[marccenter]

Here's my math,

We're planning on using the KOP, Andy Mark PG71 Gearmotor with almost 3: 1 sprocket ratio ( 22 tooth on PG71 attached via chain to 60 tooth gear). 60 tooth has arm mechanism. According to specs PG-71 stall torque is nearly 16.6 ft-lbs (Andy-Mark website) . Someone calculated that 20 foot -lbs was required to tip the bridge (CD website) based on the 2 batteries and distance demonstrated in one of the FIRST kick-off videos.

So, at 60% stall torque (10 ft-lbs) * (60/22) * favorite efficiency ratio (0.8) = 21.7 ft-lbs > 20 ft-lbs should work.

Distance from center of shaft for bridge arm is 6 inches (1/2 foot) on the 60 tooth sprocket , so theorectically placing > 40 ft-lbs on bridge at 60% full torque.

We didn't build a bridge (small team) but the bridge arm easily picks up the front end of our robot. Hope this helps.

BTW, PG71(am-0914) is powered by RS775 motor and has internal 71:1 gear set. KOP rules allow 2 units for 2012. Andy Mark also sells a 10 mm key hub that couples directly to the 10 mm output shaft (am-0985). You will need to also get a 4 mm key for the hub (am-1249).

So, for our assembly the Bill of Materials , from memory is the following:
1) PG71, 2) 10mm key hub, 3) 4 mm key, 4) 2 - 375 key hubs (am-0134), 5) 1 sprocket, (22 tooth - am0118), 6) 1 sprocket, (60 tooth - am0057). Need also 7) 3/8 shaft for the 2 - 375 key hubs (recommend 1 foot to start)- Andy Mark does not sell - McMaster-Carr and 8) 3/32 key stock for your 375 key hubs (am-1059, 2 pcs)

I have a second PG71 Gear motor ready to add if necessary (contingency plan).