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Sand casting a robot chassis
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chunkulator



Joined: 27 Jul 2016
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Gotcha. Let me have a think about how to incorporate machined rollers into my design.

Another thought I had was that maybe the axial (up-down) bearing load will be relatively light compared to the radial load. In this case I might even be able to go for 608 skate bearings (8x22x7) top and bottom so I can get the ring closer to the floor and put more weight into bigger radial bearings. I think the machined roller idea is pretty killer though. I'll see if I can work that in.

Post Tue Jan 10, 2017 9:24 am 
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Nick
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Vertical forces will be low when the ring hits another bot - unless it hits a wedge and the inclined surface pushes the ring up. The problem is when you get hit by a vertical spinner. The vertical force on the bearings will be huge.

What ever design you finally pick, the key to making it reliable is probably to make the fit as sloppy as possible while still guiding the ring. There are so many parts that will flex during a big impact and you want everything to keep turning with as little binding & friction as possible.
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Post Tue Jan 10, 2017 9:50 am 
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chunkulator



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Won't the ring get unbalanced and tear the whole thing apart at high speed if I don't have the radial bearings loaded up?

Post Tue Jan 10, 2017 2:13 pm 
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Nick
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Its all new territory - who really knows? The ring will always be riding tightly on some of the bearings, probably the ones across from the drive wheel. Its always a compromise between precision & tight fits and friction and binding. Perhaps the best option is to design the bearings to have a fairly close fit but allow room to give them extra clearance if there are problems.

Here is an update on my roller plan; it has the 6202 bearings and generously sized roller that can be machined down if the fit is too tight. I also added Belleville spring washers top & bottom to allow for vertical movement and shock absorption:


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Post Tue Jan 10, 2017 3:20 pm 
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chunkulator



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Nice! Good plan. I'll put something like this into my CAD model and see what it does to my weight. This will all end up being fairly heavy I suspect. Probably I'll only have weight for 4 of these, but I'll see.

The standard 6202 is 15mm ID. There is also a non-standard 6202-16 variant with a 16mm ID which are pretty easy to get. Generally 16mm shafting seems easier to get than 15mm shafting, so I was thinking of doing that.

Are there major benefits to tapered roller bearings that I should consider too?

Post Tue Jan 10, 2017 4:56 pm 
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Nick
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If the weight is a problem, you can probably reduce the size of the rollers; I made them large on purpose so they can be machined to increase the gap inside the ring.

Tapered rollers would be ideal for this as they can take higher loads in both planes. They have drawbacks too; they don't run at high speeds (not an issue here) and they need tensioning (often called pre-load) which is what the spring washers are for in the original design. The main issue is that I don't think they come in a smaller size to fit in a featherweight - if you find some small enough I would be interested in using them.
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Post Tue Jan 10, 2017 5:08 pm 
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chunkulator



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I noticed this when I did a quick search:
http://www.ebay.com.au/itm/like/30202-15mm-x-35mm-x-12mm-Single-Row-Taper-Tapered-Roller-Bearing-Silver-Tone/331642308882

pretty similar in size to a 6202.

Post Tue Jan 10, 2017 5:14 pm 
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Nick
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Good find, I would definitely use something like that over a ball bearing. The load bearing figures are significantly higher and they are still rated for 11,000 RPM.
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Post Tue Jan 10, 2017 5:22 pm 
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chunkulator



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One thing just occurred to me. If the ring is rotating at around 3000 RPM these bearings will be rotating at over 30000 RPM. If they're only rated for 11000 I wonder if I'm better off with ball bearings after all?

Post Wed Jan 11, 2017 5:44 pm 
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Nick
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Good point, bearing speed recommendations are for a long service life, while a match is only 3 minutes. The bearings will still last quite a while, but for lower maintenance, the ball bearings would definitely last longer.

Its a case of whether the bearings get hammered to pieces of just wear out prematurely Smile.
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Post Wed Jan 11, 2017 7:04 pm 
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chunkulator



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Can anyone suggest a flat, high power motor with high kV?

I've been making OK progress with this design based on Nick's roller suggestion. However, I'm having trouble fitting in the weapon drive motor without making the robot very tall or unusually shaped. Ideally the robot would be no taller than about 90mm so that a pair of 4" Colsons stick out the bottom and the top (so it can drive upside-down). Assuming I want about 5mm of metal over the drive mechanism the motor including shaft plus any sprockets/pulleys/gears it can be no longer than 80mm. I've been struggling to fit my existing weapon drive motor (see previous post) in vertically.

I might be able to find a way, but the alternatives I can think of are:
1) Make it taller and use, say, 5" Colsons. There will be a lot of empty space inside.
2) Make it funny shaped so it's taller at one end to make room for the drive mechanism.
3) Use bevel gears so I can mount the motor horizontally. How to fit the bevel gearing in is non-trivial.
4) Drive the ring on the upper or lower edge, rather than the inside edge. This exposes the drive wheel too much I think.
5) Use a different motor in a flatter can.

I've been looking for powerful weapon drive motors in a nice flat can and have found various helicopter motors, Neu 20xx motors and other such things, but all have kV way too low. In order to run the 280mm ID ring at around 5000 RPM with a 50-75mm drive wheel (bigger than that is hard to fit) on about 6S I want kV at least 800. Otherwise I'll need to gear the motor up before the drive wheel, which sounds inefficient and complicated. I'm not sure I'm at the stage in my robot-building career where I want to go rewinding motors to custom kV (although I suppose I'd need relatively few turns), which brings me to my question:

Can anybody recommend a powerful brushless motor (say >2000W) in a flat form factor (say < 70mm including shaft but having enough shaft to attach a drive sprocket) with kV > 800 RPM/V?

Post Tue Jan 17, 2017 8:48 pm 
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chunkulator



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I figure a picture is in order to go with my question. This is the 5" Colson solution. It looks way too tall and the upper deck is mostly empty space - all so I can fit that pink weapon motor with its drive sprocket in. There must be a neater solution.

Post Tue Jan 17, 2017 9:04 pm 
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Nick
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That motor will be really hard to find! One of the larger Hobbyking multicopter motors is the right hight and power but has a WAY too low KV. All the other motors I can think of have cans longer than 80mm Sad. Since you are using chain drive between the motor and the drive wheel, how about using two smaller motors? Not ideal, but much easier to find.
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Post Tue Jan 17, 2017 10:03 pm 
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Nick
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Quick update: This motor might just fit the requirements: http://www.scorpionsystem.com/catalog/helicopter/motors_4/hkiii-40_1/HKIII_4025_890/



The can length is only 51,8mm, the KV is 890 and the continuous power is 2700W. Its designed to run at 8s, but running at 6S will just reduce the power rating down to a bit under 2000W
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Post Tue Jan 17, 2017 10:15 pm 
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chunkulator



Joined: 27 Jul 2016
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Thanks Nick. This looks like a possibility. I'll investigate.

An alternative idea I had overnight is to see if I can cut down the hubs on the sprockets. As long as they're a tight fit on the shaft I should be able to make the hubs shorter without introducing too much runout. They just need to be long enough to hold a set screw or some other method of keying to the shaft. Then I can cut the motor shaft a bit shorter and see if I can squeeze it in. If I can loose about 5mm I'm home. If I can't make that work in CAD, time to get a Scorpion motor.

Post Wed Jan 18, 2017 5:57 am 
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