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The
United States Standard Screw Threads (1864)
William
Sellers at the Franklin Institute
Philadelphia
By
the 1850s, textile machines, metal planers, steam engines, locomotives,
rotary printing presses, and a myriad of other high-technology products
powered an era infatuated with machines. But infatuation turned
to frustration when those machines broke down. Lacked any national
or industry standards for the most basic mechanical elements, the
nuts and bolts that created a functional machine from disparate
parts. So when a bolt broke or a nut was lost, the user had to make
a new one to fit its mate or to send for a replacement from the
builder. No one could deny the value of national standards for these
essential fasteners.
Indeed
by that time the world's industrial leader, Great Britain, was adopting
a comprehensive system of screw threads promulgated in 1841 by that
nation's leading maker of machine tools, Joseph Whitworth (1803-1887).
His American counterpart, tool builder William Sellers (1824-1905)
of Philadelphia, understood the value of Whitworth's standard, a
clear improvement over the various "mongrel" threads that
U.S. machinery makers had adopted. But Sellers decided to improve
upon Whitworth's approach, creating a system of threads adapted
to U.S. needs.
 In
April 1864 Sellers laid out his proposed system of screw threads
in a paper delivered at Philadelphia's Franklin Institute. Sellers
simplified Whitworth's design by adopting a thread profile of 60
degrees (versus 55 degrees), which was easier for ordinary mechanics
and machinists to cut. In addition to this profile, Sellers offered
systematic approaches to thread pitch (the number of threads per
inch), form, and depth, as well as rules to proportion hex nuts
— for each fractional size from 1/4-inch to 6-inch diameter
bolts.
The
Franklin Institute was America's leading forum for developing the
art and science of mechanical engineering in this era before the
founding of the ASME, and Sellers was its president. On December
15, 1864, a special committee of the Institute endorsed the Sellers
or Franklin Institute threads. To aid their adoption throughout
the United States, the Institute lobbied the U.S. Army, Navy (whose
Bureau of Steam Engineering was a leading mechanical innovator),
and the master mechanics of America's largest railroads.
The
new thread standards did not sweep the nation overnight. The inertia
of old approaches was difficult to surmount, while thorough adoption
required newly precise taps and dies as well as reliably dimensioned
steel bar stock. But by the 1880s, the system had triumphed, as
machines with interchangeable parts — from typewriters to locomotives
— flooded the national economy. Known originally as the Sellers
or Franklin Institute threads, they became the United States Standard
threads. Other systems of screw threads have since come into widespread
use. But down to the present day, William Sellers' innovation remains
a ubiquitous standard. Take a quarter-inch nut from a Portland,
Maine, hardware store and it will reliably fit a quarter-inch bolt
in Portland, Oregon. The economy and simplicity of this elegantly
rational system represents William Sellers' legacy and the enduring
quality of fine mechanical engineering.
The
Franklin Institute was recognized as the site of the presentation
of screw thread standards by William Sellers in 1864 in a designation
ceremony as an ASME Mechanical Engineering Heritage Site on June
12, 2005. ASME president Harry Armen will present the bronze plaque
to the Franklin Institute.
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