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Technology Topical 20.02.2019     Author: Annette Bammert

CEROBEAR rolling bearings on seven-year Mercury mission in space

On 20 October 2018 at 03:45 CEST, the four-part BepiColombo space probe was launched in the Ariane 5 carrier rocket on its journey to Mercury. Also on board: Rolling bearings of the MinebeaMitsumi subsidiary CEROBEAR. The space mission was launched at Kourou (French Guiana). The mission is a cooperation between the Japanese Aerospace Exploration Agency (JAXA) and the European Space Agency (ESA). The spacecraft, named after the Italian mathematician Giuseppe Colombo, is expected to be on its way for seven years.

With a diameter of 4,878 kilometres, Mercury is the smallest and least explored planet in our solar system. This is due to the harsh conditions for space probes in the vicinity of Mercury and, consequently, the Sun. The solar radiation in the Mercury orbit is about ten times higher than on Earth. The long Mercury days and nights play a role too: The change between day and night takes 176 days on Mercury. The temperature on the side facing the sun is therefore extremely high at approximately 430 degrees Celsius, and extremely low at minus 180 degrees Celsius on the shadow side. This intense thermal stress requires all devices and components to meet special heat management requirements.

After Mariner 10 and the Messenger orbiter, BepiColombo is only the third space probe on a Mercury expedition. The scientists involved hope to obtain a comprehensive description of the planetary properties and history of Mercury. One of the two satellites, which will separate from the probe at the target, will examine the surface, the other the magnetic field.

BepiColombo is expected to reach its target in December 2025. Nine so-called "swing-by manoeuvres" have to be carried out along the way. The spacecraft passes Earth once in April 2020, then Venus twice and Mercury itself six times. These manoeuvres are intended to decelerate, so that the 4.1-tonne probe does not collapse into the interior of the solar system.

CEROBEAR rolling bearings are on board

Rolling bearings from the MinebeaMitsumi subsidiary CEROBEAR are used in the current Mercury mission in, among other applications, the Solar Array Drive Mechanism of the MPO (Mercury Planetary Orbiter), i.e. in the satellite that shall explore the surface. The long journey and the extreme conditions place special demands on the rolling bearings: After seven years of standstill, the bearings must function perfectly and withstand the high temperature fluctuations in the Mercury orbit. Therefore, hybrid bearings are used for this mission, where the rings are made from steel and the balls from silicon nitride. This is the only way to ensure that there is no cold welding between rings and balls during the start and the long journey, even without lubricant.

CEROBEAR's aerospace history began at the beginning of 2000 with the first flight of ceramic cylindrical rollers in the bearings of the liquid hydrogen pumps of the Space Shuttle. The ceramic rollers operated at 36,200 rpm at temperatures of minus 250 degrees Celsius, and were lubricated only with liquid hydrogen. Using CEROBEAR rolling elements improved the maintenance interval of the engines by a factor of 12, while the payload was increased by 9 percent at the same time. The maintenance times between missions were reduced from 1000 hours to less than 50 hours.

In aerospace applications, CEROBEAR bearings must be bespoke and designed for a long service life at ultra-high precision and low friction. Materials and coatings that are suitable for use in space are available for low-speed bearings, which are typically used in space mechanisms. New rolling bearing steels and ceramics are used for the extreme requirements.

CEROBEAR GmbH at Herzogenrath is a spin-off company of the Fraunhofer Institute for Production Technology, and an innovative manufacturer of rolling bearings with ceramic components. Core competences are hard machining of ceramics and hardened rolling bearing steels, development and production of customer-specific bearing solutions as well as testing of rolling bearings and their components. Innovations are the result of permanent research and development in the fields of materials technology, manufacturing technology and new bearing geometries.

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