Author Topic: Torque  (Read 3859 times)

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Offline crarmTopic starter

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Torque
« on: January 10, 2009, 03:50:02 PM »
I am new and i dont get the servo torque thing could someone explain it to me plz. ???

Offline want2learn

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Re: Torque
« Reply #1 on: January 10, 2009, 03:54:44 PM »
Torque is a measure of rotational force, usually expressed in lb/ft or N/m

Is that what you meant?
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Offline frodo

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Re: Torque
« Reply #2 on: January 10, 2009, 03:57:50 PM »
i don't get it either, how are you supposed to know lb/ft or N/m? and whats the point of torque?
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Offline want2learn

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Re: Torque
« Reply #3 on: January 10, 2009, 04:23:53 PM »
i don't get it either, how are you supposed to know lb/ft or N/m? and whats the point of torque?

Say you have a motor (doesn't matter about the specifics) and you attach a bar to its output shaft so it swings like the hand on a clock. The motor is mounted securely. Under the end of the shaft thats free to swing you put a scale, This scale will measure the force that the motor can spin with. Remembering that most electric motors give their highest output under stalled conditions (which we are replicating here).

Now if your scale weighs in pounds(lb) and ounces(oz) you would use a shaft say 1 foot (ft) long, now if your motor pushed the scale down to 10 lb then the stalled power of your motor would be 10lb/ft (if the shaft was 6 inches long (half a foot) your motor would give 5lb/ft of force (torque)

The difference between lb/ft and N/m is that lb/ft is imperial (American or old British measurements) and N/m is metric (European).

The point of torque measurements are that you can look at the data for the motor before you buy it and know if its under or oversized etc.

Hope that helped a bit.

P.S. Don't actually replicate this it's just a theoretical example
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Offline gordon.cooke

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Re: Torque
« Reply #4 on: February 01, 2009, 06:49:31 PM »
Here is a very simplified explanation of torque and what it's for in plain language.

Lets say you take your little 12v motor or servo and put it on your micromouse with a 2 inch plastic wheel.  Everything works fine and your wheel turns about once a second.  Great.  Now, imagine if you could somehow disocunenct the axle on your car. Take that same motor mounted to the frame of your car and attach the tire to the motor.  Wire it in to the 12V car battery.  Do you think it drive your one ton car at 1 rev/sec?

I hope most people would say "No, of course not!"  The problem is torque.  Torque is a measurement of how "strong" your motor is.  You only need a little bit of torque to move your little bot, and you need a lot of torque just to budge your car.

Torque is it's own spec for a motor.  It's not really related to speed or anything else.  You can't take a big high torque motor from, say, an 2,000 lb electric car, put it on a 2lb robot and expect it to go 1,000 time faster.  The speed rating and the torque rating of the motor are two different things.  Just want to point that out because when you read spec sheets you will find that they do influence each other a little bit.  You will see that as torque changes the max speed will change.  This is only within a narrow band around the specs of the motor.  Don't let it confuse you.

Key things related to torque is the weight of your bot and diameter of your wheels.  Changing the wheels will change the torque you need.  So you can't just keep making your wheels bigger to make your bot go faster.  Eventually the wheels will get big enough that the motor no longer has enough torque.  This is'nt just because of the heavier wheels, it is because of the larger diameter.

Hope thats a simple enough explanation.  If you want to get into the details and physics read the dynamics tutorial and the RMF calcualtor on this site.  They are pretty good.

Offline tjump

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Re: Torque
« Reply #5 on: February 02, 2009, 07:54:52 AM »
I am going to challenge one point that was made in an earlier post (referecence the following quote):

"Now if your scale weighs in pounds(lb) and ounces(oz) you would use a shaft say 1 foot (ft) long, now if your motor pushed the scale down to 10 lb then the stalled power of your motor would be 10lb/ft (if the shaft was 6 inches long (half a foot) your motor would give 5lb/ft of force (torque)"

If your motor exerts 10lbs at a distance of 1ft from the center of rotation it will exert 20lbs at 6 inches, not 5lbs. The key to the calculation is the length of the moment arm (the distance from the where the input/output force is perpendicular to the center of rotation). If the input arm is shorter than the output arm you esentially have a 3rd class lever, ergo a mechanical advantage (MA) less than 1 (MA = length of input arm/length of output arm...again where the forces are perpendicular to the arms). Therefore, in the example above the input arm is the shaft of the motor and the output arm is the 1 foot (or 6 inch) rotating arm. As the output gets shorter the force will go up (basic math, as the denominator gets smaller the overall number gets larger).


