chip3 actuator updates: let's choose the right filament to print with! And tolerance! [updates]
So in the last post, we learned a lot about the characteristics of what our motor might look like/perform like and we started some CAD: https://www.adim.io/post/scout3-actuator-updates-looking-at-the-cheetah-paper-and-imf-learning
Now today, what I really want to do today is figure out what material we will print with based on our current manufacturing capabilities. So doing a quick google on what materials for 3D printing are good for what things, we came across this: https://rigid.ink/pages/filament-comparison-guide
And if we look at the bottom of this list we find a really interesting filament called Carbon Fiber Reinforced Nylon...
The reason we're googling that one is since we're printing gears and a housing we're really looking for something that is STRONG. Strength is the primary concern but since we want to have a zero-backlash gearbox (minimum backlash) then we also want the material to not shrink like ABS. We also want to chose a material by a reputable manufacturer that actually tells us how the filament will shrink and one we can contact if we really need support.
So we started to look into Carbon Fiber Reinforced Nylon and took a look at these links:
We would need a 0.6mm hardened steel nozzle?
https://taulman3d6.com/910-features.htmly good, but with the required hardened nozzle and the increase to 0.6mm nozzles. All this and the fact that when we printed with carbon PLA, we noticed that this material had worse layer bonding than regular PLA, when we say this review on amazon we got to wondering:
Hmm Alloy 910 sounds really industrial and cool... what's that? After a quick google this is what we found:
And here's what we found on the website of tulman3d which is actually a really large name in 3D printer filament!
So let's summarize:
Alloy 910 has a shrinkage of 0.0031 in/in - honestly don't know if this is good or bad we'll go figure that out after this
It has a tensile strength of 8,100PSI
Can print with any nozzle size
According to the website it can be used to print "HIGH END GEAR AND CAMS"
Interesting Twitter plug, (guy's making a gearbox for a BLDC motor hmmmmm):
And the spec sheet straight from the manufacturer:
If I'm being completely honest, this thing seems like it's been designed really well to meet the needs that we are looking for. And the best part for our project is it's not the most expensive thing I've seen:
Okay... 32 bucks for a pound is a lot but HEY it's supposed to be a really really strong filament. That's 64 bucks for a 2 lb spool. Hmm we'll have to do some calculations before doing any buying. If I had to guess, this will probably come out to being 2 lbs of filament but we shall see.
So I think we can go ahead and settle on Alloy 910 because it HAS been used to print gears for things like robot gearboxes before. That's the real reason we're going with is. So let's think about total shrinkage for a second - that's 3 thou per inch roughly that's a 0.31% shrinkage rate. We might need to resize the parts a little so they fit the dimensions (so increase be 100.31% in size). Make sure we account for this when printing.
Size Increase Required by Allow 910: 100.13% for Accurate Print
So that seems like a good enough plan.
So now that we have material sorted there are a bunch of things we need to finish CAD-ing. Let's go back to this post: https://www.adim.io/post/scout3-actuator-updates-looking-at-the-cheetah-paper-and-imf-learning to find the teeth numbers. But the thing is, now we're designing the 48t gear for the planets, and to do this we really need to know something about tolerances. Because - we need to put in the right bearing holes!
So this is what we have so far. Now the question is how much tolerance do we want to give to the bearing/etc. The 60355k44 bearing says for Housing ID: 3/4"! Here's a good guide on tolerance: https://baartgroup.com/how-to-determine-bearing-shaft-and-housing-fit/
So here's what I'm getting from this:
We need to choose what kind of fit we want on both the shaft and the housing side. The shaft side likely later, the housing side now-ish. Because that's how we can determine what level of tolerance we want to give to our parts. This is different than the normal way we determine these things because we're pressing into a 3D printed part. And we will enlarge the part by 100.31% to account for shrinkage but how much extra do we want to give it? If at all?
Let's look up what these tolerances even mean...
So first let's try to see what our bearing's thickness is: and that's around 0.28125 in so we put that in in the top of this calculator. And now we want to figure out what kind of fits we want. We really want a slip-fit on the SHAFT and a very loose press on the hole. Like really really loose. So maybe we're looking at a J6 or an H8 - we don't want a clearance fit, but too close to a press fit we won't be able to press it at all.
So maybe we want like an H7/k6 tolerance on the housing and G6/h7 tolerance on shaft based sliding.
This also might be the completely wrong way to do this... let's google a little more about what these hole-based press and etc mean.
