An Exercise in CAD, or How to Draw (Internal) Gears.

Peter Shaw takes readers through his approach to drawing gears on a computer

Introduction

Some years ago, I was having trouble visualizing how epicyclic gearing would work and came to the conclusion that perhaps if I drew it out using CAD, including all the teeth, I could gain a better understanding. This I managed to do, and I produced some figures which appeared to show how it worked.

Unfortunately for me, I eventually discovered that the figure for the internal gear was wrong, even though it appeared to work on paper. I therefore resolved to attempt to improve matters. This article is the result of those improvements, and fig. 1 shows an epicyclic gear system consisting of a 40T Sun wheel, two 20T Planet wheels and an 80T annulus or internal gear wheel, all drawn using the methods outlined below.

I should point out that this article does not set out to be a definitive guide to figure gears, nor does it set out to be a definitive guide to using CAD. Instead, it is merely a description of how I managed to produce figures of gears which are based on something resembling actual practice and which helped me to understand the hows and whys. The article is offered to perhaps help others who may be having similar problems.

Literature on Gears & Gearing.

When I first looked into this subject, the only information I could readily find was that published by Ivan Law in his book Gears and Gearcutting (Workshop Practice Series (WPS) 17), together with a series of articles by Dave Lammas in Model Engineer (ME) starting 21 January 1990. Law does not cover internal gears, whilst Lammas devotes only a few paragraphs to this sub-division, and even then, it is based on how to cut the gears rather than the theory behind them. I did find a very complex set of articles by Len Snell in ME starting 26 September 1997.