It is difficult to know where to start when writing about worm gears, because this market segment has defied “standardization” within the United States. The rest of the world has been making some progress in that regard, as summarized in an article in the February issue of our sister publication, Power Transmission Engineering.
I am not too proud to admit I learned a few things from it — especially the “standard” nomenclature for the tooth flank geometry.
One of the reasons American worm gear makers avoided standardization was due to the competition between “conventional” worm gears and the “double-enveloping” worm gears produced by a major company and a few smaller competitors. The “battle” was waged in the marketplace and in the technical journals. It was perhaps one of the most bitter technical arguments in the history of engineering.
On one side was the leading gear theorist of his time; he considered double-enveloping to be one step above snake oil — and he was a very prolific writer. On the other side were lesser-known engineers who diligently worked to make Leonardo DaVinci’s and Samuel Cone’s visions into longer-lasting, more compact gearboxes.
There was a time when dozens of American shops designed and built worm gearboxes; a former employer supplied over 10,000 worm drives to the main Chicago post office from their plant less than half-a-mile away. Simple, conventional worm sets could be made quickly on common gear machines. By “conventional” I mean straight cylindrical worms — usually thread-milled, hardened, and ground — meshing with a single-enveloping bronze gear. A typical product brochure offered a series of center distance with ratios of 5:1 up to 100:1 that could be packaged into hundreds of configurations such as worm “up;” worm “under;” vertical output; vertical input; and multiple reduction stages.
Over the years, the product offers coalesced into extremely similar designs, bordering on standardization in exterior dimensions — but never in gear geometry. They might have similar numbers of threads and teeth, while lead angles, pitches, pressure angles and tooth depths remained unique. All the while, the single-enveloping vs. double-enveloping battle continued to be fought.
So, who won? The consumer! In smaller sizes — perhaps 4-inch centers and below — the lower cost of the conventional worm triumphed. The bigger the gear set becomes, the higher capacity of the double-enveloping system negated the higher cost of the gears themselves; and market share moved to the “hourglass” worm products. And the American gear companies all lost, because that convergence on mounting dimensions made them a commodity and overseas competitors could offer lower prices.
This does not mean an aspiring gear person can omit worm gears from their expertise; it just means you are more likely to purchase the gear sets instead of making them to a new design.