Looking at Parts

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Several weeks ago I saw some 25-tooth pinions from the propeller-drive reduction gear of a postwar Wright R-3350 radial aircraft engine for sale on eBay. In the dark days of 1937-1943, Wright was having a reduction-gear failure on its early 3350s about every other time a prototype or preproduction engine was run. But when that engine was completely replaced (“completely” as in every part was different) by the postwar 3350, it was given a reduction gear of brand-new design.

A Love of Gears

I have a particular love of gears, partly because they must combine extreme surface hardness with precise shape and partly from all that I learned in making transmission gears last in my 1970 Kawasaki H1R 500 two-stroke roadracer. Surface hardness, which is achieved either by carburizing or nitriding, is necessary to carry extreme tooth-to-tooth loads, which can rise above 100,000 psi. Precise shape is required so that a gear pair’s rotation is conjugate, which means that the driven gear’s motion is smooth, continuous, and always in strict proportion to that of the driving gear—not a series of variations as each driving tooth unloads and the next one takes the load. You don’t have to be a gearhead to appreciate this; museum patrons can see such precise shapes in very hard material when they view 4,000-year-old Egyptian statuary, where sculptors confidently reproduced the flowing and continuous forms of human bodies in hard stone such as granodiorite (which contains more than 20 percent quartz).

I stared at the small photo on eBay. The teeth of the advertised 2-inch-diameter reduction pinion were strangely long and pointed; very different from the coarse, stubby, robust teeth found in motorcycle gearboxes, or from the 15-tooth pinions of Wright’s original wartime reduction gear. These slender teeth reminded me of cooling fins! Last year I wrote of the change Yamaha made in its 250 production racer’s gearboxes in 1965: from a greater number of small teeth that must have appeared too often in warranty claims to fewer but coarser and stubby-looking teeth.

A Wright R-3350 radial propeller-drive reduction gear. (Mark Lindemann/)

Yet here in the eBay photo was this comparatively delicate-looking long-toothed pinion, one of 20 used to reliably transmit up to 3,700 takeoff horsepower to the prop on aircraft such as Lockheed’s graceful Constellation. A sun gear on the engine’s crankshaft meshed with the 20 planet pinions, which in turn engaged a surrounding stationary ring gear with inward-facing teeth. The planet pinions attached to a ringlike pinion carrier which in turn drove the propeller.

I had to have one.

Contact Ratio and Gear Design

I also had some reading to do! What I learned was this: For every meshing gear pair there can be derived a number, called the contact ratio, which roughly tells us the average number of teeth simultaneously carrying the load. By sharing the load among up to 2.6 teeth simultaneously, high contact-ratio gearing can greatly increase torque capacity, durability, and smoothness of power delivery, while also achieving significant noise reductions (7–11 decibels in one example) and cutting gearcase vibration by one-half to three-quarters.

Although there had been some high-contact-ratio gearing applications before WWII, no one had yet taken the plunge of applying it to heavily loaded power gearing in aircraft engines (where the requirement is always “light enough to fly, heavy enough to get there”). The contractor for Wright’s postwar R-3350 reduction gearing, Arrow Gear, was able to demonstrate that these delicate-looking long-toothed 2.2-contact-ratio pinions provided durability and strength far greater than Wright’s original design, with a contact ratio of 1.0.

The traditional gears look “right” because their teeth are stubby rather than tall and slender. But the numbers didn’t lie. When the load was shared by an average of 2.2 teeth at all times, load concentration was more than halved. On takeoff power, each of the 20 1.1-inch-wide pinions transmitted 185 hp.

When my pinion arrived in the mail I put it here on my desk to remind me that looks can deceive.

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[MikeHorton]

Great Kevin is back

[Stu]

1 hour ago, MikeHorton said:

Great Kevin is back

 

I think he has been left home alone for Christmas!