Winding down balances winding up
Canadian engineers faced the problem of designing a system to study water salinity, flow and temperature in Arctic channels which are iced over for nine months of the year.
Problem: Since the ice is moving, the floating probe has to be wound down clear of the undersides of ice ridge keels but allowed to float up close to the ice to make measurements once a day. The system can only be accessed to replace batteries in Summer – so winch power consumption had to be minimised.
Solution: The ‘Icycler’, as it has been called, places its winch in a second large float at a depth of 50m.
The winch exhibits a large diameter drum and a small diameter drum, geared to each other. The instrument float which makes the measurements is attached to the larger drum, while the cable to the mooring point on the sea bed is attached to the smaller drum.
When the cable on the large diameter drum is unwound, and the instrument float is allowed to rise, the other cable is wound in, pulling the float containing the winch downwards but by a proportionately smaller distance. The diameters and buoyancies are such that the potential energy released by the small float rising is balanced by the amount gained by pulling the large float downwards and vice versa. In this way, there is no net change in potential energy whether the sensing float cable is let out or reeled in.
Application: Designer George Fowler says that the system uses only 20 to 25% of the energy required by a conventional system.
The instrument float exerts a buoyancy force of 100N and is pulled down at 0.2m/s. Motor power is 35W and battery weight is 18kg – sufficient for 400 raising and lowering cycles.
The same principle may also be applied to lifting – by having a very heavy weight or massive amount of spring force acting on a small diameter drum counterbalancing a smaller lifted weight acting on a larger diameter drum. TS
