It was brought to my attention that the graphics used in my recent “herringbone” postings actually show double helical gears. I have a great support crew here at Gear Technology, but not all of them are aware of the subtle differences between a “true” herringbone and a double helical. At the risk of repeating things previously posted, it seems timely to post more on these differences. Actually, repetition is not a bad thing when learning a new subject. As Pastor Eggers used to remind us in catechism class fifty-plus years ago, “Repetition is the key to memorization and memorization is the best way to insure that knowledge is available to you in the future.” No Google in 1965!
Technically, a herringbone gear is a subset of the “double helical” gear type; i.e. — “double” as in right- and left-hand helices sharing a common shaft, and so the thrust forces of the helical tooth are negated. The thrust forces also cause an overturning moment, so the cancellation of the thrust eliminates the overturning moment. Both thrust and overturning moment increase the load on the shaft’s bearings. At the dawn of the 20th century, bearing capacity was limited by materials, heat treating, and accuracy. The double helical design was a key to making machines last longer while running faster.
The “error” in the graphics was the presence of a small groove between the helices. A “true herringbone” does not require any groove; it was marketed as a “continuous tooth” and the “gear with backbone” because there was no separation between the helices. This feature also made them an excellent choice for oilfield pumps-type applications, but I will save that topic for a later blog. The cutters used to generate the herringbone teeth simply met in the middle (also called the apex) and clipped the planed chip off against the opposing hand. It was a brilliant solution to obtaining a double helical part as it used well-understood technology, i.e. — the removal of material with the axial motion of a sharp blade that is harder than the material being worked.
People had been using the same motion since a caveman picked up a seashell and scraped the last bit of fruit out of a melon. It was just a hop-skip-and-a-jump from that shell to a knife — to a block plane to the spring pole lathe and the earliest of dedicated gear cutting machines. Well, it only took a few thousand years, but the point is that the herringbone process was easily understood by people who were new to the industrial arena in 1900. Because operators were not trusted to make complicated adjustments to the machines, the designers of the herringbone process built most of the important things into the machines and the cutter. All the operator had to do was load the part; install the correct cutter; put in some change gears to obtain the correct index; and center the cutting head on the desired position for the apex.
It is very simple — so long as the machines themselves are well-maintained and the cutters properly sharpened. If those requirements are not met there is an expensive pile of scrap gears created rather quickly. Over time an additional problem arose: the hardness of the steel being processed increased and cutter life decreased. Cutters are complicated to make and are very expensive. Worse yet — the machine is not making gears while you are changing cutters. There were always a few herringbone parts with a big gap between the helices so as to permit a more compact design of the gearbox. Someone noticed that the cutters lasted much longer on those parts and theorized that adding a groove to the more conventional parts would reduce cutter expenses. They were right — and within a few years it was almost assumed that chip relief grooves were permitted.
Herringbone purists were not enthused about losing that “backbone,” but eventually gear theorists came to understand that the groove eliminated a variation in tooth stiffness along the face of the part. Managing this variation is, to this day, an important area of research and very few “continuous tooth” herringbones are requested currently.
This has been almost twice my usual word count. And I’ll continue with herringbone education in my next posting.