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Topic: Screwdriver motor (Read 2162 times) previous topic - next topic

DonMilne

Feb 13, 2014, 06:01 pm Last Edit: Feb 13, 2014, 06:14 pm by DonMilne Reason: 1
In a previous discussion 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.

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.


Robin2

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

DonMilne

#2
Feb 13, 2014, 10:26 pm Last Edit: Feb 13, 2014, 10:29 pm by DonMilne Reason: 1
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.

Robin2

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

MarkT


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.
[ I won't respond to messages, use the forum please ]

cr0sh

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.
I will not respond to Arduino help PM's from random forum users; if you have such a question, start a new topic thread.

DonMilne

@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.

groundfungus

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).

Robin2

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

DonMilne

#9
Feb 14, 2014, 01:35 pm Last Edit: Feb 14, 2014, 01:39 pm by DonMilne Reason: 1
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.

Robin2


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

zoomkat

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
Google forum search: Use Google Advanced Search and use Http://forum.arduino.cc/index in the "site or domain:" box.

DonMilne

#12
Feb 15, 2014, 06:51 pm Last Edit: Feb 15, 2014, 07:07 pm by DonMilne Reason: 1

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.

zoomkat



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.
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michinyon

Is it a reversible screwdriver ?

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