Impeccably round and smooth
© Schaeffler
December 2018

Impeccably round and smooth

By Denis Dilba
No bearing would rotate without perfectly shaped steel spheres. The rolling elements exhibit variances only in the micrometer range: a manufacturing technology feat whose beginnings date back more than 130 years – and that today ensures proper rotation and smooth acceleration.

​Rolling minimizes rubbing. This, in a nutshell, describes the equally simple and ingenious basic principle of a ball bearing. Its standard design has consistently remained the same for more than two centuries: an outer ring, an inner ring and a cage that retains, with equal gaps between them, the balls that gave the bearing its name. That’s basically it. And that’s what even the first modern grooved ball bearing looked like for which the British inventor Philip Vaughan was awarded a patent in 1794. It was designed to support axles in carriages – and still an exotically new technology at the time. Today, ball bearings have long become standard components used billion-fold around the globe. Most of them are found in automobiles, aircraft, wind turbines and cranes. The smallest versions untiringly toil away in dental drills or computer hard discs. They even assist everyday items like vacuum cleaners, trolley cases, turntables or food processors in achieving the right rotary motion. Experts assume that today more than 100,000 different ball bearing designs and versions exist. In short, without these largely invisible everyday heroes the world would simply grind to a halt.

The ball grinding mill marks a breakthrough

At the core of the bearings are the balls – also referred to as rolling elements in technical jargon. They determine how smoothly a bearing runs. Although Vaughan’s first ball bearing marked a technological leap from the previous plain bearings for axle applications the hand-crafted cast-iron balls used in his bearing were still a far cry from precision-manufactured products. They had to be painstakingly de-flashed by hand and then ground to assume a shape that was as round as possible. Also, compared to today’s ball bearings, Vaughan’s invention tended to rumble rather than truly running smoothly. That type of progress only came with the invention by a man named Friedrich Fischer. In 1883, after years of tinkering in his workshop, the locksmith and turner from Schweinfurt, Germany, managed to produce the first hardened cast-iron industrial-scale spheres that were exactly of the same size and perfectly round, using the “ball grinding mill” he had developed.

Impeccably round and smooth
Ball blanks (bottom left) used to be cast. Today, thanks to advanced manufacturing technologies, they’re pressed within fractions of a second from metal wire that was previously cut into small pieces (above)
Boosted the development of rolling bearings in 1883: Friedrich Fischer’s “ball grinding mill” (bottom left)© Schaeffler
Impeccably round and smooth© Schaeffler

By employing a grinding technique previously utilized for grinding marbles, Fischer in his operation achieved accuracies of down to 20 micrometers (0.00079 inches) – a previously unknown level of precision. For comparison: The hair of a Central European has a thickness of 70 to 80 micrometers (0.0028 to 0.0031 inches). The ball grinding mill patented in 1890 resulted in the worldwide breakthrough for ball bearings – with Fischer’s precision rolling elements they were running smoother than ever before because the closer the metal balls come to the ideal geometric form the lower the friction resistance they generate. The balls of ball bearings produced by Schaeffler today even achieve accuracies in the single-digit micrometer range. For the human eye, such minimal differences in the surface quality of the spheres that gleam like silver are not discernible – not only due to the perfection of their surfaces but also because ball production takes place at such a rapid pace: today, rattling machines spew out massive amounts of metal balls at one-second intervals.

Seven steps to success

Some 30 metric tons (33 short tons) of them per day leave Schaeffler’s plant in Homburg, Germany. “We’re basically able to provide every human being on Earth with one rolling element,” says Matthias Feld, operations manager at the Homburg plant where ball diameters between 3 and 17.5 millimeters (0.1 and 0.7 inches) are produced. In the Bavarian town of Eltmann, precision ball manufacturer Umbra produces up to eleven metric tons (12 short tons) for Schaeffler– all with diameters between 18 and 200 millimeters (0.7 and 7.9 inches). The latter are used in large-scale transmissions such as those of wind turbines, among other things. In contrast to the days of ball-production pioneer Fischer, the use of cast iron technology has long ceased in manufacturing the ubiquitous rolling elements. Today, the bearing component, whether small or large, that is the most challenging one in terms of manufacturing technology is always produced in a seven-step process: wire cutting, cold heading, de-flashing, hardening, grinding, pre- and post-lapping.

