OCR A-Level Product Design structures, mechanisms and electronic systems: a complete overview

What this topic demands

This topic is the most calculation-heavy of the technical principles. It tests moments and equilibrium, lever and gear ratios, pulley velocity ratios, Ohm's law and the potential divider, each carrying marks for the method, the answer and the units, plus an interpretation. Marks are lost on inverted ratios and mixed units, and gained by stating whether an output is faster or slower, stronger or weaker, higher or lower. This overview ties the four dot-point pages together.

Structures and forces

Structures are frame (a skeleton of members), shell (a thin load-bearing skin) or monocoque (skin and structure combined). They carry tension, compression, shear, bending and torsion. The principle of moments ($\text{moment} = F \times d$) balances a structure in equilibrium when clockwise moments equal anticlockwise moments. Designers stiffen structures with triangulation, hollow sections and folding, ribbing or corrugating, all adding stiffness for little mass. See structures and forces.

Mechanisms, levers and linkages

A lever has a fulcrum, effort and load; the three classes are distinguished by which is in the middle (fulcrum, load, effort). Mechanical advantage is the load over the effort (effort arm over load arm for an ideal lever); velocity ratio is the effort distance over the load distance, and gaining force costs distance. Linkages (reverse motion, parallel motion, bell crank) change the direction or type of motion. See mechanisms, levers and linkages.

Gears, cams and pulleys

The gear ratio is driven teeth over driver teeth, and the output speed is input speed over the ratio; a larger driven gear is slower with more torque. A cam and follower converts rotary motion into reciprocating motion (rise, dwell, fall). A pulley and belt drive has a velocity ratio of driven diameter over driver diameter, with $n_1 d_1 = n_2 d_2$. See gears, cams and pulleys.

Electronic systems

Electronic systems are modelled as input, process and output blocks. Ohm's law is $V = I \times R$; resistors add in series and combine reciprocally in parallel. A potential divider gives $V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2}$, and replacing a resistor with an LDR or thermistor makes the output respond to light or temperature, which is how products sense and react. See electronic systems.

How to revise this topic

1. Drill the calculations. Moments, mechanical advantage, gear ratio, pulley velocity ratio, Ohm's law and the potential divider, with units.
2. Interpret every answer. Faster or slower, more or less torque, higher or lower output voltage.
3. Know the structure and lever types. By geometry and by which component is in the middle.
4. Keep units consistent. Convert kilohms and use perpendicular distances and matching diameters.
5. Learn the input, process, output model. With common sensors and output transducers, then attempt the quiz.