Charles D. Schultz is President of Beyta Gear Service and one of Gear Technology's technical editors.
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One of the first things I learned about gears was the importance of having a “hunting tooth” combination. This was not open to discussion. It was carved in stone in our gear technology handbook. I had no idea this was not a universally held belief until I found myself charged with designing new product lines and there was no gear technology handbook to lean on.
The available reference books had mixed advice on the importance of the hunting tooth; some saw no point in it, others agreed with my early mentors that it was as necessary as the blue sky. I decided to become a free thinker on this bit of doctrine and haven’t though much about it since.
As mentioned earlier, I bought a reprint of 1910’s American Machinist Gear Book, written and edited by Charles Hays Logue, formerly of R.D. Nuttall Company. Mr. Logue had strong opinions on hunting teeth, as expressed on page 31:
“It has been customary to make a pair of cast tooth gears with a hunting tooth, in order that each tooth would engage all of the teeth in the mating gear, the idea being that they would eventually be worn into some indefinite but true shape. Some designers have even gone so far as to specify a pair of ̒hunting tooth miter gears’ That is, one ̒miter’ gear would have, say, 24 teeth and its mate 25 teeth.
“There never was any call for the introduction of the hunting tooth even in cast gears, but in properly cut gears any excuse for its use has certainly ceased to exist.”
Strong words back in 1910. I think I would have enjoyed meeting Mr. Logue and talking about gears. His book covers a lot of topics and I am sure he rubbed some people the wrong way. Other prominent engineers contributed to the book, so he had friends, too.
The “hunting tooth” is but one of the design guidelines companies make. I’ve found some engineers were taught, for example, to never use helix angles over 15 degrees or 20 degrees. Others were shocked when they saw pinions with less than 21 teeth or gear ratios over 5:1. Now there may have been some logical reasons for that in certain circumstances, but over the years the underlying reasons were lost. Yet the guideline remained.
It reminds me of a story of Thanksgiving dinner being prepared by a multi-generational group of women. The chief cook, somewhere in the middle of the generations, was just telling the youngster present to “always cut off the last half-inch off the ham” before putting it in the pan. A junior member of the crew insisted upon getting confirmation from the oldest present. Great grandma scoffed at the idea. “The one time your grandmother helped cook the ham we had an extra big ham and it wouldn’t fit in the roaster.”
Just a reminder to you teachers and mentors out there:
Teach your children well. Don’t just give them rules. Give them reasons.