IB Design Technology Topic A3.3.10 Linkages Revision…

Linkages: Rigid Bars That Steer a Motion

A linkage is a set of rigid bars joined by pivots, and a designer reaches for one to do three things:
1. Change the direction of movement
2. Alter the magnitude of a force
3. Control the motion of system parts

Parallel Linkages

Maintains Orientation: Keeps the connected parts parallel throughout the motion.
Even Force Distribution: Distributes force evenly across the linkage.

Example

Suspension Systems: Used in vehicles to keep wheels aligned with the road.
Drawing Boards: Ensures the drawing surface remains level.

Reverse Linkages

Direction Change: Converts input motion into the opposite output motion.
Simple Design: Often consists of a single pivot point.

Example

A bicycle brake lever uses a reverse linkage, so squeezing inwards pushes the cable outwards.
A simple see-saw toy does the same: push one end down and the other rises.
The single pivot is what reverses the direction, and nothing else in the mechanism is doing any work.

Bell Crank Linkages

Angle Conversion: Alters force direction, usually at a right angle.
Force Amplification: Can increase or decrease force magnitude.

Example

Bicycle Brakes: Transmits force from the brake lever to the brake pads.
Steering Systems: Changes the direction of steering input.

Reading Any Linkage: Input, Transformation, Output

Input Motion: A force or movement is applied to one part of the linkage.
Transformation: The linkage alters the direction, magnitude, or path of the motion.
Output Motion: The transformed motion is transmitted to another part of the system.
To identify a linkage in an exam, find the fixed pivots first, because they are what the moving bars turn against.

Example question

Which linkage is used to change the direction of a force through approximately 90 degrees? (1 mark)

A. Parallel linkage

B. Reverse linkage

C. Bell crank linkage

D. Push-pull linkage

Solution

1 mark for the correct answer: C, bell crank linkage.

A bell crank pivots about a corner, so a pull coming in along one arm leaves at right angles along the other.
A reverse linkage flips the direction through 180 degrees, and a parallel linkage keeps parts aligned rather than turning a force.

Example question

A drawing board uses a parallel linkage to support its ruler. A bicycle uses a bell crank linkage at the wheel to operate its brakes.

(a) Outline why a parallel linkage is suitable for the drawing board. (2 marks)

(b) Explain how the bell crank linkage transmits the rider's effort to the brake pads. (3 marks)

Solution

(a) Award [1] for each point, up to [2 max].

A parallel linkage keeps the connected parts parallel to each other throughout the whole movement. [1]
The ruler can therefore be moved anywhere on the board and will stay perfectly level, which is exactly what a technical drawing requires. [1]

(b) Award [1] for each linked point, up to [3 max].

The rider squeezes the brake lever, which pulls the cable and applies a force along one arm of the bell crank. [1]
The bell crank pivots at its corner, turning that force through roughly 90 degrees so it acts along the other arm. [1]
This pushes the brake pads inwards against the rim, and the relative arm lengths also amplify the force so that a light squeeze produces a firm braking force. [1]

Active recall

What three things can a linkage do to a motion?
Which linkage keeps the connected parts parallel throughout the movement, and name a product that needs that.
What does a reverse linkage do to the direction of an input motion?
Through what angle does a bell crank typically turn a force, and where would you find one on a bicycle?
What should you look for first when identifying a linkage in an unfamiliar product?

Linkages: Rigid Bars That Steer a Motion

A linkage is a set of rigid bars joined by pivots, and a designer reaches for one to do three things:
1. Change the direction of movement
2. Alter the magnitude of a force
3. Control the motion of system parts

Parallel Linkages

Maintains Orientation: Keeps the connected parts parallel throughout the motion.
Even Force Distribution: Distributes force evenly across the linkage.

Example

Suspension Systems: Used in vehicles to keep wheels aligned with the road.
Drawing Boards: Ensures the drawing surface remains level.

Reverse Linkages

Direction Change: Converts input motion into the opposite output motion.
Simple Design: Often consists of a single pivot point.

Example

A bicycle brake lever uses a reverse linkage, so squeezing inwards pushes the cable outwards.
A simple see-saw toy does the same: push one end down and the other rises.
The single pivot is what reverses the direction, and nothing else in the mechanism is doing any work.

Bell Crank Linkages

Angle Conversion: Alters force direction, usually at a right angle.
Force Amplification: Can increase or decrease force magnitude.

Example

Bicycle Brakes: Transmits force from the brake lever to the brake pads.
Steering Systems: Changes the direction of steering input.

Reading Any Linkage: Input, Transformation, Output

Input Motion: A force or movement is applied to one part of the linkage.
Transformation: The linkage alters the direction, magnitude, or path of the motion.
Output Motion: The transformed motion is transmitted to another part of the system.
To identify a linkage in an exam, find the fixed pivots first, because they are what the moving bars turn against.

Example question

Which linkage is used to change the direction of a force through approximately 90 degrees? (1 mark)

A. Parallel linkage

B. Reverse linkage

C. Bell crank linkage

D. Push-pull linkage

Solution

1 mark for the correct answer: C, bell crank linkage.

A bell crank pivots about a corner, so a pull coming in along one arm leaves at right angles along the other.
A reverse linkage flips the direction through 180 degrees, and a parallel linkage keeps parts aligned rather than turning a force.

Example question

A drawing board uses a parallel linkage to support its ruler. A bicycle uses a bell crank linkage at the wheel to operate its brakes.

(a) Outline why a parallel linkage is suitable for the drawing board. (2 marks)

(b) Explain how the bell crank linkage transmits the rider's effort to the brake pads. (3 marks)

Solution

(a) Award [1] for each point, up to [2 max].

A parallel linkage keeps the connected parts parallel to each other throughout the whole movement. [1]
The ruler can therefore be moved anywhere on the board and will stay perfectly level, which is exactly what a technical drawing requires. [1]

(b) Award [1] for each linked point, up to [3 max].

The rider squeezes the brake lever, which pulls the cable and applies a force along one arm of the bell crank. [1]
The bell crank pivots at its corner, turning that force through roughly 90 degrees so it acts along the other arm. [1]
This pushes the brake pads inwards against the rim, and the relative arm lengths also amplify the force so that a light squeeze produces a firm braking force. [1]

Active recall

What three things can a linkage do to a motion?
Which linkage keeps the connected parts parallel throughout the movement, and name a product that needs that.
What does a reverse linkage do to the direction of an input motion?
Through what angle does a bell crank typically turn a force, and where would you find one on a bicycle?
What should you look for first when identifying a linkage in an unfamiliar product?

IB Design Technology Topic A3.3.10: Linkages Notes

IB Design Technology Topic A3.3.10: Linkages Notes

Linkages: Rigid Bars That Steer a Motion

Parallel Linkages

Reverse Linkages

Bell Crank Linkages

Reading Any Linkage: Input, Transformation, Output

Linkages: Rigid Bars That Steer a Motion

Parallel Linkages

Reverse Linkages

Bell Crank Linkages

Reading Any Linkage: Input, Transformation, Output

IB Design Technology Topic A3.3.10: Linkages Notes

A. Design in theory

B. Design in practice

C. Design in context

IB Design Technology Topic A3.3.10: Linkages Notes

A. Design in theory

B. Design in practice

C. Design in context

Linkages: Rigid Bars That Steer a Motion

Parallel Linkages

Reverse Linkages

Bell Crank Linkages

Reading Any Linkage: Input, Transformation, Output

Linkages: Rigid Bars That Steer a Motion

Parallel Linkages

Reverse Linkages

Bell Crank Linkages

Reading Any Linkage: Input, Transformation, Output