# Blog: Stepper Motor Sizing

## Sizing: Adding a rubber sleeve as a primary roller

Recap of the last posting In our last posting, we were comparing the inertia of a 12” long, 0.25” diameter steel rod (0.0013 lb-in^2) to a 1” long, 1” diameter aluminum coupling (0.00957 lb-in^2). We asked why the lighter weight aluminum coupling would have a larger inertia than a heavier steel rod and the person …

## Sizing: Calculating the inertia of a load shaft

In our last posting, we calculated the inertia of a helical shaft coupling. The coupling is used to connect the motor shaft to the load shaft. We calculated the inertia of: a coupling that was 0.5” in diameter and 0.5” long at 0.0003 lb-in2 a coupling that was 1” in diameter and 1” long at …

## Sizing: Determining the mass of the coupling

In our last posting, we talked about having a good quality coupling that connected the motor’s shaft to the load’s shaft. Finding the right coupling A good quality coupling would be able to accommodate a reasonable angular misalignment between the shafts as well as parallel misalignment. In addition, it should have the minimum torsional windup. …

## Torsional Windup and Load Friction

In our last posting, we talked about how a very torsionally flexible coupling could affect the ringing at the load. A rubber tube We proposed a one-inch long plastic tube that fit snugly over the motor’s shaft and the load’s shaft and that the torsional windup of this coupling was zero. The moves were snappy …

## Sizing: Load to rotor inertia matching

We’re continuing our discussion about stepper motor ringing and inertia matching. We had established the following load-to-rotor inertia ratios:

## Sizing: Ringing and inertia mismatch

I’m sorry if you Googled “ringing” because of a hearing issue and end up on this blog. You should Google “tinnitus” instead or search the WebMD site. Now where were we? Oh yea, in our last posting we were talking about load-to-rotor inertia matching. We had established the following load-to-rotor inertia ratios:

## Sizing: Load to rotor inertia matching ratios

We started our stepper motor sizing discussion with the last posting and introduced the NEMA rating for motors. The stepper sizes that we talked about ranged from the smallest NEMA 14 up to the largest the NEMA 34. Other manufacturers might have smaller (NEMA 8 &12) or larger sizes (NEMA 42), but we’ll let those …

## Sizing: stepper motor physical size

Our last posting talked about applications that had marginal torque and that caused our “Hybrid” motor to correct its position. These applications were essential “path critical” and the correcting moves the “Hybrid” control did cause the path or the move time to vary. Proper motor sizing avoids this issue.