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Thread: Stepper couplers?

  1. #1
    Join Date
    Mar 2003
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    4,395

    Default Stepper couplers?

    Question for anyone who has built or serviced a typical mini-mill conversion: What type of coupler did you use between the stepper and the ballscrew?

    I'm working on a buddy's previously-converted machine, basically rebuilding it from the ground up. It came with Lovejoy brand shaft couplers, which are two pronged couplers with a rubber "spider" in between. These particular ones aren't really meant for automation, there's too much slop between the couplers and the spider.

    I picked up some apparently fairly common one-piece aluminum ones, that have a series of saw cuts in the center, to make it somewhat flexible. They seem to be well-made, and certainly allow flex, but also appear to be kind of "squishy". That is, the center section is milled in a spiral pattern, and just twisting it by hand I can feel a small bit of "unwinding".

    I suspect it'd likely be fine for a low-force application like a laser or a 3D printer, but they seem just a little too wimpy for even a small-scale benchtop milling machine.

    Are there better choices? I see there's a similar version of the Lovejoy available, with a red polyurethane "spider", and two or three other types of the same solid aluminum coupler... Did I wind up with one simply too small for my application? Should I go with the kind with the rubber spider? Is there some other way to go? Or am I mistaken, and these little ones will actually transmit a fair amount of force?

    Doc.
    Doc's Machine. (Probably not what you expect.)

  2. #2
    Join Date
    Oct 2013
    Location
    Setubal, Portugal
    Posts
    599

    Default

    Oldham couplings are a good compromise. They handle a little misalignment with almost zero backlash[

    https://goo.gl/images/VemF58
    Helder Ferreira
    Setúbal, Portugal

  3. #3
    Join Date
    Jan 2016
    Posts
    185

    Default

    various types of couplings are a compromise between the type and the amount of misalignment.

    spiral couplings are good for both but have torsional stiffness issues.

    oldham and lovejoy couplings are essentially the same thing and both have backlash problems.

    one solution is to put the fixed-fixed ballscrew or leascrew bearing at the opposite end of the stepper motor, and directly couple the stepper to the leadscrew or ballscrew.

    you can use a sheet of rubber or plastic to bolt the stepper to the machine.. the flex of the ballscrew or leadscrew will take up the misalignment, and the rather large sheet of rubber (say 80mm square for a 50mm square stepper motor) will distort and flex but won't rotate axially very much. so it will be less backlash than a spiral coupling while still allowing for a lot of wobble.

    but for that unfortunate case where you need to couple the stepper motor directly to the fixed fixed bearing with very little distance between them you need a coupling and that will introduce either backlash, or it will introduce torsional stiffness issues.. you get one or the other. or for that case where you have run the preloaded ball nut all the way up to the stepper motor, there is no way around it, something has to move.




    another way is to use a hard coupling but use 6 setscrews on both sides of the hard coupling, so that's 12 setscrews total. it will take a bit of trial and error to get it right but you can get the stepper motor shaft completely perfectly inline with the leadscrew. then fill the coupling with loctite.. you will then need to get the stepper and leadscrew into the machine correctly aligned, easier said than done. can't simply bolt the stepper motor to a flange if the flange isn't square.. so either shim it or drill and tap the flange for set screws.
    Last edited by johansen; 03-26-2019 at 03:22 AM.

  4. #4
    Join Date
    Mar 2012
    Posts
    165

    Default

    No personal experince, but I have heard good things about bellows couplings like these: https://www.ruland.com/
    Last edited by Wheels17; 03-26-2019 at 07:39 AM.

  5. #5
    Join Date
    Sep 2012
    Location
    Australia
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    Default

    I've used the helical type couplers; in most applications the holding torque of the stepper is lower than the force required to substantially 'unwind' one. Either way, the coupler acts as a spring; it restores the leadscrew to the right index once the stopping force is removed. IMHO they are the go-to choice for direct drive applications.

  6. #6
    Join Date
    Jan 2007
    Location
    Minnesoa
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    Default

    Hi,

    Toothed belts work very well with steppers when you need accurate positioning. They are also cheap and easy to implement.
    If you think you understand what is going on, you haven't been paying attention.

  7. #7
    Join Date
    Apr 2010
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    Canada
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    Default

    Couplings are often the spiral or helical type, but when used with motors, the S.S. type should be used, the aluminum are more suitable for encoders etc, where there is less stress.
    Max.
    Last edited by MaxHeadRoom; 03-26-2019 at 10:33 AM.

  8. #8
    Join Date
    Jan 2013
    Location
    Alaska
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    453

    Default

    Doc,

    I think wheels17 post nailed it for you. I've used small (1" Dia) bellows style for precision position sensing in hot areas and beam style to transmit motion drive rotary switches etc. Both have virtually zero backlash and it's not hard to make the beam style.

  9. #9

    Default

    I would go helical. Using sst will make it about 10 times stiffer for the same size.

    Sent from my SAMSUNG-SM-G891A using Tapatalk

  10. #10
    Join Date
    Oct 2013
    Location
    Setubal, Portugal
    Posts
    599

    Default

    I still think that Oldham couplings are a good choice but, if you want a good, easy solution that you can build yourself, a disc type coupling like the one in the link could be the answer
    https://goo.gl/images/pGzWr1
    Helder Ferreira
    Setúbal, Portugal

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