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Using Arduino => Motors, Mechanics, and Power => Topic started by: DonMilne on Feb 13, 2014, 06:01 pm

Title: Screwdriver motor
Post by: DonMilne on Feb 13, 2014, 06:01 pm
In a previous discussion (http://forum.arduino.cc/index.php?topic=207394) in which I outlined my need for a DC motor that can lift and hold a weight, Zoomcat suggested that I buy a 6V cordless screwdriver powered by 4xAA batteries (http://www.amazon.co.uk/gp/product/B000ZO0E48/ref=oh_details_o08_s00_i00?ie=UTF8&psc=1).

Well I did, and I took it apart, and I have to say that the result is most interesting. When it arrived the first thing I did was confirm the high torque - I can't stop the spindle by hand pressure.

Inside the plastic case was an RS380 6VDC motor (that's what the label on the side reads). This motor stands out from the other hobby motors I've bought in that it has very strong permanent magnets just inside the case - I have to be careful where I lay this motor down.  Does anybody know what "RS380" mean?  I.e. is it just a form factor or does it only apply to motors that perform exactly like this one?  Do motors with very powerful permanent magnets have special attributes, e.g. higher torque?

In the previous thread Zoomcat said that he was unable to turn the spindle from the outside - and I can confirm this. The question for me is "how come"?  Inside the black nose cone I found a two stage planetary (epicyclic) gearbox, which turns out be a delight to take apart and put back together. There's only one screw, and the construction is quite obvious. If you have access to a metal lathe then it will be trivial to replace the screwbit holder with something that suits your project.

Both the motor spindle and the planetary gears can be reverse driven quite easily when the two are separate. But put the two together and it all locks solid, it can't be reverse driven at all. I already learned from the previous thread that worm gears cannot be driven in reverse. Is the same thing true of planetary gears?

Comments welcome.

Title: Re: Screwdriver motor
Post by: Robin2 on Feb 13, 2014, 08:08 pm
As far as I know RS380 is a form factor though I don't know what it measures. Google probably knows.

There is no reason why planetary gears can't be reversed. I suspect it's just that the enormous gear ratio multiplies the motor "friction" beyond what you can turn by hand - especially with powerful magnets. If you don't mind the possibility of damaging something try using a lever to turn the spindle.

...R
Title: Re: Screwdriver motor
Post by: DonMilne on Feb 13, 2014, 10:26 pm
Thanks for your reply, however I must admit that it doesn't "feel" right to me. As I mentioned, both the motor and gearbox can be turned quite freely when apart, there is no great friction. It is only when brought together that they're locked solid, just a little play (backlash). It feels very similar to my attempt to turn a worm gear in reverse.

If I had to guess I'd say that when driven normally the planetary gears are pressed against the outer (annular) ring and turn freely, but when driven in reverse the planetary gears squeeze together onto the central (sun) gear, past the required tooth clearance, and hence it locks up instead of turning - and since the sun gear is on the motor spindle, this doesn't happen when motor and gearbox are apart. This is a pure guess like I said, but that's what it feels like.
Title: Re: Screwdriver motor
Post by: Robin2 on Feb 13, 2014, 11:37 pm
I can turn my cheap battery drill/screwdriver by hand.

Unless the gear train is specifically designed not to reverse (which seems unlikely) I would be quite sure it's just friction or "sticktion"

...R
Title: Re: Screwdriver motor
Post by: MarkT on Feb 14, 2014, 12:35 am

Thanks for your reply, however I must admit that it doesn't "feel" right to me. As I mentioned, both the motor and gearbox can be turned quite freely when apart, there is no great friction. It is only when brought together that they're locked solid, just a little play (backlash). It feels very similar to my attempt to turn a worm gear in reverse.


