Motor Calculations

Now that we have a preliminary computer model and the testing done, we need to calculate the sizes of the motors. Since we've got the final force, we can work backwards to find the torque needed from each motor. I briefly went through the process already, but we'll go over it in more detail here.

First off, we need to generate 5000 lbs of force at the crush piston. That force then gets transferred along the Acme threaded rod to the Acme threaded nuts that turn the shaft. The nuts then transfer the force to torque from the sprocket turning the nuts. A neat thing about Acme threaded rod is that it acts as a very long, shallow incline slope. From a physics standpoint, it is easier to push something up a shallow slope than it is to lift the same thing straight up. So the 5000 lbs at the nuts get multiplied by .073 (for 3/4-6 Acme rod) to convert to the needed torque. Also, while this magnifies our power output, it also drastically reduces the speed of the machine. 

So 5000 x .073 gives us 365 in-lbs of torque needed to turn the nuts. Now, we are using two sprockets, one small one on the motor, and a larger one turning the nuts. These will be connected by roller chain. The larger sprocket will be 6" diameter, and the smaller one a 2" diameter, which provides a gear ratio of 3:1. This gearing allows us to divide the torque by 3, and reduces our speed even further. So now our required torque gets transferred along the chain and is divided by the ratio, which gives us a required torque at the motor of 121 in-lbs of torque.

We repeated this process for the other two motors, but with smaller initial forces. The results are below. Something of note here is that motor usually has high torque or high RPMs (speed) but not both. So by having a motor with less torque, we can have a faster motor. While speed is not our primary concern the extra RPMs will be nice, especially on the longer 8.5" stroke.

Click to enlarge