Further Modifications

You might be thinking this conversation would logically move to other tooth modifications. Gear Technology has and will continue to publish many fine, scholarly papers on micro-geometry, and I encourage you to read them. Other than explaining some of the key concepts, however, I will not offer guidance on amounts of modification or methods for determining those amounts.

I am a fundamentalist with regard to design. In some circumstances tooth modifications can greatly enhance gear performance and service life, but the best route to good gear design is through proper macro-geometry and well-engineered mounting systems. Good design works over a wide range of loads, but modifications — not so much.

Tooth modifications are used to adjust the contact pattern in response to defections and misalignments, both of which are load-dependent. You pick a certain load — or range of loads — and calculate the amount of metal that must be removed to eliminate a “hard contact” or end loading condition.

Until the advent of computer -controlled flank grinding, it was very difficult to obtain micro-geometry modifications. Profile modifications required special cutting tools with the changes built into them. Even a simple crown, i.e. — lengthwise curvature — required a template for a machine tool equipped to use it. If you did not like the results you had to modify the cutter or the template. This took time and money.

Early grinding machines allowed you to dress flank modifications into the wheel, but it was often frustrating to have to re-dress the wheel just when you got the form right. Crowns frequently required grinding the gear, then setting the helix angle a bit “off“ and grinding just a portion of the face width. There was a reason it sometimes took days rather than hours to get a part completed.

Just because the computer-controlled grinder can quickly produce whatever modification you specify is not an excuse to do so. A group of experts can argue for hours over the amount or form of flank modification; tip chamfer or tip round or parabolic 1 or parabolic 2 are all valid choices. The root relief or no root relief debate is always fun too. And lead modifications can lead to bedlam on the shop floor; they might spend an entire shift tweaking the results — only to be told the direction was 180 degrees off.

Tip and root relief are great ways to reduce “hard contact” on mesh entry and exit, but they are not a substitute for selecting the correct pitch. An offset taper crown will centralize tooth contact under some loads, but it will concentrate the load under other loads; it is also no substitute for proper face/diameter ratio and stout bearing support.

Not to mention that micro-geometry modifications are not generally available in unground gears; and yet there are still millions of unground gears made every year. So, my advice is to get your basic design right and then determine the appropriate level of modifications using the methods described in the excellent papers published in our magazine.

About Charles D. Schultz 463 Articles
Charles D. Schultz is President of Beyta Gear Service and one of Gear Technology's technical editors.

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