The well windlass: wheel and axle
Lift a bucket of water from a well by turning a crank. Change the length of the handle, the thickness of the axle, the bucket and whether the axle is oiled, and work out the VR, MA and efficiency.
Core concept
A windlass is a wheel and axle: its velocity ratio is the handle's radius ÷ the axle's radius; friction makes the mechanical advantage smaller than the VR, and efficiency = MA ÷ VR × 100%
Topic: Simple machines
Many village wells have a windlass: a rope wound round a wooden axle, turned by a crank handle. It is a wheel and axle. Your hand moves round a big circle (the handle's radius) while the rope winds onto a small one (the axle's radius), so your hand moves farther than the bucket rises. The velocity ratio (VR) is the distance moved by the effort ÷ the distance moved by the load, here the handle's radius ÷ the axle's radius. In a perfect machine the mechanical advantage (MA = load ÷ effort) would equal the VR. But friction: A force that slows things down when two surfaces rub together. All key words at the axle wastes some of your work, so the MA is smaller. Efficiency = MA ÷ VR × 100% tells you how much of your work is useful.
What you will learn
- Identify the effort, load and fulcrum of a wheel and axle.
- Calculate the velocity ratio of a windlass.
- Calculate the mechanical advantage and efficiency from the load and effort.
- Explain why no real machine is 100% efficient.
Before you start
- Know that MA = load ÷ effort.
Materials
- A model windlass (a pencil through a cotton reel, with a cardboard crank), string, a small bucket
Equipment
- A spring balance to measure the effort
Procedure
- Record the effort to lift a 200 N bucket with a 30 cm handle and a 5 cm axle.
- Record with a 20 cm and a 40 cm handle.
- Record with a rusty axle.
- Record with a thicker (10 cm) axle.
How this simulation is simplified
- The rope's thickness, and the weight of the rope and handle, are left out.
- An oiled axle wastes 20% of the work, a rusty one 50%.