Rotary/Linear motion

Mechanisms: Crank and Slider
A crank and slider mechanism changes rotary to reciprocal motion or vice versa. In the car engine (illustrated) the reciprocating motion of the piston caused by exploding fuel is converted into rotary motion as the con-rod moves the crankshaft around. An air compressor uses this principal in reverse - an electric motor turns the crankshaft and the piston moves up and down to compress the air.


Mechanisms: Quick return mechanism
The 'Whitworth' quick return mechanism converts rotary motion into reciprocating motion, but unlike the crank and slider, the forward reciprocating motion is at a different rate to the backward stroke. At the bottom of the slotted arm, the peg only has to move through a few degrees to sweep the arm from left to right, but it takes the remainder of the revolution to bring the arm back. This mechanism is most commonly seen as the drive for a shaping machine.

Mechanisms: Cams and Followers
The cam and follower is a device which can convert rotary motion into linear motion (movement in a straight line). A cam is a specially shaped piece of material, usually metal or hard wearing plastic, which is fixed to rotating shaft. The cam can have various shapes eg. round, oval, heart shaped. A follower is a mechanism which is designed to move up and down as it follows the edge of the cam.Many machines which have moving parts use cams. A good example is the motor car engine which has cams to open and close valves and contact breaker points and operate fuel pumps.



Mechanisms: Rotary Cams
There are several different types of cam but most of these can be placed into two groups - rotary and linear. Rotary cams change rotary motion into reciprocating (backwards and forwards) motion. As the cam rotates, the follower moves accordingly. The exact distance it moves depends on the shape of the cam.