Really poor torque from motor, can't work out why

Using Arduino Motors, Mechanics, Power and CNC
#1

I bought some micrometal gear motors (1006:1 ratio) specified to have a stall torque of about 5.3kgcm, able to handle 0.8kgcm long term and managing 2kg*cm for reasonable periods. I've run them through a 3D printed worm gear system which slows by 45:1 and should give a correspnding increase in torque of a factor 45, if the efficiency is poor (say 50%) for the gear train then maybe only 22 fold but still a pretty big factor.

My application has torques of around 90kg*cm but does not need much speed, not much at all.

I'm using four motors working together to provide the torque at the start of the gear train, they are in two pairs. In each pair two motors drive a shaft together, then the shafts from the two pairs are combined via a kind of system a little like an automotive differential.

I've tested the torques and my system is producing, at the slow end, only about 18kgcm of torque, a fifth of that needed, and about a 20th of what I reasonably calculate the system should do, what with the motors managing, according to the datasheet, about 2kgcm for periods.

I could beleive losses of 50% overall to various inefficiencies, but I'm losing 20 fold the torque I expect. Any idea what to check for?

My printed gearing is smooth when turned by hand (although this is when the slow end isn't connected to anything heavy it has to move, as when it does get connected there isn't any way to reach the gearing to hand turn it so motor power is the only option then).

The motors seem to be working as well as when new, no apparent damage.

My motors are getting enough voltage, I've checked, and there shouldn't be any trouble with current limits on the supply. The motors don't take much current overall as despite high torques they are slow, so the power isn't that much.

The gearing is taking the stress ok, it's multi-perimetered internally solid ABS done on a high quality printer.

I'm at a loss as to where the torque is all being lost to.

Thanks.

#2

It would not surprise me if friction from 3D printed gearing is a large part of the problem.

#3

Have you measured the motor output torque directly?

Or the input and output torques of your gear system?

That would narrow-down where the problem is.

Why have you kept the datasheet of the motor a secret? If you want help about some piece of
hardware post its full details, ie a link to datasheet or product page.

BTW 0.8 x 22 = 17.6, and you see 18, which is in good agreement as far as I can tell. If you want the
2kgf-cm value you'll need to run at full voltage, winding resistance saps torque at lower speeds with
small DC motors as they have very poor speed regulation.

#4

As a general rule, motor poor performance can be due to the motor being undersized or under powered for whatever it is trying to do.

#5

If you are using a L293 for a driver that may be a big part of your problem. Also it sounds like the gears are not meshing properly under load. Measure the motor voltage under load, you might get a surprise!

#6

Sorry totally forgot datasheet, here it is. https://cdn.shopify.com/s/files/1/0389/1025/files/JL-12FN20-1006-0614_X19-7-Model.pdf?18050302546138261866

As for drivers I'm not using L293D having previously discovered just that problem. I'm using TB6612 so don't get any siginificant voltage drop within the H bridge.

I can take a motor out and test it with a lever arm and known weight to check motor torque, but how can one easily measure both input and output torques of a gear train at once?

3D print friction, surely not enought to lose 95% of torque though? Any ways I can run some mreo tests in this regard.

#7

A good approach to torque measurements is drum and thread - fix a drum of some sort to the axle, wind some thread around it. Now you can tie some weights to the thread to provide a torque load, or investigate the torque
needed to start something moving against friction. Nuts and washers or something similar are convenient for
threading together as a load weight.
But you can mount an arm on an axle (make it symmetric so its weight is factored out), and hang a load on it
at various distances from the axle.

Worm gears can be very inefficient as they have sliding contact unlike most gear arrangements.

#8

Post a detailed drawing of your gear train, if not done right, 1 or more of the motors may not be contributing to torque and just “riding along”.

#9

And a possibly dumb question: Are you positively SURE that all motors are driving in the same direction? :slight_smile:
Put your Ammeter in series with the motor power supply and driver(s) and record the current. With power OFF, disconnect one motor at a time and with the other 3 driving check the current. If the current does not decrease about 25% then that motor is not contributing.
Ignore the proceeding dumb suggestion. :-[

#10

I've been reading through some forums and Facebook groups lately about stepper motors and issues people have with them. That in no way makes me an expert. (I'll have to watch a few more YouTube vids to get the certification for that).

One thing I've noticed is several folks with torque issues had their wiring wrong. And in most, if not all, it was on 4 wire steppers and the 2 middle wires were reversed.

Is that the case with yours? I don't know. But it's worth taking another look at.

#11