Load cell technology boosts hardness tester's accuracy

DN Staff

September 22, 1997

3 Min Read
Load cell technology boosts hardness tester's accuracy

Canton, MA--Rockwell hardness testers essentially measure hardness by determining the depth a steel ball or other penetrator moves into a specimen's surface under certain fixed test loads. Like the Brinell, Vickers, and Knoop tests, the Rockwell test defines a material's resistance to permanent indentation.

Although indentation hardness is not a fundamental material property, it has become one of the more reliable controls for the heat treatment and quality of manufactured parts. Unfortunately, correlating hardness test data taken with mechanical deadweight systems is challenging. Dirt, friction, and other mechanical parameters cause variations from tester to tester.

"Maintaining accuracy and repeatability means frequent disassembly, cleaning, and lubrication in the field," says Instron Corp.'s manager of hardness tester engineering, Jack Merck. By replacing mechanical deadweight mechanisms with load-cell technology and closed-loop servo controls, Instron engineers claim they've achieved the highest user-to-user and tester-to-tester displacement-measurement accuracy and repeatability ever attained.

Gauge Repeatability and Reproducibility (GR&R) is what General Motors uses to determine a given tester's contribution to error. The Wilson Series Rockwell 2000 Hardness Tester offers less than 7% GR&R with plus or minus 3 Rockwell-points tolerance. Significant reduction in test errors is evident from the GR&R certificate shipped with every tester. "No one in the industry does that," Merck asserts.

Series 2000's design eliminates all moving parts between the penetrator, force sensor, and displacement transducer. This arrangement defeats key error-contributing mechanisms found in traditional deadweight-type testers. Although Instron's engineers had experience designing load-cell-based tensile testing systems, they needed to create new algorithms to modify their load-cell technology for hardness testing.

A patent application was filed for the in-line servo system design. The load cell and displacement transducer mount directly above the penetrator; there are no levers, dashpots, or pivot points.

Texas Instruments' TMS 320-32C DSP controls the servo loop and provides operator interface functions. It uses load and position feedback to adjust the input current to the dc servo motor (manufactured by Worcester, MA-based K&D Magmotor). The motor drives a ball screw to apply load normal to the penetrator. This structure compensates for dirt, friction, and other mechanical contributors to error, and ensures application of the exact pre-set load to the specimen. Rockwell number determination relies on accurate displacement measurements obtained from a Heidenhain linear glass scale.

A concurrent design team used Silicon Graphics workstations with SDRC IDEAS to design the product for manufacturability. "Reducing parts count from 390 to 160 not only reduced inventory and lead time, but improved reliability so much that we doubled the warranty to two years," says Merck.

Other Applications

  • Tensile testing

  • Fatigue testing

  • Spring testing

Additional details...Contact Jack Merck, Instron Corp., 100 Royal St., Canton, MA 02021-1089, (617) 828-2500.

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