Constant hook height with 3 section boom

[deehtha]

For my Sbrick contest entry, I created a truck with what was essentially a 3 section crane boom mounted horizontally. One of the difficulties I ran into was extending the boom without changing the hook height, something it seems like many cranes ignore. After much trial and error, I figure out a way.

What I did was count the total number of studs that the final pulley (the one the hook dangles from) on the boom moved. At the halfway point of that travel, I mounted a pulley (circled in red in the images below). From this pulley the string then goes to a pulley at the end of the second boom, and then onto the final pulley. The red pulley then connected to the boom (through some other pulleys, because of the odd mounting of the spool).

The green in the pictures represents the path of the winch string.

Booms retracted:

Booms extended:

I hope this helps someone.

[DrJB]

Very interesting. I wonder how this is done in real life. From watching cranes operate, typically the boom only swings left/right under load. It is not supposed to extend/retract while loaded. The kinematics/design of these machines are the subject of intensive research and patents, and your find certainly confirms that. Great Job!

[Epic Technic]

Wow this is really smart, I never would have thought of it! :)

[Nazgarot]

This is well known method from the reel world. But nice work to find it out by yourself! Congrats!

Similar systems are used with tower cranes as well, in order to make the hook stay at the same height when the carriage is moved along the boom.

I researched a system to keep the hook in same height on a mobile crane with a extendable boom when the boom is extending at an angle. I hoped to simplify it by using a similar technique, but quickly found that because of the delta created by varying angle I had to find a dynamic solution. I ended up trying to use a RCX to multiplex motors to do it, but had to give it up because of a lack of programming skill. When I have time to learn more/better programming in C++ I will do a new attempt. By the way, this problem in a mobile crane is solved in the same way, a electronic unit that multiplexes the movement of the winch with the movement of the boom. Mobile cranes with this kind of setup can be found in construction sites replacing tower cranes. In order to move a load laterally like a tower crane they have to keep the hook level while changing the angle of the boom, or extending the boom.

In a crane where the boom have vertical movement (angular movement) your solution would not work, but it's not without value as it can be applied to hoses for pneumatic cylinders, allowing the hoses to extend with the boom just as the cable in your implementation.

-ED-

[Tadej]

The simultaneous turning of the winch used by the 8421 is also a good system.