I have designed a 2 NEO PTO gearbox for a drivetrain with 6" wheels that I want to make in the offseason. The shifter has a spacer and a machined aluminum pulley on it. The pulley would be used to wrap string to extend ‘stilts’ for a HAB climb. Feedback would be greatly appreciated.
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Two things that I see from a quick look:
- When you shift to climb your wheels will still be engaged. Depending on how your climbing system is set up that could be fine or it could be a big problem.
- The pulley is only supported on one side. If you’re lifting a whole robot’s weight through that pulley, you really want it to be well supported, otherwise it will significantly bend.
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Definitely an appropriate use of PTO, provided you resolve Ari’s issues.
Not being able to see the grabcad, what is your reduction to the wheels? What are your reduction and spool/pulley diameter for the climb? How heavy is your robot?
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The reduction is 84:9 and the pulley diameter is 1.5. @AriMB The pulley is supported on the other side by this 1/4" plate. There is a bearing in that plate behind the large encoder gear that supports the other side of the shifter.
I see that, but I’m afraid the whole pulley, plate, and spacers will rack when put under the kind of forces you’re talking about. Ideally you’d like to see that bearing plate supported more significantly, and in a way that keeps it from parallelogramming.
We’ve actually changed how we do PTO’s now. Rather than integrating them into the gearbox, we install them in a different location. We used this PTO:
http://www.wcproducts.net/217-3636
We use that PTO as our real wheel axel. Our drive transmissions are located in themiddle of the robot or front, and this at the rear. Not having it be integral to the transmission gives us more flexibility: we actually dropped the PTO and climb assembly then put it back on as the design evolved. It’s nice because you can just “slide” it in place of one of your drive axles and you have a PTO with a minimum of work.
Back to the topic at hand - I really like the design and how lightweight it is. It looks a lot like the transmissions we are prototyping for a lightweight (single reduction) drivetrain.
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I mean, wouldn’t you have the same concern with the mounting of the whole gearbox then? Both the gearbox and the secondary plate are mounted by 4 8-32 screws with either aluminum or onyx spaces around them.
The front plate of the gearbox is (assumedly) mounted solidly to the chassis. The rear plate, on the other hand, is cantilevered a few inches off the chassis and only supported by 4x 8-32 screws. With the forces on that pulley so high, I’d be afraid of the cantilever bending.
If I ran this in season, I’d probably include standoffs across the width of the drive base (from gearbox to gearbox) to support that climb cantilever from the other side.
Would it be better if I changed the support for the secondary plate to three 1/4-20 bolts instead of four 8-32 screws? I am using 4 standoffs with 8-32 screws to connect the gearbox to the drive rail right now, should I change that?
Speaking strategic design, This is the most correct solution to PTO, which I’d argue is usually unnecessary in modern FRC anyway (under unlimited motors rules)
We’ve done single gearbox drive+PTO and got burned badly in comp when any of the many features started failing and we couldn’t drive anymore, plus even when it was working we couldn’t align the climb easily because whenever we ran the winch we were stuck driving straight out of position…
But OP, this is still a sweet design exercise. I like the peanut mounting, your fasteners are going to be fine since you’ve got all that contact area reacting the moment loads.
We did this on our climb in 2017 and learned that you need to be conscious of your loads when you do this. Often you won’t see drive component failures because they are traction limited and before you exceed the amount of torque a belt, chain, bearing, axle, frame, etc can handle you just slip the wheels. If you are putting your drive train in a situation where it is no longer traction limited, but instead latched onto something with a hard stop, you generally have enough torque to destroy some drive line components. You’ll also want specific feedback here that is not just drive train encoders in order to ensure the system doesn’t over travel.
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Good point. We actually did overtravel once or twice, and it was the climber components that failed and not the drive train. We beat it up pretty badly on our practice bot and never saw any major failures.
I was planning on having a separate encoder on the climb systems to account for any error while shifting.
It looks like the wheel is going to be on the other side of the chassis tube. The weight of the robot on that wheel will apply a twisting force on the standoffs that attach the big plate to the chassis tubes. Those standoffs are placed very close to each other. The section of chassis tube supporting the standoffs will have a hard time resisting that twisting force and may buckle.
There isn’t much material in the small plate and it may deform under force too.
The standoffs are actually one standoff that will be 3d printed. I can enlarge their size if necessary.
They really need to be spread further apart in the vertical dimension
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