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IndustryArena Forum > Mechanical Engineering > Linear and Rotary Motion > Query with Ball Screws and Lead Screws
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  1. #1
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    Query with Ball Screws and Lead Screws

    Hello,

    I am currently working on a multipurpose CNC machine and would like to be able to drive 2 (or more) lead / ball screw nuts on one lead / ball screw. The nuts on the same ball / lead screw cannot be able to move at the same time and so I was wondering if there is anything out there that would be able to lock a single ball / lead screw nut onto the ball / lead screw without locking the whole ball / lead screw so that the other nut can move and then as soon as that nut has finished moving, it can be locked, the other nut can be unlocked and then moved up and down.

    Many thanks in advance for any help with this rather odd situation.

  2. #2

    Re: Query with Ball Screws and Lead Screws

    Sure. Fix the ball screw and turn the nuts with the drive motors. Or in the case of an Acme thread lead screw, use a split half-nut like a manual lathe.
    Jim Dawson
    Sandy, Oregon, USA

  3. #3
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    Re: Query with Ball Screws and Lead Screws

    Hi Jim,

    Thanks for your reply. I really like the idea about using a split half-nut however the lead screws that I was going to be using would of been mounted vertically and so as soon as the split nuts disengage from the lead screw they would probably just fall down. Is there any way to overcome this???

  4. #4

    Re: Query with Ball Screws and Lead Screws

    Quote Originally Posted by maxbest View Post
    Hi Jim,

    Thanks for your reply. I really like the idea about using a split half-nut however the lead screws that I was going to be using would of been mounted vertically and so as soon as the split nuts disengage from the lead screw they would probably just fall down. Is there any way to overcome this???

    A separate mechanical brake or ratcheting stop system would make sense. Without knowing a lot more about the proposed system it would be hard to get more specific.
    Jim Dawson
    Sandy, Oregon, USA

  5. #5
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    Re: Query with Ball Screws and Lead Screws

    Quote Originally Posted by Jim Dawson View Post
    A separate mechanical brake or ratcheting stop system would make sense. Without knowing a lot more about the proposed system it would be hard to get more specific.
    Thanks again,

    Attached is a quick mock-up I did on SolidWorks, so there are two nuts on one lead screw and I want to be able to move each one independently by somehow locking and unlocking them to the lead screw. Not in the picture, however there will be a Nema 23 stepper motor at the bottom, turning the lead screw and causing the nuts to move up and down. The part I am stuck at is being able to make a mechanism that will be able to engage or disengage a nut on the lead screw whilst allowing for it to remain in place and still hold the load on each nut.

  6. #6

    Re: Query with Ball Screws and Lead Screws

    Based on what you have modeled, the easiest way to achieve the action that you want is to use seperate steppers to turn the nuts on a fixed screw. This method might be the best in any case because you will always know the location of the devices. I did something like this a number of years ago for a high speed rip saw with moving blades, the difference was that the system was horizontal, but the same principle applies.
    Jim Dawson
    Sandy, Oregon, USA

  7. #7
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    Re: Query with Ball Screws and Lead Screws

    Quote Originally Posted by Jim Dawson View Post
    Based on what you have modeled, the easiest way to achieve the action that you want is to use seperate steppers to turn the nuts on a fixed screw. This method might be the best in any case because you will always know the location of the devices. I did something like this a number of years ago for a high speed rip saw with moving blades, the difference was that the system was horizontal, but the same principle applies.
    The only reason I don't really want to use separate steppers is because that would mean needing to buy 8 steppers as well as the space they take up for a relatively small application. Is there no other possible way to lock the nuts in place whilst not engaging with the lead screws, possibly using mini solenoids or something???

  8. #8

    Re: Query with Ball Screws and Lead Screws

    In that case, each device would have to have some kind of brake to prevent movement. The brake could be air actuated as you suggest and would have to contact a fixed frame member to be effective.

    One thing to concider when using split nuts is that there only discrete positions that the nuts can re-engage the lead screw. For instance, if you are using a 10 pitch screw, then there are only 10 positions/inch that the half-nuts can engage. This problem might be overcome with some creative programming and use of sensors. I don't know what positioning accuracy is needed in your system.
    Jim Dawson
    Sandy, Oregon, USA

  9. #9
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    Re: Query with Ball Screws and Lead Screws

    Quote Originally Posted by Jim Dawson View Post
    In that case, each device would have to have some kind of brake to prevent movement. The brake could be air actuated as you suggest and would have to contact a fixed frame member to be effective.

