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Thread: A little help with Torque measurements?

  1. #1

    Default A little help with Torque measurements?

    First off i hope i am posting in the right forum for this question, seeing as it pertains more to mechanics i thought that this would be the best place to ask it??

    So, my understanding of a torque measurement is this. Lets say a servo has 11 Kg/ cm of torque, which equates to roughly 156oz/in? Does that mean that if the servo had a "arm" that was one inch long, that it could conceivably lift a weight that was 156oz's? Or if the arm was 10 inches long, it could lift 15.6oz's?... I am working on a robot arm and i want the arm to be able to extend as far as a human arm and to have a few degrees of freedom, while still being able to lift 5 or 6 ounces? The servos i bought are rated for a stall torque of 11 Kg/cm which is quite a bit, but i don't know if it will quite do it. Should i double up on the servos?..... Thanks in advance guys
    I got the best hair around...Guaranteed!

  2. #2

    Default Re: A little help with Torque measurements?

    Ravenkallen,

    I think you have the general idea correct.

    The units of torque aren't Kg/cm and lb/in. They are Kg*cm and lb*in.

    So your 11Kg*cm servos should be able to lift 110g with a 1m arm. That's less than the 5oz you're after. I think 110g is between 3oz and 4oz.

    Let's call 1oz 30g (I think that's close). So you want to lift 150g with your 11Kg*cm servo.

    11,000g*cm / 150g = 73.3 cm (over 28 inches).

    I don't think you'd want to have the servos running at max torque all the time though. It's looking like doubling up is a good idea.

    BTW, I used to be high school chemistry amd physic teacher. Units are your friends. They can help you see if you set the equation up correctly or not.

    Duane

  3. #3

    Default Re: A little help with Torque measurements?

    Torque is force x distance.
    Leon Heller
    G1HSM

  4. #4

    Default Re: A little help with Torque measurements?

    It is not unusual to find official servo documentation from the manufacturer showing torque units written kg/cm and oz/in. Yes, it's wrong and it bugs me every time I see it.

    I won't even get into N-cm vs. kg-cm.

  5. #5

    Default Re: A little help with Torque measurements?

    Generally the posted torque spec is stall torque or holding torque. My rule of thumb with servos is to multiple the rated torque spec by 2/3.

  6. #6

    Default Re: A little help with Torque measurements?

    Quote Originally Posted by Ravenkallen View Post
    ...Does that mean that if the servo had a "arm" that was one inch long, that it could conceivably lift a weight that was 156oz's?...
    While that might be theoretically correct, keep in mind that the shaft itself will be susceptible to shear forces, and whatever bearings it has will be susceptible to the friction. In other words, mathematically it works out that such a system should be able to lift a weight of 1560 oz on a 0.1 inch arm, or 15600 oz (975 pounds) on a 0.01 inch arm, etc, etc. but in reality such a force would likely shear off the shaft or deflect it beyond usefulness. Or at least create so much friction on the bearing that the shaft would not turn. So the only time these torque numbers really mean much is when the "arm" length is of reasonable size.... but if the arm is too long, then you must take into consideration the weight of the arm, too. You can try to balance out an arm with a counterweight so the torque needed is just the difference, but again you have to consider what your total shear forces are, etc.

    Also note these torque ratings are generally for quasi-static loads, so if you're really swinging things around, then you must take into account dynamic effects (moments of inertia, etc.).

  7. #7

    Default Re: A little help with Torque measurements?

    Thanks for all the replies... I didn't know the units of measurement(kg/cm and oz/in) were wrong? I saw it on datasheets all the time so i assumed the servo manufacturers knew what they were talking about. There are many other variables that i didn't include such as the weight of the arm, so i will have to factor those in to. I think doubling up will be my best bet. The quoted torque is the maximum amount of force that can be applied before the quoted rotation speed will begin to degrade, right? Like the servo will still be able to rotate, but just not as fast? Could that cause damage?
    I got the best hair around...Guaranteed!

  8. #8

    Default Re: A little help with Torque measurements?

    On the other end is there a way to measure how much torque you are going to need?

