Tower Crane

A few months ago, I bought and built the Lego crawler crane. That is a very good set which comes with lots of very useful parts and two alternative models, both of which are nicely designed and full of neat touches. I was determined to use the new pulleys, strings and spindles with my NXT to create a working tower crane. This is the result.

I have taken a slightly different approach to most of the cranes I have seen on NXTLog. Rather than focussing on lifting heavy loads and making a working crane, I have tried to make a good-looking model that looks as much like a real tower crane as possible.

Lots more details, photos and a video after the link…

Crane Actions

The first problem I faced was that in order to get the full range of movements (hook up and down, hook forwards and back, rotating the jib) I would need to mount the motors on the jib (the horizontal part at the top of the crane). This would then mean I would need to mount the NXT itself on the jib, so the cables don’t get tangled. All of this would be very heavy and the supporting structure would have to be very large. I simply don’t have enough Lego, or space in my house, to do that!

The solution was to only provide two motorised actions, which are raising/lowering the hook, and rotating the jib. This enabled me to move the motors to the base of the crane, so the jib could be kept as light as possible. One motor rotates the jib via a set of joined together axles running up the side of the mast with small gears at the top driving the turntable the jib is mounted on. The other motor directly drives a spindle which holds the string the hook is hanging from. This is routed up through the middle of the mast, through the hole in the turntable and along the jib.

The base of the crane, showing the worm gear drive to the jib turntable on the outside and the hook string spindle in the centre

The centre of the jib, showing the turntable drive gears and the hook string running along the jib

It is still possible to move the hook along the jib, but this action isn’t motorised. The trolley part can be moved by hand along the jib, and rests on the rubber pieces from the NXT kit to stop it slipping. 

The front of the jib, showing the adjustable trolley

Stability

Keeping a structure like this stable is very difficult. A real tower crane keeps upright by being bolted into a concrete slab in the ground. The closest I could manage with Lego was to use a large grey base sheet to spread the load, with studded beams attached to it. The base of the mast is attached to these beams (you can see this in the photo of the base above). Because the beams are longer than the mast is wide, and there are many points of contact with the studs, the structure is actually very rigid.

The second aspect to keeping the crane stable is the balance of the jib. The jib is held flat by the strings running to the triangle shaped section in the middle. These are the support strings from the crawler crane that have round plastic studs at each end. Additionally, the weight of the front of the jib, trolley, hook and load is counterbalanced by the weighted bricks from the crawler crane set. This is adjustable by sliding it back and forth along the back end of the jib, to match the change in balance when the trolley is moved.

The back of the jib, showing the adjustable counterweight

The final aspect of stability is making the jib move smoothly. The turntable that the jib is mounted on was not particularly smooth at first, so I disassembled it and lubricated it using vegetable oil. I used vegetable oil as it is less likely to damage the plastic than some other things such as Vaseline. This made a significant improvement.

Control

The crane is controlled with the NXT intelligent brick as a kind of wired remote control. I guess I could extend it to use Bluetooth from my mobile phone, but I didn’t really see much point in that since it doesn’t move at all!

The controller 

The arrow buttons on the NXT are used to raise and lower the hook. The motor attached to the side of it is used to set the desired position of the jib. The program reads the rotation sensor on that motor and uses that to work out how many turns the jib motor has to go through to match it.

Finally

There you have it. The final version of the crane has a metal hook from an old Technic crane so it swings less, and the crawler crane’s cab is attached upside-down to the jib for the finishing touch!

 The driver’s cab

Here is a video of it in action