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Micro Magnum, Team Magnum, NSW
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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And btw if this robot is ever gonna need a mascot... It will probably be a mixture of these two...

http://imgur.com/QKxw3VP

http://imgur.com/2LrH48o

Post Tue Nov 29, 2016 9:27 pm 
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Nick
Experienced Roboteer


Joined: 16 Jun 2004
Posts: 11802
Location: Sydney, NSW


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OK - is that a bronze bearing in the centre of the pulley? If it is, then the pulley will need to be very well attached to the drum hub. I'm not quite sure why there is a shaft collar between the pulley and the inside of the bulkhead when there appears to be a huge shaft locking clamp on the other side of the bulkhead. Most people just use a bronze thrust washer as a spacer or you can get flanged bearings that space the rotating parts away from the bulkhead.
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Post Tue Nov 29, 2016 9:32 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Well, actually that was supposed to be a deep groove ball bearing, but since I don't have the skill and time required to build a model for a ball bearing, I took the shortcut and just had a ring in there. (dimensions are the same as the actual one, its just me being lazy XD)

The shaft collar I mentioned was not on the weapon shaft, but on the front wheel shaft, which is right under the weapon shaft. But for now I think the gap is a bit bigger than I thought it was and the shaft collars with a set screw are much smaller than I thought, so I guess I'm fine for now.

Post Tue Nov 29, 2016 9:41 pm 
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pilleya



Joined: 31 Mar 2016
Posts: 91
Location: Sydney


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I think it will be easier to machine the drum from solid, rather than trying to use a tube, machined endcaps, pulley and then an axle. You should be able to have a very large thrust bearing inside the rim of the drum( on the OD of the bearing) and then have a bearing block which goes into the ID it will be a lot stronger and simpler that way.

Post Tue Nov 29, 2016 9:44 pm 
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pilleya



Joined: 31 Mar 2016
Posts: 91
Location: Sydney


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You don't need to model the bearing, you can just pull the CAD model from McMaster Carr or another supplier

Post Tue Nov 29, 2016 9:46 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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That is certainly way stronger... Problem is the weight, it's surely heavier than what I currently have and I might be having like a 14.5 kg overall weight.

Post Tue Nov 29, 2016 9:47 pm 
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pilleya



Joined: 31 Mar 2016
Posts: 91
Location: Sydney


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But more weight is good lol, you should aim for a drum which is about 10lb's in weight, or if you don't want more weight only have one tooth on the drum and spin it up a lot faster. If your going to put all this effort in you might as well make it ridiculous.

You could also put the weapon motor inside the drum if you are that way inclined.

Don't use shaft collars for your axles, use a round bearing with an ID that is smaller than the shaft and turn the end down accordingly, this will reduce weight and overall complexity.


Last edited by pilleya on Tue Nov 29, 2016 9:59 pm; edited 1 time in total

Post Tue Nov 29, 2016 9:49 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Well... I would say that drum weight might not be the single most important aspect of a drum, due to the moment of inertia equation giving the radius a power of 2.

Solid drums are great because they are sturdy, but they also will have a lower MOI to weight ratio, or in other words a bigger hollow drum can achieve the same MOI at a lower weight, saving weight for other components, at the cost of structural strength.

Post Tue Nov 29, 2016 9:57 pm 
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pilleya



Joined: 31 Mar 2016
Posts: 91
Location: Sydney


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Yes, but you are limiting yourself to a 3-4inch diameter drum so it makes little difference. A solid piece of 3.5inch diameter bar which is 10 inches in length doesn't even weigh 10lbs. You also need to maximise your bite.

Post Tue Nov 29, 2016 10:02 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Yeah using a bearing with a smaller ID and doing a friction fit would be great, but right now I just don't see enough info such as if it's a tight fit, any idea on sites where I can find the exact ID of bearings?

Post Tue Nov 29, 2016 10:09 pm 
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pilleya



Joined: 31 Mar 2016
Posts: 91
Location: Sydney


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You don't need a friction fit if you are supporting it with similiar bearings on both sides.

Post Tue Nov 29, 2016 10:10 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Well... The drum body (the cylinder part) has an OD of 11cm, which is more close to 4.5 inches, and the teeth reach out to about 13.5cm, which is more than 5 inches, if I was to go for a solid drum, I am pretty sure I'm running into some severe overweight problems.

Post Tue Nov 29, 2016 10:11 pm 
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pilleya



Joined: 31 Mar 2016
Posts: 91
Location: Sydney


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Sorry for any confusion I don't mean use a solid drum, I mean machine the drum from a piece of bar into a tube with a wall thickness of about an inch. It will be easier to machine it from scratch than trying to get a bought tube to work.

Post Tue Nov 29, 2016 10:14 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Ah I see what you mean there, but I think that pretty much is what I planned to do first (my first design was more complex, but in terms of making the part, both my initial design and what you've just said would require some long and stiff tooling, which might be hard to come by)

Post Tue Nov 29, 2016 10:22 pm 
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MoonSet416



Joined: 25 Sep 2016
Posts: 436
Location: Sydney


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Well... It's been a while since I last posted here. What happened was I realized that my old design was too hard and expensive to make, so I made several attempts to simplify the design.

I just made a very crude 2D sketch of what I think might work (just to make sure there are no big issues with the layout) (will be doing a 3D sketch afterward): a two-wheel drive invertible eggbeater bot.

So far at least from the 2D sketch, I might be designing a tall (instead of wide) egg beater. I'm wondering if anyone has seen such kind of design and if there are problems associated with it.

Post Thu Mar 23, 2017 12:17 am 
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