[Andrew Blair]

One important thing to note is that it doesn't matter how much torque your gearbox can output, but how much the chain is ever going to see. The force of friction multiplied by the radius of your wheel tells you the torque that that wheel can transfer and the maximum load (theoretically) that the chain will experience. Divide the torque by the radius of your sprocket, and you have the force the chain will experience. Multiply that by at least two or three to maintain a decent safety factor- perhaps more. Compare that to the breaking strength of the chain, and decide if you are too close you are to the rating, and need to bump up the size.

Ex:

Coefficient of friction of wheel (u)- 1.5
Robot weight (N)- 150 lbs
4WD
6" wheels
Single chain run per side
Radius of wheel sprocket- 1.5"
Force of friction= uN

Force of friction entire robot can exert- 225 lbs

Force a single side may exert(Ff)- 113 lbs

Torque on wheel= (Force of Friction)*(Radius) *We may use the Ff of a side as total torque because we have a single chain per side

Torque- 339 lbs-in

339lbs-in/1.5in= 226 lbs of force on chain

226 lbs*2 (safety factor)= 452 lbs


Now, I couldn't find the ultimate tensile strength of #25 chain, but Mcmaster lists it's working load at between 140-300 lbs working load. I seem to recall the ultimate tensile strength to be at or over 1000 lbs. In my example, if you had well aligned sprockets and decent chain, #25 would probably be pretty safe, considering the lifespan of a FIRST robot. However, know that you would be operating the system above the recommended working load, and could break chains. If you want the weight savings, bring extra chain, but you would be relatively safe, barring more extreme circumstances.