# How to negate rotation of a rotating linear guide rail on a carriage?

We would like a carriage mounted element (CME) on a rotating linear guide rail (LGR) that negates the LGR rotation and thusly remains rotationally stationary relative to the LGR's pivot affixation surface.

Here is a diagram to illustrate what is desired for just three example angles and three example distances from the pivot superimposed (but all angles and legal distances should work - even right up to the pivot itself, though that is less important). P denotes the pivot and the CME has a notch on it to indicate constant orientation relative to the surface the pivot is affixed to:

The axis of rotation is the same for both the LGR and CME in question. This axis is the axis you'd intuitively imagine when imagining a LGR rotating, which is to say the axis that pierces through the top of the carriage and into the LGR, although that is only describing the direction of the axis, the origin/pivot doesn't follow the carriage; it is in a constant position near the end of the LGR.

I've already attempted various solutions, but most fail because of their inability to function with a dynamic distance from the pivot due to the carriage's linear freedom of movement.

One solution that could account for the carriage's dynamic distance from the pivot was to use sensors to measure the rotation of the LGR and a servomotor to rotate the CME to the desired angle. I will begrudgingly use this solution if nothing better is found, but I'd much prefer something with good old gears/pulleys/chains/etc. to ensure there's no "lag" and to keep costs down, etc.

Sorry for being cryptic, I am working on a prototype for something and my partner in crime is paranoid that their idea will be stolen. If you have any queries relevant to finding an answer I'll be happy to answer them.

edit: There will be two carriages on the LGR, each with their own CME, differing to the diagram. Both needing to negate the rotation of the LGR as already described. This is one of the things my partner would have rather withheld if you are wondering why I didn't say it sooner.

Not knowing the distances or latency, you could use a 2nd LGR in parallel with your green rail with another CME. Connect the two with an extension spring and hooks. Or mount the whole thing vertically (pivot axis is horizontal) and let gravity maintain orientation of your CME.

• Just to clarify, there's no green LGR, as I say above the diagram there are 3 superimposed example orientations of the LGR, the green one is just one example, the blue and red ones are the same LGR but at different orientations. The distance as I said needs to be dynamic. And I also said in the question that I'd like effectively no latency, using gravity would be too slow by far, and wouldn't work if someone were to grab the CME and rotate it. Could you explain your parallel LGR, extension spring, and hooks solution in more detail, I don't understand it. Dec 21, 2020 at 20:42
• Updated original answer with a crude MSPaint drawing. You'd have another LGR (black) with a CME (just one, color coded to match the different positions in your example) with locked rotation. An extension spring connects that to the point on your existing CME that you want oriented nearest to the 2nd LGR. You'd orient the 2nd LGR perpendicular (x axis for reference) to the axis you want these oriented towards (y axis for reference).
– jko
Dec 21, 2020 at 20:51
• Thanks for the diagram, very understandable, sadly because the springs will pull the CME it will heavily interfere with the CME's position itself, both along the LGR and the LGR's rotation also. In other words it'd heavily tend to the top and centre to minimise the tension in the spring. However if I had 4 of these I could even it out a little, but it'd tend towards the centre. Certainly a good attempt and only failed because of my cryptic requirements! But it is a new angle I will explore, and I think it may lead to an answer that fits all our needs, even the ones I forgot to say! Dec 21, 2020 at 20:58
• If you think there'd be too much pull force with a spring, you could just have a solid bar (instead of a spring) that travels in the "y" direction through a slot in the black CME. There would be no forces applied in that direction.
– jko
Dec 21, 2020 at 21:02
• you're breaking my balls dude... intuitively I feel like some sort of differential mechanism would provide an adequate solution. Check out "south-pointing chariots", the Chinese managed to do this thousands of years ago.
– jko
Dec 22, 2020 at 12:40