DiffBot - Differential Screw-Driven XY Gantry

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JoergS5
This a very interesing idea.

If you suffer from backlash in the gears, you can try a friction drive (with 90 degree pivoted), or the idea of [www.youtube.com]

Looking forward your further development!

Thanks! I'm really trying to stay away from flexible materials entirely, that's one of the goals here (as well as simplicity and low part count/high temp materials), so I'm not looking at 90deg belt drives and similar.

Friction drives are possible, as well as some more complex gear tooth profiles, but I don't actually think the gear backlash part is much of a real concern. Here's my reasoning:

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Any tooth-tooth backlash doesn't translate directly to linear motion in X or Y, because only the nuts move the axis. That means any backlash in the gears is divided into the screw/nut resolution, which is backlash/360 * lead.

For example, 1º of backlash would be: 1/360 rev * 8mm/rev = 0.022mm linear backlash, or 22 micron The worst backlash possible on my 20T (tooth) gears is 360/20 = 18º, and that's like, missing a whole tooth. So practically speaking, let's say it's about half, or 10º for simplicity. That would be 0.2mm linear (200micron), which is certainly not great, but I just want to illustrate that even with extremely poor meshing, the gear -> screw -> nut relationship makes it far better than it seems at a glance.

In comparison, 20T belt driven XY is often quoted at the calculated 25micron resolution that gets you, but as we all know when it comes to belt systems, there's often variations in tension, alignment, concentricity of idlers/pulleys/shafts, and a variety of other factors that reduce the resolution. So it's not like my design is wildly worse in comparison, even in a worst-case tooth mesh scenario.

Of course, there's considerations and concerns in mine as well, such as linear guide alignment, stiffness, straightness, deflection tolerance, screw straightness, etc. But those are present in a belted design as well, in terms of the mechanical gantry.

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Now all *that* said, my videos had some gear mesh issues and also some linear rail/frame squareness issues. I saw and have improved them since then; I replaced the gear sets and adjusted spacing and can see/feel no discernible tooth-tooth backlash, and I replaced one of the linear rails as the old one was relatively rough and loose, and I printed an X carriage to further connect and stiffen the X screw/X rail portion. I'll have new video soon, but the behavior seems to have improved a lot.

Sorry for the lengthy response, thanks for your thoughts!

Edited 1 time(s). Last edit at 09/29/2019 02:08PM by Apsu.

I offered the ideas only because in your second video [youtu.be] at second 18, when you change from X motion to Y motion, there is a jerk in the gears which may produce X direction errors. I thought this is due to backlash, but this may have another reason.

I agree that using flexible material will create other problems.

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JoergS5
I offered the ideas only because in your second video [youtu.be] at second 18, when you change from X motion to Y motion, there is a jerk in the gears which may produce X direction errors. I thought this is due to backlash, but this may have another reason.

I agree that using flexible material will create other problems.

Yeah, I saw it as well. Some of it was definitely backlash between the gear teeth. I have since changed them out for a tighter spacing, and that effect is gone. However, there is some twisting in the rail clamps underneath the gearboxes that I'm investigating. There's no longer a jerk as backlash is taken up, but there are some small inconsistencies on direction changes and differential transfer because the X rail is rotating a little in its clamps. I have some ideas to minimize or fix it, so we'll see what I can come up with.

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cozmicray
Perhaps a sprial bevel gear

[www.thingiverse.com]

they are very smooth and used in Hi torque applications (differentials)

?What is the best (smoothest) right angle gear configuration?

Have to also consider the locking of the gear?

plus 1

i no longer remember details. i know there is a better gear setup out there.

at moment both Apsu, and what you link. the gears will either want to jump "up" or push "up. one spun one way, and then reversed they will want to push "down" right were the teeth mesh together. this causing wear on teeth and putting vibration down the screws rods.

