Author Topic: Electric motor question  (Read 2552 times)

Offline chipenter

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Re: Electric motor question
« Reply #25 on: June 24, 2017, 12:59:31 PM »
It's probably an electric brake on the end of the motors , I have a pair very similar .

Jeff

Offline AdeV

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Re: Electric motor question
« Reply #26 on: June 24, 2017, 01:55:26 PM »
mega-3D-printer!

...or maybe a ceiling-mounted plasma cutter....!  :lol:

Hi Jeff - could be... although I think it's a worm drive gearbox, looking at the shape of it (good excuse to open her up!), would a worm drive need a brake?

Edit to add: Yep, you're spot on, it's an electromagnetic brake, just like the one on the top of my CNC mill... That should reduce the required current somewhat!
Cheers!
Ade.
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Offline Noitoen

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Re: Electric motor question
« Reply #27 on: June 26, 2017, 04:37:55 PM »
I don't think it's a worm drive. It does not work well as wheelchair motor. To acheive the 90 setup they usually use bevel gears.

Offline AdeV

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Re: Electric motor question
« Reply #28 on: June 26, 2017, 07:26:14 PM »
Right then... tomorrow I will take the lid off and have a look!

I've been doing some calculations... with a 4mm TP screw (or an 8TPI Acme, if I can get imperial anywhere), I need around 576rpm to achieve my desired 600mm rise/fall within about 20 seconds. If I can get the wheelchair motor to turn at 64rpm (around 18-20V should do it), I can use off-the-shelf bicycle sprockets to gear up to exactly that RPM. So the only question then is, is one motor powerful enough to turn all those gears & cogs & idlers, to overcome the friction of 4 leadscrews, and still lift a sensible weight?

I calculate I will need: A large main drive sprocket (will have to make the bushing to fit it to the motor), driving a small cog, which is rigidly attached to another large cog, which in turn supplies drive to small cogs on each leg (so 2 chain rings, and a minimum of 6 small sprockets. To reduce the individual chain lengths, I had wondered about adding an additional large/small pair (so each driver pair drove 2 legs), so that's 3 chain rings & 7 sprockets. Each driver (except for the one on the motor) will need to run in bearings, the top of each leadscrew will also need bearings, so at a rough guess that's 8 bearings so far. Then the chain will require tensioners; 3 options here: Fixed plastic "slide" tensioners ala a car cam chain (which introduces possible wear issues), but these would be very simple to make & fit; plastic guide wheels (more bearings), or additional sprockets (more bearings AND sprockets)... Then there's the chain, of course. Plenty of it, certainly more than on your average bike...

Everything's actually pretty cheap, except for the chain rings (around 12 each) and the lead screw/nuts (but I could potentially make these from bar, which would be cheap). Actually, the lead scews can be had for less than a tenner, but the nuts are ferociously expensive, even from China, so maybe I'll buy the German leadscrews & just make the nuts myself. Of course, one option to save a few pennies is to buy a job lot of broken bicycles, and rob the sprockets off them... but then I end up with a pile of dead bicycles to dispose of into the bargain. Plus, who knows how much wear & rust will be on these things?

Still.... I'm tempted... not ready yet to abandon the idea to a central hydraulic hand-operated jack & latching legs... but I do have a deeper appreciation of why these things cost so much.

Oh - I looked into 2nd hand rise/fall desks - they're certainly available on the 'bay, but the cheapest I saw was 200, and no indication of maximum load. I'd love to find a broken one so I can see what they use for a mechanism...
Cheers!
Ade.
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Offline awemawson

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Re: Electric motor question
« Reply #29 on: June 27, 2017, 02:21:47 AM »
Perfect excuse for you to get a CNC plasma table. You could cut your sprockets exactly as you want them  :clap:
Andrew Mawson
East Sussex

Offline AdeV

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Re: Electric motor question
« Reply #30 on: June 27, 2017, 12:52:38 PM »
Ha - indeed... but the project to make or buy/fettle a plasma CNC table is currently beyond my meagre means :( Not to mention the space in my workshop is currently rather chaotically taken up with Too Much Stuff...

