Mindboards

I have a project I'm working on that requires rotating a small 3-5 lbs object and I'm looking for suggestions or examples of ways the NXT (motors) can handle that.

Since I don't have to actually grab anything, it seems like a simple project, but have found a few issues. Mainly handling the weight is the problem.
I have built a basic frame for what I need, and have revised it several times already. Sorry, cannot go into detail as I'm hoping I can use this at work.
But some basic concepts I am using / coming to grips with are;

* Center of gravity. Keeping item centered to avoid balance problems.
* Using more than 1 motor. Already having issues with controlling power with 1 motor.
* Rotational momentum. Think I need to gear down to control this. I'm at 1:2.33 now, and still have issues. More than 1 motor may solve this issue.
* Holding item down? I'm hoping gravity will do the work for me. Some rubber parts should keep the thing in place.
* Struts/caster supports. I have some good struts in my frame, but may need additional support for the weight. Thinking about adding casters the the bottom to keep it level...

Any examples/ideas along these lines would be great.

Ok...so after more research, and build revisions I have found the following;

~40lbs movable on lego bearings on youtube.

I added a 2nd motor to my turntable and run both at 50% power. Seems to work well for 3lbs.

I still have some work to do on my frame, and this will take some time....

If it is acceptable why not wrap a Lego chain round it and drive that using a standard gear?

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I am considering something along those lines. The object is cylindrical and smooth, so I could use a couple rubber treads to grab onto it. My only concern is if it slips. Since the object is smooth it is susceptible to slipping and as I am moving it frequently, any loss of movement is unacceptable.

So right now I have it balanced over two large wheels about 3 inches apart which are directly driving the object. This is working OK, but because of the weight of the object I need to support the entire assembly a little more.

I have also considered "clamping" down on the object to keep it in place, but I am still hoping gravity will do that work for me.

I am concerned about the long-time wear on the plastic gears. With two motors driving 24t gears to a turntable (56t) at 50%, I think it is OK.

I am also concerned about direct drive on a 3-6 lb. object (just spinning it). But so far, the two large wheels are doing it. If I can take more pressure off the wheels (and motor in return), then it should help.

I had a similar situation although the forces involved were not nearly so great.

I rolled out some BluTak into a long worm about 3-4mm diameter and applied that to the surface, then squeezed Lego chain down over that.

The chain was driven by a standard gear which was turned using a worm drive, giving a very low gear ratio so it may be too slow for your purposes.

To minimise wear, I applied graphite grease to the worm drive, but this runs only intermittently so I don't know how long the gears will last.

To avoid over-straining the chain, the speed could be ramped up slowly.

If non-Lego parts are acceptable, there are a variety of metal or heavy duty plastic chains available, for instance.

http://www.smallparts.com.au/search/res ... 3A11&hl=en

I also saw a device where a Spanish Windlass was used, i.e. a long string wrapped round the object and a small axle. The tension helps to keep the string from slipping.

That device used a circular wooden turntable resting on 3 wheels for support.

String went round a groove in the edge.

I agree with h-g-t. If you can use non-Lego parts. I would definitely go with metal for the gears and chains.

I am not exactly sure how you are trying to shape your robot but I would look at this framing.

http://www.nxtprograms.com/trailer_pull/steps.html

(One problem is that this robot cannot turn. But it can be modified to turn.

Hope that helps.