501 needs your help – P60k 256:1 Banebots Transmission

[fox46]

It's not that teams have done a poor engineering job or are overrating their components. BB clearly states that the gearboxes are not designed to take more than 35 lb-ft of torque. The mechanism on our robot is neutrally biased by a gas shock- you can lift the arm with one finger however, the gearboxes are just not robust. They work for a bit and then parts start wearing, the aluminum galls and things come apart.

Their gearboxes, especially those 128:1 and greater produce a TON of torque. The only think keeping the ring gear from spinning free are four 3/32" roll pins pressed into aluminum, loaded in bending against two small flats! its like trying to tighten a 12 sided aluminum bolt with a crecent wrench. This could have easily been avoided by keying the faces between the end plates and ring gear.

The output shaft although 1/2", transmits torque through a 3/8" diameter shoulder with two flats machined on it. The stress concentration is so high at the edges of these flats that it distorts the final reduction carrier that it fits into and rounds it out. I don't understand why they wouldnt machine the final reduction carrier and shaft as one and avoid this problem all together.

[MrForbes]

Originally Posted by fox46

The mechanism on our robot is neutrally biased by a gas shock- you can lift the arm with one finger

How much inertia does the arm have? how much load is on the transmission when you change arm travel direction?

Before using a specific gearbox, should the team take one apart and see if the design is sufficient for what they're going to do with it? especially if they read that the torque rating of the gearbox is less than what it can generate with the motor they're using?

It's a tough situation, and a good way to show how things work in real life, unfortunately.

[fox46]

How much inertia does the arm have? how much load is on the transmission when you change arm travel direction?

This is exactly why we are having issues but that comes back to the marketing of a gearbox for a motor which it can't handle. The load on the motor is almost nothing but the speed at which they start and stop is what is wearing them out. We have the programmers working on a ramp function / soft start function for them which should help.

This is still a highschool competition and these machines are being built by highschool students, not engineers.... or at least thats how it should be. When we got our gearboxes, they were given to a couple of students who were told to disassemble these, figure out how they work, then reassemble them with grease. Of course they aren't going to be able to identifty the weaknesses in a design- they are just beginning to learn this stuff. I just would have preferred they learn about it in a different way in this case.

[AdamHeard]

Originally Posted by fox46

This is exactly why we are having issues but that comes back to the marketing of a gearbox for a motor which it can't handle. The load on the motor is almost nothing but the speed at which they start and stop is what is wearing them out. We have the programmers working on a ramp function / soft start function for them which should help.

This is still a highschool competition and these machines are being built by highschool students, not engineers.... or at least thats how it should be. When we got our gearboxes, they were given to a couple of students who were told to disassemble these, figure out how they work, then reassemble them with grease. Of course they aren't going to be able to identifty the weaknesses in a design- they are just beginning to learn this stuff. I just would have preferred they learn about it in a different way in this case.

Banebots posts maximum torque ratings for the gearbox on the page they sell them on, and based on those ratings the 256:1 is a poor choice for anything that will see stall loads.

I would stick to the 64:1 or less (4:1 for us), and make/get a different style gearbox for the comp bot, even if you take measures to alleviate the load, the potential for failure with the 256:1 at full motor stall is always there, and could totally wreck your season.

[MrForbes]

Originally Posted by fox46

This is still a highschool competition and these machines are being built by highschool students, not engineers.... or at least thats how it should be.

That's debatable. If you have an engineering mentor who understands how to design this stuff, that mentor should be helping the students figure out how to make a workable design--that includes making sure the parts are strong enough for the application.

I realize that many (most?) teams don't have a mechanical engineer helping them, the result is that we kind of need to have indestructible parts available. That's a tall order.

[sanddrag]

For those of you with failing BaneBots planetaries, you may want to look into Dewalt Drill gearboxes, and/or a hypocycloidal lug nut torque multiplier. Ours has been holding out so far, but it hasn't seen a lot of heavy use. We have the 256:1 with a steel ring gear. In our design, it should see no more than about 15 lb-ft, and it is well supported on the end. I hope it lives, but I'm prepared to deal with it if it doesn't.