    I looked at force gauges but wow they where expensive.

    I have a Halloween project that I what to move.

    http://www.facebook.com/video/video....50269867955550

    Been looking for non pneumatic options.

  9. #9

    Default Re: A little help with Torque measurements?

    Quote Originally Posted by Ravenkallen View Post
    ...The quoted torque is the maximum amount of force that can be applied before the quoted rotation speed will begin to degrade, right? Like the servo will still be able to rotate, but just not as fast? Could that cause damage?

    I think for most electric motors the torque rating is the stalled torque, which means the motor will be providing the maximum torque it can provide, but the motor will be stalled - in other words, the motor won't be moving at all at its stall torque. At stall torque, motors are generally sucking up the maximum amount of current they can use, so that means the motor windings are generating their maximum heat. So stall is the most likely scenario at which a motor might overheat, if it can overheat. So running a motor at or near stall is, generally speaking, probably not the wisest thing.

    Note that stall torque is not the same thing as maximum power, which is achieved at a peak that is related to the product of speed and torque. At stall torque you are theoretically getting zero power because you are achieving zero speed.

    Keep in mind that these are ideal numbers, so you will want to take friction into account or wind, etc. if such is the case.

    To Ragtop: not everyone has been subsumed into the Facebook borg. Also, you're somewhat hijacking Raven's thread here, so how about starting a separate thread of your own and post your project in a way that does not require a Facebook account. That would help make suggestions on your project easier to manage. :-)

  10. #10

    Default Re: A little help with Torque measurements?

    Hi:

    Save your money, hobby servos for what you want to do will not work for the project you have in mind.

    First off a company wants their rating to look good. So if the servo is rated at 156oz/in it means that it will stall at that torque; which in turn means your servo will break with in secs!

    Also "double up on the servos" may sound good but it is not. Each servo takes small steps as it turns. Not all servos are going to take the same size step. So the servos will fight each other just a little. This will waste power or if they are really out of step by a lot; break a servo.

    You are going to need more than hobby servos if you want to build an arm that extends as far as a human arm with human like joints. The only way you can get an arm that extends as far as a human arm using hobby servos is to build a linear arm. They don't use human like joints.

    Al

  11. #11

    Default Re: A little help with Torque measurements?

    Quote Originally Posted by ElectricAye View Post
    I think for most electric motors the torque rating is the stalled torque, which means the motor will be providing the maximum torque it can provide, but the motor will be stalled - in other words, the motor won't be moving at all at its stall torque. At stall torque, motors are generally sucking up the maximum amount of current they can use, so that means the motor windings are generating their maximum heat. So stall is the most likely scenario at which a motor might overheat, if it can overheat. So running a motor at or near stall is, generally speaking, probably not the wisest thing.

    Note that stall torque is not the same thing as maximum power, which is achieved at a peak that is related to the product of speed and torque. At stall torque you are theoretically getting zero power because you are achieving zero speed.

    Keep in mind that these are ideal numbers, so you will want to take friction into account or wind, etc. if such is the case.

    To Ragtop: not everyone has been subsumed into the Facebook borg. Also, you're somewhat hijacking Raven's thread here, so how about starting a separate thread of your own and post your project in a way that does not require a Facebook account. That would help make suggestions on your project easier to manage. :-)
    It seems similar enough to me. Before buying a motor, how do you determine how much torque your application is going to need?

  12. #12

    Default Re: A little help with Torque measurements?

    Quote Originally Posted by Ragtop View Post
    It seems similar enough to me....
    To what does your it refer here? Your motor spec? your configuration? Your topic?

    Quote Originally Posted by Ragtop View Post
    ...Before buying a motor, how do you determine how much torque your application is going to need?

    One way to do it would be to construct an arm of the desired length, then figure out how much force is applied at the end of that arm tangential to the circular movement. To figure out the force, you could use a spring scale similar to one of these:

    http://www.carolina.com/product/pull...by=bestMatches

    http://www.mcmaster.com/#spring-scales/=cj2oap

    If you don't want to use a spring scale, you could use a weight at the end of the arm, but make sure your test has the arm in the horizontal position and the force of that weight is acting straight down, at a right angle to the arm. Water makes a good weight because you can make a container from a lightweight cup and easily adjust the weight by adding or subtracting water. You can easily look up the density of water so all you need is a good way to measure the volume of water.

