Mini metal motors

hello ben,

i am actually planning to order a mini metal gear motor but i have a doubt whether the product in this link is pololu made
it looks the same as pololu mini metal… but specs indicated for this product at sparkfun has pushed me to a confusion …especially the torque it indicates 5oz per inch (in our country we use kilograms majorly as a weight factor)
so please tell me how to pick a motor if my robot weighs about 400-450 grams with power supplies onboard…
can the motors in this
( )
link handle a weight of 400-450grams …assuming that am using a 32mm wheel (same as 3pi) or 42*19 mm pololu wheel
i am aware of the fact that torque also depends on wheel diameter …and ground resistance (lets assume i want the robot to move on a white wooden tabletop with a black insulation tape to make a track)
and i would be using 2 motors

am asking this question at pololu forum because at sparkfun… they are telling that this( ) is pololu made and the torque to be a mere 2.6oz-inch(which is just not tallying with the specifications given for this… product at pololu website)… as i think the quality of pololu products are really high and prefer pololu than other brands…as i also have a 3pi robot which is a clear indication of pololu’s high quality products…

my purpose of motors is making a good line follower like a 3pi with good speed (as some competitions dont allow company built robots)so am looking for a motor with good rpm and also good weight handling capacity(torque) …
so that i can come at least 50or60% as close to 3pi

so please tell me for my application which is the better motor…

thank u,

Hello, Technoholic. I’ll let Ben or someone else address the majority of your questions, but I would like to clear up your confusion about torque. The website of the motor you linked to from Sparkfun (ROB-08913) says “5 oz inches torque” which means

  • 5 ounces multiplied by 1 inch
  • 5 oz*in

This is very different from what you said in your post, which was “5oz per inch”, which means:

  • 5 ounces divided by 1 inch
  • 5 oz/in

Torque is always expressed as a force multiplied by a distance, not divided.

You can convert this torque to metric very easily using Google Calculator. Just try going to Google and searching for 5 ounce inches in gram centimeters.


thanx for your reply david,

and i am waiting eagerly for the reply to the questions i have asked in the above post…and that was a lot of help from
you… now that makes me a bit more clear…on torque conversions
i still want a rough figures of the capacity of pololu motors(motor links which i have mentioned in the above posts) as you would have tested them and a rough theoritical figure from pololu experts can put me to a assumption based on my weight accomodations to the robot and would help on deciding the right motors…
but i still want to stick to the fact on using a pololu made metal gear motors rather than some other brand looking the same…as pololu’s products for me what matters is performance and quality and cost effectiveness of the product


I apologize for my delayed reply. Sparkfun initially got their metal gearmotors from us, but it now seems like they have started getting them from another supplier (probably directly from China), and I do not know how similar they are to the ones we carry (the specs didn’t change, but I don’t know how much you can trust that).

Have you seen Jan’s blog post on forces and torque? You might find the information there helpful in computing the torque you will need for your robot. If you know the parameters of your robot, such as motor torque and wheel radius, you can convert torque into acceleration and determine how quickly your robot will be able to accelerate and what kind of inclines it will be able to climb.

For a 0.5 kg robot, I would consider our high-power micro metal gearmotors. Specifically, I think the 50:1 micro metal gearmotor HP could work well for you if you have a motor driver that can deliver enough current. Basically, you should consider how much speed and torque you need (higher torque means higher acceleration and better responsiveness) at the voltage you plan on supplying, and then search for the motor that meets those criteria.

By the way, just to give you some constructive criticism, your lack of capitalization and excessive use of “…” make it difficult for me to read your posts and understand what you’re asking. Can you try using commas and single periods instead of variable length ellipses (…)?

- Ben

i apololgise for the way i have asked the questions ,
i thought reader would understand the question better if i asked in above manner any way next time ill be staight forward and make it as easy as possible for the reader

i just took a look at those 50:1hp micro metal motors and maybe ill see to that the rpm also suits my application
and yes jans post help me figure out how to calucate the force at the edge of the wheel to run the robot forward
very informative.

and thanks for all the help,

and yes am totally clear on how to select motors after reading jans post on torque .

thanks to pololu team,