Hmm... maybe if I knew someone who had a CNC plasma table, and rather enjoyed using it............ anyone?  :lol:

Actually - seriously for a minute, if I bought you the sheet of metal, hows about it?
Cheers!
Ade.
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Offline awemawson

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Re: Electric motor question
« Reply #31 on: June 27, 2017, 01:01:42 PM »
I'm sure that we could work something out  :thumbup:
Andrew Mawson
East Sussex

Offline Pete W.

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Re: Electric motor question
« Reply #32 on: June 27, 2017, 02:14:09 PM »
Ade,

I reckon you'd better take a shovel and a pair of wellies with you!!   :lol:   :lol:   :lol:   :lol:   :lol:   :lol:   
Best regards,

Pete W.

If you can keep your head when all about you are losing theirs, you haven't seen the latest change-note!

Offline AdeV

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Re: Electric motor question
« Reply #33 on: June 27, 2017, 03:45:02 PM »
Haha! Unfortunately, I'm allergic to hard physical labour, it makes me break out in a nasty sweat, causes muscle aches and general weakness  :lol:
Cheers!
Ade.
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Offline awemawson

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Re: Electric motor question
« Reply #34 on: June 27, 2017, 04:13:42 PM »
Wimp  :lol:
Andrew Mawson
East Sussex

Offline hanermo

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Re: Electric motor question
« Reply #35 on: June 28, 2017, 12:50:48 PM »
These are my thoughts / experiences for 2..

ANYTHING threaded rod in common/normal use is crap.. wears out, binds, rusts, leaks..
anything whiny or with noise, you built, will annoy You forever..
at the loads, I would aim for about 20-30 mm D threaded rod ... AND with acme/trapezoidal, if only using one rod, and no linear guides.
Friction is HUGE with small threaded rod under load.

It is not the load rating ... its the cantilever effect causing the binding.
(If you were lifting a rigid bit from the center, then the effect is vastly reduced, perhaps 20-fold.)

Cheap small air-cylinder counterweights are available for around 5-10 each.
They can maybe reduce torsion/cantilever effects 20x or more, for not much money.

Perhaps .. fix one or more cylinders next to legs, try it, if it is good, add more as needed, then bolt them inside the vertical legs for looks, not to bother, protection, "neat".


Offline vtsteam

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Re: Electric motor question
« Reply #36 on: June 28, 2017, 04:19:46 PM »
ANYTHING threaded rod in common/normal use is crap..

Hmm, I guess that would include my Gingery lathe built 15 years ago, its milling attachment, the CNC hot wire foam cutter I built 5 years ago, and an automated filter sampler I built for a Cary 5000 spectrophotometer for a high tech optical firm that I worked for 12 years ago.  :dremel:
I love it when a Plan B comes together!
Steve
www.sredmond.com

Offline AdeV

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Re: Electric motor question
« Reply #37 on: June 30, 2017, 10:12:59 AM »
I don't think it's a worm drive. It does not work well as wheelchair motor. To acheive the 90 setup they usually use bevel gears.

OK, so I finally got around to looking inside the gearbox last night. It's still not 100% certain, because the thing was so full of black grease that I couldn't easily find the input shaft, without washing the grease out... however, the fact that there's a large gear with "throated" teeth suggests strongly to me that it is worm drive. The large worm-driven gear is attached to the same shaft as a small spur gear, which is then transmitted to a larger cog which is on the output shaft. The gear can be slid along the output shaft to disconnect it from the drive spur, a spring pushes it back into mesh when required.

I didn't power up with the lid off, as I didn't fancy getting sprayed with grease!
Cheers!
Ade.
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Location: Wallasey, Merseyside. A long way from anywhere.
Or: Zhengzhou, China. An even longer way from anywhere...
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