IB Design Technology Topic A3.3.8 Cams and Motion…

Cams: The Shape Is the Instruction

Definition

Cam

A mechanical part that converts rotational motion into linear motion.

How Cams Work

Cam Profile: The shape of the cam determines the motion it imparts.
Follower: A component that rides on the cam's surface, translating its shape into motion.
Camshaft: The rotating axis on which the cam is mounted.
Crank: In some systems, cams work alongside a crank, which converts rotary motion into reciprocating motion.

Types of Cams and Their Motion

Pear Cam

Shape: Resembles a pear, with a rounded end and a tapered section.
Motion: Provides a smooth rise and fall with a dwell period (no movement).
Uses: Automated machinery where a pause in motion is needed, such as textile machines.

Example

In a sewing machine, a pear cam controls the needle's movement, allowing it to pause at the top and bottom of each stitch.

Circular Cam

Shape: a circular disc, turning about a point that is offset from its centre.
Motion: a smooth, uniform rise and fall, with no dwell at any point.
Uses: Simple applications where consistent motion is required, like rotary pumps.

Note

A disc turning about its own centre gives the follower no motion at all, because its radius never changes.
It only works as a cam once the axis is offset, and a circular cam mounted this way is also called an eccentric cam.
The syllabus lists both names, so use whichever the question uses.

Triangular Cam

Shape: Triangular, with rounded corners.
Motion: Creates a sudden rise and fall, with sharp transitions.
Uses: High-speed machinery where quick, repetitive actions are needed, such as printing presses.

Tip

When analysing a cam, pay attention to the transition points.
These are critical for understanding how the cam will impact the follower's motion.

Eccentric Cam

Shape: Circular, but the rotation axis is offset.
Motion: a smooth, even reciprocating motion in the follower.
Uses: Piston engines and valve systems, where precise control of movement is essential.

Analogy

Think of an eccentric cam like a wobbling wheel.
The offset center creates a gentle up-and-down motion as it rotates.

Oval Cam

Shape: Elliptical or oval.
Motion: Provides a gradual rise and fall, with varying speeds.
Uses: Complex machinery where variable motion is needed, such as automated assembly lines.

Note

Oval cams are ideal for applications requiring smooth acceleration and deceleration.

Snail Cam

Shape: Spiral or snail-like.
Motion: Gradual rise followed by a sudden drop.
Uses: Hammer mechanisms and timing devices, where a sudden release of energy is required.

Common Mistake

Don't confuse snail cams with pear cams.
While both have dwell periods, snail cams are designed for sudden drops, whereas pear cams provide smooth transitions.

Types of Followers

Flat follower: Simple and stable; may wear unevenly
Knife-edge follower: Precise point contact; high wear
Roller follower:Reduces friction and wear; common in high-speed systems
Spherical follower: Balanced contact for smoother motion

Reading a Cam: Rise, Fall and Dwell

Rise: The follower moves upward as the cam rotates.
Fall: The follower moves downward.
Dwell: The follower remains stationary.
Dwell is the feature that makes cams special, because it lets a machine pause without stopping the motor that drives it.

Example question

Which cam profile produces a gradual rise followed by a sudden drop? (1 mark)

A. Pear cam

B. Oval cam

C. Circular cam

D. Snail cam

Solution

1 mark for the correct answer: D, snail cam.

The spiral profile lifts the follower slowly, then the sharp step lets it fall suddenly.
That sudden release is why snail cams are used in hammer and striking mechanisms.
A snail cam can only turn in one direction. Reverse it and the follower jams against the step.

Example question

A sewing machine uses a pear cam to drive its needle up and down. The fabric must be moved forward between stitches.

(a) Explain why a pear cam is more suitable than a circular cam for this application. (4 marks)

(b) State one advantage of using a roller follower rather than a knife-edge follower. (1 mark)

Solution

(a) Award [1] for each linked point, up to [4 max].

A pear cam gives a rise, a fall and a period of dwell, during which the follower does not move. [1]
The dwell holds the needle clear of the fabric while the fabric is advanced, so the stitches are evenly spaced. [1]
A circular cam gives a continuous, uniform rise and fall with no dwell at all. [1]
The needle would therefore never pause, so the fabric would have to move while the needle was still in it, tearing the material or breaking the needle. [1]

(b) A roller follower reduces friction and wear, because it rolls over the cam surface instead of dragging across it, which suits the high speed of a sewing machine. [1]

Active recall

What are the three parts of a cam mechanism, and what does the follower do?
Define rise, fall and dwell.
Which cam gives a gradual rise followed by a sudden drop, and what is that used for?
Why does a circular disc only work as a cam when its axis is offset?
Which follower would you choose for a high-speed machine, and why?

Cams: The Shape Is the Instruction

Definition

Cam

A mechanical part that converts rotational motion into linear motion.

How Cams Work

Cam Profile: The shape of the cam determines the motion it imparts.
Follower: A component that rides on the cam's surface, translating its shape into motion.
Camshaft: The rotating axis on which the cam is mounted.
Crank: In some systems, cams work alongside a crank, which converts rotary motion into reciprocating motion.

Types of Cams and Their Motion

Pear Cam

Shape: Resembles a pear, with a rounded end and a tapered section.
Motion: Provides a smooth rise and fall with a dwell period (no movement).
Uses: Automated machinery where a pause in motion is needed, such as textile machines.

