Motorized Monorail Switch
Version 1
MOC by Brian Alano
Use of gear rack inspired by: XXX
Use of sliding worm gear inspired by: XXX
Thanks to Eric Smith and Brian Darrow for loaning me their monorail track.
This model allows for the remote or automated switching of monorail points. It could be connected to an RCX running LACC (http://XXX) and be controlled through DCC (http://www.nmra.org).
Evolution and principles:
I finished the first motorized monorail switch, but I was distrurbed by the fact that the train would derail if it met the switch from the "wrong" side. What was needed was a way to disengage the drive train once the point had been switched by the motor. My first attempt was with a differential used first to drive the point, then to slide the 8t gear off the rack. That turned into a monstrosity. The next iteration employed a sliding worm gear to drive then disengage from a 24t gear, which is linked to the rack which drives the point. I used rubber bands attached to a 1 x 3 liftarm on each end of the worm gear to hold the worm gear in place under a 24t gear. The bands held the worm gear centered until the point reached the end of its motion. At this point the torque required for the worm gear to turn the 24t gear was greater than the torque required for it to oppose the rubber bands, so the rotating worm gear slid screw-wise along the 24t gear and onto a fixed 8t gear. Once it engaged the 8t, it pulled away from the 24t completely, freeing the point to be moved by the train. It worked very well, but there were two problems. First, four rubber bands were used, and each had to be tensioned equally to keep the 1 x 3 liftarms from jamming and to keep the worm gear centered on the 24t gear. This was inconvenient, but not a show-stopper. The main problem was constructing a frame that was compact but didn't interfere with the travel of the liftarms. This I could not do, so I looked for an alternative to the rubber band/liftarm combination. The solution, presented here, uses the same principle as above, but uses springs (from the small shock absorbers) in place of the rubber bands and liftarms.
Another note, to inform those who might wonder, and forewarn those who might try:
I tried using an 8t gear in place of a 24t, because it made the structure more compact, and it made the geometry of the frame much simpler. However, the extra torque required to turn the 8t gear was enough to overcome the spring tension. The result was that the worm gear disengaged before the points were switched.
Good features:
* reliable
* not a large footprint
* can still be manually controlled
* can be removed by sliding off the 8t gear and 1 x 2 brick with axle hole.
* the train can still move the switch automatically (as long as the point is not in the middle of being switched by the motor)
For future improvement:
* It takes 3 to 5 seconds to switch. Maybe using the older ungeared motor would speed this up.
* The motor is not completely protected from stalling. (the worm gear usually, but not always, slips over the 8t gears at the extremes of motion.)
* Occasionally the uncompressed spring will slide directly under the 24t gear. This can impede the switch if it is being forced to move by the train, possibly causing a derailment. Some way to anchor the springs would solve this.
Notes on the building instructions:
The 8t gear that is shown in step 14 and attached to the 5L axle in step 15 is shown removed in steps 18 through 23. There's no reason for this. The proper position of the 8t gear is shown in step 24.
The belt which was attached in step 15 is no longer attached in step 21. There's no reason for this, either.
Critical parts:
geared motor (though other motors might work)
gear rack
8t gear
worm gear
12L axle
2 springs from small shock absorbers
1 x 2 technic brick with cross-axle hole