Hardness testing of metals has been a major judge of quality since the early 2000s. In the past, dead weights have been used to apply the test force in open loop systems, as they are relatively inexpensive and easy to manufacture.
The main problem with dead weight hardness testers is that they are difficult to manage. Since the weight being applied is considerable, and the indenter (the small diamond or ball that actually presses against the material being tested) is very small, the force must be administered through a complex system via a number of smaller weights and levers.
The force increases to 3000 kgf for the Brinell hardness test, making the challenge to control the dead weight even greater. Since there is no feedback from the surface in the open loop system, stopping the weight with precision is also difficult. In addition, these levers require a number of parts which produce friction and instability, which will only increase with wear and tear. As a result, these systems have relatively high maintenance costs and are often impractical for use in real world applications.
The load cell – a sensor that measures force and delivers an electrical signal as output – is used to turn material deformation into an electrical signal. The feedback is used to adjust the motor that applies the force.
This increases the precision of testing and applies the correct weight on the sample, reducing human error in testing. It is also a more reliable testing system, as automatic loading and unloading provides repeatable and dependable test results.
A closed loop system constantly measures the force applied and a feedback loop is part of the tester, making the test force highly accurate.
There are fewer mechanical parts in these systems, and the load cell is installed as close to the indenter as possible. This makes the closed loop system more stable in design and reduces risk of mechanical imperfections, unplanned movement and other disturbances. The simple electronic calibration procedure also speeds up test procedures considerably.
The closed loop system is often more expensive, and also depends on electricity for its operations. However, it has a mature control system and arrives at the closest to true value, regularly and reliably.
That’s why, today, almost all electronic tensile and compression instruments use closed loop systems exclusively. Blue Star Engineering & Electronics recommends the use of load cell based closed loop hardness testers in all use cases. Contact us to learn more.