Unit 2.26 Conversion of motion

Key points about conversion of motion

A cam converts rotary motion into reciprocating motion in the follower
Eccentric cams create a gentle up-and-down motion
Pear cams create a quick rise and slow fall motion
Heart cams create uniform motion with constant speed
Snail cams create a slow rise followed by a sudden drop
Crank and slider, rack and pinion, screw threads, and ratchet and pawl mechanisms convert different types of motion

Conversion of rotary motion to reciprocating motion using cams

A cam is a rotating piece that converts rotary motion into reciprocating motion in the follower. Think of it like a spinning shape pushing something up and down.

A cam mechanism showing directional arrows to demonstrate a camshaft which rotates and a follower which moves up and down. — Figure caption,
A cam mechanism

Eccentric/off centre cam: this is the simplest cam - imagine a wheel mounted off-centre. As it rotates, it creates a gentle up-and-down motion, like what you might find in a simple mechanical toy.

Circular cam, a follower and slide. Directional arrows show the movements of the cam and follower in the slide. — Figure caption,
An eccentric/off-centre cam

Pear/egg cam: shaped like a pear, this creates a quick rise and slow fall motion. You might find this in machinery where you need something to lift quickly but lower gently.

Pear cam, a follower and slide. Directional arrows show the movements of the cam and follower in the slide. — Figure caption,
A pear cam

Heart cam: this special shape creates a uniform motion - meaning the follower moves at a constant speed up and down. This is useful in machinery where smooth, consistent movement is important, like in sewing machines.

Heart-shaped cam, a follower and slide. Directional arrows show the movements of the cam and follower in the slide — Figure caption,
A heart-shaped cam, a follower and slide

Snail/drop cam: creates a slow rise followed by a sudden drop. Imagine a door closer mechanism - it closes the door slowly but lets it open quickly.

Snail cam, a follower and slide. Directional arrows show the movements of the cam and follower in the slide — Figure caption,
A snail or drop cam, a follower and slide

Types of follower

Flat follower sitting on a pear cam. The flat follower is a tube with a flat bottom. Directional arrows show the movement of the flat follower and the pear cam. — Figure caption,
A flat follower sitting on a pear cam

Flat
Flat followers have a flat bottom that sits on the cam. These cope well under load but aren’t very accurate and have a lot of .

Point follower sitting on a pear cam. The point follower is a tube with a pointed bottom. Directional arrows show the movement of the pointed follower and the pear cam. — Figure caption,
A knife (or point) follower sitting on a pear cam.

Knife
Knife followers have a narrow point that sits on the cam. These are very accurate and low friction but are quick to wear away the pointed edge.

Roller follower sitting on a pear cam. The roller follower is a tube with a ball at the bottom. Directional arrows show the movement of the ball follower and the pear cam. — Figure caption,
A roller follower sitting on a pear cam

Roller
Roller followers have a roller such as a ball bearing attached to the bottom of the following. These are accurate, low on friction and can withstand load, but are more costly to produce.

What is a crank and slider?

Showing a mechanical device called a crank slider and the moving pivot directing the movements of the other components. — Figure caption,
A crank and slider mechanism

A crank and slider converts rotary motion to reciprocating motion (or vice versa). Picture a train's wheels and connecting rods - as the wheel (crank) turns, it makes the piston (slider) move back and forth. This is also how a petrol engine converts the up-and-down motion of pistons into the rotating motion of the .

A green steam locomotive with red wheels — Image caption,
Steam train wheels are an example of a crank and slider

What is a rack and pinion mechanism

Showing a rack (a long sheet of ridged metal) with a pinion (a ridged gear wheel) sat atop it. — Figure caption,
A rack and pinion mechanism

An example of this is in a pillar drill, where the table bed is moved up and down.

A pillar drill with a rack and pinion arrangement that moves the table bed up and down — Image caption,
A pillar drill uses a rack and pinion mechanism to move the bed up and down

The mechanism of an electric gate showing the toothed rack — Image caption,
An electric gate makes use of a rack and pinion

A Rack and Pinion converts rotary motion to linear motion using a gear (pinion) that meshes with a toothed bar (rack). When the gear turns, the rack moves in a straight line. This is used in car steering systems - turning the steering wheel (rotary) makes the wheels turn left or right (linear). Can also be used to open and close an electric gate, with the motor (rotary) makes the gate move left or right (linear).

Conversion of motion using screw threads

Screw threads convert rotary motion to linear motion by using spiral grooves. When you turn a screw (rotary), it moves forward or backward (linear). Examples include machine vice in workshops, jar lids, and bottle caps. The thread angle determines how far the screw advances with each rotation.

A blue vice labelled with linear and rotary motion — Figure caption,
Screw thread mechanism on a vice

What is a ratchet and pawl?

A ratchet and pawl mechanism allows rotation in one direction while preventing reverse motion. Think of a bicycle's free wheel - you can pedal forward, but the wheel won't spin backwards. The ratchet is a wheel with angled teeth, and the pawl is a finger that fits between them. It's also used in socket wrenches and hand winches.