Tower crane hook travelling
Crane hook travelling along the jib of a tower crane can be achieved by a single wire spooling off and on two drums on the same shaft, driven by one motor.
The wire comes from 1 drum (grey), runs via the trolley (black), where it is attached to, runs to the return sheave (yellow) back via the trolley not attached) back to the other drum.
In reality this is the same, however the wires are spooled with 1 layer only onto a grooved drum. The diameter and length of the drum is chosen such that the trolley can travel the full length of the jib, without the need for a second layer of wire on the drum.
Lego has no grooved drums so I needed to find a way to cope with the different speed of the wires, while 1 drum part grows thicker and thicker with wire, while the other parts gets thinner. This is clearly visible at the photo below.
The result of this is, if no additional measures are taken, that one of the wires will always be slack. This is acceptable to a certain level, as can be seen in real tower cranes, but for my model, given the small diameter of drum compared to the large distance to be travelled, would result in a wide loop of slack wire hanging below the jib.
For the Bailey crane model I introduced spring loaded return sheaves in the tip of the jib.
With the trolley about half way the jib, the wires were wound evenly onto the both sides of the drum. Then the wire ends which were meeting at the trolley were tightened such that the springs were about half way compressed.
The result was a much less slack wire when nearing the end of the way of travel.
Here you can see the way the wires are tightened on the trolley. Both wires are spooled onto a shaft, which is secured by the black wedge belt wheel and a pin. The other wire is passing over the trolley, without connection.