“On Brinell, Rockwell, Vickers, Shore?.. On MET!”

“On Brinell, Rockwell, Vickers, Shore?.. On MET!”

S. Aleksandrov, “Technology for the Youth”, 2001

4 STATIONARY HARDNESS TESTERS IN ONE POCKET

Machines break down! Operating errors, non-planned external factors, incredible coincidences… And, of course, their own unreliability. But what determines reliability?

Turns out it’s a complex integral probabilistic characteristic that can’t be directly measured. But it can be calculated from other parameters of machine parts. The most important of them is hardness. It determines how resistant the working surfaces of the parts are, and that’s where the destruction of machines is most likely to begin.

Hardness can be measured, but… how exactly? A diamond pyramid, a conus or, say, a steel ball is pressed into the tested surface. If we count the ratio between the load and the indentation diameter of the steel ball, we will get the hardness value according to the Brinell method. In the Vickers hardness test, the indentation surface of a four-sided diamond pyramid is measured. Of course, the sizes of balls and pyramids, as well as the applied force are standardized. During the Rockwell test, two different loads are applied sequentially, and the indentation depth increment is calculated. Finally, according to the Shore method, hardness is determined by the rebound height of a pin that drops freely onto the sample from a fixed (standard) height.

In each case, the values are compared to the reference numbers obtained in the same manner, and the result is recorded as follows: “hardness equals this or that Brinell (Vickers, Rockwell, Shore) number”.

However, none of the four methods is perfect. Despite all enhancements, mechanical devices are bulky, demanding in terms of metrologists’ qualifications, and unsuitable for use in hard-to-reach places (a horizontal open-top surface is the ideal condition).

The diamond pyramid is taken from the Vickers method, it is pressed into the sample by a fixed force of a calibrated spring. But that’s where the similarities with its predecessors end. The pyramid here is an acoustic resonator of an ultrasonic generator. In full compliance with the laws of acoustics, other things being equal, the natural frequency of the resonator depends on how the pyramid indenter has penetrated into the tested surface. Which, in turn, clearly depends on the hardness of the latter. Relative changes in the resonator frequency are transformed by the electronic module into hardness values of a chosen scale.

The electronic module performs the measurement, monitors the penetration of the pyramid into the sample and the change in the resonator frequency. The electronic module microprocessor calculates the sample hardness for the scale chosen by the operator. In fact, the electronic module is a specialized microcalculator storing in its memory both individual features of the probe (its calibration) and the main properties of the hardness scales.

The hardness tester is calibrated against hardness reference test blocks verified on the state hardness primary standard machines on Brinell, Vickers, Rockwell and Shore D scales, which guarantees high accuracy of measurements. The hardness tester allows to work on any standardized scale, since the values obtained by different methods are connected by certain dependencies. You can calibrate three additional scales by yourself – that’s important if you need to work with, say, aluminum, copper or any other materials. Finally, it is possible to store up to 100 measurement results, pre-process them, and, in case you’re using the MET-U1, copy them on your PC via RS-232C interface.

But most importantly, compact devices (slightly bigger than a microcalculator or a cell phone) guarantee operative hardness control of almost any surface and part, and in any direction.