Back Driving Problems
 

My team has been having problems with back driving on our winch system that is used to reload our launcher. We have thought of a few possible solutions to this problem, but have encountered some difficulties implementing them. We have attempted introducing a ratchet into our gearing system, but any shafts that we could use to stop back driving issues are completely round, making it difficult to attach a ratchet to. We have also considered a gate latch system using a piston as a hard stop to eliminate back drive, but we have very few pneumatic components and this sort of rig would take away our ability to execute variable shots. I was just wondering if any other teams had found creative solutions to stop back driving issues in super shifters or catapults in general or had any advice on this subject.

Thanks in advance,
Shane

Re: Back Driving Problems
 

Yes, we did something a little different that may work for you. We mounted a 36 tooth sprocket onto the side of our winch using bolts. As the winch spins, the sprocket spins with it. We then have a pneumatically powered lever with a steel pin mounted to it. When the lever is engaged, the pin rests inside one of the grooves/teeth of the sprocket preventing the winch from back driving. It works very well. If you are considering this, then I can post a picture.

I don't know what your setup looks like, so it may be hard to fit a sprocket on your winch but it worth a try.

-Keegan Harrington

Re: Back Driving Problems
 

A picture would be greatly appreciated! If I am understanding you correctly, then I think we may be able to implement something similar on our winch using this concept. Thanks so much!

As a side note, I would still be interested in seeing other creative ways that teams have solved this same issue!

Re: Back Driving Problems
 

To expand a little bit more on TheKeeg's explanation. The pin s controlled by the motor activation. When the motor is requested to move, the pin is released, when it stops, the pin is placed back in the sprocket. All automated in the software.

Re: Back Driving Problems
 

I will post one to this thread as soon as I can. It may be a few hours though.

Re: Back Driving Problems
 

So, when the motor is requested to move is the pin retracted via the piston, the motor moves, and then the pin is replaced in order to prevent back drive?

Awesome! I look forward to it.

Re: Back Driving Problems
 

Exactly.

Re: Back Driving Problems
 

In 2010, we used a Dewalt Drill Transmission to drive our kicker mechanism, and it almost impossible to back drive it. There is a white paper on how to attach it to most of our motors. You can also buy a Dewalt transmission from Dewalt directly for somewhere around $40 (can't find the link). They work really well.

The BIGGEST warning I have about them is to work carefully with it and follow the instructions from the white paper. They are extremely complicated and very hard to reassemble if they become disassembled (such as dropping it on the ground like we did at championships. Fortunately, there was some from Dewalt standing nearby who reassembled it for us in 15 seconds.) We just tried to service it, it took us two days to figure out.

Re: Back Driving Problems
 

What are you using to retract your winch? If you use the right gearbox/motor combination, it won't backdrive at all. For example, our winch was originally retracted by a BAG motor in a 100:1 planetary gearbox, and when the motor was unpowered, it backdrove extremely slowly (we used a talon SR to control the motor so it automatically applied brakes when it wasn't powered). At this point all it took was to program the motor to servo to a specific position and it would hold it there without any other mechanical lock. The BAG motor was capable of holding this position for several minutes and barely even got warm. Recently we switched to a 50:1 planetary, which backdrives faster and requires more power to hold, but the BAG still holds the position fine for us. We use an encoder on the winch shaft to keep track of our loaded position and a VEX ballshifter to shift from engaged to neutral, so we just hold the winch in place then disengage the shifter to fire. This also allows us to vary our shot power, because we can just change the winch position target and the motor will hold it there.
There are other options, too- some gearboxes fight backdriving by design, such as a worm-drive similar to the window motor gearboxes. However, all of these ideas hinge on whatever method you use to fire.

Re: Back Driving Problems
 

Having enough torque on the system to prevent back driving would be the ideal situation for our winch, but we are very limited on funds and basically only have what has been donated to us, which in this instance is a supershifter. Do you have any suggestions for a relatively cheap alternative to the ratio that we get from the super shifter? We are able to allocate two CIMs to our winch, which would theoretically give us enough power, we just don't seem to have enough torque through the super shifter's gear ratio.

Re: Back Driving Problems
 

I'm not too familiar with super shifters, but do you know what ratio you are running? How fast does it winch back all the way? What type of speed controllers are you using for the CIMs? Unfortunately, CIMs draw so much power that it's probably not practical to try to hold position with them, or they might get hot and drain your battery. If this is the case, you will have to investigate one of the mechanical locks discussed in this thread. With the lower gear ratios that super shifters use, they are significantly easier to backdrive then a 50:1 or 100:1 gearbox, so holding your winch with the motor is probably not your best bet. However, don't take my word for it- do some testing if you have the chance.

Re: Back Driving Problems
 

Take a look at a disc brake used for mountain bikes and a cylinder to pull the cable. You could use a left of right hand 1/4-20 threaded screw on the end of a shaft

Re: Back Driving Problems
 

We made a custom gearbox that uses a worm gear, which inherently does not back drive.

Re: Back Driving Problems
 

If you're shifting into neutral with a supershifter, that means at least two gears aren't being used. You can pull those gears out, expose some hex, and stick in a ratcheting wrench.

Re: Back Driving Problems
 

My team used a one-way bearing with a manual disengage via piston. It's the most consistent part of our robot. Probably not the most elegant, but it works and was relatively easy to do.