Gear horsepower calculation

[whlock]

I did this design calculation and come up with a relatively low HP. Please help me see if I did anything wrong.

Problem:
A carriage weight 1500 lbs driven by a star ball rail system. It travels 6.67 FT in 3 seconds. I need to figure out the horse power needed to make this travel.

My attempt:
Gear pitch diameter is 3.5 in

Feet Per Min, FPM = 6.67/.05 = 134 FPM

RPM of gear = FPM/ (pi*3.5) * 12 = 146 RPM

Coefficent of friction of rail system is .003
Force = 1500 x .003 = 4.5 lbs
Radius = 1.75 in/12 = .1458 ft

Torqque req'd = 4.5 * .1458 = .6561

HP = T.RPM/5250 = .02 HP which is very low.

Is this correct? please help

[JAlberts]

Your HP calculation is basically correct once the vehicle is moving at the design speed due to your low friction factor; at the same time, I anticipate that it will be necessary to overcome the loaded carriage's inertia to accelerate it to speed and this must be taken into consideration in the driver selection.

For example, if you are using an electric motor for power, then the motor's locked rotor torque rating (1500 * .1458 = 219 lb-ft) must be taken into consideration and this, rather than the moving carriage required HP, can be the controlling factor in the sizing of the motor. Ultimately, discussing your requirements with a qualified motor supplier is your best route.

P.S. This is also the torque value that should be used in the design load and stress calculations of your gearing and drive assembly (+ appropriate safety factors of course).