This project is similar to the 4WD car, not only in that the student is working with wood and electrics, but also that the concept of drive ratios is involved. The crane takes this a little further: one can observe how the winch diameter affects lifting speed and power, and the very useful principle of mechanical advantage gained by the two pulleys. When this is taught in schools, it is normally not easy for kids to really get it as a concept; how the addition of pulleys increases the lifting power and reduces the speed. More often than not they only wind up memorizing formulas so they can pass the test and forget about it. But by making and playing with one themselves, they can see first-hand what actually happens when you add pulleys. They no longer need to memorize anything – they just know it.
This is also a lot of fun to play with.
Here are some shots showing the winch construction:
and from underneath
The motor is a very cheap one, around US$1. It runs on 1.5-3.0 volts DC and is commonly found in hobby shops. The black thing on the shaft is a stick-on rubber foot with a hole poked through it, of the kind you put on the bottom of electronic project boxes and such. It’s to prevent the rubber band slipping off the shaft. Here is a better shot of it:
A three inch, 3/16″ screw holds the winch together. The bearing surfaces are two small brass bushes that came out of the pulleys (plastic curtain pulleys).
The orange plastic piece was something I found in the D.I.Y. shop. When my Chinese has improved, I might ask the shopkeeper what it is. The blue wheel is the lid off a jar of peanut butter. This was definitely an exercise in making do with whatever was available.
This is the construction of the forward/reverse switch. The contacts are brass things I found in the DIY shop. I think they come out of sliding doors. Drawing pins would also be OK, and easier to solder too; they just don’t look quite as “high-tech”.
The metal contact strips were cut from a tin can and nailed on with small brass nails. The nails, being a bit too long, had to be cut off on the top side and filed flat. Under the ends, there are pieces of thick double-sided tape to give it a “springy” feel. One of the strips is slightly curved upwards at each end so that a definite pressure is required to turn it on.
The switch arm pivots on a length of steel wire.
and the hole for it was drilled using a length of the same wire.
The lifting arm has to have a space cut in for the main pulley.
The main pulley can then be put in using a suitable screw.
The bottom pulley is made with a small right-angle bracket. It’s drilled in the middle first with a 3mm drill:
Then we take the center out of a suitable pop rivet.
And bend it into a hook (easier said than done)
Then pass the hook through the hole that was drilled in the bracket, and squash it a little wider than the pulley.
The aluminum pop rivet can then be used to hold in the pulley. (It needs to be hammered over on the end so it doesn’t fall out.)
Lastly, a bent nail was used for tying the string under the lifting arm.
With the motor used here, the crane can lift a little over 0.5 Kg.
Update 12 March 2014
Here is a nice version of the crane done by Jeff and his son in the U.S..