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General Concepts

10.1.1 Define mechanical advantage, velocity ratio and efficiency.

Mechanical advantage
This is the factor that the machine multiplies the force put into it
Velocity ratio
A measurement of force amplification
Efficiency
Mechanical efficiency is the effectiveness of a simple machine

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10.1.2 Calculate mechanical advantage (MA), velocity ratio (VR) and efficiency for simple mechanical systems.

MA = load/effort
VR = distance moved by effort/distance moved by load
Efficiency = MA/VR

Levers

10.1.3 Describe first-, second- and third-class levers.

Identify load (L), effort (E) and fulcrum (F) in first-class levers (EFL, for example, see-saw, crowbar, scissors), second-class levers (ELF, for example, wheelbarrow, bottle opener, nutcracker) and third-class levers (LEF, for example, tweezers, broom, fishing rod).
1st 2nd 3rd

10.1.4 Discuss the relevant efficiencies of the three classes of lever.

Class 1 & 2 levers are common as they provide a good mechanical advantage, on the other hand class 3 levers provide a MA near or below one.

10.1.5 Explain that, when a lever is in equilibrium, the net moment is zero.

When there is more than one force acting on a lever there will be more than one moment and a net moment can be calculated. If the the lever is not in motion i.e. in equilibrium then the net moment will be zero.

In a see-saw the moments (turning affect) at the fulcrum will be clockwise as well as anti-clockwise, therefore when a see-saw is balanced then it is said to be in equilibrium and the moments will be zero.

10.1.6 Calculate mechanical advantage and effort for first-, second- and third-class levers.


Activity: Find two design contexts where levers are used. Explain how they are used. Identify what class of lever they are.


Gears

10.1.7 Describe gear systems.

10.1.8 Calculate velocity ratio for gear systems.

10.1.9 Describe the function of different types of gears in a range of objects.

Use rack-and-pinion, bevel and worm gears.

Rack and pinion Bevel gears Worm Gears

10.1.10 Explain a design context in which a compound rather than a simple gear train would be appropriate.

Consider the gearing system on a metal lathe designed to be changed to cut a specific type of thread. Consider ratios, mechanical advantage and changes.

10.1.11 Discuss the function of different types of gears in a range of objects.

Use rack-and-pinion, bevel and worm gears.

Belts

10.1.12 Describe a belt or chain drive system.

Consider profile, load, changes in load, and speed.

10.1.13 Calculate velocity ratio for belt or chain drive systems.

For chain drives...

  • Calculate VR as you would for gears ... just ignore the chain.

For belt drives ...

  • Velocity ratio = Diameter of driven pulley/Diameter of driver pulley

10.1.14 Compare belt or chain drives and gear systems.

Consider profile, load, changes in load, and speed.
Compare/Contrast Belt and Gear Systems Belt or chain drives Gear systems
Profile
Load The belt and chain tension strength is its limiter Can provide or accept greater loads than belts or chains
Changes in load Has a large range within which it can work. Change in load can occur quicker. Change in load can occur at anytime in any way within reason.
Speed Greater range in speeds than gears, possibly faster as well. Gears can keep a constant speed but are not as fast, nor is it as friendly to changes in speed as belts system.

10.1.15 Design a system to provide belt torsion to a belt-and-pulley system.


Inclined plane

10.1.18 Describe an inclined plane.

Consider inclined planes, screw threads and wedges.
Inclined plane screw thread Wedge

10.1.19 Explain the advantage of an inclined plane.

It allows a worker to raise a heavy load easily.

It is believed that the Ancient Egyptians used this simple machine in building the pyramids.


References

Bulleted list and italicised paragraphs are excerpted from Design Technology: guide. Cardiff Wales, UK: International Baccalaureate Organization, 2007.

Images are clickable links to its location.

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Page last modified on March 24, 2014, at 07:01 PM