Back Driving Problems

[eli2410]

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.

[Joseph Smith]

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.

[shanek21]

Originally Posted by Joseph Smith

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.

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.

[Joseph Smith]

Originally Posted by shanek21

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.

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.

[roystur44]

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

[Pratik Kunapuli]

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

[Gregor]

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.

[cadandcookies]

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.

[TheKeeg]

Here is a picture of the lock

[Team3266Spencer]

We actually used a wrench in our pull back mechanism to prevent back drive but it will still back drive when the robot is powered off or disabled without assistance from the CIM driving it. In our particular setup the wrench does not seem to do much or any good.

[fox46]

Our reloader mechanism for our catapult, although it doesn't operate the way you describe, utilizes a gearbox from an ATV winch. These particular units are fitted with a mechanical brake which locks up the gearbox when the motor is not driving to prevent them from backdriving and dropping your ATV back into the hole from whence you are trying to free it. So far it has proved to be a great safety feature in that if we remove power from the motors during a reload cycle the mechanism just stops in place instead of backdriving. (During which there is about 50 ft-lbs of torque on it).

The gearbox is from a SuperWinch T-2000 ATV winch. It is a previous model of the LT-2000 which is currently being sold commercially. The difference between the two is that the T-2000 has a pinion gear that is separate from the motor output shaft and connects with a simple D interface. The Cybergnomes build their robot out of an ATV and Snowmobile wrecker's shop so we were able to tear through 3 different winches found in the yard before we found one that could be easily interfaced with our motors. I was able to find two brand new T-2000s on kijiji for $100 and some places like amazon may still have stock. Just remember the LT-2000 pinion gear is part of the motor's shaft and will take some magic to make it work. The motor used on the T-2000 is the same diameter, bolt layout (and I postulate, same design) as a CIM motor. Our initial prototypes used a Mini CIM with the shaft cut down and one side milled flat which fit right into the pinion gear. The faceplate of the Mini CIM could actually be removed and the motor bolted right up to the gearbox however, some would contest this violates motor rules. The T-2000 provides a gear reduction of 153:1. Our robot uses a 4:1 Versaplanetary to feed the T-2000 box providing 612:1. The versaplanetary is fed by two RS775 motors in a 2into1 Versaplanetary mount.

Edit: see this pdf for parts breakdown: http://www.google.ca/url?sa=t&rct=j&...62788935,d.b2I

The output ring gear we were able to bolt a VEX hub right up to using pins to fit into holes in the ring gear.

[TheKeeg]

Originally Posted by Team3266Spencer

We actually used a wrench in our pull back mechanism to prevent back drive but it will still back drive when the robot is powered off or disabled without assistance from the CIM driving it. In our particular setup the wrench does not seem to do much or any good.

Could you describe this more? I cannot visualize it based on your description.