Chief Delphi - Redesign, 36mm question.

As to falling arms, this is a non issue if you do the right thing and counter balance your arm.

NOTE TO EVERY TEAM WITH AN ARM:

DO YOURSELF A FAVOR -- COUNTER BALANCE YOUR ARM WITH SOME LATEX TUBE.

Seriously. This is the single best thing you can do to make your arm more controllable -- More significant than increasing your gear ratio. Really. Design in a way to wrap 20 or 30 loops of latex tube on the back side of your arm pivot and you will be much much happier. Here is a rule of thumb for you: plan on about 7lbs of force per strand of latex tube -- deciding how many loops you need is as easy as dividing by 7.

Example: Trying to hold up an arm that weighs 15lbs and has its CG at 30"? That means you need 450in-lbs (=15x30) of counter balance. If you can get a counter balance bar 6" behind the arm pivot, that means you need to pull on that bar with 75lbs (=450/6). To get 75lbs of force from latex tubing you'll need 11 strands of latex (=75/7) or 5-6 loops.

As to the 36mm gearbox, I have made a prediction about them failing with 100in-lbs of torque with repeated cycling. I predicted that the 64:1 gearboxes will fail giving situations where they see stall loads and dynamic loading. While I stick by this, the number of cycles is quite high (many hundreds in my very abusive testing).

Bottom line: Given what I know now and given good counterbalance and given that you don't make a habit of ramming your arm at high speeds into the floor or the end stops, I think that the BB 36mm gearbox could be a good choice. I would use as big of a ratio as I could in going from the gearbox to the arm (say a 96T on the arm from IFI and an 8T on the BB gearbox for a total ratio from motor to arm of 768 -- that would make your arm's freespeed be something like 20 RPM -- fast but not too fast. Using 85% per planetary stage and 95% for the chain you would have 125N-m (1100in-lbs) of torque at your arm. That would give you 22lbs at 48" -- Not too shabby. BUT don't forget that you should not design your mechanism to routinely stall these motors or they will become copper slag heaps. You should really only routinely use about 20-25% of the stall torque of these motors -- this would mean that you could only use about 5-7lbs out at the end of the arm -- not all that great -- but you could use 2 BB gearboxes and be back in business OR you could go to the 125:1 gearboxes (but I don't really like the high ratios so you have to pick your poison).

If your still nervous about the impact load replace a couple of sections of chain with spectra string (one on each side of the drive sprocket) and you'd effectively have an infinite life mechanism.

Joe J.