A3.3C Combining motion types (HL only)

Combining Mechanisms: Building a Chain of Motion

1. Real machines almost never use one mechanism on its own, they chain several together.
2. The rule that makes a chain work is simple: the output of one stage becomes the input of the next.
3. The four motions and the five mechanisms are the alphabet, and combining them is the language a designer actually writes in.

Following the Chain Through a Bicycle

1. The rider's legs supply reciprocating motion, pushing down alternately.
2. The cranks turn that into rotary motion at the pedals.
3. The chain and sprockets carry that rotation to the rear wheel, and the gear ratio decides whether it arrives as speed or as torque.
4. The wheel converts rotation back into linear motion of the whole bicycle along the road.
5. Four different motions, in one machine, and each stage exists only to feed the next.

Why Designers Combine Mechanisms

1. Conversion: the power source almost always spins, but the job almost never needs spinning, so something must translate.
2. Compounding: mechanical advantage multiplies along a chain, so two modest stages can achieve what one huge stage could not.
3. Control: combining a cam with a linkage lets a machine produce a movement with a precise shape and a precise timing.

Effects Multiply Along the Chain, and So Do Losses

1. If one stage gives a mechanical advantage of 3 and the next gives 4, the system gives 12, because the stages multiply rather than add.
2. Efficiency multiplies the same way, and that is the catch: every stage adds friction, so a chain of stages that are each 90 percent efficient is far less than 90 percent efficient overall.
3. This is the trade-off at the heart of every compound system, and calculating it is the work of B3.3.

The Cost of Complexity

1. Every extra stage adds friction, wear, weight, cost and one more thing that can break.
2. So the best mechanical design is usually the one that reaches the required motion in the fewest stages, not the cleverest one.