Yes the gears are multiplying the torque needed to turn the motor, and they are high-ratio.
Title: Re: Screwdriver motor
Post by: cr0sh on Feb 14, 2014, 01:18 am
Regarding the "RS380" bit - here's a bit:

http://www.ezonemag.com/pages/faq/a414.shtml

...in other words - it could be somewhat "standard" - or no standard at all, and just what the company decided to label it with. If you really start to dig into it (google "rc brushed motor sizing" for instance), you'll basically find the same thing - that these numbers used to mean something, but over time manufacturers have either exaggerated claims, marked the motors in such a way for marketing purposes, or the numbers don't mean anything.
Title: Re: Screwdriver motor
Post by: DonMilne on Feb 14, 2014, 01:32 am
@Robin2: if you can turn yours by hand then it clearly isn't the same design. I gave a link to the specific device I had in mind, I never said that all cheap cordless drills work that way. I was equally sceptical when Zoomcat described the tool, but my opinion has been changed by actually getting my hands on one. I see you're in the UK, so if you're interested then perhaps you might invest the eight quid to check it out. It's a decent price even compared to buying a bare hobby motor.
Title: Re: Screwdriver motor
Post by: groundFungus on Feb 14, 2014, 01:37 am
I had a B&D cordless drill that worked the way the OP describes.  When you wanted to tighten the chuck it would lock to reverse turning so you could get the chuck nice and tight.  Maybe a sprag clutch on some sort?  The only one I've seen like that and wanted a replacement since I broke it (dropped too far).
Title: Re: Screwdriver motor
Post by: Robin2 on Feb 14, 2014, 10:06 am
That's not a sophisticated screwdriver. But it probably has a very high gear ratio to allow it to work with 6v and low current to give it a reasonable run-time. And there is no chuck to tighten that might justify the expense of a clutch.

If you can turn the gearbox from the screwdriver end when there is no motor connected and with a gentle (finger tip) load on the motor end then there is no clutch mechanism.

Also, feel how much effort it takes to turn the motor shaft with your fingers and imagine that multipled a couple of hundred times. And remember that the friction within the gearbox increases as the movement is multiplied when driven from the screwdriver end.

...R
Title: Re: Screwdriver motor
Post by: DonMilne on Feb 14, 2014, 01:35 pm
Just in case anyone comes along and gets confused, when I talk about forward and reverse I'm not talking about the direction of rotation of the screwdriver bit, I'm talking about driving the gear train from the intended (motor end) versus the unintended spindle end.

And no, there's no clutch, I've taken it apart and there is only the two stage planetary gearing that I described. There is also no typical drill chuck as standard, just a hex holder for a screwdriver bit. I can see where the locking behaviour of the gearbox might be useful if you fitted one of those secondary quick-change chucks.  It would also be useful if you wanted to use the screwdriver manually.

I'm not convinced that the radical change from free rotation of both halves to totally locked is solely due to friction - in fact I thought epicyclic gearing was low friction by nature - but I can see that this thread is just going to degenerate into data-less speculation, so there's probably no point in continuing unless anyone here is claiming to be an expert in epicyclic gearing in all its forms.
Title: Re: Screwdriver motor
Post by: Robin2 on Feb 14, 2014, 03:35 pm

I'm not convinced that the radical change from free rotation of both halves to totally locked is solely due to friction - in fact I thought epicyclic gearing was low friction by nature


I wasn't talking about friction within the gearbox but about the resistance to turning that is inherent in any electric motor with permanent magnets. When that resistance is multiplied by the overall ratio of the gearbox it won't be trivial.

And I agree, I don't have a definitive answer.

...R
Title: Re: Screwdriver motor
Post by: zoomkat on Feb 15, 2014, 06:04 pm
Quote
In the previous thread Zoomcat said that he was unable to turn the spindle from the outside - and I can confirm this.


Your reference to my statement is totally incorrect. No where in the below discussion did I say the shaft could not be turned from the outside. In fact I can grip the B&D screwdriver shaft with a rubber jar lid opener and rotate the shaft in both directions fairly easily. What I did say was with a specific weight suspended from the spindle of the screwdriver, the shaft did not rotate. Correct technical reading and comprehension is essential for getting things correct.


http://forum.arduino.cc/index.php?topic=207394.msg1528816#msg1528816
Title: Re: Screwdriver motor
Post by: DonMilne on Feb 15, 2014, 06:51 pm

Correct technical reading and comprehension is essential for getting things correct.

But, an understanding of the norms of politeness is, apparantly, not so essential.  You could have corrected me without the patronising comment - I'll resist the temptation to respond in kind.
Title: Re: Screwdriver motor
Post by: zoomkat on Feb 16, 2014, 12:35 am


Correct technical reading and comprehension is essential for getting things correct.

