October 12, 2022
Few designers would feel comfortable with a face to pinion pitch diameter ratio of 2.00 anymore. Some still prefer to select face width by face contact ratios, often aiming for integral values. When designing a product with a range of gear ratios, I have found it best to use a .3 to .4 limit instead of .5 on the face to center distance ratio. This keeps the face to pitch diameter ratio comfortably low for ratios less than 5.50:1. We used to routinely use single stages with up to 7.6:1 ratios. Eventually we came to understand that just because something was allowed by a rating standard did not mean it was a good idea. Very seldom do the extremes of a formula result in outstanding field performance. Limiting ratios to 3.5 to 5:1 with a .3 to .4 face to center distance ratio results in face width to pitch diameter ratios that are in the 1.00 to 1.25 range. These “short, stubby” pinions tend to be very robust. By robust we mean resistant to both torsional and bending deflection. Robust gear sets do not need lots of “optimization” and tolerate a range of loading conditions very well. A competitor’s salesman once told a customer that my new product line was obviously imported from Europe because it had metric center distances and those short stubby pinions. So unlike his legacy designs with “inch” centers and wider face widths. I was not offended by his gossip but wrote a memo for my salesman to give to the customer describing the design process that lead us to the “same” conclusion as our European competitors. The rating formulas work the same in Pittsburgh as they do in Munich. Computer modeling allows the designer to test a wide variety of proportions before committing to a specific layout. Wonderful technology is available in every personal computer so you do not need to take my word for what works best for your application. Just be careful about designing yourself into a corner by using the extremes of the standard’s formulas.