[MrForbes]

The key would have to support the load along the length which is contacting each sprocket. So, if the key is fully under each sprocket, and each sprocket is (guessing) 1/2" inch thick, then the load at each one is spread over

3/32" x 1/2" = .0047 sq inches

you have 30 ft lbs torque, at 3/16" radius, so the shearing force on the key is

30 x 12 / .188 = 1900 lbs

so the stress on the key at each sprocket is

1900 / .0047 = 400,000 psi which is a bit much!

so it looks like keys won't do the job. Could you drill some holes in the sprockets and bolt them together? might put the load out at a larger radius which could help.