But, an understanding of the norms of politeness is, apparantly, not so essential.  You could have corrected me without the patronising comment - I'll resist the temptation to respond in kind.


I would suggest this be a technical learning moment. The "and I can confirm this. The question for me is "how come"?" lead somewhat to chasing a phantom issue. The up side might be that more is known about an inexpensive high torque gear motor.
Title: Re: Screwdriver motor
Post by: michinyon on Feb 16, 2014, 04:18 am
Is it a reversible screwdriver ?
Title: Re: Screwdriver motor
Post by: michinyon on Feb 16, 2014, 04:22 am
I'd also make the obvious observation,   that the speed and torque requirements of a cordless drill and an electric screwdriver,   and,  specifically,   the ability to start turning while under load,   are quite different.

Your practise may vary,  but I find it is always better to turn the drill on before I attack the piece of wood with it,   but the screwdriver works better if you put the tip of the screwdriver into the slot or star-shaped hole on the end of the screw,  before you turn the screwdriver on.
Title: Re: Screwdriver motor
Post by: DonMilne on Feb 16, 2014, 02:09 pm

I would suggest this be a technical learning moment.

Just so's you know, I'm a professional engineer with 34 years experience. Though admittedly very little experience with motors. You can suggest anything you like. But, I personally have lost all respect for your opinions. You are a self important buffoon.
Title: Re: Screwdriver motor
Post by: DonMilne on Feb 16, 2014, 02:18 pm
@michinyon: Yes, the cordless screwdriver turns in both directions. Obviously a drill and a screwdriver may behave differently as they're different applications. The attraction of this particular motor is that it runs on 6V, specifically 4xAA batteries, and it has very high torque - at least with the screwdriver gearbox attached. Unfortunately I don't have a pulley that fits this motor, so measuring the motor torque by itself is tricky. The other noticeable feature of the motor is how powerful the permanent magnets are. My other motors may have magnets inside, but none of them will pick up objects off my desk. I can't find any references that say that (large) permanent magnet motors have higher torque, but that seems intuitive to me. And of course the gearbox will magnify that... though I'm not certain how to calculate the gear ratio for a two stage epicyclic gearbox (*).

(*) Each stage has three planets with 19 teeth each. The motor pinion has 6 teeth. The annular ring has 48 teeth I think.
Title: Re: Screwdriver motor
Post by: Robin2 on Feb 16, 2014, 05:43 pm

though I'm not certain how to calculate the gear ratio for a two stage epicyclic gearbox (*).


As usual Wikipedia knows but I don't have the will to work through the complexities.

...R
Title: Re: Screwdriver motor
Post by: DonMilne on Feb 19, 2014, 01:48 pm
I've seen the formula on Wikipedia, and while I could apply it blind I would prefer to understand it better. Frankly, I don't trust Wikipedia on things like this.  I guess I won't understand it until I write a software simulation, but that would be a fair bit of work and I have only so much free time!
Title: Re: Screwdriver motor
Post by: DonMilne on May 01, 2014, 02:17 pm
Hi folks. I thought I might bring you up to date on my efforts to repurpose a cheap screwdriver motor. I have to say that it looks like a complete success. See the attached picture below. I have only tested it with 1kg and 2kg weights since I don't have any larger water bottles, but the motor easily handles that load and supports it without power at any intermediate height (in the photo the bottle is suspended about 600mm off the ground). I have no doubt that the motor would be happy with 3kg (my original design wish), though the same might not be said of my temporary mount - it isn't fixed down so needs a heavy counterweight to avoid any risk of tipping.

Oh yeah, tips for others about the importance of the spindle!  I made my spool on a small metal lathe I have. You want one which has as narrow a diameter as possible, consistent with the load, as that maximizes motor torque. The bevelled sides resist the potential of the thread jumping off the end. I also found it worked much better with that blue guide beneath the spool, which steers the thread back towards the center, otherwise it'll want to keep winding in one direction. It would actually work better if the guide was a small hole instead of a V, I'll do that next time. Oh, and the "thread" is actually 100lb (45kg) braided fishing line, only 0.25mm thick - you want narrow thread to keep the spindle diameter down and torque up.

(http://s29.postimg.org/q68unlsif/01052014152.jpg)