Closing the Gap

Double helical gear courtesy of Kapp Niles.

We left the gear tribe arguing over how to solve the problems that were cropping up with the new-fangled double helical or herringbone designs. Faster and better machines were developed to appease accounting; the old practice of “running in” with an abrasive compound to better distribute load was revived, but the whiners wanted more progress.

The fundamentalists in the spur gear faction bragged about their high tech hobbing machines being more accurate. Then they moved on to grinding the actual tooth flanks so the gears got “better” yet. “You can’t do that on your shaping machines!” they taunted.

“We can if we do the two halves separately or have a wider gap,” the double helical people replied, while developing a machine to allow a rotary wheel to follow that helix angle.

The shaping machines had guides to match the helix angle. Some were 15 degrees, others 20 or 23 or 30 or even 45 degrees. [Just to avoid patent litigation, a few were 29 degrees and 59 minutes, but that is getting off topic.] So the new universal hobbing machine needed to be able to handle the entire range of helix angles from zero (spur) to 45 degrees (crazy-high but not patent-infringing).

Once those game changing differential mechanisms were introduced, the old questions about dealing with thrust and overturning moment resurfaced. Anti-friction bearings (a delightfully old school name for rolling element bearings) were getting better and better to keep up with faster rotating speeds. Surely they were now strong enough to tame physics’ dangerous forces.

The Anti-friction bearing tribe, recently kicked out of the venerable and ancient Bronze Bushing tribe, agreed but only if the “numbers” were right. Complicated formulas were proposed. Test programs were required. New and more complicated forms of rollers were introduced. Strange concepts like fatigue limits, static and dynamic capacity, average life, and limiting speed were introduced. Suddenly, a squirt of congealed animal fat was no longer acceptable lubrication. All this “science” took time and money.

And the re-united gear tribe is still left arguing just how much helix angle is enough and how much is too much …

About Charles D. Schultz 660 Articles
Charles D. Schultz is President of Beyta Gear Service and one of Gear Technology's technical editors.

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