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Thread: What is the best way to remove the horn of the Dynamixel?

  1. What is the best way to remove the horn of the Dynamixel?

    Hi, I inserted a horn on the X-series motor shaft. Then, I found that I was supposed to put a washer in between first. In this case, should I leave the motor as is? Could removing the horn easily damage the motor? What is the best way to take off the horn?

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    Re: What is the best way to remove the horn of the Dynamixel?

    For the MX-series, I used a spare horn to remove them, but a piece of aluminum ~3mm or thicker with a few holes drilled (and one tapped) would work just as well. The plate needs a center hole tapped for a screw at least as large diameter as the center horn retention screw (M3 or larger; #6-32 or larger), and at least two more clearance holes drilled to match the mounting nuts/holes in the horn so that the plate can be attached to the horn through these holes using longer screws than would be used to attach a frame to the horn.

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    The process is roughly:
    1) replace the center horn retention screw with one ~2X longer than necessary when fully tightened down (want the horn to be able to freely slide off the spline without encountering the head of the screw) (also want the threads of the screw fully engaged with the thread in the output gear/spline to prevent stripping them)
    2) align the plate over the horn and use long screws into the horn mounting nuts/holes through the clearance holes in the plate
    3) use a screw through the center tapped hole in the plate to push on the center horn retention screw (a ball bearing sandwiched between the head of the center horn retention screw and the end of the screw in the plate might help to prevent the screw in the plate overtightening the center horn retention screw)

    As you tighten down the screw in the plate, it will push on the center horn retention screw which is attached to the output gear/spline of the servo while pulling on the horn with the screws in the horn itself. This pulls the horn off the spline.

    Alternatively, you could use a much longer screw as the center horn retention screw through an unthreaded center hole in the plate and use a nut that you turn to push the plate/horn off the spline. This is possibly less likely to damage the thread in the output gear/spline since you can use a hex wrench in the head of the screw to counter the torque when turning the nut instead of having one screw driving into the head of another and possibly getting stuck and stripping the thread in the spline.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
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  3. Re: What is the best way to remove the horn of the Dynamixel?

    Thanks tician. I don't know if the X-Series is similar. What do you think of the tool used in this video? The idea seems to be similar.

    https://www.youtube.com/watch?v=0Xof5-JkpU8

    The width of the motor is about 3cm. There are so many products on Amazon. Some of their descriptions are not clear. Hard to decide which one to get. Asked Robotis which product they use in the video but they told me to try different ones by myself.
    Last edited by Snoopy; 04-02-2020 at 05:42 PM.

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    Re: What is the best way to remove the horn of the Dynamixel?

    That video is infuriating. Pulling the center screw out like that before pushing on it with the bearing puller can destroy the threads in the output gear/spline.

    You do not need a bearing puller. Just a piece of metal or thick plastic with three or five holes, then just follow the directions I outlined above. It literally combines the extra horn and bearing puller into one piece of scrap.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    [git][mech][hack]
    gives free advice only on public threads

  5. Re: What is the best way to remove the horn of the Dynamixel?

    Thanks. I need to slowly digest your procedure. Maybe 3D print a thick plastic with three or five holes.

    I don't have M2 screws with me so I will need to order online. Giving the outbreak situation, better not to go around places to look for components.

    I expressed concern about removing the horn could damage the motor. I asked them for a replacement as their manual did not mention about inserting the washer before the horn. They said that removing the horn is a "fairly common process and should be possible without damage to the DYNAMIXEL". They asked me where I heard that removing the horn would damage the motor!

    As I recall, the MX106 had a larger gap between the horn and the motor box. The X-series has a very very thin gap.

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    Re: What is the best way to remove the horn of the Dynamixel?

    I do not know how it could damage the motor. Improper methods can destroy the threads in the output gear/spline, or break the spline off the output gear, or crack the front face panel of the servo housing. Fortunately, the center horn retention screw is pretty redundant so stripping the threads is not the end of the world. Even if you are using it like the shoulder pitch or hip yaw in Bioloid Humanoids or DARwIn-OP/HROS-1/5 where everything simply hangs off the horn without using support from the back side of the servo. In that usage, the horn might possibly be pulled off the spline over time without the center horn retention screw to keep it secure on the spline, but it will take time and heavy axial loads.

    The gap on the MX-28/64 has always been 0.5~1.0mm (1/32" thick PTFE thrust washer) to prevent the screws in the horn scraping on the case and provide a bit more support to the ball-bearing against bending moments.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    [git][mech][hack]
    gives free advice only on public threads

  7. #7

    Re: What is the best way to remove the horn of the Dynamixel?

    What I do for MX series servos, which works great, is:
    1) Remove the center screw.
    2) Screw in a bracket into the horn, using 4 screws at each 90 degree cardinal direction.
    3) Pull on this bracket until the horn comes off.
    You don't need custom anythings -- the brackets that are intended to go on the horns do well.

    This has worked great for years for my MX series servos.
    The X manual doesn't suggest the X series is any different. It looks almost exactly the same in section 4.1.1: installing the horn.

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    Re: What is the best way to remove the horn of the Dynamixel?

    I only add the center screw to make it easier/safer to pull the horn off the spline (just tighten the screw and it gently pulls the horn off without any force on the bearings in the front case). I'm not known for my coordination or motor control, so if I just tried to pull it off myself there is a fair chance I would break the spline off sideways or crack the front case or damage the bearing AND hurt myself somehow.

    For AX servos, I just pry the horn up a little bit with a screwdriver then pull on the horn while rocking it a bit.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    [git][mech][hack]
    gives free advice only on public threads

  9. Re: What is the best way to remove the horn of the Dynamixel?

    Quote Originally Posted by jwatte View Post
    What I do for MX series servos, which works great, is:
    1) Remove the center screw.
    2) Screw in a bracket into the horn, using 4 screws at each 90 degree cardinal direction.
    3) Pull on this bracket until the horn comes off.
    You don't need custom anythings -- the brackets that are intended to go on the horns do well.

    This has worked great for years for my MX series servos.
    The X manual doesn't suggest the X series is any different. It looks almost exactly the same in section 4.1.1: installing the horn.

    Thanks jwatte. I have successfully removed the horn. When I did that, I heard a pop sound. Hope it was just a sound caused by the horn being separated from the shaft.

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