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Easily testing if a motor has the torque needed?

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  • Easily testing if a motor has the torque needed?

    I got some scrap stepper motors recently and I've been thinking about adding them to my surface grinder for X and Z feed automation, as well as an electronic lead screw for my lathe, and probably replacing the power window motor that I use on my mill for a X feed, since I can sense it is not healthy.

    One issue I have is that I don't know if the steppers can deliver the torque I need for the different applications, and would rather not find out after I build the thing. I was wondering; are there any tricks to get a motor roughly connected to test an application?

  • #2
    Hold it in a vise, and make a lever arm 12" long (or whatever you have). Use it to lift weight until it can't anymore. Easy math from there to figure out the torque.

    Assuming there's not specs, or # on the motor you can look up of course.

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    • #3
      Originally posted by Dan Dubeau View Post
      Hold it in a vise, and make a lever arm 12" long (or whatever you have). Use it to lift weight until it can't anymore. Easy math from there to figure out the torque.

      Assuming there's not specs, or # on the motor you can look up of course.
      The second side of the issue is the unknown torque on the other end of the application. I don't know how much torque it takes to turn the handle of the mill, for example. However your approach would probably work to determine the torque required on that side as well.

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      • #4
        Another method is wish a fish/spring scale and a pulley etc of a known dia. for Inch/lbs - Nm etc.
        You also need a specialized stepper drive that counteracts the loss of torque at higher rpm, typical characteristic of a stepper.
        If using high rpm etc.
        Max.

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        • #5
          Originally posted by mattthegamer463 View Post
          The second side of the issue is the unknown torque on the other end of the application. I don't know how much torque it takes to turn the handle of the mill, for example. However your approach would probably work to determine the torque required on that side as well.
          Work in reverse. Put a lever arm, say 12" on the handle and start hanging weights till it moves. They call them foot lbs for a reason

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          • #6
            Drive maximum current trough one of the windings and measure the locked rotor(holding) torque with fish scale or lever arm and weights.

            adjustable lab power supply is sufficient and you dont need to attempt to measure torque from rotating parts
            Last edited by MattiJ; 01-15-2020, 02:25 PM.

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            • #7
              Thanks, the fish scale idea seems like a good way to experiment.

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              • #8
                You might be able to use a torque wrench or direct reading torque meter:

                https://www.banggood.com/1_5-1000NM-...r_warehouse=CN ($50)

                The #1 model reads 0.3 - 30 N-m which is about 3 - 300 kg-cm. A NEMA23 stepper may have holding torque of 4.2 kg-cm (1-5/8 long, 1 pound) to 21 kg-cm (3-1/4" long, 2.6 pound).
                http://pauleschoen.com/pix/PM08_P76_P54.png
                Paul , P S Technology, Inc. and MrTibbs
                USA Maryland 21030

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                • #9
                  Originally posted by mattthegamer463 View Post
                  The second side of the issue is the unknown torque on the other end of the application. I don't know how much torque it takes to turn the handle of the mill, for example. ...
                  Determining the feed torque required is a bit tricky, as it depends on the cut being taken. A deep cut & fast feed takes a lot more torque than shallow & slow. The way that I did it for my lathe was to set up the worst-case cut and drive the lead screw with a cordless drill. I adjusted the drill's clutch until it just slipped on the fastest feed that I needed. Then I measured the drill's torque on the bench with a lever and scale. Not terribly accurate due to the difficulty of holding the drill's speed for the feed desired while adjusting the clutch. But close enough.



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                  • #10

                    Originally posted by mattthegamer463 View Post
                    The second side of the issue is the unknown torque on the other end of the application. I don't know how much torque it takes to turn the handle of the mill, for example. However your approach would probably work to determine the torque required on that side as well.


                    Well aren't you in luck. Master machinist Robin Renzetti is doing the same exact thing, and he posted an instagram video that might give you some ideas.
                    https://www.instagram.com/p/B61MRqPJphM
                    21" Royersford Excelsior CamelBack Drillpress Restoration
                    1943 Sidney 16x54 Lathe Restoration

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                    • #11
                      I always build the necessary adaptors and install the motors first. If it works I brag to everybody how smart I am. If the motors don't have enough torque to work properly, I say nothing---Just go out behind the house quietly and bash my head on a rock!!!
                      Brian Rupnow

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                      • #12
                        Torque at at a given speed is what you need to determine, torque decreases as speed increases.
                        There will be a speed range where maximum torque is developed, you will need a dynamometer to measure that.
                        A simple dynamometer may be easily built at home for low torque/low rpm motors

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                        • #13
                          Electric motors generally have fairly constant torque over a wide speed range, from locked rotor on up. Stepper motors have a "hold torque" which is the maximum it can produce at rated current. As speed increases, the inductance of the windings causes current to drop with a fixed voltage, so the voltage can be increased somewhat to obtain rated current and torque. But there is a fairly steep decrease in torque as speed increases over a fairly low level, to perhaps 20% at maximum speed.

                          https://www.orientalmotor.com/steppe...er-motors.html

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                          https://www.controleng.com/articles/...torque-basics/

                          https://www.motioncontroltips.com/fa...olding-torque/

                          http://pauleschoen.com/pix/PM08_P76_P54.png
                          Paul , P S Technology, Inc. and MrTibbs
                          USA Maryland 21030

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                          • #14
                            Originally posted by mattthegamer463 View Post
                            One issue I have is that I don't know if the steppers can deliver the torque I need for the different applications, and would rather not find out after I build the thing. I was wondering;


                            are there any tricks to get a motor roughly connected to test an application?
                            Ok. Here is a long standing "trick" that you need. Dont spread it around.

                            Grab hold of the shaft of the electric motor then apply power. Did it feel like it might move the table?

                            There you go. JR

                            My old yahoo group. Bridgeport Mill Group

                            https://groups.yahoo.com/neo/groups/...port_mill/info

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                            • #15
                              There is much wisdom above this post. But somehow I like this one. You have used the machine and know what the hand wheels feel like when taking a heavy cut. Put a hand wheel of similar diameter on the motor and power it up. See how much it takes to rotate it against the torque it develops. You will want a motor that gives you around 5X to 10X that torque for reliable operation and at operational speeds. I don't think grabbing the shaft is a good way to do this. You need something similar in size and feel to the hand wheels on the machine.

                              Others have stated that stepper motors will have less torque at higher speeds. That is true. But it is not all. When you apply a resisting torque to a stepper that is operating, the stepper will not just suddenly jump from the center of one step to the center of the next step. It will move in increments toward the failure point where it jumps. So you need to consider just what positional accuracy your application needs. If anywhere +/- 1/2 step is OK, then all you need is a motor that never skips a step. BUT, some systems use micro-stepping and if the accuracy of fractional steps is needed, then you need a LOT more torque to prevent a movement of 25% or 10% or 5% of a single step. This is in addition to the need for more powerful motors for higher speeds.

                              All this being said, if it looks and feels like it should work, it probably will.



                              Originally posted by JRouche View Post

                              Ok. Here is a long standing "trick" that you need. Dont spread it around.

                              Grab hold of the shaft of the electric motor then apply power. Did it feel like it might move the table?

                              There you go. JR
                              Paul A.
                              SE Texas

                              Make it fit.
                              You can't win and there is a penalty for trying!

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