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Concept
Rockwell scale
Summary
The Rockwell scale is a hardness scale based on indentation hardness of a material. The Rockwell test measures the depth of penetration of an indenter under a large load (major load) compared to the penetration made by a preload (minor load). There are different scales, denoted by a single letter, that use different loads or indenters. The result is a dimensionless number noted as HRA, HRB, HRC, etc., where the last letter is the respective Rockwell scale. Higher numbers correspond to harder materials.
When testing metals, indentation hardness correlates linearly with tensile strength.
The differential depth hardness measurement was conceived in 1908 by Viennese professor Paul Ludwik in his book Die Kegelprobe (crudely, "the cone test"). The differential-depth method subtracted out the errors associated with the mechanical imperfections of the system, such as backlash and surface imperfections. The Brinell hardness test, invented in Sweden, was developed earlier – in 1900 – but it was slow, not useful on fully hardened steel, and left too large an impression to be considered nondestructive.
Hugh M. Rockwell (1890–1957) and Stanley P. Rockwell (1886–1940) from Connecticut in the United States co-invented the "Rockwell hardness tester," a differential-depth machine. They applied for a patent on July 15, 1914. The requirement for this tester was to quickly determine the effects of heat treatment on steel bearing races. The application was subsequently approved on February 11, 1919, and holds . At the time of invention, both Hugh and Stanley Rockwell worked for the New Departure Manufacturing Co. of Bristol, CT.S.W. Kallee: Stanley Pickett Rockwell Stanley Pickett Rockwell - One of the Inventors of the Rockwell Hardness Testing Machine]. Retrieved on 21 November 2018. New Departure was a major ball bearing manufacturer which in 1916 became part of United Motors and, shortly thereafter, General Motors Corp.
After leaving the Connecticut company, Stanley Rockwell, then in Syracuse, NY, applied for an improvement to the original invention on September 11, 1919, which was approved on November 18, 1924.
Official source
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