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Old 02-23-2012, 08:22 AM   #11 (permalink)
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seems to be hard to get a broken down beast of grinders... I'll try my luck on the fleahmarket this weekend. I saw ppl selling old/broken power tools...

I might just end up with a friction drive. At least with that I'd get a picture of the RPM under load. Planning the drivetrain would be straightforward after that.

About the electrics.
I'm planning a precise charger for the batteries. I found a charge controller, UC3906, which seems to be up top the task pretty well.

My problem is that i did not find a schematic which would be able to charge bigger batteries, and I would want something more than 1-1.5A maximum current. (battery pack will be 2x 24Ah @ 20h 12 batteries in paralel)

I got the datasheet for the batteries and with the datasheet of the UC3906 I'm able to plan the thing, but I'm a bit lost at the power switch part. It was too long ago I last dealt with transistors... So If somebody could tell me taking a look at the datasheets, weather the chip will be able to control 10A with a BDW94C darlington, I'd really appreciate that.
I guess it can handle it, but better not fry the chip, right?

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Old 03-08-2012, 09:37 AM   #12 (permalink)
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Finally I managed to get a half big grinder for chips on the local fleamarket.

I have no idea what machine it could be but probably some 1.5 - 2.3 kW model.

I made a little "plan" in Paint about mating it together with the motor.



My concerns:
The small cog was originally put on the shaft of the grinder only by the friction force what a well tightened nut gave to it. Withouth machining tools I'm a bit reluctant to try to create an insert for the motor shaft to use the same solution.
The idea: I cut a little, 4mm wide, 4mm deep slot on the motor's shaft which is 18.8mm in diameter. Also I cut out a fitting piece from the bottom of the cog, so they will sit together and this joint will transfer the torque.

the motor output shaft has a hole drilled into it, some 11mm in diameter, with tiny teeth inside. The cog has a bore inside of 10mm diameter, so what I thought is that I have a 10mm steel bolt (I happen to have one already cut) which I could fit into the hole on the motor shaft using some soft metal ring and loctite. This would center the cog perfectly and the thread on the end would keep the cog escaping. Hopefully.

What I have problem with with this two joints is: I have no idea how strong they will be...

anyone with some mechanics - machining knowledge could give me a hend here?
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Old 03-10-2012, 08:10 PM   #13 (permalink)
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The shear strength on the 4mm x 18mm area will depend on the material of the cog. Would it be possible to put a steel sleeve over the outside of both the cog and motor shaft then use four 6mm set screws to hold it in place?

Any improvements in aerodynamics will reduce power requirements.
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Old 03-13-2012, 06:58 AM   #14 (permalink)
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Hi, thanks for the idea.

The cog has a 10mm diameter bore in it. It was for the original motor shaft, and the cog was held on it only by a simple nut and the friction force what this tightened nut gave to it. So the actual face of the metal is not 4x18mm... It's smaller due to the hole in the middle of the cog. The shaft of the motor I use also has a hole in it with tiny grooves. I figure I could drill that up to 12mm or so, get a little shaft welded into the cog and drill and tap a few small screws into the motor shaft to hold the shaft of the cog...? Would that work? Would that be any stronger then the 4x 10-ish slot?
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Old 03-13-2012, 10:10 AM   #15 (permalink)
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let me see if I get this. The hole in the cog is 10mm. The hole in the motor shaft can be drilled up to 12mm. Could you press a shaft into the cog? Could you press that whole thing into the motor shaft? maybe some combination of pressing and welding? Small screws would probably break, its hard to get any strength out of them.
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Old 03-13-2012, 10:59 AM   #16 (permalink)
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Yes, I could make a shaft, which is something like this:



instead of the nut I can actually weld the cog to the 10mm shaft.... this way the only thing to do is to ensure that it will stay aligned with the motor and that it will be capable of transmitting the power...
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Old 03-13-2012, 08:04 PM   #17 (permalink)
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I like the looks of that much better. Pressing the peices together, done properly, should transmite the torque in the amounts your dealing with.
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Old 03-28-2012, 10:27 AM   #18 (permalink)
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No really update here... I found a mechanic workshop who could do the trick of machining the pieces together, however they are exactly opened during those hours when I work...
probably next week I can pop in there though.
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Old 03-28-2012, 11:35 AM   #19 (permalink)
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Old 03-29-2012, 02:30 PM   #20 (permalink)
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A good press fit will be needed to secure the parts. Using a hydraulic press on a motor shaft can be tricky as you don't want to bent the shaft at all. Either thread the adapter to 12mm and use a jam nut or grind two flats on the adapter 100 grads apart then secure with 6mm 9.8 grade set screws. You may need to separate the gear from the motor at some point. The people at the mechanic shop may have some good ideas too.

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