*bah* life of me i do not remember names of the joints.
1. differential on a car/truck for the drive shaft that feeds power to both rear tires. i know there is some extra "gear and then gear sets" to allow for rear wheels to spin more freely of each other. but there was some different types of differential setups that had a more better setup for this perhaps?
2. or CV joints for front drive wheels. but most likely not.
3. 2 additional gears toss in (current set of 2 gears then other side of each gear parallel them) but that just combines additional vibrations, backlashing, jamming.
4. there are few other "joints" that do not use gears. but with the main lead screw going all the way through, and the other rod going 90 degrees. those joints can not be used. without cutting the main lead screw in half.

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JoergS5
This a very interesting idea.

If you suffer from backlash in the gears, you can try a friction drive (with 90 degree pivoted), or the idea of [www.youtube.com]

Looking forward your further development!

Apsu you could do what JoergS5 shows. it has merit. with ability to pull the 2 top pulleys "upwards" for a belt tension device.

what cozimycray, shows there are some good amount of math for "teeth profile" that can get really nasty. some teeth profiles better for certain directions, while others for reverse direction. while others forward and backwards. at moment your gears will have major wear at the "sharp / 90 degree" like corners. what cozimycray shows is more of the tooth will touch and mesh together resulting in longer term use with less wear.

there is a wave pattern for the teeth vs 90 degrees or 45 degrees offset. but i no longer remember the details and might be placing the wave pattern teeth profile for some other gear setup.

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since you do not have "weight". you have noted you have added some springs inside to help reduce backlash. by keeping a force on things.

i really could not see detail of springs you videos and/or animation videos.

if you went bearing -> washer/s -> spring -> washer/s -> gear..... then screwed the main shaft all the way through. the single spring may not do much. thing of a "sew saw for kids" the were teeth mesh, the spring diameter, and then the lead screw. each of them are acting as a point on a simple lever / fulcrum. and you have a very big kid on very long length of lever. while the little spring still a knee high baby. trying to lift the the big kid on the short length of lever not really going anywhere. "yes yes" things are spinning and turning. but you are still obtaining a lever / fulcrum points, and example was out of proportion. but hopefully idea gets across.

i don't even remember name. i want to say an old style "clutch pack". instead of single spring in center. there is smaller springs around the shaft. the springs set in a "disc" and were the springs are placed the disc is partially drilled out to hold the spring in place. same thing is done for another disc that holds other side of springs. then there some screws between the 2 disc's that put some tension on the springs and keep the 2 discs from coming part. both disc's with springs between them then looks like a huge ugly washer. with bearings as well. Goal is more springs = not needing as heavy duty of single spring. but each spring adds up over all. and it moves the fulcrum point towards the "big kid" above and gives the knee high baby some room to move on a simple lever doing.

what others are saying for different tooth profiles, or belt pulley's type setup. is trying to reduce the "back lash". the spring / "so called clutch pack" is more of a band-aid, vs going more at the direct issue of causing the back lash in the gears.

remember 1 major ugly factor for your system. moving x,y at same time. means one motor spinning faster than other motor and making high speed sudden reverse of current direction. those with belts will see it in belt vibration along with belt expanding contracting. you will see it on other hand as teeth wear and vibrations going through the lead screws. and at moment to me, your gear profiles are highest point of "failure" and/or cause of a worst 3D print that you could improve and have good effect on your system vs other things.

Edited 1 time(s). Last edit at 11/19/2020 06:02PM by boggen79.

@boggen79

This thread has been dead for more than a year !
Let bad ideas, unsubstantiated claims rest in peace for fear that like vampires they pop up again

Edited 1 time(s). Last edit at 11/20/2020 04:35AM by MKSA.

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"A comical prototype doesn't mean a dumb idea is possible" (Thunderf00t)

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MKSA
@boggen79

This thread has been dead for more than a year !
Let bad ideas, unsubstantiated claims rest in peace for fear that like vampires they pop up again

You should consider talking to someone about your feelings. Carrying a weird unsubstantiated grudge for over a year isn't healthy. Best of luck in your healing!