    Under circumstances like the above, your torque = (the length of the arm) x (the weight at its end).

  13. #13

    Default Re: A little help with Torque measurements?

    ElectricAye, I don't know what is up your crawl, but READ title of post.

    I don't see any difference talking about understanding torque measurements and determining how much torque you need.

    But thank you that is what I was looking for.

  14. #14

    Default Re: A little help with Torque measurements?

    Quote Originally Posted by Ravenkallen View Post
    ...The servos i bought are rated for a stall torque of 11 Kg/cm which is quite a bit, but i don't know if it will quite do it. Should i double up on the servos?.....
    11 Kg-cm = (24.25 pounds)(0.39 inch) = about 9.5 in-lb. = 152 oz-in. In other words, to hold a weight of 5 ounces, the arm could be 152/5 = about 30 inches long. But remember this is the stall torque, so nothing is moving at that torque. Also, you must consider the weight of the arm, whose weight will appear to act at its center of gravity. Assuming the arm is of constant cross section all along its length, and the arm is operating in the horizontal position, and let's say the arm weighs 8 ounces, then a 30 inch arm will require (8)(30/2)= 120 oz-in of torque just to lift it. You can use a counter weight to balance things better, but keep in mind how much total shear force that starts to add to the shaft and how that might start to bind the bearings, flex the motor housing, etc.

    I agree with al1970 that you can't just "double up" a motor to get more strength. Generally speaking it's just not a good idea. I'm sure it's been done in some circumstances but the two motors would need to be precisely coordinated so they truly work together, and that isn't easy to do.

  15. #15

    Default Re: A little help with Torque measurements?

    Quote Originally Posted by Ragtop View Post
    ElectricAye, I don't know what is up your crawl, but READ title of post.....
    Ragtop,

    because you posted your own project and were talking about options to pneumatics and so forth, I thought it would be more courteous to Raven and more helpful to you for you to start a new topic here so people could look at your specific project and provide inputs.

  16. #16

    Default Re: A little help with Torque measurements?

    Ravenkallen, I'm not sure what kind of servos you have. I personally haven't tried doubling up on normal hobby servos but I have doubled up with Dynamixel AX-12 servos. The AX-12s aren't to hard to use in pairs.

  17. #17

    Default Re: A little help with Torque measurements?

    These are the servos i bought. They were really cheap and they had a high torque rating, so i thought they would be ideal
    http://www.hobbypartz.com/kahaoubrmo14.html

    So i guess i am in another pickle. Couldn't someone just use a little clever programing to make up for the slight difference in the servos? Like if one is to far to one direction could you just tell that one to go back slightly? Linear actuators are nice, but they are really pricey. I am trying to do this on a budget
    Thanks for all the help!!
    I got the best hair around...Guaranteed!

  18. #18

    Default Re: A little help with Torque measurements?

    Ravenkallen,

    Heck, for that price I'd say go ahead and try doubling them up. If you could monitor the current to each servo you should be able to see when they are fighting each other. I'm pretty sure those are analog servos so if their position is off a little they wont draw a lot of current trying to move.

    Just make sure and post your progress somewhere on the forum (if you want to). I'd like to see what you do.

    Duane

  19. #19

    Default Re: A little help with Torque measurements?

    @Daune.... Yeah, i am going to try it out on two of my three dollar servos first, just to see if it will work... The finished project is not going to be perfect, but it is going to be really cool. Think wearable third arm. I will definitely be posting it up once i finish it
    I got the best hair around...Guaranteed!

  20. #20

    Default Re: A little help with Torque measurements?

    I'd imagine using multiple servos would be ok if you used some form of mechanical mixing. Or you might be better off removing the position feedback circuit in your servos, and implement your own with sensors somewhere on the arm (or you can still use the potentiometer in the servos).

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