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Offline Charizard

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Re: Torque
« Reply #7 on: February 08, 2009, 02:43:02 AM »
I was thinking, in normal case (like the clock pointer), torque can be found by getting the moment. But what if the load doesn't has distance (shown in diagram if you can't understand my point)? How can I know how much torque I need for my servo?

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Re: Torque
« Reply #8 on: February 08, 2009, 02:50:48 AM »
Quote
torque can be found by getting the moment. But what if the load doesn't has distance (shown in diagram if you can't understand my point)? How can I know how much torque I need for my servo?
Torque with zero distance is just a force - nothing rotates.

However the image you put up *does* have a distance, which is the radius of that thing that is rotating.

Offline want2learn

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Re: Torque
« Reply #9 on: February 16, 2009, 01:10:05 PM »
I am going to challenge one point that was made in an earlier post
If your motor exerts 10lbs at a distance of 1ft from the center of rotation it will exert 20lbs at 6 inches, not 5lbs.


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Offline TrickyNekro

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Re: Torque
« Reply #10 on: February 26, 2009, 07:34:37 PM »
Well.... Torque is physics.... At least, I see know how my profession can come in handy!!! ::) ::)  ( kidding....)

I'll begin, explaining what is a force....
A force in a Newton system is the cause of acceleration... This simply means that you have to
apply a force at an object to start moving!!! Now, the acceleration of an object is equal to the resultant of the applied
forces over the mass of this object.... It has the same direction as the resultant and it's proportional to it....

Torque is a bit more complicated...
For torque to exist, objects must not only have masses but also dimensions... In the real world everything has dimensions...
So what is torque....
A torque in a Newton system is the cause of angular acceleration... This meas that in order to have an object spin around
a certain point you have to apply torque.... But torque can be also explained as a force... It is the force applied in a system
able to rotate around a certain point to make the system start rotating... It is proportional to the force and the distance
between the the applying point and the rotation center....
For a motor torque means that at a certain distance from it's shaft a certain vertical force can be applied to that point....

I can't really teach you what I have been studying for almost two months with some simple lines....
But I can at least hope that I have helped you with the heads up process...

I'll try posting some pictures that shows what really torque is.....

Regards,
Lefteris
« Last Edit: February 26, 2009, 07:57:35 PM by TrickyNekro »
For whom the interrupts toll...

Offline newInRobotics

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Re: Torque
« Reply #11 on: November 28, 2010, 07:24:44 PM »
Ok, related question.
From this guide: http://www.societyofrobots.com/actuators_dcmotors.shtml
"My personal rule is if you have 2 motors on your robot, make sure the stall torque on each is enough to lift the weight of your entire robot times your wheel radius."

So, "two wheel balancing robot" has a frame that weights ~4kg, with all electroics and bateries I assume It should weight ~ 4.5kg; it also has wheels with 8cm radius. So, 4.5kg * 9.81 * 0.08m = 3.5 N.m is the required torque of one motor. Is it even possible (taking into account that required motor is 12V rated)? That looks like a big number...
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Re: Torque
« Reply #12 on: November 28, 2010, 07:28:04 PM »
Oh, that was just a rough rule of thumb.

You should calculate it here:
http://www.societyofrobots.com/RMF_calculator.shtml

Offline beautifulsmall

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Re: Torque
« Reply #13 on: December 09, 2010, 07:47:33 PM »
Torque for wheelie,
ive been trying to work this and lost. basic assumptions . lumped model , any help appreciated.
At a basic level i assume zero starting velocity. max (stall) motor torque. no slip.
forward velocity calculation is easy but wheelie needs centre off gravity figure, and this should be measurable on the static machine. and then i get lost....

 


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