Tiny 6mm geared motor

[Andy]

Hi all, i've just ordered one of these tiny little geared motors from ebay - - www.ebay.co.uk/itm/131285034976?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT



At 6mm dia., it's so small, you could literally hide it inside an axle housing. You could have one motor per axle, giving drive to both wheels as well as a differential effect allowing easy turning. Or even better, you could use a model with all wheels the same size like the Universal Hobbies MF 1200 (below) and have a separate motor independently driving each wheel. True four wheel drive AND diff effect per wheel - - How cool would that be?



The trouble is, i don't know what such a tiny motor would produce in torque, so iv'e ordered just one to test. It's coming from China, so i'll have to wait a while for it to get here.

The specs are as follows:


Voltage: 3V
　
Current: 30mA
　
Noise: 40db
　
Rotation speed: 47r/min
　
Rotation direction: CW/CCW
　
Reduction ratio: 700
　
Torque: 20mN.cm
　
Length: 21.05mm

As you see, it has a heck of a reduction ratio, but i don't know what 20mN.cm of torque means - - i tried to Google it without much luck. Maybe RC Tractor Guy could throw some light on what these figures mean?

Anyway, i'll let you know my findings as soon as i test the motor. Meanwhile, i'll leave you with a picture of it's tiny gearbox.



 

[RC Tractor Guy]

Hi Andy, being able to achieve 4wd with differential effect would be brilliant. I just ordered a few motors yesterday to do another motor review video including this and some stepper motors. I was thinking the 6mm might be useful in my JCB build for small screw mechanisms. I hadn't seen the specs for the motor though, very interesting.

I notice the rpm is maybe half that of a SIKU control 32 motor, I recorded a video last weekend about the motors speeds and I think it was around 100 rpm at 3 volts but sacrificing speed for maneuverability in corners with a differential and traction from 4wd would be worth it I think. I made a small calculator on the site to work out what rpm motor you need to achieve the 1:32 scaled version of the 1:1 real machine top speed. Here it is if anyone is interest, just enter the real vehicle top speed and the model wheel diameter Motor rpm calculator.

Warning, I may have over taught this a bit so skip the next three paragraphs to go straight to the torque comparison with a 50 rpm N20 motor unless you're a unit conversion aficionado

Just for those who don't know, the Si unit for torque is a Newton meter (N.m). So lets say you attached a 1 meter long moment arm extending perpendicularly from a shaft and then you applied a constant 1 Newton of force perpendicularly to the very end of the moment arm, the shaft would now have a torque of 1 Newton (the force applied) per meter (the length of the moment arm). A dot in a unit usually denotes multiply so if we look at the unit again N.m we can remember is a the force applied multiplied by the length of the moment arm (the distance to the center of the pivot point). 20 N applied at 0.5 m is 10 N.m, 10 N applied at 1 m is 10 N.m, 5 N applied at 2 m is 10 N.m.

Now back to Andy's question, this motor has torque of 20 mN.cm so it can provide 20 mili Newtons of force if you put a 1 centimeter moment arm on it. A model wheels has a diameter around 55 cm so the moment arm is 27.5 cm so one motor can provide around 20mN.m/27.5cm = 0.73mN of force. Force = mass x acceleration so that's enough power to accelerate 0.73 grams at 1 m/s^2 and I've no idea how that helps us understand anything

There is another "unit" used when manufacturers describe servos, this is kg.m which means the torque from 1 kg of weight on the end of a 1 m moment arm. In this case the force is the mass multiplied by acceleration due to gravity but they don't mention the acceleration in the units which means it only makes sense if the load is hanging from the moment arm experiencing gravity only. To convert N.m to kg.m you divide N.m by acceleration due to gravity which is around 9.806 m/s^2.

Using some ebay specs a 3 volt 50 rpm N20 gear motor which has a stall torque of 0.2 kg.cm which is 0.002 kg.m. That is 0.019612 N.m = 1.9612 N.cm = 1961.2 mN.cm. That seems very big, if I've made a mistake let me know.

Based on that calculation it would seem to be lacking torque but I could have made a mistake, ebay specs are notoriously inaccurate also so only some real world testing will let us see what we can use them for. Looking forward to seeing your findings.

[Andy]

Hi RC, thanks for the reply. Wow, those are some pretty mind bending stats i'll have to get my head round about torque and newtons!

