Rookie Team Redesign

[joelg236]

Originally Posted by IKE

Shocks or impacts are likely what is killing your gearboxes. Every bump causes really high torque back into the arm. My recommendation is to remote the gearbox from the arm. A low pivot "hip joint" on the front side would reduce loads. Using sprockets to drive it. Or possibly setting it up as a winch off the back.

A winch would limit our range of motion (since the arm needs to go all the way back for bins). I'm intrigued by the idea of reducing shock, since we have lots of spare belts and some sprockets, so it would just be a question of mounting. I'll be trying this today.

[seg9585]

Originally Posted by joelg236

A winch would limit our range of motion (since the arm needs to go all the way back for bins). I'm intrigued by the idea of reducing shock, since we have lots of spare belts and some sprockets, so it would just be a question of mounting. I'll be trying this today.

A winch would not have to limit your range of motion. From your picture, mount the winch above the drive CIMs and attach a tension cord to the extreme back of the mechanism (where the tubing is connected). That should give you full range of motion.

[IKE]

Originally Posted by joelg236

A winch would limit our range of motion (since the arm needs to go all the way back for bins). I'm intrigued by the idea of reducing shock, since we have lots of spare belts and some sprockets, so it would just be a question of mounting. I'll be trying this today.

Shock loading is a really tricky thing to improve, but awesome when you get it right. What you are looking for is something that can support your nominal load fully, but then flexes a little bit if you exceed that. Using a coupling like these spider couplings might work:
http://www.mcmaster.com/#spider-couplings/=vxqhqt

It looks like your arm has about a 3 foot reach for center of mass of a Tote. I think this would equate to about 25 ft*lbs or 300 in*lbs. I personally would size the coupling and buy 3 spiders (different material). They have urethane ones rated for 290 in*lbs, and a Buna Nitrile the same size rate for 140 in*lbs, and a Hytrel rated for 400 in*lbs. I think you might be happiest with the Buna Nitrile (I personally would order 2 of those to have a spare). I suspect that the torque rating is a Max Continuous at operating RPM, and for your application would add a nice cushion to the lifting and bumps. Should it be undersized, then the same geometry urethane would be a good alternate.

All this being said, you are outside of VEXPROs load rating for that gearbox. They do their rating chart based off of motors and ratios as the motors have the capability to apply enough torque to do damage to the gearbox in those scenarios.
http://content.vexrobotics.com/vexpr...s-20150106.pdf
IE, any of the above fixes, and you are still operating outside of the recommended limits of the gearbox.

For this, my overall recommendation would be get a new 5:1 ratio box, and rebuild it to be 50:1 ratio, and then use a chain loop for the arm. I personally like big sprockets on arms as it reduces the tension in the chain and thus compounded loading effects Something like a 36 tooth sprocket or larger attached to the arm, and then driven by a 12 to 15 tooth sprocket.

the same 300 inch*lbs needed by the arm will equate to about 150 lbs of chain tension (radius of a 36T #35 chain sprocekt is a bit over 2 inch).