### Author Topic: Robot horse neck  (Read 2077 times)

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#### SvdSinner

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##### Robot horse neck
« on: December 07, 2020, 12:44:56 PM »
My 14yo daughter and I are building a robot horse.  I'm working on the mechanism to allow the neck to raise, lower, and move side to side.  My idea is to use 3 linear actuators in a triangle to create the motion.  the ends of the actuators will be free to swivel.  I am constraining the lower actuators to not point down (from the torso) and the upper actuator from not pointing up.   See the attached image of my first iteration of the mechanism.  (I'm new and couldn't figure out how to display the attached image inline)

Upon completing this first drawing, I worry that the weight of the neck might damage the stepper motors or the threaded rods due to high radial and axial loading.  (The neck and head frame weigh 3.9lbs bare and the head will eventually add 5-7 servos and a speaker, and it will eventually be covered in "fur".  The torque of just the frame is currently 3.3ft/lbs.  The center of the head and the weight of its future mechanisms is about 18" from the base of the neck, which adds an additional 3ft/lbs torque.  That means a static axial load of 8ish pounds on the top threaded rod and 4ish pounds on the bottom two.  Dynamic forces as the horse moves will be much higher.)  I am trying to figure out how to better support the weight of the front without hurting the steppers or the threaded rod.  I'd like the joint to be strong enough to prevent it from breaking the first time a child tries to pull down on the head.

Some ideas I've thought of (but am not fully sure of their value):
• I can move the mounts on the torso further towards the top and bottom (while keeping the mounts on the neck in the same position) to trade off some radial load for more axial load.
• I can add a 608 bearing to the lead screws (mocked up on the bottom two steppers) to increase support.
• I could/ "hang" the neck with some tension springs to off-set the weight of the head/neck.  I don't have much experience making custom springs, but I realize they would need to have a certain tension so that they don't make the axial forces worse.

1)  Does anyone know the rough limitations of Nema 17 steppers as far as axial/radial load?
2)  How much radial would 608 bearing to be able to handle?  (It obviously isn't a thrust bearing, but is designed for some axial forces.)  For cost reasons, I'd rather use 1 (or more) cheap 608 bearings than dedicated thrust bearings, if possible.
3)  Any suggestions on how best to bear the weight and torque of the neck/head in a mechanism like this?
4)  Any major flaws of this mechanism that I'm not seeing?

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