Statistics

Height:                                    16.5cm
Length:                                    3.6cm
Weight:                                   1.36kg
Maximum load:                        1.46kg
Lifting and lowering speed:        0.2cm/s
Travelling speed with full load:    9cm/s
Travelling speed with full load:    2.5cm/s
Number of motors:                     2

How it works

The actual lifting mechanism (the big box on top) is just a box of bricks, the gearbox and a wheel. I noticed that the large (thick, not wide) Lego MindStorms wheels all have a very small hole in them. I threaded some fishing line through that hole and tied it in place, so that when the wheel was rotated, it would pull on the cable and wrap it around the wheel. The fishing line was then passed through a 2x1 beam's hole to guide it so that it would wrap around the wheel and not the axle holding it in place. The wheel was built into a gearbox (constructed from three 8 tooth gears and three 40 tooth gears) and the motor was connected to the gearbox input. I don't know how to calculate the gear ratio but there is a fairly large torque and speed difference between the input and the output. At the end of the fishing line, there is a plastic bag, secured to the line by looping the line through itself and inserting a thin plate into the loop.

The entire lifting mechanism was positioned on top of a reinforced chassis (I had to reinforce it since it used to just break apart when the lifting mechanism was at full load). This chassis had three pairs of wheels, the large(ish) driving wheels in the middle, some smaller wheel at the front of the lifting mechanism to stop the crane dragging on the floor and some very small wheels at the back to absorb the shock if the fishing line were to snap, thus protecting the RCX. The RCX was placed at the opposite end to the lifting mechanism to try and spread some of the weight. The driving wheels were driven by a single motor which was reduced using an 8-to-40 tooth gear mechanism. This provided the required power to move both the maximum load and the crane itself as well as having some to spare if it hit any minor obstacles. The fishing line was thread through the chassis so the load was positioned directly underneath the centre of the crane. This was done to balance the moments acting on the crane, therefore eliminating the need for counterweights.

The two motors were hooked up to the RCX and the remote control was used to move the motors forward and backwards. Only one problem was encountered during the use of the crane: when loaded over the maximum, the 8 tooth gear closest to the lift wheel started to wear down very rapidly as it slipped but that is why it is given a maximum load specification so in reality, this wouldn't be a problem. Once the gear was slightly worn, the gearbox became less reliable and in real life, the gear causing problems would have to be replaced but I discovered that I could pretty much nullify the slipping by adding a small beam on the side to hold the axles firmly in position and allow no movement or flexing to take place.

In the final experiment, the maximum load was used (the book). It subjected the crane to the worst case scenario (a crane wouldn't normally be pushed so close to its limit do to the increased risk of failure) and the crane showed no signs of  difficulty. It could have gone further when performing the driving experiment but my makeshift test platform (just a sheet of plywood balanced on the back of a chair) began to tip over : ) In theroy, it could've lifted the mass higher too but the desk and the length of the fishing line didn't allow it.

Pictures