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Unread 02-16-2016, 02:53 PM
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Compact Joint Help Needed

Wanted to check a design and some numbers past folks to make sure we don't waste what little time we have left.

Will the GEM Gear boxes be able to handle the load?
Do we need any structural reinforcement?
Will the arm axle be able to handle the load?
Is there anything we should change about this design?



The structural members are 1x2" and are .1 thick.
The arm that is being rotated is 20lbs total and is 33" log so the numbers in JVN are increased over reality.
The extra gearbox location is in two different locations in the two images and is only being used to house an encoder.
Here is what the arm looks like:



We have blown up a few lift joints so far this season and want to make sure this one works. http://team1389.com/9-days-left/ Here is the 2nd failure https://www.youtube.com/watch?v=BjIZBcQOSe4
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Unread 02-16-2016, 03:08 PM
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Re: Compact Joint Help Needed

I can't recall the reduction in our GEM gear boxes for our arm, but we have a shamefully heavy intake system that is at the end of an arm that pivots at the back of the robot. It can lift the intake fairly well, as well as the portcullis.
This is the practice session we had yesterday and you can see that we have a similar design using two sets of GEMs and CiMs. We didn't do much lifting during practice since we didn't have our encoders and limit switches programmed yet, but it does lift. PM me and I can get back to you on gearing specifics.

It would be worth giving this a shot. Do you have any sort of lifting assist? Our arm has two 40 pound gas springs keeping it up, along with the plate springs on the intake pivot. I highly recommend a lifting assist of some sort. The shock absorbing effect of the gas springs and the plate springs also help the arm come down gently as you can see when we get some airtime on the moat. You might want to consider the stresses that your arm will endure when doing some bumpy driving. I can almost guarantee it will pop up and slam if you go too fast over a defense. More than just your hearts will be broken.
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Last edited by bEdhEd : 02-16-2016 at 03:20 PM.
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Unread 02-16-2016, 03:25 PM
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Re: Compact Joint Help Needed

I honestly think this is way overkill but then again...

I will be interested to know if you see binding inside of the GEM gearboxes the way we did last season. If overloaded then they are prone to internal binding from our experience with them... granted we were lifting a dynamic 20 pound load at 70 inches so you know... don't do what 900 does.
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Unread 02-16-2016, 03:26 PM
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Re: Compact Joint Help Needed

Agreed on the spring assist of some sort. We had one student bring in some elastic spear gun tubing and that stuff works wonders - much better than the kit surgical tubing. It will be featured on our robot this year for our climber spring.

Was that duct tape in that video??
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Unread 02-16-2016, 03:33 PM
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Re: Compact Joint Help Needed

5803 is running 2 GEM gearboxes with MiniCIMs in a 200:1 overall reduction for a similar weight and length arm and they've been fine for us so far. The GEM should be capable of handling the load as long as you keep the sprocket close to the base of the shaft (which it looks like you are).

I would be concerned about the amount of torsion going through the hex shaft though. It might be OK, but I would recommend trying steel hex there if that's not already the plan. Aluminum is likely to start yielding there, if not from the dead weight than from the shock loading of the arm bouncing while you drive around. If there's any way to flip it around so the sprocket is bolted to the arm structure and the axle is a dead axle and free-spinning, that would alleviate the concern too.

I can't tell from the picture, are you using #25 or #35 chain there? I would highly suggest #35 for this application, the loads are high enough (especially if the MiniCIMS end up stalled driving the arm into something) that you will probably break #25 chain.

Best of luck! Arm joints can be tricky to do right, especially with a heavy load on the end.
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Unread 02-16-2016, 03:49 PM
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Re: Compact Joint Help Needed

Quote:
Originally Posted by OccamzRazor View Post
Was that duct tape in that video??
It was, After it broke the first time they re-riveted the motor mounts and ducttaped the motor to the frame. Then we tried hose clamps and it bent the structure. Then we beefed up the structure and destroyed two versa planetaries. So now this is where we try one last time, with a lot more smarts under our belt and some help from CD and make sure it works.

Thanks for all of the tips so far, we are updating the drawings as we speak.
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Unread 02-16-2016, 04:23 PM
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Re: Compact Joint Help Needed

Looks great. With regards to the hex shaft and its loading capability, I've used 7075 alumimum hex for a similar purpose on a longer arm and it held perfectly fine even with bouncing.

+1 for #35 chain.
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Unread 02-19-2016, 10:39 PM
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Re: Compact Joint Help Needed

The Joint is working much better. Thanks for all of the input. https://www.youtube.com/playlist?lis...lmrXS81A8KXs7d
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