I believe you tried this motor in a review (the one you said a Lego part might fit) - - www.ebay.co.uk/itm/141510396948?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT



The specs are as follows:

Voltage :1.5V
　
Current:30mA
　
Noise: 37db
　
Rotation speed: 29r/min
　
Rotation direction: CW/CCW
　
Reduction ratio: 171
　
Torque: 8.67mN.cm
　
Length: 18.3mm

It has less than half the torque (or so they say) of the 6mm motor i just bought, and i thought you found that quite difficult to stall in your review? It also has a lot less reduction. The thing is, trying to stall it while gripping the axle is all well and good, but trying to stall it with a wheel attached will be a lot easier given the leverage. Like you say, only a real world test will show it's true capabilities. Remember, one motor per axle will give 4x the torque, but would that be enough? Also, you would want to tow a trailer/implement and also potentially go up hills with it needing extra torque.

As regards Siku tractors, i think they appear a bit too fast. Maybe OK for more modern tractors, but if you were going to do a tractor like an MF 1200 that's from the 1970's, it will be slower. I think an MF 1200 probably only does about 25mph flat out, some modern tractors can do close to 40mph. Don't forget, tractors only go flat out when on the road, if you're doing actual farming with your model (and that's where the fun is) you'll want something slower and more controllable. As my personal preference, i would want to model tractors from the 70's 80's era, as that's the type i drove at college (showing my age now!).

When i was thinking about what speed my model should do, i went to youtube and looked at a 45rpm record going round and turned the image of the record into a tractor rear wheel (in my mind, that is) and thought "yeah, that looks about right" Hardly a scientific approach, to to me it looks right.

Hmm...another point to think about is 47rpm on the 6mm motor is bound to be a 'no load' speed, so actual speed under the weight of the model will certainly be slower. Plus the 10mm motor above does a lot less at 29rpm.

Plus, i would intend to run these motors at 3.7v, that being a standard for LiPo batteries. An extra .7 of a volt won't cook the motor (i don't think!), but it would increase it's speed by almost a quarter, so that's something else to take into consideration.

Swings and roundabouts, or i think trial and error would be a better phrase.

Did you say you ordered one of these motors too? Perhaps we could hear your thoughts too when it arrives.

Another way to alter gearing is a Delrin chain drive (Delrin is a type of tough self lubricating plastic), But of course when you increase speed, you decrease torque.....hmmm


Delrin mini chain drive.

Well, those are my thoughts for now. I'll have a look at that motor rpm calculator later, to see what i can make of it.

Anything anyone can add to this conversation would be much appreciated.

All the best, Andy.

[tomek]

I have to admin, we have a very interesting topic.

RC Tractor Guy wrote:
Now back to Andy's question, this motor has torque of 20 mN.cm so it can provide 20 mili Newtons of force if you put a 1 centimeter moment arm on it. A model wheels has a diameter around 55 cm so the moment arm is 27.5 cm so one motor can provide around 20mN.m/27.5cm = 0.73mN of force. Force = mass x acceleration so that's enough power to accelerate 0.73 grams at 1 m/s^2 and I've no idea how that helps us understand anything

I do not know how that helps us, but I have an idea.
Our administrator has a John Deere 9650R with Tamiya 70189 set to second lowest speed. Torque of that motor is 866 gf*cm and this is 0.084925589 N*m = 85mN*m = 8500mN*cm. If my calculations are correct this means that tamiya motor is 100 times more powerful than 4 motors mentioned by Andy.

Second question is if a John Deere 9650R made by our administrator is overpowered and if yes, how much. Dear RC Tractor Guy, can you please check the weight of that Tractor, and can you please check how heavy trailer it can pull with a reasonable speed and acceleration? If you can please also send a diameter of the wheel maybe we will be able to estimate pulling force and torque we need per for example 100 grams of the model.

[Andy]

Actually tomek, at 8500mN.cm it's 1000 times more powerful! Anyway, at 8.67mN.cm, that's for the second motor i talked about, similar to the one that RC tested. The 4 i thought about using in a model are 20mN.cm each, making a combined 80mN.cm of available torque, which would be ten times more torque than the motor similar to the one RC tested. To quote RC he said at 3v it had quite a bit of torque, and at 6v plenty of torque in his video review of cheap motors.

One way you could test the capability of motors without ruining models, or trying to sus out the science would be to build a test bed. A simple flat chassis made of sheet plastic fitted with the motor and wheels you wish to use. Drill a couple of holes where you want to mount the axles and simply zip tie them onto it (not too tight as to restrict movement). You wouldn't even need to install radio gear, just use it wired to a battery. To this you could add the weight of the model you wish to convert, as well as the weight of a trailer or implement. If it could easily move the weight, you know the motor is suitable. You could use those little gram weights they use on mini balance scales.

