Atwood Speed help

I’m planning on using an Atwood in an Inventor model that I’m doing for my engineering class and I came across a question; The spec sheets have “Normal load” in the performance chart, but what is normal load? is it weight specifically for this motor, or is there a standard load, say 150 lbs the motors are tested on? Any help would be greatly appreciated. If it matters, I’m trying to figure out the RPMs under different loads to calculate how fast it can move.

Basically, you have two data points… free speed and stall torque. Draw a line between them on a torque vs. speed axis. At the free speed, there’s no torque applied to the motor shaft, and at the stall torque, there’s no movement by the motor shaft.

For any given load (torque) you can find the speed that the motot will spin at.

There are multiple posts out there that have a lot of data on this sort of thing. I busted onto Chief Delphi pretending to know a lot about this about 3 months ago… it took a few friendly heart to hearts with Joe Johnson to set me straight. Feel free to post your application and I’ll run through the math for everybody.

Atwood and Drill Data in English Units, geared at free speed of drill motor.

Here are a couple of graphs that show legitimate motor data for both the drill and atwood motors geared down to the speed of the speed of the drill motor.

Look at the post, I honestly no longer agree with some of the technical arguements the author made back when it was posted.

Yep, I was the author. Live and learn.


The “normal load” is usually a reference for that motor only. There is no standard load for motors but there is a standard method for testing motors. It would help if you post the site you are referencing so everyone could see what we are discussing. The normal load for this motor likely refers to the fact that it is designed to be used with a trailer jack. The specifications for the weight range of trailers is likely what the sheet is referring to and that takes into account the gearbox to which the motor is coupled.
If you look at just the motor curvesor charts, you will see that there is a distinct correlation between motor speed, power out, torque, current and efficiency. A designer will perform black magic as he/she chooses where on the chart to operate. A low a speed will increase torque but sacrifice efficiency and raise current. A high speed will sacrifice torque but minimize current. Somewhere in the middle will optimize all of the factors, giving reasonable speed, torque and output power while using acceptable currents. Paul Copioli might be a better one to jump in here and explain. You might try searching his posts and see if he hasn’t already posted an explanation on this.