B3.3 Mechanical systems application and selection (HL only)

Welcome to Mechanical Systems Application and Selection (HL Only)

In the previous theory chapter (A3.3), you learned how machines work. Now, in Design in Practice, we look at how designers actually select and apply these systems to real-world products. Think of this as the "decision-making" stage: Why choose a gear over a pulley? Why use a specific lever for a prosthetic limb? This chapter is all about making the right choices to ensure a product is functional, efficient, and safe.

Mechanical systems are the "muscles" of a design. They take an input (force) and transform it into a useful output (motion or increased force). Don't worry if the math seems a bit scary at first—we will break it down step-by-step!

Quick Review: The Four Types of Motion
Before we select a system, we must know what motion we need:
• Linear: Moving in a straight line (e.g., a paper trimmer).
• Reciprocating: Back and forth in a straight line (e.g., a sewing machine needle).
• Rotary: Moving in a circle (e.g., a bicycle wheel).
• Oscillating: Swinging back and forth in an arc (e.g., a pendulum clock or a playground swing).

1. Criteria for Selecting Mechanical Systems

When you are designing a product for the IB DP, you can't just pick a mechanism because it "looks cool." You must justify your choice based on several factors:

A. Required Output Motion: Do you need to change the type of motion? For example, a car engine uses a crankshaft to turn the reciprocating motion of pistons into rotary motion for the wheels.

B. Mechanical Advantage (MA): Do you need to lift something heavy with very little effort? If so, you need a high Mechanical Advantage. The formula is:
\( MA = \frac{Load}{Effort} \)

C. Velocity Ratio (VR): Do you need something to move very fast? Velocity Ratio is the relationship between how far the effort moves compared to the load. The formula is:
\( VR = \frac{Distance\, moved\, by\, effort}{Distance\, moved\, by\, load} \)

D. Space and Weight: In a handheld product like a power drill, the mechanism must be compact and lightweight. Heavy steel gears might be too bulky, so a designer might select high-strength plastic gears or a compact planetary gear system.

E. Maintenance and Durability: Will the product be used in a dusty environment? If so, a belt and pulley might be better than a chain and sprocket because belts don't require lubrication, which attracts dust.

Key Takeaway: Selection is a balance between Force, Speed, and Context. You usually have to sacrifice speed to get more force (and vice-versa)!

2. Application of Levers and Linkages

Levers are the simplest mechanical systems, but choosing the right class of lever is critical for user-centred design.

The Three Classes of Levers (Mnemonic: PLE or FLE)
1. Class 1 (Fulcrum in the middle): Best for reversing direction or gaining high MA (e.g., a crowbar or scissors).
2. Class 2 (Load in the middle): Always gives a mechanical advantage greater than 1. Great for moving heavy weights (e.g., a wheelbarrow).
3. Class 3 (Effort in the middle): Used when you want the load to move a long distance or at high speed, even if it requires more effort (e.g., a fishing rod or a pair of tweezers).

Linkages: These are series of levers connected together to change the direction of force or provide a specific "path" of motion.
• Parallel Motion Linkage: Keeps the output moving in the same direction as the input (e.g., a toolbox or a folding drying rack).
• Bell Crank: Changes the direction of motion by \( 90^{\circ} \) (e.g., bicycle brake levers).

Selection Tip: If your user has limited strength (like a child or an elderly person), you should select a Class 1 or 2 lever to maximize Mechanical Advantage.

3. Selection of Drive Systems: Gears, Belts, and Chains

How do we get power from a motor to a wheel? Designers choose based on the specific needs of the product.

Gears

Gears are used when you need a positive drive (no slipping) and high torque.
• Spur Gears: Straight-cut teeth, used for parallel shafts. Efficient but can be noisy.
• Bevel Gears: Used to change the drive through \( 90^{\circ} \) (e.g., a hand drill).
• Worm and Wheel: Used for massive speed reduction and to prevent the system from "back-driving" (e.g., a guitar tuning peg).
• Gear Ratio Formula: \( Gear\, Ratio = \frac{Number\, of\, teeth\, on\, driven\, gear}{Number\, of\, teeth\, on\, driver\, gear} \)

Belts and Pulleys

Selection Case: Use these when the two shafts are far apart. They are quieter than gears and can "slip" if the machine jams, which acts as a safety feature to prevent the motor from burning out (e.g., a pillar drill or a washing machine).

Chains and Sprockets

Selection Case: Use these when the shafts are far apart but you cannot have any slipping (e.g., a bicycle or a motorcycle). They are very strong but require regular oiling.

Did you know? V-belts are shaped like a "V" to increase the surface area in contact with the pulley, which uses friction to prevent slipping without needing teeth!

4. Changing Motion: Cams and Followers

Cams are used to convert Rotary motion into Reciprocating motion. This is essential in automated machinery.

Selection Factors:
• Cam Shape: A Pear-shaped cam allows for a long "dwell" (period of no motion), while an Eccentric (circular) cam provides a smooth, continuous rise and fall.
• Follower Type: A Roller follower reduces friction and wear (good for high speeds), while a Flat follower can handle higher loads but wears down faster.

Common Mistake: Don't forget the return spring! A cam only "pushes" the follower up. You need gravity or a spring to pull it back down to follow the cam's profile.

5. Mechanical Efficiency

In the real world, no machine is \( 100\% \) efficient because of friction. Friction turns some of our input energy into heat.

Calculating Efficiency:
\( Efficiency\, (\%) = \frac{Mechanical\, Advantage}{Velocity\, Ratio} \times 100 \)

Design Application: To improve efficiency, designers select:
• Bearings: To reduce friction in rotating shafts.
• Lubrication: To allow parts to slide easily.
• Materials: Like Nylon or PTFE (Teflon) which are naturally "slippery."

Quick Review: Summary Checklist

• Identify the input and desired output motion.
• Calculate the required Mechanical Advantage (do we need more force?) and Velocity Ratio (do we need more speed?).
• Evaluate the environment (is it wet, dusty, or high-pressure?).
• Select the mechanism:
- High force, short distance? Class 1/2 Lever.
- Transfer power over distance without slip? Chain and Sprocket.
- Convert rotary to reciprocating? Cam and Follower.
- Large speed reduction in small space? Worm Gear.
• Justify the choice based on the user's needs and the product's function.

Top Tip for Paper 2: When you see a product in an exam question, look at the moving parts. If it has a handle that moves further than the part doing the work, it's designed for Mechanical Advantage. Mention this to get those AO3 marks!