I’m making a gearbox for a climber with 3 neos. The neos will have 9t press fit pinions. The reaction will be two stage with 84:9 and then 84:18. I’m using vex 20dp gears. If I need 150 fr lbs on the output, will the gears be able to take it? From the math I’ve been able to do it seems reasonable but wanted to get some other opinions for more experienced people
Before going too far in here, why are you using 3 neos for this climber? Im assuming this is for climbing this year?
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It’s a clone of 118s
idk how many motors they use, but just feel like its probably preferable to not run the motors close to their smoke point if possible so is there a reason 3 is a bad idea and not just overkill?
Are you planning on doing the entire powertrain for your 118 climb with gears? If so, I’d heavily recommend you reconsider. 118 themselves are using chain for the final reduction. Chain is great for that purpose because it can take shock loads much better than gears, shocks loads like the arm will see when weight is suddenly transfered from one rung to the next.
To answer your original question, the smaller gear you use the more load is transfered to less material. If I were you I’d consider using 10, 11, or 12 tooth pinions which will be stronger.
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No, its not necessarily a bad idea, but you need to do the math to find out.
To calculate the bending stress on a tooth please read this:
https://www.engineersedge.com/gears/lewis-factor.htm
The material for Vex gears is 7075-T6 Aluminum. The material properties are readily available on the web. Here is one example from a vendor:
https://asm.matweb.com/search/SpecificMaterial.asp?bassnum=ma7075t6
I can’t seem to find a reference to the type of steel, but it might be this stuff:
or this stuff:
I recommend not exceeding 50% of the yield strength of the gear.
Why don’t you run the numbers and post your results here?
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thats a good insight! you happen to know the reduction/motor configuration they used?
No idea, I am not affiliated with 118 at all. Just happen to be a mentor for a team that learned the hard way how to not design high torque arms during the 2019 season.
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As an academic exercise, have fun running the numbers. In reality, there are only disadvantages to aluminum gears when steel ones are available, cheap, and weigh only a tiny bit more.
Ask me how I know… This (drive system) gear lasted 8 matches. New team rule… all steel all the time. Steel gear: $12. Cost of one lost match: ~$400, excluding comp performance and intangibles like team morale.
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wow thats impressive. How many motors were on that gearbox? what reduction?
This happens often in any gearbox that has shock loads or more than a single high-power motor. I recommend doing the Lewis calcs, but any gear under 26T should really be steel for safety. 2016 drivetrains are a good example of how even a couple of CIMs on modest reductions can do a number on small gears in the 14-20T range.
Derate your Lewis calculations based on shock load, and add a factor of safety to compensate for the aggressive undercut Vex puts on their gears. Climbers won’t see a ton of shock load, but it’s best to plan for the worst. If you can get away with it, use 1 or 2 motors until your feel like your design sense has evolved enough to know what’s safe.
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isnt this going to put more load onto each of the very low tooth count pinions on the motors? i dont know much about Lewis factor yet, but from what i understand, 9t gears would be pretty horrendous in this regard. Is it a bad idea to just trust software to be able to limit the current on the spark maxes so the gears each see less torque while to total output torque is the same?
E: i get what youre saying though, keeping things lower power is probably a smarter idea.
Ive got another question - when choosing a reduction for this type of thing, is the proper thing to do to gear the motor down so that the mechanism will run properly (ie with enough torque) while the motor is at the peak of its power curve?
also, the pinions im planning to use are 9t press fit pinions - should i be concered about these enough to replace them or will they manage fine given i design everything else right?
also, when yall say chains are better for absorbing shock loads than gears is that because chains are able to stretch a bit to sorta extebd the time of the impulse, or is it simply that they’re run less close to failure (in frc) than gears so they can take bigger shock loads before failure?
Chains spread load over many thick steel pieces, where a gear has only single-line contact, which is much weaker.
I hate press fit pinions because they are hard to remove and install. Stick to 10T keyed if you can.
Aim for a current draw of under 20 amps using something like JVN Design Calculator. This gives you a big factor of safety on overheating and tripping the 40A breakers. Look at motors.vex.com for information on stalling motors.
Yes. However, the profile shift on the <12T gears makes them stronger than, say, a 16T Vex gear. Plus the smaller pinions are all steel as well and have the least force on them in your gear train.
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Just happen to be a mentor for a team that learned the hard way how to not design high torque arms during the 2019 season.
I figured I might as well elaborate on this. Some lessons we learned:
- 1/2 hex is a terrible choice for high torque axels. It will will twist or even shear entirely. Additionally the fit between hex shaft and hex bore can be sloppy which can lead to positional issues over the length of the arm.
- Dead axel pivots work better for high torque situations than live axel pivots since the force is translated directly to the member you want to move rather than transitively through the axle
- Larger diameter round tubing makes for excellent high torque pivots, see 2471’s 2018 CAD for a good example
- Chain on the final reduction of the arm will tolerate shock loads better than gears since the load is spread across ~50% of the sprocket teeth as opposed to just a few gear teeth meshing
- You can double up gears to help spread load
- Steel gears are better than aluminum gears, the weight penalty is worth it
Here’s our 2022 climber pivots where we employed some of these lessons.
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5/7, would recommend this climb
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how large we talkin here? 2in? 1in? 6in?
I elaborated about that in the linked post.
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