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Author Topic: testing torque for motor selection  (Read 1241 times)

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Offline jlgTopic starter

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testing torque for motor selection
« on: October 16, 2011, 02:01:07 PM »
I need to drag a ring across a surface by pulling it with a string attached to a motor. The motor will simply coil the string, or the motor will be attached to a pulley and the ring will be tied on the rope. My key problem: the surface is not conventional so there is no friction coefficient.

Basically, I know very little about science but I've done a lot of research on this topic so I hope someone can follow my thought process and point me in the right direction. Thanks in advance.

At first, I was comparing the strength of motors based on calculating the watts. I have a hand mixer that will pull the ring across with no problem, but I have a 5 hobby motors that failed. There seemed to be no correlation to the amount of watts used by the motor. After some research, I realized torque was the missing element. But without a friction coefficient I'm not sure how to determine my required torque.

Since I need to buy the cheapest motor that will do the job, it would be nice to know the exact amount of torque required to drag this ring across the surface. I pulled out a luggage scale and measured the resistance of the ring to the surface. This involved tying one end of the rope to the scale and the other to the ring. Then, I held the surface with my left hand and the scale with my right and simply pulled the ring across. The scale showed 2 pounds of resistance.

I then measured the hand mixer that had worked so well and the scale showed 5 pounds. The hobby motor that came closest to working provides .25 inch pounds of torque according to the manufacturer.

From what I understand about measuring torque, it is simply the length of the lever multiplied by the force. Since I didn't have a lever for this exercise, I need a motor with 2 inch pounds of torque, and my hand mixer has 5 inch pounds of torque. In other words, whatever was on the luggage scale is my required torque. So I need to look for a motor with more than 2 inch pounds of torque, correct?

I hope I've provided enough information. I would really appreciate any help you can provide on this topic. Thank you.

Offline Soeren

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Re: testing torque for motor selection
« Reply #1 on: October 16, 2011, 04:13:51 PM »
Hi,

How fast do you need to drag the ring?

Gearing down the 0.25 lb motor by a factor of eight (or more) would do the trick, but slow the motor down a similar amount.
Small gearhead motors that you just need to mount a pulley wheel on can be bought ibn just about any online robotics shop.
A larger diameter of the pulley needs a larger torque of course.

Take a look at the gear motors here
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline jlgTopic starter

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Re: testing torque for motor selection
« Reply #2 on: October 16, 2011, 04:47:05 PM »
Speed is basically irrelevant. Am I right to approximate torque with the luggage scale or is that off base as well? I'll look into those gear motors. Thanks for your help.

Offline waltr

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Re: testing torque for motor selection
« Reply #3 on: October 16, 2011, 06:51:11 PM »
The scale will measure the force needed to pull the ring. Torque, as you mentioned, is a force time the moment arm which is the radius of the pulley or drum or shaft that the 'rope' is being wound onto. This you haven't told use for the mixer or motors you tried.
Example:
    5 pounds (force) pulled on the rope with the mixer.
    1 inch Diameter of drum on the mixer (adjust this to what you measure)
Torque = 5 * 1 = 5 in-lb

Was the drum that winds up the rope on the small motor the same diameter? If so then, as Soeren said, gear reduce the small motor by a factor of 5:1 or more.

Offline jlgTopic starter

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Re: testing torque for motor selection
« Reply #4 on: October 16, 2011, 07:18:13 PM »
Ok great, so this is actually a reasonable method to calculate the torque of the hand mixer.

The diameter of the shaft for the mixer is 1/4th of an inch. So, the hand mixer produced 1.25 in-lb of torque. I know from the specs of my small motor that it provides .25 in-lb of torque. Therefore, I need to be shopping for a motor or some kind of gear box that will ultimately give me a torque in that range, but whatever I do I need to make sure it exerts at least 2 lbs of force on the string.

Correct?

Offline Soeren

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Re: testing torque for motor selection
« Reply #5 on: October 17, 2011, 01:14:56 PM »
Hi,

Therefore, I need to be shopping for a motor or some kind of gear box that will ultimately give me a torque in that range, but whatever I do I need to make sure it exerts at least 2 lbs of force on the string.
If the motor have a rating for the stall torque of say 2 lbf-inch, a pulley with a diameter of 2" will have a max pull of 2 lb (as the radius is 1") if the pulley is 1" in diameter, it will have a max pull of 4 lb etc.

Mind you, the stall torque is the point where it gives up and can't move, so you need to have a stall torque higher than the torque you need to pull the ring.

Since the speed doesn't matter, any motor can be geared to pull the ring.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline jlgTopic starter

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Re: testing torque for motor selection
« Reply #6 on: October 17, 2011, 01:42:01 PM »
One last question, I drilled a hole in a 2 inch pvc pipe cap and securely inserted the motor shaft into the hole. Then, I attempted to coil the string around that 2 inch cap, but it did not seem to provide any additional torque. Why?

Offline newInRobotics

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Re: testing torque for motor selection
« Reply #7 on: October 17, 2011, 02:29:30 PM »
One last question, I drilled a hole in a 2 inch pvc pipe cap and securely inserted the motor shaft into the hole. Then, I attempted to coil the string around that 2 inch cap, but it did not seem to provide any additional torque. Why?
Because You reduced lifting power of the motor by putting 2 inch pulley on it. If You tried to wound string on motor shaft itself, You would get more power. As Soeren mentioned, the smaller the pulley, the more You can lift, the slower it goes and vice versa.

Try lifting bucket of water with Your arm straight and then try same thing with bent elbow. You will find out that the longer lifting arm You have the less weight You can lift.
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Offline jlgTopic starter

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Re: testing torque for motor selection
« Reply #8 on: October 17, 2011, 02:34:16 PM »
I understand. Thanks very much this has been a great help!

Offline waltr

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Re: testing torque for motor selection
« Reply #9 on: October 17, 2011, 05:30:26 PM »
Torque is on the shaft from the motor/gears. To get the force at a distance from the center of the shaft (1 inch for your PVC cap) then you divide the torque by this distance (the moment arm). If the mixer's torque is 1.25 in-lbs then the force at the circumference of the PVC cap is 1.25 lb.

And do remember what Soeren said about 'stall torque' and choosing a motor (gear-motor).

 


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