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Thread: How much will these motors carry?

  1. #1
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    How much will these motors carry?

    Hi guys, this is my first post here.

    I am designing a 3D printer and need to know how much weight these motors will support:
    Motor specs:
    - holding torque of 4.3 kg*cm
    - rated torque of 4.3 kg*cm
    - Max rpm of 2344RPM

    Set up:
    I will have two motors lifting a platform vertically. Each motor will have a belt tying it to two of four screws (one at each corner). The ratio is 1:1 from the motor to the screw and the screw has a lead of 5mm. I'd like to aim for a speed of 150mm/second at a fair acceleration (what ever that may be).

    I'm really not sure what the calculations are to determine what this set up will support as far as weight for a platform (ignore friction). In the image, I am looking to see what weight i need to aim for for the motors to move the yellow parts up and down at the specified speeds. Solidworks is giving a mass of roughly 10kg.

    Thanks for your help
    Attached Images Attached Images
    Last edited by eyezaak; 07-07-2013 at 08:53 PM.

  2. #2
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    Hi and welcome to the forum. The motors, I assume because of the "holding torque" statement, are stepper motors. You need to be more precise if you want to get any useful information. Stepper motors are at their *maximum* torque when holding. When driving the torque figure drops rapidly with speed.

    Also, the modern stepper drivers can supply much greater current than the design specs so it is impossible to accurately calculate the torque available when working at speed. The figures can be approximated when the controller and motor performance is known.

    So let's get back to "precise," as "two of four screws" means nothing to us. We would need to know the diameter, the pitch, if they are ball-screws, Acme or just plain old threaded rod. Also having two motors driving two sets of screws is a recipe for disaster as stepper motors can notoriously lose steps and there is usually no feedback to the controller to tell it to catch up. This will cause jacking and jamming of the platform. Not "might cause," but "will cause."

    You need to spend more time learning and understanding the mechanism you are trying to design otherwise you will waste an awful lot of time, effort and maybe money on something that will disappoint.

    Happy to help more, but you need a lot more, reading, learning and thinking before you buy those steppers.

  3. #3
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    Thank you for the reply. I've decided to take your advice and will replace both steppers with one large motor. I will get back to you on the motor specs later tonight. For now, would you suggest I put a larger pulley on the motor as to enlarge the ratio so that the stepper doesn't have to turn as quickly? Similarly, should I redesign the X/Z-axis as to have a larger ratio for the same reason? I was aiming for simplicity and adding a gear system to the X/Z-axis would complicate the build slightly, is it worth it?

  4. #4
    Technical Fellow jboggs's Avatar
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    1) Larger pulley on the motor increases the torque it sees by the same ratio that it lowers the speed.
    2) I've never heard of an "X/Z" axis. What does that mean? Is it X (normally horizontal), or is it Z (normally vertical)?
    3) Use ONE motor per axis of motion. For three axes of motion (X, Y, and Z), you would have three motors.

  5. #5
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    I meant x/z as in "x and z axis" (both horizontal axis, by my convention)

  6. #6
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    As JB suggests, larger pulley on the motor means reduced torque so stay with a small (maybe 12T to 15T) on the motor. With efficient controllers (Gecko??) you should be able to peak at about 3,000rpm, but quite low torque so aim for around 1000rpm to 1500rpm for about 50% of holding torque. Do some more research please otherwise we are just going to be repeating a zillion search results and I for one will tire of that quickly.

    Standard interpretation of axis for pretty much anything CNC (GCode) when looking from the front of the machine is...
    X-axis is Horizontal and Left to Right with Positive = Right
    Y-axis is Horizontal and Front to Back with Positive = Back
    Z-axis is Vertical and Up Down with Positive = Up
    Last edited by PinkertonD; 07-08-2013 at 02:34 PM.

  7. #7
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    Thanks, I'll do that. 1500 rpm is just what I'll need. I know this is a vague and hard to answer question with the info that you have, but how many steps would you say I can expect the motor to skip at those rpms and a good amount of weight? I'm just looking for a very rough estimate. I realize how vague the question is. Also, would you all agree with the equations found on page 82 of the link for torque and acceleration calculations?
    http://www.cbmind.com/linear/thk/pdf...scriptions.pdf

  8. #8
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    Quote Originally Posted by eyezaak View Post
    how many steps would you say I can expect the motor to skip at those rpms and a good amount of weight?
    ...and that's why I say do some research on what you are using and trying to do. Skipped steps is a bad thing and they cannot be made up or compensated for during a complete printing run.

    If you design this thing correctly, fully understanding all the ramifications of the mechanics and electronics you are using, your printer should never lose steps -- EVER!

    Depending on what you are aiming at that would make this printer design different from a thousand others, there are better ways than using stepper motors for movement. Steppers are cheap and use simple electronics, but with that comes a plethora of compromises.

    I really am at the end of what I can help you with as you still, it seems, have not fully learned about the equipment you are trying to use. This is like sitting you in a 747 cockpit and telling you "this steers the plane, this adjusts the seat, now fly me to Florida." Until you understand what you are working with, you are going to be wasting a lot of time effort and money.

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