### Author Topic: Motor selection - trying to understand stall torque  (Read 358 times)

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#### pterrus

• Jr. Member
• Posts: 37
##### Motor selection - trying to understand stall torque
« on: August 07, 2014, 11:08:28 AM »

The short story is I need to replace a motor because my previous motor stalls and no longer drives my heavier robot.  I bought a new motor based on RPM alone, and now I'm concerned that I should have paid more attention to the stall torque.  I don't really care how fast the robot goes, but it needs to not stall!

I googled the previous motor, RS-360SH-2885, and it looks like it has a stall torque of 420 g*cm.

The new motor has a stall torque of 700 g*cm.  I'm using a 9.6 V battery rather that the 12V maximum rating of the motor so that makes my actual stall torque even lower, right?

Other relevant information: the robot currently weighs about 4.5 lbs (2041 g) but I plan on adding an arm and other equipment that should be around 2 pounds for a grand total of 6.5 lbs (2948 g).  The wheel radius is about 7 cm.  There is a gearbox between the motor and the drive shaft with gear ratios of 10:34, 12:36, and 14:43, for an total ratio of 0.03192.

So if I take 2948g * 7cm I get 20638.8 g*cm.  Factoring in the mechanical advantage of my gearbox, I'm down to 903.7, which is well above the stall torque.  But that's not quite right because I'm not exactly lifting the whole robot right?

Basically, should I be worried here?  I want to make sure I have margin before I go pressing a gear onto this motor shaft.  Any insight would be appreciated.

#### Billy

• Full Member
• Posts: 120
##### Re: Motor selection - trying to understand stall torque
« Reply #1 on: August 08, 2014, 01:02:41 PM »
As you pointed out, the calcs show you have almost enough torque to climb a wall.
Assuming you are not climbing a wall but instead rolling over a mostly flat level surface, you have plenty.

But drop the calcs and think about it. The old motor had a stall toque of 420 and worked for a while. The new motor has 700. It should be good.

I only question how you determined the motor was the issue and not the drive circuit.

#### pterrus

• Jr. Member
• Posts: 37
##### Re: Motor selection - trying to understand stall torque
« Reply #2 on: August 08, 2014, 03:00:16 PM »
What do you mean by drive circuit, the electronics?

Re: 420 vs 700, part of my concern is I'm not sure it's an apples to apples comparison.  The old motor data sheet has 7.2 V nominal, versus the new motor sheet which appears to be based on 12V.  Am I reading all of this right?  I'm assuming that stall torque varies appreciably with the actual voltage applied (that's true, right?)

The old motor only worked when it was significantly lighter.  By the time everything I want to add is on there, the weight will probably be double what it was.  Hence the concern that I didn't actually add enough margin.

#### jkerns

• Robot Overlord
• Posts: 266
##### Re: Motor selection - trying to understand stall torque
« Reply #3 on: August 08, 2014, 04:10:28 PM »
Torque for a given motor will be proportional to current which will be, in turn, proportional to the applied voltage at any given speed.

So, if your motor has a stall torque of 700 at 12 volts, at 9.6 volts it will be about 700*(9.6/12) = 560 (ignoring any losses in the drive circuits, etc. )
I get paid to play with robots - can't beat that with a stick.

http://www.ltu.edu/engineering/mechanical/bachelor-science-robotics-engineering.asp