Gear performance under non-integer distance

As far as I know, VEX gears have an involute profile, which is supposed to mesh decently even when the distance between the gears isn’t exact.

How’s your experience with having non-exact distance, non-multiple of 0.5", between gears? How do they perform in terms of friction, backlash, or part longevity?

dont do it. just dont.

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Sprockets and chain kinda live for this. I’d use them. I’ve never tried this, but it seems to be the type of thing you literally have to seek out to do. I wouldn’t try it because if you do try it then it probably is a more complicated solution than chain, regardless of how well it actually works.

And the whole fact that the teeth are like 3/32 of an inch tall so if you are able to mesh them on a non integer distance it can only be like 1/32 off at most, offering no benefit. If it is any farther then it likely will just skip once there is any load on the gears.

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I imagine chain and sprockets is better option here

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I am implementing this on my robot, and although they can work really great, it’s a nightmare to optimize, and most people never do. My actuated intakes work decently with this, but its not perfect, and i run into friction issues, I worked a ton on them.

But to optimize this for maximum efficiency, which you prolly won’t try, it’s requires so much perfect bracing that in most high-load or high-speed or really anything that needs an AMA remotely close to IMA…you know what, don’t try it.

Yeah I am actually curious if this question was just because he was wondering randomly or if he actually has an implementation for it. Cause I can’t think of some bizarre vex application complicated/obscure/specialized enough to make this beneficial.

It depends on what mechanism you are implementing this on, but the teeth on Vex gears are quite short. If you don’t keep to 0.5-inch intervals you with have issues with gears skipping or worse not moving at all. Chain and sprockets might be a better option in cases where you don’t have proper spacing for gears. If you were using washers as a way to make your spacing you could probably get away with a small difference but you will have problems if the system is experiencing high torque.

In my experience, you never want the gears to be too close. Too far is probably okay. I ran into this issue alot when meshing gears spinning on screw joints. The slight shifting of the position of the screw within the hole can cause massive frictional loses. Too close causes the gears to bind and bow away from eachother.

Edit: too far = skipping issues and teeth breaking under high loads. The gears will still work and probably be smoother than proper spacing but with more slop. One thing to note is that skipping is almost purely due to axles bending. So if you support such a setup properly, skipping should not be an issue.

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You won’t run into this issue at all if you build a screw joint correctly; that is, with bearings on the joint. You are just talking about putting a screw in a hole freely, which is likely not how most teams do it and is certainly not the best way. My teammates and I did it this way on a six bar we built once, and although this six bar only stayed together for a matter of weeks it had worn out the holes in the c-channel by then, basically making it useless.

Here is (what I believe to be) the correct way:
PilonsScrewJoint
This is the Pilons’ ITZ robot.

I suppose depending on the application it might be beneficial to add a second bearing. Also joint styles slightly vary, just make sure to pin down the all the necessary factors in whatever exact design you might choose.

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Here is an image from 929u in the zone which used screw joints with 1 bearing on the lift. I believe that a screw joint without a second bearing is better since I like to have the screw mounted securely and not free spinning which is only dependent on the keps nut. Some variants have the keps or locknuts slightly unscrewed to allow free spinning of the screw but I find this does not decrease friction. The reasoning is that the moving components will slip past one another at the point of least resistance. The free spinning screw will cause 2 moving interfaces instead of 1 but both are the same exact level of friction(screw to bearing friction) so it is pointless. Sliding on one interfaces is not preffered over the other. The reason why 1 bearing only is better because the 2 bearing variant introduces extra slop from the slight untightening and adds extra weight, and decreases the effective screw length.
Screenshot_20201005-092015_Photos

edit: If you are using a second bearing purely for centering the screw in the hole, then I think that is a okay use but not needed. It does ensure nothing gets moved during collisions and use but a tightened keps nut should not come loose and could easily be retightened.

edit2: I will concede that there truly might be a reason why screw joints with 2 bearings is better but I have yet to see it. On the 929U in the zone robot the joint between the RD4B and the lift towers is a 2 bearing joint but with no keps/locknut at all??? They also put these metal one bar flanges on the bearings which completely eludes me.

edit 3: I found it. Here’s why. Not really relevant to discussion but it does address edit2: https://www.youtube.com/watch?v=zHS9VIAxNNM

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Okay I think I figure it out, putting a 2nd bearing is just to ensure the screw stays centered in case the keps nut comes loose. I do know that screw joints come loose over time an need to be retightened throughout competiiton. The reason I will still not endorse this method though is because it is trivially easy to retighten the keps nut into the perfect position by feeling for the sweet spot. By shifting it around when it’s loose you will naturally settle into the point of lowest friction.

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Yes the second bearing may or may not be beneficial depending on what you are using the joint for. Regardless, how was there a shift in the screw which was damaging to gear mesh if you build all your screw joints correctly? I have never had a screw joint shift around unless it wasn’t built right.

Yep you are right it is totally dependent on how you want to build, how much space is available, and if you want to have to retighten or not.

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The screws weren’t shifting after I tightened them. It just that you have to pay attnetion to the positioning before you tighthen. for example, I put in one screw joint that ended up on the very edge of the hole which caused the meshing gear to always be too close since the hole in the aluminum for the 2nd screw joint doesn’t allow it to be far enough. Then I’d have to untighten the first screw joint setup to accomadate the 2nd one.

edit: I know that logically this doesn’t sound correct since offsetting by0.5 inch increments should always yield a valid situation no matter the offset. It might be that my metal was warped or the ideal gear distance is slightly more than the one dictated by 0.5 inch increments.

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Ok that makes perfect sense. If you had at least 1 bearing how did it allow this since bearings are automatically centered?

Back to the original thread, the 2 free spinning gears on screw joints set up did not use any bearings at all. It just used the free spinning inserts to center the gear on the screw joint.

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That explains it, I bet it would be spaced on an integer distance if a bearing was added to keep it in the center.

I was thinking more on the lines of not putting gears vertical or horizontal to each other. For instance, putting a gear at (0, 0) and another at (3, 1), which will make the distance between them 10^0.5.

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wait are these 2 gears parallel still? Because if they are still parallel, only the perpendicular distance between the 2 gears matters. In this case the distance will still be 3 assuming your gears are in line with the xyz axis. If you have somehow mounted the gears to be at a titlted angle, then what I said in the first post applies and you should avoid putting the gears too close together at all costs.