotary Motion


Rotary motion is motion in a circle. It is the starting point for many mechanisms. 


Rotary motion is measured in either angular velocity, the number of degrees turned in a given time, or in revolutions per minute (rpm). 
The direction of turn, either clockwise or anti-clockwise is also part of the measurement of rotary motion.
The strength of rotary motion is known as the torque, the turning force. Torque is measured in Newton Metres defined as the force of one newton acting at a perpendicular distance of one metre from the axis of rotation.


Rotary motion to:



Linear Motion Wheels.
Rack and pinion.
  Increase / Decrease Gears.
Worm gear.
Reciprocating Motion Piston.
Geared mechanism.
Cardan gear.
  Reflect Gears.
Oscillation Crank.
Quick return.
  Rotate Bevel gear.
Intermittent Motion Geneva Stop.      
Irregular Motion Cam.      

Worm Gear

A worm is used to reduce speed. For each complete turn of the worm shaft the gear shaft advances only one tooth of the gear. In this case, with a twelve tooth gear, the speed is reduced by a factor of twelve. Also, the axis of rotation is turned by 90 degrees. Unlike ordinary gears, the motion is not reversible, a worm can drive a gear to reduce speed but a gear cannot drive a worm to increase it. As the speed is reduced the power to the drive increases correspondingly. Worm gears are a compact, efficient means of substantially decreasing speed and increasing power. Ideal for use with small electric motors.


Make your own working worm gear model.
Turn the handle and the worm turns! The 24 tooth gear turns at 1/24 of the speed of the handle.

Quick Return

Oscillating motion is motion which moves along a path, then returns along that same path backwards and forwards, backwards and forwards.
In this example the drive wheel is used to power a waving machine, notice how the left to right movement is slower than the right to left. This is because that left to right motion takes place over a longer part of the drive wheels turn.
By moving the drive wheel closer to the pivot point this effect can be exaggerated. The same mechanism is used in mechanical saws to provide a quick return after the cutting stroke.

Download and make your own working quick return mechanism. Use it find out how the quick return mechanism works first hand or as the starting point for your own model. More details...

Geneva Stop

The Geneva stop is named after the city of its invention where it was used in the construction of clocks.
The Geneva stop is used to provide intermittent motion, the orange wheel turns continuously, the dark blue pin then turns the blue cross quarter of a turn for each revolution of the drive wheel. 
The crescent shaped cut out in dark orange section lets the points of the cross past, then locks the wheel in place when it is stationary. 
The Geneva stop mechanism is used commonly in film projectors to move the film on one frame at a time.



Gears are used to change speed in rotational movement. In the example above the blue gear has eleven teeth and the orange gear has twenty five. To turn the orange gear one full turn the blue gear must turn 25/11 or 2.2727r turns.
Notice that as the blue gear turns clockwise the orange gear turns anti-clockwise.
In the above example the number of teeth on the orange gear is not divisible by the number of teeth on the blue gear. This is del

iberate. If the orange gear had thirty three teeth then every three turns of the blue gear the same teeth would mesh together which could cause excessive wear. By using none divisible numbers the same teeth mesh only every seventeen turns of the blue gear.

Rings of Gears and the Two Pound Coin

Gears turn in alternate directions. In the example, left the blue gear turns anti-clockwise, the two green gears on either side of it are then driven clockwise, these in turn drive the yellow and purple gears anti-clockwise and so on...
A ring of gears containing an even number of gears turns freely, as you can see, but imagine a ring with an odd number of gears , image that the blue gear is removed, you will see that the two green gears be turning the same direction and so would not be able to turn, the gears would lock together. 
Now, look at the ring of gears around the centre
of the British two pound coin. Nineteen gears...!
Check out the gears model to download and make in the shop!

Reciprocating motion


This mechanism is used to convert between rotary motion and reciprocating motion. In this mechanism the straight line motion stays at a constant speed throughout the full length of the throw.



The watch escapement is the centre of the time piece. It is the escapement which divides the time into equal segments. 
The balance wheel, the gold wheel, oscillates backwards and forwards on a hairspring (not shown) As the balance wheel moves the lever is moved allowing the escape wheel (green) to rotate by one tooth. 
The power comes through the escape wheel which gives a small 'kick' to the palletes (purple) at each tick.

Chain and Sprocket

Chains are used to connect gears. They work in a similar way to pulleys but with a positive drive rather than a reliance on friction. Gears which are connected by chain turn in the same direction unlike gears which mesh against each other.


Check out the updated page here:

Chain & Socket

Cardan Gear

Invented by Girolamo Cardano in the 16th century the Cardan gear is a way of converting rotary motion into straight line motion. Watch how the red dot on the inner purple gear exactly follows the vertical dotted line. The outer gear has a diameter exactly twice as large as the inner gear. In the above example they have 40 and 20 teeth respectively. Cardano also invented a type of universal joint and investigated the mathematics of probability. Understanding the mathematics of risk helped him make a living from gambling until eventually he could find no-one to gamble with and had to move onto new pastures...

Bevel Gear

The bevel gear is used to change the axis of rotational motion. By using gears of differing numbers of teeth the speed of rotation can also be changed.

You can download and make your own working bevel gear in the shop