Can duet help me with planetary gear backlash?



  • Good afternoon,

    I started working on building a CoreXZ printer called Voron Switchwire.

    This printer uses a counterweight (via a retractable keychain) to lift up the X assembly since the motors driving the CoreXZ system are not able to maintain the weight.

    Although unrelated, I know that Duet has advanced features such as pressure advance.

    To simplify the setup, I am thinking about using a stepper motor with integrated planetary gear reduction mechanism (5-to-1) and forgo the retractable keychain.

    From the specs sheet, however, I see that these mechanisms come wikth a "less than 1 degree" backlash.

    I do not understand how does this backlash could effect the movement of the axes.

    I guess my question is two folds;

    1st) Can duet make up for this backlash the same way it magically handles pressure advance?

    2nd) If I used the 5:1 geared motors to drive the axes, what kind of an affect could they have on the prints with their less-than-1-degree-backlash specs?

    I know many will say just build the the printer as specced but I was very excited to learn about the planetary gear system and I would love to have an actual understanding about these things.

    Thank you for your time, I am looking forward to learning about this.

    Cheers

    P.S.

    This is my first message on the Duet boards. I apologise in advance for any possible mistakes. I am posting this under General discussion. Please feel free to let me know or just move the thread to the appropriate board if it should belong elsewhere. Thanks again 🙂



  • @Philn said in Can duet help me with planetary gear backlash?:

    1st) Can duet make up for this backlash the same way it magically handles pressure advance?

    no.
    see
    https://forum.duet3d.com/topic/4665/backlash-compensation

    2nd) If I used the 5:1 geared motors to drive the axes, what kind of an affect could they have on the prints with their less-than-1-degree-backlash specs?

    why not go for the 5:1 geared solution from the voron 2.4?

    however this is not a good solution, as it will severely limit your printing speed on the x axis.



  • Hi! Thank you for the extremely rapid response. I couldn't believe my eyes.

    I read the 1st thread. I understand that compensation is a no-go.

    And I get your point regarding the 2nd question. Even if backlash wasn't an issue, things become slower..

    Do you mind if I trouble with you with a 3rd question?

    Bed-slinger printers are usually quite slow as far as I can tell if you want to get a decent print.

    If I was printing, say at around 60mm/s on avarage, would reducing the gear be an issue?

    Understanding acceleration/jerk vs speed was hard enough, when combined with torque I have no idea where to begin just to get the numbers.



  • @Philn

    it depends on your setup

    put in the numbers into the emf calculator
    https://www.reprapfirmware.org/

    there is no support for gear reduction, so you will have to multiply the speed with the reduction.

    so for a 60mm/s the motor would have to go at 300m/s to achieve the same speed.

    see if that motor looses torque at that speed



  • @Philn said in Can duet help me with planetary gear backlash?:

    From the specs sheet, however, I see that these mechanisms come wikth a "less than 1 degree" backlash.

    StepperOnline has two lines of steppers with gear boxes.

    The better units list the backlash spec as "<=15arcmin".

    Also notice that the "economy" line lists the "5 to 1" units as actually being "5 & 2/11 to 1" which works out to 5.18181818... to 1

    I started with the economy units but that odd ratio is messing with my calculations.

    I just received the better units but they need a different gear - one that has a keyway.

    I don't know what the backlash is on "belt and pulley" setups but there are 100 tooth pulleys out there that could be used along with the typical 20 tooth pulleys to get a true 5-to-1 ratio.

    However the setup would not be as simple as a stepper with gearbox. It would likely be more expensive.

    Frederick



  • @fcwilt said in Can duet help me with planetary gear backlash?:

    20 tooth pulleys to get a true 5-to-1 ratio.

    the voron uses 16 tooth pulley with 80 tooth pulley givin 5:1



  • @Veti said in Can duet help me with planetary gear backlash?:

    the voron uses 16 tooth pulley with 80 tooth pulley givin 5:1

    Yes that would work. I've always used 20 tooth so that is why the 100 tooth came to mind.

    Frederick



  • i would ask in the voron discord why they did not go with the gear based setup.
    i am almost certain that they have evaluated that possibility.



  • @Veti
    Backlash 🙂

    What I'd really like to know now is why they haven't used bigger motors with higher torque. I feel bad wasting their time with my questions though. They are already doing so much and I feel like abusing their kindness.



  • Many thanks @fcwilt

    This is great to know it will help a lot in the future I bet. The one I have is not the better version it seems though which is bit of a sad news.

    Do you mind if I ask what is your usecase for these motors? Are you doing a CoreXZ or similar?



  • @Philn said in Can duet help me with planetary gear backlash?:

    Do you mind if I ask what is your usecase for these motors? Are you doing a CoreXZ or similar?

    Quite some time ago I got the parts to build a DBOT (CoreXY).

    Long story short - didn't like it - not the CoreXY part - just the overall design - "wheels on rails" instead of rods or linear guides.

    I got the urge to try a MarkForged type of printer and built one using many of the frame parts from the DBOT.

    Recently I decided to "upgrade" it to use triple Z steppers and give "auto-leveling" a try.

    While I was at it I noticed someone here talking about using belt drive for the Z axis.

    So I decided to try that as well - leading to the purchase of the "economy" steppers but not noticing the odd gear ratio.

    In any case I quite like the belt drive setup - simple and quiet - much quieter than the previous lead screw drive setup.

    And the auto-leveling is a great fun to watch in action and it beats manual leveling.

    So when I have the needed pulley for the new "precision" steppers with true 5-to1 I will make the needed changes to the stepper mounts and install the new steppers.

    And my math problems will be a thing of the past.

    Frederick


  • administrators

    It may be that the weight of the print head that the motor is lifting will be sufficient to preload the gears enough to remove the backlash.


Log in to reply