I think that finding a suitable motor has more to do with finding one the right size and speed for your model, and one that will also pull a good load, than the sciency facts and figures stuff. The only way to do that is to do what RC has done, which is to just buy a load and test them in the real world.

[RC Tractor Guy]

Hi Guy's, very good point I did and do think the 10 mm motor has a good bit of torque for it's size but like you say trying to stall the output shaft is not a great test really could be a very different story with the leverage of the wheel involved. I'm envisaging it in my Ford 7810 model which is pretty light. I was actually looking at that motor and the 7810 yesterday because of our discussion on Friday funnily enough. I was trying to think how I could attach it to the wheels.

Another thing to thing to consider is whether the gear train can take the strain, Tomek mentioned the 70189 motor and I did a bit of a test of four motors last weekend where I mounted the motor on a piece of wood with part of the Fendt 936 model to weight the front down. I used an electronic scales for weighing luggage and basically set the motor going to see what weight it registered for each motor. Basically the motor was trying to pull an immovable object so the motor would either need to exceed the frictional coefficient between the siku tires and the table or pull with it's stall torque. A gear in the 70189 sheared a few teeth so it may not be as good of a choice as I had taught despite its great torque reading and it seeming to work well in the John Deere model, although I haven't finished the model yet so it's only pulled a trailer full of electronics round a flat carpeted floor so far. Another strange thing is I would never have taught the 70189 had anywhere near the torque of the N20 motor yet the numbers say it should be stronger.

About the 3.7 volts, don't forget you'll have losses in your motor driver so it probably will end up around 3 volts even with a very good motor driver.

I like the chain drive idea but I will say I tried a belt drive on the Massey 8680 originally because I was trying to use a servo to drive it and the plastic gears kept breaking so I was hoping the belt might slip before the gear box broke. It worked but the belt drive took up a lot of space in the model because the motor was offset from the axle so just be aware that it might be difficult to fit electronics in later in the build.

Tomek you asked for John Deere figures the weight is 1050g and the wheel diameter is 69 mm. Like I said earlier I damaged the motor last weekend but it did comfortably pull itself and a 600g trailer around on the carpet. I'll try do some testing with the John Deere 8360RT as it is the only model I have working at the minute and see if I can learn anything from it.

I like your test bed idea Andy, if you used a pulley to suspended the weight so that you were pulling it up against gravity you could get a force which you could compare between different models. All I need now is to finish a few models and we could get some real data

[tomek]

I was thinking about differential effect achieved with separate motors for each wheel. I think, that if motors will be too powerful, the effect will be gone. Anyway I want to try that with John Deere 9650R and 4 N20 motors. So far I wait for a model which I ordered today. I have to verify if there is enough place for those motors. It should be. Worst case scenario the gearbox will be located somewhere inside the inner wheel.

[tomek]

RC Tractor Guy I am confused. For JD9650R you have used Tamiya 70189 with second lowest speed and this is 20 RPM. I have checked with your calculator that tor this diameter of the wheel and speed 40km/h should be about 100 RPM. My guess is, we cannot follow the calculator, because it will be too fast.

[RC Tractor Guy]

Four low rpm N20 motors would be super powerful in that big model, 8 wheels and the weight give them traction should give it plenty of pulling power. I'd like to see how you mount the motors, inside the inner wheels is a very good idea.

My John Deere 9560R model was pretty slow compared to the other models but it seemed right for the big model, the siku motors in the other models are over 100 rpm if I remember correctly which makes them more realistic for the models they are used in like the Fendt 930. Like Andy said though how often would you be doing 40kph in a tractor the size of a 9560R in reality probably 10-20kph pulling something like a huge plough behind it is more realistic.

I recorded a video about the motor rpms a couple of weekends ago just haven't got round to editing it, hopefully get that done this week ready for Saturday upload.

[Andy]

Mounting the motors inside the inner wheels is a very good idea tomek, i think they'll be too wide side by side otherwise. Another idea is have one N20 driving front left wheel and another N20 driving rear right. You would still get plenty of power, and an even amount of torque on each side. Plus no worries with the diff effect. Plus only having the two motors, your battery would last twice as long, another thing to consider.

I also ordered two N20 motors when i ordered the 6mm. A 60rpm and a 100rpm. This should be an interesting side by side comparison of the N20s for torque vs speed. The 60rpm has a wheel attached. I got this one for stall testing purposes, i dare say the wheel can be easily removed.



