On February 11, 1919, Hugh Rockwell and Stanley Rockwell received a patent for the Rockwell hardness tester, a device for measuring the resistance of metallic materials to force. Though they were unrelated, both men worked for the New Departure Company in Bristol. An aviator and automobile enthusiast, Hugh was the son of Albert Rockwell, one of the company’s founders, and Stanley worked as a metallurgist.
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Founded in 1888, the New Departure Bell Company began by making doorbells. It then patented a new type of coaster brake and by 1908 had developed the double row bearing. An evolution in design, the bearing consisted of a ring-shaped track containing a double row of freely revolving metal balls. These bearings played an increasingly important role in the construction of automobiles—a new and rapidly expanding industry—and New Departure became the leading bearings manufacturer. By 1916, the company had become part of United Motors.
The hardness tester developed out of the need for a machine that could quickly and easily measure the effects of heat treatment on the bearing raceway, or track. (Determining hardness provides information on the durability, strength, and flexibility of the material being tested.) A common method of measuring hardness involved attempting to indent the material being tested by pressing a sharp object into its surface using increasing amounts of pressure. Hugh and Stanley expanded upon the traditional indention test by using a conical diamond for the indention media and basing the test results on displacement. The main advantages of the Rockwell test were the much smaller area of indentation needed to obtain a measurement and the fact that the test could be performed on curved as well as flat surfaces.
Both Hugh and Stanley had moved on from the company before receiving approval for their patent application. In 1924, Stanley Rockwell began the commercial production of Rockwell testers with an instrument manufacturer in Hartford, Charles H. Wilson. This company grew into the Wilson Mechanical Instrument Company. The Rockwell hardness test remains the most efficient and widely used hardness test today and is recognized as one of the 20th century’s metallurgical innovations.
The Rockwell hardness test measures hardness in the simplest way possible, by pressing an indenter into the surface of the material with a specific load and then measuring how far the indenter can penetrate. Generally, the indenter is made of either a steel ball or a diamond.
In February 1919, Hugh Rockwell, and Stanley Rockwell, were awarded a patent for the Rockwell hardness testing device, an apparatus designed to measure the resistance of metallic materials to an applied force. Though they were unrelated, both men worked for the New Departure Company in Connecticut and were respectively an automobile enthusiast and a metallurgist.
Illustration courtesy of the United States Patent Office
The Rockwell hardness tester was developed out of a need for a machine that could quickly and easily measure the effects of heat treatment on bearing raceways, as establishing hardness provided information on the durability, strength and flexibility of the material being tested. A commonly used method of measuring hardness involved attempting to indent the material being tested by pressing a sharp object into its surface using increasing amounts of force. The two engineers augmented the traditional test method by employing a conical diamond for the indention device and basing the test results on the depth of indenter penetration. Important advantages of the Rockwell test were the much smaller test area needed to obtain an accurate measurement and the fact that the test could be performed on both flat and curved surfaces. In 1924, Stanley Rockwell began the commercial production of Rockwell hardness testers with Charles Wilson, an instrument manufacturer based in Connecticut.
Rockwell hardness remains the most efficient and extensively used hardness test and is recognized as one of the 20th century’s metallurgical innovations. The Rockwell test method is used on all metals, except in circumstances where the test metal structure or surface conditions introduces too much variation, for example, the indentations would be too large for the application or where the sample size or shape would prohibit its application.
ZwickRoell Rockwell hardness testing machine
The Rockwell method measures the permanent depth of indentation produced by a force on an indenter. First, a preliminary test force, commonly referred to as preload or minor load, is applied to the test sample using a diamond or ball indenter. This preload breaks through the surface to reduce the effects of the sample surface finish. After maintaining the preliminary test force for a specified dwell time, the baseline depth of indentation is measured.
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After the preload, an additional force, called the major load, is added to achieve the total required test force. This force is held for a predetermined amount of time or dwell time, to allow for elastic recovery. This major load is then released, returning to the preliminary load. After holding the preliminary test force for a specified dwell time, the final depth of indentation is measured. The Rockwell hardness value is derived from the difference in the baseline and final depth measurements. This distance is converted to a hardness number and the preliminary test force is removed and the indenter is retracted from the test sample.
Test method illustration
A = Depth reached by the indenter after application of the preload or minor load
B = Position of indenter during total load, minor plus major loads
C = Final position reached by indenter after elastic recovery of test sample material
D = Distance measurement taken representing the difference between preload and major load position. This distance is used to calculate the Rockwell Hardness Number.
A variety of indenters may be used, for example, conical diamond with a round tip for harder metals to ball indenters with diameters ranging from 1/16” to 1/2” for softer materials.
Range of Rockwell hardness test indenters
When selecting the appropriate Rockwell hardness, a general guide is to adopt the scale that specifies the largest load and the largest indenter possible without exceeding defined operation conditions and accounting for conditions that may influence the test result. These conditions include test specimens that are below the minimum thickness for the depth of indentation, a test impression that is too close to the edge of the specimen or another impression or testing on cylindrical specimens. In addition, the test axis should be within two degrees of perpendicular to ensure precise loading and there should be no deflection of the test sample or test machine during the loading application from conditions such as dirt under the test specimen or on the elevating screw column. It is also important to keep the surface finish clean.
Conversion charts are available that allow conversion from one method of hardness to another, but it must be remembered that these conversion charts cannot precisely convert from one to another.
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