Mechanical Devices & Movement
Mechanical Devices & Movement
Many products need to move, or need to change the direction or size of a force. Designers use mechanical devices — levers, linkages, cams, gears and pulleys — to achieve this. In the exam you may be asked to identify a mechanism, explain how it works, or calculate values such as mechanical advantage, velocity ratio or output speed. Always show your working when a question asks you to calculate — marks are awarded for the correct method as well as the final answer.
Types of Movement
Mechanisms produce one of four types of movement. Linear motion travels in a straight line in one direction, such as a drawer sliding open. Rotary motion turns in a circle around a central point, such as a wheel or a motor shaft. Reciprocating motion moves backwards and forwards in a straight line, such as a sewing machine needle or a piston. Oscillating motion swings back and forth along an arc, such as a pendulum. Many mechanisms exist purely to convert one type of movement into another.
The four types of movement are linear, rotary, reciprocating and oscillating. Cams, cranks and linkages are used to convert one type into another.
Levers and Mechanical Advantage
A lever is a rigid bar that rotates around a fixed pivot called the fulcrum. Levers are used to multiply force, multiply distance, or change the direction of a force. The three classes of lever are defined by where the fulcrum, load and effort sit along the bar.
Where the fulcrum, load and effort are positioned determines whether a lever multiplies force or multiplies distance and speed:
Class 1 — the fulcrum sits between the effort and the load, e.g. a seesaw, crowbar or pair of scissors. Moving the fulcrum closer to the load increases the force multiplication.
Class 2 — the load sits between the fulcrum and the effort, e.g. a wheelbarrow, nutcracker or bottle opener. These always multiply force because the effort moves further than the load.
Class 3 — the effort sits between the fulcrum and the load, e.g. tweezers, a fishing rod or the human forearm. These always multiply distance and speed, but need more effort force than the load.
Two calculations describe how well a lever performs. Mechanical advantage (MA) compares the load to the effort needed to move it: $$MA = \frac{\text{Load}}{\text{Effort}}$$ Velocity ratio (VR) compares how far the effort moves to how far the load moves: $$VR = \frac{\text{distance moved by effort}}{\text{distance moved by load}}$$ Efficiency compares the two, showing how much energy is lost to friction: $$\text{Efficiency} = \frac{MA}{VR} \times 100\%$$ A perfectly efficient lever has an efficiency of 100%; real levers are always lower because of friction at the pivot.
MA = Load ÷ Effort. VR = distance moved by effort ÷ distance moved by load. Efficiency = (MA ÷ VR) × 100%. Show every step of a calculation to earn method marks.
An effort of 50 N is applied 0.8 m from the fulcrum to lift a load of 200 N positioned 0.2 m from the fulcrum. Step 1: \(MA = \frac{200}{50} = 4\). Step 2: \(VR = \frac{0.8}{0.2} = 4\). Step 3: \(\text{Efficiency} = \frac{4}{4} \times 100\% = 100\%\). Showing each step like this earns full method marks even if the final answer is later found to be wrong.
Linkages
A linkage is a system of bars connected by pivots that changes the direction of a force or movement. A bell crank changes the direction of movement through 90 degrees — pulling a bicycle brake lever pulls a cable, which a bell crank converts into a pushing force that presses the brake pad onto the wheel rim. A reverse motion linkage reverses the direction of a push or pull — push one end in and the other end pulls out.
Cams and Followers
A cam is a shaped disc mounted on a rotating shaft. As the cam turns, a follower rests against its edge and is pushed up and down, converting rotary motion into reciprocating motion. The shape of the cam controls the pattern of movement produced.
A pear-shaped cam lifts the follower, lowers it, and then holds it still (a long dwell) for about half of each revolution — as used to operate engine valves. An eccentric (circular) cam is an off-centre circle that gives a smooth, gradual rise and fall. A drop (snail) cam raises the follower gradually then lets it drop suddenly. Followers also affect performance: a roller follower reduces friction and wear, a knife-edge follower follows the cam profile most accurately but wears quickly, and a flat follower is simple and durable but cannot follow sharply-shaped cams as precisely.
Pulleys, Belts and Gears
Pulleys and belts transmit rotary motion between shafts that are some distance apart. A V-belt grips the pulley more securely than a flat belt, reducing slipping. The velocity ratio of a pulley system compares the diameters of the two pulleys: $$VR = \frac{\text{diameter of driven pulley}}{\text{diameter of driver pulley}}$$ and the output speed is found by dividing the input speed by the velocity ratio. A crossed belt reverses the direction of rotation; an uncrossed belt keeps both pulleys turning the same way.
Gear trains use toothed wheels to transmit rotary motion. A simple gear train has gears meshing directly; a compound gear train fixes two gears of different sizes to the same shaft to produce a bigger speed change. An idler gear is placed between the driver and driven gear — it reverses the direction of rotation, or bridges a gap, without changing the overall gear ratio. Bevel gears transmit rotary motion between shafts set at 90 degrees to each other. A rack and pinion converts rotary motion into linear motion, as a circular pinion gear rotates along a straight toothed rack — used in car steering systems.
A driver gear with 20 teeth rotates at 300 RPM and drives a gear with 60 teeth. Step 1: gear ratio \(= \frac{\text{driven teeth}}{\text{driver teeth}} = \frac{60}{20} = 3\). Step 2: output speed \(= \frac{\text{input speed}}{\text{gear ratio}} = \frac{300}{3} = 100\) RPM. Because the driven gear is larger, it turns more slowly than the driver but with greater torque, and its direction of rotation is reversed.
Cranks and Sliders
A crank and slider mechanism converts rotary motion into reciprocating motion, or the reverse. A rotating crank is connected to a slider by a connecting rod; as the crank turns full circles, the slider moves back and forth in a straight line. This mechanism is used in engines, where the reciprocating motion of a piston is converted into the rotary motion that turns the wheels.
Gear ratio = driven teeth ÷ driver teeth. Output speed = input speed ÷ gear ratio. A larger driven gear always turns slower than a smaller driver.