Sensors on board
Impeccably round and smooth© Schaeffler

For a long time, rolling bearings were a classic exam- ple of components that were inherently analogous. This changed with the introduction of so-called sensor bear- ings. Schaeffler’s VarioSense bearing – a product that has won an award this year – provides several different mea- surements for machine and process monitoring – and as a connecting link between mechanical and electronic sys- tems enables Industry 4.0 (aka industrial IoT or smart fac- tory) solutions. Readings obtained in this way of rotational speed, temperature or force load make remote monitoring of component assemblies possible. As a result, impending defects can be detected due to noticeable changes in the signal curves before they lead to costly downtimes of an entire machine. In addition, continuous digital monitoring can reduce previously required maintenance intervals. At the moment, the VarioSense bearing launched last year is only available as a grooved ball bearing – however, VarioSense cylinder rolling bearings and tapered rolling bearings are currently in development.

Where steel balls reach their limits
Impeccably round and smooth© Schaeffler

Steel balls in bearings for electric motors are prone to wear compara- tively fast. “The currents generated there can flow from the outer ring of the bearing via the metal ball to the inner ring, and over time may damage the ball surface,” explains Schaeffler product manager Markus Seis. Therefore, ceramic balls are used for such applications as well. They’re produced from ceramic pow- der that’s compressed into shape and fused in a smelting furnace. The material has insulating properties, so damage by electric current is excluded. “In addition, ceramic balls, Extremely tough but not exactly cheap: Cronitect hybrid rolling bearing due to their higher hardness com- pared to steel spheres, have better emergency running properties,” says Seis. Even without a lubricat- ing film they allow for a bearing to run slightly longer without suffering damage than steel balls. However, due to their more expensive pro- duction, ceramic balls cost about ten times more than their steel rel- atives, but offer longer maintenance intervals. Consequently, typical applications for ceramic balls are bearings in wind turbine generators where they help achieve savings in the costly deployment of cranes.

Impeccably round and smooth
The Triondur-C-coated Schaeffler ball with a roundness of 160 nanometers (6.2992e-6 inches)© Schaeffler
21.26 meters (69.75 feet)

This is the distance across which the roundest of all Schaeffler spheres rolled. The high-tech product with a weight of exactly 500 grams (17.64 ounces) and a diameter of five centimeters (1.97 inches) set this record during a Japanese television show billed as “handwork vs. high tech.” In keeping with the show’s name, the Schaeffler team, led by Thomas Kreis, Vice President Competence Center Bearings, and Andreas Bohr, Vice President Development Ball Bearing, competed against a glass sphere crafted by an artisan factory in Yokohama. Although the Triondur-C-coated Schaeffler ball with a roundness of 160 nanometers (6.2992e-6 inches) was defeated by the glass ball from Japan, which achieved 30 meters (98 feet), it set a new record: „A steel ball never rolled this far in previous shows“, says Kreis: “A resounding success. Schaeffler received a lot of attention in Japan,” says Shinzo Yotsumuto, Managing Director of Schaeffler Japan, who is not at all upset about the “defeat.” It was to be expected: the form of the path and its lower weight gave the glass ball an advantage.

Impeccably round and smooth
Schaeffler experts explain the fortes and technical details of their high-precision bearing ball to a Japanese television crew© Schaeffler

The perfect rolling element starts out as a steel wire. Wound on large rolls, the wire is placed on top of a ball press, fed into the machine and cut off. Subsequently, two semi-spherical dies, pressing against each other with the force of about ten metric tons (11 short tons) give the wire sections the shape of a ball, resulting in a blank with a so-called Saturn ring and a pole. In the next step – de-flashing – these irregularities are removed and the ball becomes rounder. After that, the unfinished balls are hardened by heating them in a furnace and subsequently cooling them down again in oil. This arranges the atomic structure of the spheres in a way that makes them stronger. During grinding, pre- and post-lapping, the balls are machined using high-tech materials like ceramics or even diamond powder. For quality assurance, roundness and roughness are checked on randomly selected balls in special measuring rooms. This is followed by washing, a surface inspection – of every single ball – and finally by packaging.

From Homburg or Eltmann the balls are then shipped to the customers’ sites where in diverse applications they invisibly and dependably do their job – and will keep the world in motion going forward.