    One thing to concider when using split nuts is that there only discrete positions that the nuts can re-engage the lead screw. For instance, if you are using a 10 pitch screw, then there are only 10 positions/inch that the half-nuts can engage. This problem might be overcome with some creative programming and use of sensors. I don't know what positioning accuracy is needed in your system.
    I have just been thinking and was wondering if it would be possible to use a mechanism like in a ratchet wrench. I'm finding it rather hard to visualise how it would work but I could make a bearing that fits over the nut that then the outer ring attaches to the red holder and so if I managed to create enough friction between the nut and the lead screw, causing it to stay in place, it would be able to rotate with the lead screw whilst the load is staying in a constant place and could be moved up and down with a manual switch or something that moves the pawl on the ratchet from side to side???

    I'm really not sure if this would be possible and there is also the problem still of locking the nut to the lead screw, I was wondering if with a lead screw there is already enough friction between the threads that it won't turn the load on it because it will be able to move due to the bearing???

    Edit:

    I've just had another thought, instead of mounting the ratchet bearing system onto the nut, I could make a nut that is threaded on its inside ring, I'm still not sure if this would fix the problem of it not being able to lock to the screw thread though.

  10. #10

    Re: Query with Ball Screws and Lead Screws

    Quote Originally Posted by maxbest View Post
    I have just been thinking and was wondering if it would be possible to use a mechanism like in a ratchet wrench. I'm finding it rather hard to visualise how it would work but I could make a bearing that fits over the nut that then the outer ring attaches to the red holder and so if I managed to create enough friction between the nut and the lead screw, causing it to stay in place, it would be able to rotate with the lead screw whilst the load is staying in a constant place and could be moved up and down with a manual switch or something that moves the pawl on the ratchet from side to side???

    I'm really not sure if this would be possible and there is also the problem still of locking the nut to the lead screw, I was wondering if with a lead screw there is already enough friction between the threads that it won't turn the load on it because it will be able to move due to the bearing???

    Edit:

    I've just had another thought, instead of mounting the ratchet bearing system onto the nut, I could make a nut that is threaded on its inside ring, I'm still not sure if this would fix the problem of it not being able to lock to the screw thread though.

    I think what you propose would work. If the nut was free to turn then it would not move the device. Once it was engaged then the device would move. There should be enough friction between the nut and the screw that with good bearings in the mounting the nut should just rotate with the screw. The engagement mechanism could be a pin that engages the nut, or any of several other systems. Could be air or electric solenoid actuated.
    Jim Dawson
    Sandy, Oregon, USA

  11. #11
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    Re: Query with Ball Screws and Lead Screws

    OP - Max...
    define it better.
    If 8 stepper motors costs are a critical path, you will struggle to get anything remotely useful for less.

    Precision and high resolution is the basis for steppers and motion control hw in general.
    Everything mechanical and not mass market and actuated by something, to the typical 0.01 mm resolution of steppers, +/- 1-2 units with no slippage, will cost many multiples of one stepper motor and controller.

    A good electromagnetic brake will cost much more than the stepper+controller, and might need precision fitting of the brake as an aftermarket.

    Sometimes some relatively cheap-seeming mechanical solutions provide great results.
    Some toolchangers with ratchet-stops for lathes had decent results, and decent life, apparently.

    If you needed few movements on one axis drive, and only high accuracy overall/second, a secondary pitch screw at one end might get excellent rigidity, repeatability, and low cost but be slow, with optional optical probing.

    The cheap ballscrews are not too accurate, and you need to oversize them for rigidity and reliability.
    And build better bearings in the mounts.

    How many axes, what is the expected/desired speed of each drive, resolution, repeatability, accuracy ? Cost per axis ?

    About 7 years ago someone wanted a 1/10.000 degree angle drive for astronomy, with fast tracking to any angle in some seconds, aka -80 - +80 degrees, more or less.
    Mechanically possible via exotic solutions, and lots of money.
    This can also be done by very direct expensive drives, and aggregate drives, and lots of money.
    Lots === 1M$ at that time.

    But 2 separate cheap servo drives, and some sw, can provide the near the same (1/1000) for less than 3000$, with notches in the gross movement.

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