So I think we're going to stay away from the interference fit because those we might not be able to get into the part. However, I think the similar fit in the transition fits seems good, assembled and disassembled with a rubber mallet or the Location fit because those two are easy to assemble but very CLOSE because what we're really trying to do is avoid slop. Right?
And from my understanding we need to know the fit of the SHAFT before we can calculate the fit of the housing because they work TOGETHER to determine how a bearing fits. It's not independent.
So actually I think Wd on McMaster is the DEPTH/THICKNESS of the bearing so we put the wrong number into the calculator but nvm we can change that.
So I don't know enough about bearings to know where to go with this at the moment - so that means it's back to research (again).
So the last thing I want to add to this log is should we really use a herringbone gear? OR can we use a straight helical gear. Let's actually calculate the tangential force a helical gear gives, determine is it always in one direction or the other no matter the direction of rotation (I don't know) and then can we design for that force instead of using a herringbone gear? Herringbone gears would be hard to install. But let's see!
First let's actually look at a angular/helical gear in a planetary gearbox and what are the forces on the gear itself from the RING and from the SUN.
And now let's look at a gear itself...
I think this image most clearly illustrates what we're looking at here and that's on the TOP of the helical gear, the teeth slant in one direction and on the bottom they slant the opposite way.
So that's the calculation we end up with the NET tangential force that comes from the angle of the gears themselves is actually zero so the gear should theoretically stay in place. The problem here is slightly different and that's - there's now a MOMENT on this whole gear. And let's do some math to see what that moment actually is. At max torque, the teeth of any gear in the system that has the highest tangential force are the planetary gears in the second stage. There are three gears... so if the max output torque is 90Nm, and the distance to the center of those gears is 45mm, each gear will be experiencing a force at the center of the gear of around 666N. That means the teeth will be seeing a force of around 333N. Given we're designing for a helix angle of around 30deg. Here's what the tangential forces would look like (it's almost like a banked curve problem where you need to find the centripetal force).
STRIKE--So each of those gears has around a 17Nm moment on them.-- The question is can our bearings take that. Note that the bearing doesn't have to take this moment alone because there's other supporting structures. But let's assume worst case and the bearing is taking ALL of that moment: 60355k44. Since I can't find it on the website - I guess we will email them!
I'm a little shook that they really mean to get back to us within 30 minutes. So I think it's time to wait 30 minutes! And then we might have an answer wether 17Nm is OK for a bearing like this.
ACTUALLY REALLY AFTER A REALLY QUICK CALCULATIONS REVIEW, THE IS ACTUALLY HALF OF THE 17 WE CALCULATED, WE SELECTED THE WRONG NUMBER OF TEETH AGAIN. IT'S 30 NOT 60. SO THAT MEANS WE'RE LOOKING AT AROUND 8.5Nm.
Let's make a gear diagram so we can stop being confused on what gear is what number of teeth or what size - it's a little annoying now to have to go look for it and then mess up math because of it.
So this diagram might be easier than looking through tables to try to figure out which gear size is what.
So McMaster already got back to us and this is what they said... they said that this bearing is not rated for moment load. So I guess that means the manufacturer never rated it for moment load? So I think we have a few options here:
Stick with this bearing a design the shell and the rest of the parts to take that moment load instead of the bearing itself.
Do more research on this bearing specifically and see if it can handle 8Nm of load at max.
Find a new bearing.
So I don't know enough about bearings to know where to go with this at the moment - so that means it's back to research.
Also, as a side note. Look at what showed up today! It's our Talon FX and Falcon 500 Motor!
Packaging is really disappointing but everything else is there. It's a nice piece of hardware feels really solid. I really like that spline shaft. We're going to start running some tests with this when the CANABLE shows up.
LINK TO THE COST SPREADSHEET: https://docs.google.com/spreadsheets/d/1TeCY5FDDwC3g7JXBc2GAc7i7bRKCexIneKGe3p7A3Rc/edit#gid=0
Questions we Have:
Will a ball bearing take any form of a moment load or is it really not rated for it at all?
How does bearing tolerances work in general? How do we set the size of the hole the bearing will fit into given a shaft that is going to go through the bearing?
Semi-Final Bearing Requirements: Static Load Capacity >= 149.723 lbf Max Speed Rating: 3190 RPM Moment Load Rating: 9 Nm Axial Load Rating: N/A
Now I want to clarify no individual bearing in the system will be dealing with all of these at once. But this is the max reading at all parts of the system for the bearings because we don't want to use 5 different types.