Mechanical devices used to produce different sorts of movement, including the four types of motion, levers and linkages, rotary systems such as gears, pulleys and belts, and cams and followers.

What this dot point is asking

This is part of AQA section 3.1.5. AQA wants you to know the four types of motion and the mechanical devices that produce or change movement. You need to describe levers and linkages, rotary systems such as gears, pulleys and belts, and cams and followers, and explain how each changes the size, direction or type of a movement. On Paper 1 (the written exam) this appears both as short recall questions in Section A and as a calculation, usually of mechanical advantage or velocity ratio.

The four types of motion

Being able to classify the motion at the input and the output of a product is the first step in almost every mechanisms question. A mechanism is judged by what it does to motion: it can change the type (rotary to reciprocating), the direction (clockwise to anticlockwise, or up to down) or the size (more speed but less force, or more force but less speed). You can rarely increase force and speed at the same time, because a mechanism transfers energy rather than creating it.

Levers and linkages

Levers fall into three classes by the order of the effort, load and fulcrum. A first-class lever (see-saw, scissors, pliers) has the fulcrum between the effort and the load. A second-class lever (wheelbarrow, bottle opener) has the load between the fulcrum and the effort. A third-class lever (tweezers, fishing rod, the human forearm) has the effort between the fulcrum and the load and trades force for a greater range of movement.

The force advantage of a lever is its mechanical advantage, the ratio of load to effort. It follows from the law of the lever, which balances the turning moments on each side of the fulcrum:

A long effort arm and a short load arm give a large mechanical advantage, which is why a tyre lever or a crowbar is long. Linkages combine levers: a reverse-motion linkage makes the output move opposite to the input; a bell-crank linkage changes the direction of a force through ninety degrees (as in a bicycle brake); a parallel-motion linkage keeps two parts moving together (as in a toolbox or an angle-poise lamp).

Gears, pulleys and belts

• Gears are toothed wheels that mesh so the teeth cannot slip. They transmit rotary motion between shafts and change speed and torque. A small driver turning a large driven gear reduces output speed but increases turning force (torque); this is a gear-down arrangement. Meshing gears always rotate in opposite directions; an idler gear placed between them makes the input and output turn the same way without changing the overall ratio.
• The velocity ratio (or gear ratio) of a simple train is the teeth on the driven gear divided by the teeth on the driver. Output speed equals input speed divided by the velocity ratio.
• Pulleys and belts transmit rotary motion between shafts that are far apart, using friction rather than teeth. A belt can slip, which protects a mechanism from overload but loses some drive; a toothed (timing) belt prevents slip where exact timing matters. As with gears, a small driver pulley turning a large driven pulley reduces speed and increases torque, so the velocity ratio uses the diameters: driven diameter divided by driver diameter.

Cams and followers

The profile of the cam controls the pattern, called the motion, of the follower. A pear cam gives a smooth rise, a brief dwell at the top, then a smooth fall, and dwells for about half a turn (used in engine valve gear). A circular (eccentric) cam gives smooth, continuous rise and fall with no dwell. A snail (drop) cam gives a slow gradual rise then a sudden sharp drop, useful where a part must be released quickly. Followers can be knife-edge, roller or flat-faced; a roller follower reduces friction and wear at higher speeds.