plastic hubs and drill motors

FIRST General Forum
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  1. When we recently test drove our robot on asphalt the molded plastic wheel hubs on the side opposite the sprocket eventually cracked and broke on all four wheels. We used the three 1 1/2 inch long screws to attach the sprocket to the outer hub. We used the nylon spacers between the sprocket and inner hub. The nuts were inserted in the outer plastic hub. We did not over tighten the nuts. We only used 3 screws as per a diagram from Small Parts. The hub kit has 1 inch screws and 1 1/2 inch screws. Should we be using 6 long screws to tie everything together or maybe 3 long screws for the sprocket and 3 short screws that would connect the inner and outer hubs together. We’re using the smaller Skyway wheelchair wheels.
    When the hubs broke we made a large aluminum washer for the side opposite the sprocket that would hopefully spread out the force on the outer plastic hub. But maybe we should be using 6 bolts either 3 long and 3 short or 6 long (from sprocket to outer hub) and no outer washer. What do you suggest?

    1. Also, we noticed one of our drill motors metal outer casing was discoloring - it was really getting hot. We had no fan on the motor and we were running at the high speed. Could the fan have prevented this or might there be something wrong with the motor? Any suggestions or recommendations.
      We’re using tie wraps to keep the speed changer in place. Maybe we should be using hose clamps.

    2. The last item is that the indicator on the the Robot controller “No Data Radio” flashes on and off every so often. We think the robot kinds of stops and goes when this happens. We think this also happens when it is on the tether. Could one of the motors be generating too much current and the circuit breaker opens and then closes or the controller shuts down and then restarts when the breaker opens and closes. Not sure how this relates to “No Data Radio”
      Any recommendations.

thanks,
Will, Team #751

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Now, I couldn’t really follow what you said, but I think that if you’re breaking hubs, you have too much play. Thus, why are you even asking whether to use 3 screws or 6? Put in 6, and tighten the sucker up. See how that works. Too much friction can cause the motors to heat up, trip the breakers, draw too much current, and consequently drain the battery and cause your radio to cut out.

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I’ll answer in order:
First, I would agree with Chris that you should be using 6 screws instead of 3, longer ones. Your set up sounds pretty solid as a whole, but the failure could be the result of uneven loading from things not being tight enough. So, make sure they are tight enough (not too tight) and consider using Loc-Tite.

You should very strongly consider using fans on the motors, the absolutely will overheat, and eventually fail, without them. We use two fans per motor. Always. Also, tie wraps should be fine for keeping the shifter in place, these gear boxes are pretty good at staying in gear.

Finally, you probably should just check the connections on both the OI and the robot controller. It’s possible that you are having some sort of freak overloading problem, but more than likely you have a bad connection somewhere, and that is why you are getting the “no data radio” light.

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The way the drill motor housing was made this year allows for great mounting of fans. We have two fans on either side of each drill motor and it creates a great airflow upward to get most of the heat out. Our motors lasted about 5 min in high gear before getting hot :yikes:

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*Originally posted by Will Camisa *
**1. We used the nylon spacers between the sprocket and inner hub. The nuts were inserted in the outer plastic hub. We did not over tighten the nuts. We only used 3 screws as per a diagram from Small Parts. suggest?

Will, Team #751 **

We had the same problem 2 years ago when the Skyway Hub Adapters came out. What we found is 1)Ditch the nylon spacers!! 2) Make a .125 thick spacer out of aluminum with the Hex hole pattern in it. 3) Use a bolt through the gear aluminum spacer, and adapter, utilizing the nut location on the oppisite side. 4) Use a Nylok style nut and DO NOT USE LOCTITE!! Loctite seemed to react with the plastic, causing it to crack. We are using the Skyway Adapters on this year’s machine and haven’t had a problem with them in 2 1/2 years. Hope that helps. :cool:

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