Our materials laboratory: This is how we examine defective rings

Do you know why rings that fail a material test with us are in for a really tough night in the lab?

Being steel at Rothe Erde is no easy job: our colleagues in production heat it to more than 1,200 degrees, compress it with a force of up to 60 meganewtons, then pierce and roll it seamlessly. After that, they machine the ring by turning, drilling, and grinding it. Once it has been brought into its final shape, our colleagues assemble it with other components into a Rothe Erde bearing – provided it passes all quality tests.

Important: The correct hardness of a ring

One thing we pay special attention to during quality testing is the correct hardness of the ring. After hardening – for example, inductive hardening – our production employees check the achieved hardness level.

Various mobile handheld devices and methods, such as the Leeb rebound hardness test, are used for this. Our colleagues ensure that predefined limits and tolerances are adhered to. These handheld measurements are non-destructive, meaning the measurement does not damage the bearing.

When things go wrong, the materials lab steps In

If the required hardness is not achieved and cannot be reached even after reworking, the job of our colleagues in the materials laboratory begins. Their mission: find out what went wrong.

Bad news for the ring: it will never become a Rothe Erde bearing. Here, destructive testing is performed. The employees cut the ring and examine its parts in the laboratory. Several methods are used here as well. One of the most precise techniques – our focus in this article – is the Rockwell test performed with a fixed testing machine.

A hard night for the ring, at the mercy of the diamond tip

Our lab colleagues clamp the ring segment into the Rockwell hardness testing machine. This machine “pokes” (or more correctly: applies force to) it automatically 40 to 50 times at different points using a diamond tip – usually overnight.

The process may sound almost cruel, but it has a good reason.

Rockwell testing: accurate to a hundredth of a millimeter

Here is how the Rockwell method works: first, the testing machine lowers a conical diamond tip onto the ring’s surface. The diamond tip is subjected to a preliminary load of 98.1 newtons, pressing it onto the surface.

This preliminary load is relatively low, comparable to a weight of about 10 kilograms. It remains on the test piece throughout the entire procedure and ensures that tiny irregularities in the steel are evened out. It also determines the zero point of the penetration depth. A diamond is used by the way because it is harder than our hardened surfaces.

In the second step, the main load is applied to the diamond tip. When testing hardened surfaces, this load is 1,373 newtons (roughly equivalent to 140 kilograms). For softer materials like bronze, the load is much lower, starting at around 588 newtons.

The main load causes the diamond tip to penetrate the hardened material. After a few seconds, the machine removes the main load and the material itself pushes the diamond tip back out slightly. The device automatically measures the difference in penetration depth between the main and preliminary load. This distance – typically in the range of hundredths to tenths of a millimeter – is converted into the Rockwell hardness value.

Using the hardness profile to identify the issue

The machine performs this test over several hours at many points to map the hardness profile. From this, the lab employees can determine, for example: is the heat of one or both induction coils insufficient overall or fluctuating? Or is the material the problem? With the help of additional testing methods, the lab always discovers the cause, allowing the error to be corrected.

This test is also performed not only when there are production defects but also when a new machine is being set up. This ensures the correct performance and allows calibration if needed.

Many more methods ensuring quality

In addition to hardness testing, the materials laboratory includes a whole range of other testing procedures, all aimed at determining why a material behaves the way it does. These include non-destructive tests like ultrasound, eddy current, magnetic particle, or the dye penetrant testing, where cracks become visible through dye. There are also tensile and impact tests as well as metallographic examinations, where samples are prepared, etched, and examined under a microscope to analyze grain size and purity, for example.

All lab methods have one thing in common: they ensure that Rothe Erde bearings do reliably what they are meant to do – often over decades. And sometimes, that means a material has to endure a very uncomfortable night under the stern gaze of a diamond tip.