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I have a rod mounted inside of a 2x4 that is allowed to freely spin inside of its hole. Attached to that rod's head is a range finder. I want the rod to reliably point down at the ground at all times to accurately read the 2x4's distance from the ground. The rod cannot be fixed in place. The 2x4 will be hoisted into the air and lowered by hand to about 8 ft max. My first idea was to weight the range finder so that it spins around and points down, however playing around with it, it's not as consistent as I would like. The 2x4 allows for free spin but isn't frictionless and I have to tap it a bit by hand to get it to fully position itself downward. I thought about adding a vibrator or something to the rod. Are there any other suggestions I may not be thinking of?

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You mean it is NOT frictionless? You should try to get it frictionless and then use a weight. Probably one on each side of the wood to reduce friction from tilting inside the hole.

One way would be to add some ball bearing between the wood and the rod, you could ask at the local bike store or get some hightech thingy. Also depends how long you are going to use it, you will need to oil them or get a version that works without oil.

If you want to go with just the wood and rod, maybe you can polish both and use some lubricant, but that would need constant maintenance.

Do you need the wood at all? you could just use a thread and a wellbalanced weight at the end. Or attach a thread to the end of the rod?

Feel free to edit confusing wording, english is not my native language.

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  • $\begingroup$ haha, I thought this is the diy page, still the ideas apply for real engineering. if you have to waste a lot of budget, try something with gravity sensors and actuators and some AI to optimize the pointing downwards factor. :p $\endgroup$
    – Martenson
    Mar 9 '19 at 0:58
  • $\begingroup$ wouldn't it be easier to put the range finder on the ground and measure the beam from there? $\endgroup$
    – Martenson
    Mar 9 '19 at 0:59
  • $\begingroup$ i saw a kinect camera used for a similar task. it gives you a 3d height measurement and you can find the beam by object recognition. $\endgroup$
    – Martenson
    Mar 9 '19 at 1:00
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    $\begingroup$ Look at model-making websites and use some proper bearings. A couple of small (e.g. 10mm outer diameter) ball bearings intended for servo motor shafts etc will only cost about $10 and do the job properly. A second-best solution would be nylon bushes which would have less friction and better control of the clearance fit with the rod, a hole in a piece of wood. $\endgroup$
    – alephzero
    Mar 9 '19 at 12:52
  • $\begingroup$ maybe the bearing at the end of the rod and a rope around it would be best/easiest? $\endgroup$
    – Martenson
    Mar 10 '19 at 0:38
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A great solution is to increase the moment of inertia of the rod by attaching another rod pointing downward and perpendicular to the original. At the end of that rod attach a mass with considerable weight so it will require a lot of torque to spin the original rod off your desired position, and every time it spins by maybe human contact or strong wind, it quickly find it way back by oscillating around the position.

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  • $\begingroup$ Adding more weight won't make any difference. The friction angle (the tangent of the coefficient of friction) between the rod and the wood won't change just because you make all the forces bigger. If it takes "a lot of torque" to spin the rod one way, it will take the same amount to spin it back again. $\endgroup$
    – alephzero
    Mar 9 '19 at 12:54
  • $\begingroup$ The OP is trying to keep it stable pointing downward. And the mass is added some distance away from the rod to increase the moment of inertia of the system not the friction. $\endgroup$
    – TechDroid
    Mar 9 '19 at 12:59

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