I also ordered one of these:



At £1.28 freepost it was just too damn cheap to resist.

I already have the single shaft version of this motor, it's tough, slow and fairly smooth. It would be up to the job, but like RC said in his review, it's bulky.

What you said about stripping gears was a bit of a concern RC, no problem with the metal gears on the N20, but that tiny plastic planetary gearbox on the 6mm probably won't be up to much. Perhaps the 6mm might be a bit flimsy for main drive, but useful for other things.

As regards the Delrin chain drive, you could still use it with motors that already have the correct gearing by using two tiny sprockets of the same size, with a very short length of chain between them. That way it would take up the minimum amount of space, and you could mount the motor without disturbing the rear axle mounts and simply slip a drive sprocket onto the axle. I think it would be a lot less work to cut a motor mount space in front of the rear axle, rather than removing the rear wheels, cutting into the axle assembly, and re-spacing and remounting everything to mount the motor directly onto the wheel. I don't know for sure yet, but it's something i intend to try. Also, i don't think you would strip the sprockets, Delrin is pretty tough stuff.

I'm off to watch a few more of RC's excellent videos......

[RC Tractor Guy]

I have seen those wheels for the N20 but haven't bought one yet, I was thinking it could be useful to cut out the inner part of the wheel hub and screw that inside a siku wheel. I did something similar with the wheel hub you get with the tamiya motors and it worked out very well.

Well only testing the 6mm motor will tell us if the gears will stand up to the load, at least they are fairly cheap so it doesn't break the bank to do this testing. I'd be pissed if the €30 + shipping siku motor broke but the €3-€4 6mm one, not so much.

Good point about the chain drive, a lot of cutting to even fit the siku motors into the siku models so you may well be onto something with that idea.

[Andy]

Yeah, imagine simply drilling a 6mm hole thru the mudguard and inserting the motor, maybe wrapped in a bit of tape for insulation, push fit a couple of sprockets, run a bit of chain round, and away you go! Simples! Well maybe not THAT simple, but you get the idea.

Fitting an N20 would be a bit more involved, you'd have to square out your hole, but same principle.

If you could buy those N20 wheels separate, you would have a cheap source of wheel centres for conversions. That's a good idea RC, you could mount the new centre in using epoxy putty.

Infact, that's probably a better idea to mount the N20 straight onto the wheel, rather than using the Delrin chain, as the bigger N20 would be more hidden directly behind the wheel

[RC Tractor Guy]

Do you have the Massey model yet? Sometime there isn't much space between the mudguard and the inner face of the wheel. Very tight on my Massey 8680 if I remember correctly I had to have the wheel sticking out slightly from the mudguard to get the belt in.

I just ordered some of those wheels from here and also a few D shaft arms for N20 motors which I hadn't seen before which might be useful, here is the link.



They don't look wide enough to support the wheels but they might be useful elsewhere.

[Andy]

Nice one RC, just added those to my watch list. If they're not wide enough, you could always glue two together. At only $2.99 for 10 they're mega cheap.

Do you mean have i got the MF 1200? No, don't have that one yet, i can't find it cheaper than £40 - odd, want to do something cheaper as my first conversion, so i just bought one of these:



At £12.99 free P+P it was really cheap - - a Britains Ford TW20. Cute, aint she? Here is the ebay link if you want to grab one - - www.ebay.co.uk/itm/171705435031?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT  most others are selling them nearer £20 - still quite cheap for a model of this detail.

As you can see, there seems to be quite a bit of space between the mudguard and the wheel to get a Delrin chain drive in. I don't know what kind of drive i'll do yet, i'll assess the model when it arrives. It'll be here before the motors arrive anyway. At less than thirteen quid, i won't be too upset if i make a balls of it!

Not too sure how much 'stash' space for electronics, but if it's anything like the UH model, the bonnet will be removable, and if needs be i can grind off the engine block and replace with sheet plastic sides with some engine detail made up and glued on. That'll give me a space inside to put speed controller and receiver ect. Sounds quite harsh, but it's nothing i haven't done before - railway models you know...

[RC Tractor Guy]

Going with a cheap tractor is a good idea to start with, I made a lot of changes to my Massey 8680 during the learning process, still making changes in fact!!

If you haven't already guessed from the website logo, I do like the Ford TW tractors The lack of cover over the engine in my TW35 model is slightly annoying but like you say I can mill out part of the engine if needs be. I'd say working on railway models should help you, the tractors are probably hugely spacious compared to a train although the train wouldn't have to handle too steep of an incline so maybe drives didn't need to be as strong.