by Edward Tobolski

When it comes to hardness testing, instruments that use the newer closed-loop systems are proving most effective for performing a wide range of tests. Their inherent design features offer benefits that can significantly improve performance compared to the dead-weight testers widely used in industry today. These older testers can’t match the repeatability, stability and flexibility of a tester with a closed-loop system. Thus, if your business requires a hardness tester that provides the best possible test results, you should definitely consider one that uses a closed-loop system.

For more than 75 years, the most common hardness testing instruments have used dead weights to apply test forces. The reason for this is fairly simple: Dead weights are relatively inexpensive and easy to manufacture to the degree of accuracy required by commonly used test methods. The problem is that the force must be applied to the test piece through some type of small indenter. Transferring any dead-weight force to the tip of a small diamond or ball indenter—especially a force as great as the 150 kg used for a Rockwell HRC scale test, for example—is difficult to accomplish. The large size and mass of a 150 kg weight requires designers to use smaller weights with levers to intensify the force to the desired levels. Levers require pivots, guides and other friction-producing elements that can introduce errors. Although instrument manufacturers have done an excellent job trying to control these sources of error, any friction point in the system will have a negative effect that increases during use.

It’s also difficult to control the weight’s application in a dead-weight system. Because the dead weight must be moved to apply the test force, stopping it quickly without overload and oscillation is problematic. Many older testers use dashpots to control the application; however, these are prone to serious variations in test results because of seal wear and temperature changes. Many newer designs have replaced dashpots with motors. Although this eliminates some dashpot problems, the need to perform tests quickly makes motor speed critical, and as a result, force overshoot and oscillation are frequent problems.

Open- and closed-loop systems

Instruments that use dead weights are normally open-loop systems. In other words, the forces are applied based upon the calculations of the weights, lever ratios and the like. Typically, a manufacturer will perform an initial calibration on a dead-weight hardness tester to ensure that the forces applied are within tolerance. They do so by using an independent measuring device, usually an electronic load cell. The instruments are seldom checked again because it’s assumed that forces remain consistent as each test is performed. Although dead-weight systems have proven to work very well in many applications, including hardness testers, there’s always a performance level that typical (i.e., affordable) dead-weight systems can’t surpass because of inherent problems.

During the 1950s, closed-loop systems came into common use on tensile testing instruments. Closed-loop systems differ from open-loop systems in that they electronically measure the force being applied during every test and then feed—or loop—the information back to the control system. This system is designed to use the feedback to adjust the force application mechanism to apply only the desired force. Closed-loop systems work so well that today all electronic tensile/compression instruments exclusively use closed-loop control.

Why are closed-loop systems better for hardness testers?

Not only is a closed-loop system able to constantly measure the test force being applied, but the components used in a closed-loop system inherently lend themselves to a much simpler design than those in a dead-weight system. As mentioned, dead-weight systems require levers, pivots and other friction-inducing components to function efficiently (see Diagram A). The indenter, the only part of the system in contact with the test sample, is detached from the weight, separated by the levers, pivots, etc.

In contrast, a closed-loop system’s main component is a strain-gage load cell. This compact, low-weight device provides an electronic output proportionate to the force applied to it. Load cells come in many different shapes; it’s therefore possible to design a hardness system with the indenter attached directly to the load cell (see Diagram B). With this design, sources of error between the indenter and the test force are eliminated.

These designs use actuators to apply the test forces, and although these actuators have bearings and sliding surfaces that may introduce friction, the design isolates these negative influences above the load cell so they don’t affect the critical test force. If, for example, friction in the actuator is so excessive that the desired force isn’t applied to the indenter, the load cell won’t indicate the correct force; therefore, the system will abort the test rather than give an incorrect result. In this way, the system constantly checks itself to make certain that only the correct test forces are applied to the indenter. The actuator’s mass can easily be controlled because of the feedback loop.

How well do these systems work?

A common way to measure the performance of a hardness tester is to use gage R&R techniques. This method quantifies a measuring instrument’s performance by comparing variations from the instrument against the total variations allowed for the part being measured. The result is a percentage that indicates how much of the tolerance is being used up by the instrument itself. The smaller the percentage, the better the instrument’s performance. Typically, those who use this method want to obtain gage R&R results of 10 percent or less, although 30 percent is acceptable in some situations. Hardness testers frequently fall into the 30-percent category because they often don’t perform well, and variations within the sample consume a percentage that’s difficult to quantify.

Depending on the hardness tester’s age and design, gage R&R results from standard dead-weight Rockwell scale testers normally range from 12 percent to 25 percent. Under tightly controlled, ideal conditions, the 10-percent target has been reached. Under the same conditions, a properly designed Rockwell tester using a closed-loop system can routinely achieve less than 7 percent, and tightly controlled units have achieved results as low as 2 percent. (Note: 2 percent is considered the lowest attainable percentage because of the nonuniformity of test samples.) In addition, closed-loop systems have proven to be more stable, which increases the test data’s reliability.

Benefits of increased performance

How important are test results? If you’re simply trying to verify whether a part has been heat-treated, achieving 10 percent of gage R&R improvement may not be important. However, if you’re working to specific tolerances, any reduction in the uncertainty of your results can save money by minimizing the possibility of either rejecting a good part or accepting a bad one. A better knowledge of the hardness value makes it possible to adjust processes for the most economical operation.

Uncertainty is the buzzword today for people in charge of calibrations. Anyone working to ISO 17025 must provide an uncertainty statement with most calibrations performed, including hardness. It’s a logical extension that customers may someday ask for uncertainty statements with every test performed. Although this will be a difficult value to determine accurately, the calculation will be influenced significantly by the hardness tester’s performance. The better the hardness tester performs, the lower an uncertainty will be.

Applications

Hardness testers using closed-loop systems are available for Rockwell, Vickers, Knoop and Brinell testing in a variety of test force ranges. Load cells typically have force-range limitations of 100 to 1. In other words, if the lowest force were 10 kg, the highest force would be 1,000 kg. This is normally a greater range than most dead-weight testers provide. Additionally, a closed-loop system allows the use of any incremental force within the acceptable range. Dead-weight systems, on the other hand, are restricted to the discrete weights. Some of the newer load cells can exceed the 100-to-1 limitations.

Another benefit derived from closed-loop systems is inherent flexibility. Because the force application process is controlled by a microprocessor, test cycles can be changed easily. Not only is this feature desirable for special testing requirements, but it also guarantees that a tester can be easily modified to meet any new or revised test method. This can be very helpful if you’re interested in having, for example, a Rockwell tester that can match the time cycles used on the new Rockwell hardness standards from the National Institute of Standards and Technology.

About the author

Edward Tobolski is a 27-year veteran of the hardness testing industry. He served as the engineering manager of Wilson Instruments before becoming the company’s president in 1990. Since the 1993 acquisition of Wilson Instruments by Instron Corp., Tobolski has held the executive position of general manager of hardness products. Letters to the editor regarding this article can be sent to letters@qualitydigest.com.