34:1 Metal Gearmotor 25Dx67L mm MP 12V with 48 CPR Encoder

Hi All
Riddhi186182 has just posted a query re shaft size on this unit. I have some other non related comments.

The title does not say if the encoder is on the final shaft or the motor shaft but looking at the pic it is fairly apparent it is probably on the motor shaft (although it doesn’t have to be). This is confirmed by going to Pololu web site and looking up this motor.

What is also noted and is a bit disturbing is that although the gear ratio is quoted here and on their chart as 34.1:1 it is actually 34.014:1 providing counts per final shaft revolution of 1636.8 (34.1) and 1632.67 (34.014), a difference of 4.13 counts per final rev. Now this does not sound much but remember this is for EVERY revolution so it would not take many revs to add up to something embarrassing.
This could mean that if you need to move an object (like the movement of an X/Y table) and you did all the calculations based on 34.1 gearbox ratio you could end up with a sizeable error if your movement involved a lot of revs.

This brings up another point. These numbers of PPR do not seem to bear any direct mathematical relationship to 360º or anything else I can think of. Calculating pulley sizes, gear ratios etc to move something exactly where you want it to go with a precise number of counts (with no fractions) could become a nightmare.

Why is it so ??? Professor Sumner-Miller might know but I certainly don’t. There must be a reason for these odd figures. The only reason I could think of is the whole thing has been put together with available off-the-shelf parts and not been designed from scratch. I think it boils down to the gear ratios and nothing can be done easily which makes the rotary encoder not much use if tight precision is required (this would depend eventually on the number of revolutions required to do a job) but repeatability might be OK.
Cheers Bob

EDIT: I am by no means trying to put off anyone purchasing this unit. For the modest price it seems pretty good value. I was pointing out the 2 different gear ratios quoted and asking the question WHY would you do that as not all applications are “close enough is good enough”.

I also question the actual ratios but there could be a very good reason for this, cost being one of them. It just means selecting the right things like final drive ratios, pulley size, belt tooth pitch etc to do a particular job with acceptable tolerances in mind.

Re gear ratios and odd PPR of the final shaft: It is possible these odd numbers may suit some of the threads used in screws feed applications. I haven’t gone into this nor do I intend to do so until I need to, someone else might want to investigate.


Hi Bob,

Excellent post! It looks like a few of the ratios are rounded both in the table and the product name, I’ve added a note to make sure that its clear in the description!

It looks like the manufacturer of the motors use it as sort of a plug and play system, where there is the core motor and then an encoder or gearbox (a few varying ratios exist) can be added, here’s one with just the encoder which confirms your suspicion of it being attached to the motor shaft (I’ve also used a couple in projects/experiments).

The PPRs are very odd, I imagine that your reason would be spot on!

I can attest to them being great value motors the range of gearboxes makes finding a torque and speed spec pretty easy.
I’ve only had to use these motors in an instance where there is another form of feedback from an IMU and use the encoder as a way to verify that the motor isn’t stalling or back-driving plus some non-critical distance measurement.


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