written and edited by Bernard Martin
When discussing Screw Threads, it may be helpful to understand a little of the history behind them.
Let’s start with helical forms. Records dating to around 250 BC establish that it was Greek mathematician Archimedes who explained the mechanical principle of the screw as a form of wedge. He went on to formulate the mathematical characteristics of a helix. This was a precursor to the invention of the “water screw”, which provided a means to move water for irrigation, and as a method for ships to evacuate bilge water.
It is apparent that other great civilizations contributed to the development and use of this tool, but the Greeks apparently had better people in the PR Department, as they get most of the historical credit. There is some evidence the water screw may have been used in Egypt before the time of Archimedes. The helical screw form was also used in presses by the Romans to make olive oil and wine, and later in printing presses like the first used by Gutenberg in the mid 1400’s.
As time went on the screw-form used as a wedge became an alternative to bindings and rivets as fasteners. Even though it offered the advantage of faster assembly and disassembly, manufacturing methods were primitive. Mating threads were matched to each other by hand, one at a time. Not until the 17th Century, with the development of lathe technology, did manufacture of precision threads become possible.
Even then, there were no standards for thread dimensions or form to assure performance and interchangeability of parts produced. Precision threads were being used in design of measuring instruments and manufacture of new technology. Railroads were being built and factories required new machines for mass production of goods. Increasing demand required a solution. Steps toward solving that problem would not come until the next century.
Joseph Whitworth and William Sellers - Two to Lead the Way
In 1841, a British engineer named Joseph Whitworth devised a set of standards for screw threads to address the need for uniformity and quality of performance in threaded parts. These standards prescribed a flank angle fixed at 55 degrees and standard thread pitches for given diameters.
In 1864, an American engineer named William Sellers presented another set of thread standards. Sellers proposed a 60-degree flank angle with flatted thread crests and roots. Like Whitworth’s standards, his proposal assigned standard thread pitches (threads per inch) for given diameters.
The British standards were accepted worldwide, including the United States, but Sellers thread-form was easier to manufacture. Measurement was also simpler, as the form was based on the angles of an equilateral triangle. Some would argue the absence of radius on the thread crest and root left a weaker thread. Years of use would prove Seller’s thread-form more than adequate for the vast majority of applications.
Even today, both of the thread-forms described above are in use. New standards have been assigned to address ever expanding uses. The addition of specialized forms like Acme, buttress, ballscrew, worm thread, and self-locking thread have been refined for function- specific use. There are threads for assembly of parts, fastening, creating motion, measurement devices, lifting, and fluid and gas sealing. The uses are almost endless.
There is a screwdriver in every house and business. Isn’t that a great indicator of the impact of screw threads on the world? Although most forms have been assigned standard specifications by governing bodies, there is no limit placed on the imaginations of engineers to fine-tune fit and function as well as to find new uses for the simple helical form developed centuries before.
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