Example

In a sewing machine, a pear cam controls the needle's movement, allowing it to pause at the top and bottom of each stitch.

Circular Cam

Shape: a circular disc, turning about a point that is offset from its centre.
Motion: a smooth, uniform rise and fall, with no dwell at any point.
Uses: Simple applications where consistent motion is required, like rotary pumps.

Note

A disc turning about its own centre gives the follower no motion at all, because its radius never changes.
It only works as a cam once the axis is offset, and a circular cam mounted this way is also called an eccentric cam.
The syllabus lists both names, so use whichever the question uses.

Triangular Cam

Shape: Triangular, with rounded corners.
Motion: Creates a sudden rise and fall, with sharp transitions.
Uses: High-speed machinery where quick, repetitive actions are needed, such as printing presses.

Tip

When analysing a cam, pay attention to the transition points.
These are critical for understanding how the cam will impact the follower's motion.

Eccentric Cam

Shape: Circular, but the rotation axis is offset.
Motion: a smooth, even reciprocating motion in the follower.
Uses: Piston engines and valve systems, where precise control of movement is essential.

Analogy

Think of an eccentric cam like a wobbling wheel.
The offset center creates a gentle up-and-down motion as it rotates.

Oval Cam

Shape: Elliptical or oval.
Motion: Provides a gradual rise and fall, with varying speeds.
Uses: Complex machinery where variable motion is needed, such as automated assembly lines.

Note

Oval cams are ideal for applications requiring smooth acceleration and deceleration.

Snail Cam

Shape: Spiral or snail-like.
Motion: Gradual rise followed by a sudden drop.
Uses: Hammer mechanisms and timing devices, where a sudden release of energy is required.

Common Mistake

Don't confuse snail cams with pear cams.
While both have dwell periods, snail cams are designed for sudden drops, whereas pear cams provide smooth transitions.

Types of Followers

Flat follower: Simple and stable; may wear unevenly
Knife-edge follower: Precise point contact; high wear
Roller follower:Reduces friction and wear; common in high-speed systems
Spherical follower: Balanced contact for smoother motion

Reading a Cam: Rise, Fall and Dwell

Rise: The follower moves upward as the cam rotates.
Fall: The follower moves downward.
Dwell: The follower remains stationary.
Dwell is the feature that makes cams special, because it lets a machine pause without stopping the motor that drives it.

Example question

Which cam profile produces a gradual rise followed by a sudden drop? (1 mark)

A. Pear cam

B. Oval cam

C. Circular cam

D. Snail cam

Solution

1 mark for the correct answer: D, snail cam.

The spiral profile lifts the follower slowly, then the sharp step lets it fall suddenly.
That sudden release is why snail cams are used in hammer and striking mechanisms.
A snail cam can only turn in one direction. Reverse it and the follower jams against the step.

Example question

A sewing machine uses a pear cam to drive its needle up and down. The fabric must be moved forward between stitches.

(a) Explain why a pear cam is more suitable than a circular cam for this application. (4 marks)

(b) State one advantage of using a roller follower rather than a knife-edge follower. (1 mark)

Solution

(a) Award [1] for each linked point, up to [4 max].

A pear cam gives a rise, a fall and a period of dwell, during which the follower does not move. [1]
The dwell holds the needle clear of the fabric while the fabric is advanced, so the stitches are evenly spaced. [1]
A circular cam gives a continuous, uniform rise and fall with no dwell at all. [1]
The needle would therefore never pause, so the fabric would have to move while the needle was still in it, tearing the material or breaking the needle. [1]

(b) A roller follower reduces friction and wear, because it rolls over the cam surface instead of dragging across it, which suits the high speed of a sewing machine. [1]

Active recall

What are the three parts of a cam mechanism, and what does the follower do?
Define rise, fall and dwell.
Which cam gives a gradual rise followed by a sudden drop, and what is that used for?
Why does a circular disc only work as a cam when its axis is offset?
Which follower would you choose for a high-speed machine, and why?

IB Design Technology Topic A3.3.8: Cams and Motion Notes

IB Design Technology Topic A3.3.8: Cams and Motion Notes

Cams: The Shape Is the Instruction

How Cams Work

Types of Cams and Their Motion

Pear Cam

Circular Cam

Triangular Cam

Eccentric Cam

Oval Cam

Snail Cam

Types of Followers

Reading a Cam: Rise, Fall and Dwell

Cams: The Shape Is the Instruction

How Cams Work

Types of Cams and Their Motion

Pear Cam

Circular Cam

Triangular Cam

Eccentric Cam

Oval Cam

Snail Cam

Types of Followers

Reading a Cam: Rise, Fall and Dwell

A. Design in theory

B. Design in practice

C. Design in context

A. Design in theory

B. Design in practice

C. Design in context

IB Design Technology Topic A3.3.8: Cams and Motion Notes

IB Design Technology Topic A3.3.8: Cams and Motion Notes

Cams: The Shape Is the Instruction

How Cams Work

Types of Cams and Their Motion

Pear Cam

Circular Cam

Triangular Cam

Eccentric Cam

Oval Cam

Snail Cam

Types of Followers

Reading a Cam: Rise, Fall and Dwell

Cams: The Shape Is the Instruction

How Cams Work

Types of Cams and Their Motion

Pear Cam

Circular Cam

Triangular Cam

Eccentric Cam

Oval Cam

Snail Cam

Types of Followers

Reading a Cam: Rise, Fall and Dwell