All it would take is a heavy duty wave washer 
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That would deform/compress over time, no?
If you can find one that’s 100kN/m it will hold everything pretty secure, I think the best setup you’ll get would be…
Motor mount - wave spring - thrust washer - thrust bearing - thrust washer - spacer - pinion - spacer - thrust washer - thrust bearing - thrust washer - gearbox cover
…on the left side where the pinion is pushed into the gearbox cover, wave spring would be between the thrust washer and gearbox cover on the right side where the pinion is pushed into the motor mount, that way the majority use case has the spring unloaded by the pinion (will still have compression from the gearbox cover, could do like 50lbs of preload and 0.5mm of compression with max of 100lbs of load and 1mm of compression or something)
Motor would have to be keyed but since the system is secured once on the shaft you don’t actually need any thread locker or retaining compound to hold it in place, and the key would be held captive by the washers so a 10mm machine key would be perfect
To keep this from being a nightmare to install the motor shaft would have to be longer than the stack and sit in a slot on the gearbox or you’ll loose the last washer, but that’s not too big of an issue
But a heavy duty wave spring should last a really long time, even if it relaxes a bit it’s a $1 part and you can replace it every 1k miles or however long it is until you clean the grease in the gearbox
Each thrust stack is 4mm and the compressed washer is 1mm, so the total stack is 19mm which is almost the same as the BN AT gearbox at 17mm deep. If I had the time to make parts I’d really like to design a drivetrain but sadly I got too much shit to do already
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That sounds like a ton of tiny parts for the end user (and Kevin) to have to juggle. The more stuff like this that’s added, the less idiot-proof these gearboxes become (i.e. one of the parts out of order in the stack, and you end up with a nonfunctional gearbox). That means more customer service calls for Kevin, taking time away from designing cool new shit, packing orders, or eating dominos.
The way I see it, helical gears are a no-go for a general-purpose DIY gearbox like these. To implement helical gears correctly, you end up with a system which requires very specific hardware to function properly (i.e. having a circlip groove and stepped motor shaft at the exact right position, etc.).
Kevin seems to be marketing his products for as much compatibility as possible, so that his drives can end up on as many DIY builds as possible, using parts the average DIY’er might already own. That’s an excellent strategy to sell a lot of drives, and it means that his stuff needs to be as hardware-agnostic as possible.
Which brings me back to my earlier point:
That’s my 2¢ anyway.
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STRAIGHTCUT FOR LIFE
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There is not straight cut options from BN right?
I would be willing to buy the parts if available for the peace of mind…
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Straight cut are a no go for me. I’d love an approved upgrade path or herringbone gears for both my m1 and m1-at drives. I’d be happy to pay more for herringbone gears.
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IMO the main issue with herringbone is that we would need much more precise axial alignment of the gears than the drives currently really support. The system would still end up reliant on finicky stacks of spacers unless something about the design was changed.
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@Boardnamics the people have spoken 
How so? My Avio MK2 drives used herringbone gears with no issues. @DerelictRobot’s prototype falcon drives too.
On the Avio’s, the motor pinion was free-floating on the motor shaft, no loctite. That’s because the gears are self-aligning. As long as your motor bolts dont come loose (messing up the backlash) or the key shears in the keyway, there’s not really anywhere for the gears to go, no matter the load. Unless there’s something I’m not understanding?
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I think the real difficulty is getting them manufactured for a decent price
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Yes, this is certainly one solution. I think in theory this could/would accentuate other issues in misalignment leading to increased noise and wear but in practice this effect might be negligible. I don’t think i would be comfortable with it but yeah maybe testing reveals I’m incorrect about this.
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What if you had it free floating by design but still used green loctite? You could apply the loctite, then assemble the drive, and spin the wheel a few times to self- align it
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There are 3.95 and 3.0 straight cut options, but you need to ask Kevin for it. Also be aware, you’ll need to dremel a few mm off the motor mount plate for the 3.95 option to be able to set the backlash correctly. If you are comfortable dremeling aluminum as if it was routing wood, the straight cuts are legit.
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I’m rocking the 4.6
Since this board is also the Tow Truck I can’t give up on all that torque…
Dangit.
And I have taken a file or Dremel and hacked up pretty much every BN part I have… So I am good with that part of it…
But I need the high gearing.
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STRAIGHT CUT FOR LIFE
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Herringbone gear profile… that is not cheap to make and you can’t straight hobb it, (trust me i use to make them in multitasking machine before)
Cheapest way is to sell spur gear set separately as a upgrade over helical.
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Lol we’ve come full circle and now straight cuts is the part to upgrade with…
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I heard a rumor that there may be 5.50 straight cut gear ratio coming out.
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RIP Helical era. Back to straight cuts everyone.
Dumb question maybe - everyone talks about how expensive machining herringbone gears are. Can’t we just slap two mirrored helicals together with JB weld and call it a herringbone gear?
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