I am not really buying my calculations of a gearboxes rpm for some conveyor applications. First conveyor has to travel 30 feet in 4.5 minutes with 2" rollers. If I divide 30' by 4.5minutes I get 6.667feet/minute and with a 2" diameter the gearbox should be about a 12:1 ratio. The other is a conveyor to travel 28 feet in 20 seconds with 2" diameter rollers. IF I did this correctly I divided 28ft by 20 seconds to get 1.4ft/sec, which would give me 160rpm. Thanks for checking my work.