Calculating efficiency
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Question
State the efficiency formula.
Answer
Efficiency = MA ÷ VR × 100%, or useful energy out ÷ energy in. VR is what the geometry promised; MA is what was delivered.
Question
Why is a worm drive much less efficient than gears?
Answer
Gear teeth roll against each other; a worm thread slides across the wheel teeth, and sliding friction is far larger. A worm can be at 50% where a spur pair is at 97%.
Question
What happens to efficiency along a chain of mechanisms?
Answer
It multiplies. Three stages at 95% deliver about 86%, so removing a stage is often worth more than improving two.
Question
When is low efficiency deliberate?
Answer
In a screw jack and a worm hoist, where friction stops the load running back when the operator lets go. Making them efficient would need a separate brake.
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Topic 7.3 hub
Mechanical systems applied
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