After many years in the business, I recently used a servo drive with rotary motion for the first time. The application was a turret for a winder and I needed to rotate exactly 180 degrees forward for each new roll.
In doing this, I learned something new about gear ratios. First of all, the gear ration on the nameplate is nominal only. It was necessary to contact the gear manufacturer for the exact tooth count on a three-stage gear. Then there was a 4th gear driving the turret itself. In all there were 4 gear meshes with a total ratio of approximately 330 plus change.
First attempts at entering a decimal gear ratio with many decimal points were unsuccessful. Throwing math out the window and carefully tweaking the gear ratio allowed about 10 rotations before the turret was out of position.
The correct solution was to enter the gear ratio as an integer numerator and denominator. That was done by counting the input gear teeth and output gear teeth for each o the meshes and multiplying them together. The numerator worked out to:
The denominator worked out to:
24*9*15*40*360 degrees = 46,656,000
Unfortunately, the servo would not take tooth counts this large. I needed to search for a common factor and was able to divide both the numerator and denominator by 144 resulting in 297517 and 324,000. These numbers were successfully entered into the servo resulting in turret that can be indexed exactly 180 degrees indefinitely.