Introduction to Mechanical Devices

Welcome to one of the most exciting parts of Design and Technology! In this chapter, we are going to explore how we can use mechanical devices to change movement and force. Whether it is a bicycle, a car engine, or even a simple hand-whisk, these mechanisms are everywhere. We will look at how cams, pulleys, and gears work, and how we can use simple math to work out exactly what they are doing.

Note: This chapter focuses on mechanisms that change motion. To learn about the four basic types of movement (linear, reciprocating, rotary, and oscillating), please check out the "Types of Movement, Levers and Linkages" chapter.

1. Cams and Followers

A cam is a shaped piece of material (usually metal or plastic) attached to a rotating shaft. As the cam rotates, it pushes a follower up and down. This converts rotary motion (spinning) into reciprocating motion (up and down).

Types of Cams

The shape of the cam determines how the follower moves. You need to know these three:

1. Pear-shaped Cam: Shaped like a pear. The follower stays still for half a turn (the "dwell" period), then rises and falls smoothly. These are often used in engines to open and close valves.

2. Eccentric (Circular) Cam: This is a perfect circle, but the shaft is joined "off-center." It creates a very smooth, continuous up-and-down movement (oscillation).

3. Drop (Snail) Cam: Shaped like a snail shell. The follower rises slowly and then suddenly drops. These only work in one direction! They are used in things like mechanical hammers or clocks.

Types of Followers

The follower is the rod that rests on the cam. Different shapes have different pros and cons:

  • Knife-edge follower: Very accurate because it has a sharp point, but it wears away quickly due to friction.
  • Roller follower: Has a tiny wheel at the end. This reduces friction, so it lasts longer and stays cool, but it is more expensive to make.
  • Flat follower: Has a wide, flat bottom. It can handle heavy loads but creates a lot of friction and isn't very accurate for complex cam shapes.

Quick Tip: If a question asks why you would use a roller follower, the answer is almost always "to reduce friction and wear."

2. Pulleys and Belts

Pulleys are used to transmit rotary motion from one shaft to another, often over a distance. A belt connects two pulley wheels.

V-Belts: Most modern machines use "V-shaped" belts. The "V" shape fits into a groove in the pulley, which increases the surface area and friction. This stops the belt from slipping when the motor starts or when moving a heavy load.

Calculating Velocity Ratio (VR) for Pulleys

If the two pulleys are different sizes, the speed will change. We use the Velocity Ratio (VR) to describe this.

\( \text{VR} = \frac{\text{Diameter of the Driven Pulley}}{\text{Diameter of the Driver Pulley}} \)

The "Driver" is the one attached to the motor. The "Driven" is the one being moved.

Calculating Output Speed

To find out how fast the driven pulley is spinning, use this formula:

\( \text{Output Speed} = \frac{\text{Input Speed}}{\text{VR}} \)

Key Takeaway: If a small pulley drives a large pulley, the large pulley will spin slower but with more force.

3. Gears

Gears are like pulleys but with teeth! The teeth interlock (mesh) so that there is zero slip. They are used to change speed, direction, or the angle of motion.

Types of Gears

  • Simple Gear Train: Two or more gears meshed together. If you have two gears, they will spin in opposite directions.
  • Idler Gear: This is a third gear placed between the Driver and the Driven gear. Its job is to make the Driven gear spin in the same direction as the Driver. It does not change the gear ratio!
  • Compound Gear Train: This is where two gear wheels are fixed onto the same shaft. This allows for massive speed changes in a very small space (like in a car gearbox).
  • Bevel Gears: These are cone-shaped gears that allow motion to be transferred at a \(90^{\circ}\) angle. Think of a hand-powered drill.
  • Rack and Pinion: This system changes rotary motion into linear motion. A circular gear (the pinion) moves along a flat toothed bar (the rack). This is used in car steering systems.

Gear Calculations (RPM)

In the exam, you will likely need to calculate the speed of the gears in RPM (Revolutions Per Minute).

\( \text{Driven RPM} = \text{Driver RPM} \times \frac{\text{Number of Teeth on Driver}}{\text{Number of Teeth on Driven}} \)

Example: If a Driver gear has \(10\) teeth and spins at \(100\) RPM, and the Driven gear has \(20\) teeth:
\( \text{Driven RPM} = 100 \times \frac{10}{20} = 100 \times 0.5 = 50 \text{ RPM} \)

4. Mechanical Advantage, Velocity Ratio, and Efficiency

Don't worry if these sound complicated—they are just ways of measuring how much a machine "helps" us.

Mechanical Advantage (MA)

This is how much the machine multiplies the force you put in. It helps us lift heavy loads with very little effort.

\( \text{MA} = \frac{\text{Load}}{\text{Effort}} \)

Example: If you lift a \(100\text{N}\) load using only \(20\text{N}\) of effort, the MA is \(5\).

Velocity Ratio (VR)

While MA is about force, VR is about distance. There is always a trade-off: if you want to lift a heavy load easily (high MA), you have to move your hands a much further distance.

\( \text{VR} = \frac{\text{Distance moved by Effort}}{\text{Distance moved by Load}} \)

Efficiency

In a perfect world, MA and VR would be the same. But in real life, friction wastes energy as heat. Efficiency tells us how much energy is actually doing the job.

\( \text{Efficiency \%} = \left( \frac{\text{MA}}{\text{VR}} \right) \times 100 \)

If a machine is \(100\%\) efficient, MA equals VR. Most machines are around \(70\text{--}90\%\) efficient.

Common Mistakes to Avoid

  • Units: Always make sure you use the same units (e.g., all millimeters or all meters) when calculating ratios.
  • Driver vs. Driven: Always double-check which gear is the "Driver" (the input/motor) and which is the "Driven" (the output). If you swap them, your answer will be upside down!
  • The "Idler" Trap: Remember, adding an idler gear changes the direction, but it does not change the speed or the gear ratio.

Quick Review Summary

1. Cams: Pear (dwell), Eccentric (smooth), Snail (drop).
2. Followers: Roller (low friction), Knife-edge (accurate), Flat (heavy loads).
3. Pulleys: Use V-belts to stop slipping. Large driven pulley = slower speed but more torque.
4. Gears: Bevel = \(90^{\circ}\). Rack and Pinion = rotary to linear. Idler = same direction.
5. Math: \( \text{MA} = \text{Load} / \text{Effort} \). \( \text{VR} = \text{input distance} / \text{output distance} \). Efficiency is never \(100\%\) because of friction.