Thank you for your interest Sinclair. If a picture is worth 1000 words, a video is worth 1000 pictures....go to 8:14 on the video linked here to see the bridge in action.
The towers are free standing and not connected in any way other than the moveable span. There are 2 Erector motors, the newer ones that have the gear train built into a transparent case, on the top of each tower.
They are directly connected to each other with a long driven shaft that has 6 Erector pulleys on it. The bridge has 6 "cables" on each end that are strung over the 6 pulleys on each tower. The cables loop over the driven pulleys, as well as 6 idler pulleys to help line things up, and are connected to counter weights within the towers. The weight of the bridge and the counter weights offset each other so the load seen by the motors is small.
I originally struggled with how to install limit switches to stop the bridge at its extremes but this looped cable system is flawless. The cables just slip when the bridge is all the way up or down. I operate the power to all 4 motors manually with one momentary-on DPDT (DC reversing) toggle switch.
Please let me know if you have any further questions and good luck with your project!
TJ
Originally Posted by sinclair:
Originally Posted by TonkaNut:
I built two towers for a custom lift bridge from Erector sets. The movable span is from Atlas.
I use four Erector motors driving Erector pulleys on the tops of the towers.
You can never have too many Erector sets or parts!
TJ
I would like more details about this lift bridge, as it looks a lot like the one I need to design and build for the entrance to my train room.