Tribology

Vacuum centifuge test facility

With more sophisticated, longer life and higher reliability mechanisms being developed for current and future spacecraft needs, it is important to remember that certain fundamental aspects remain, including the phenomena of friction and wear and the need for lubrication (the area of science known as Tribology).

Spacecraft mechanisms typically require lifetimes in-orbit of 7-10 years or more of extremely reliable and smooth operation. Yet during launch they are subjected to severe vibration and space itself is a hostile environment in which the absence of oxygen encourages metallic surfaces to weld together and extremes of temperature are experienced. If not effectively lubricated mechanisms cease to run smoothly and ultimately wear out or jam, yet conventional oils and greases would freeze at the coldest space temperatures or evaporate into the vacuum of space.Given this, the lifetime and performance of spacecraft mechanisms is critically dependent upon effective lubrication and minimisation of the wear which will ultimately result in either premature failure or a planned end-of-life for the mechanism. Therefore application of the principles and a sound understanding of space tribology is required in the design, procurement, assembly and qualification of mechanisms and their components, as well as the selection and application of their specialised lubricants.

So that payload, platform and launch vehicle mechanisms can meet their critical performance requirements, ever-greater demands are placed on the lubrication approach used in them. This in-turn requires more demanding tests to be carried out under simulated thermal vacuum conditions of space which prove that the mechanisms will perform to expectations.

Experience has shown that careful attention to space tribology issues at an early stage can bring considerable cost and schedule savings to all involved. It was for these reasons that ESA developed the European Space Tribology Laboratory (ESTL), a centre of excellence in space tribology, which provides an independent and confidential service to European designers and engineers involved in the design, development and testing of spacecraft mechanisms.

Typical lubricants

Oils and greases are often grouped as liquid lubricants.

Oils: Oils are very effective lubricants at sufficiently high speeds as they tend to create rather stable friction conditions and resistive forces/torques (i.e. low noise). However, they tend to evaporate (though at a relatively low rate compared to oils for non-vacuum applications) and to migrate by surface creep. Therefore, oils are suitable especially for mechanisms, which are sealed like for example reaction wheels. It is important when designing the mechanism to take into consideration degradation of the oil, which takes place particularly with PFPE oils like Fomblin Z25. The Fluor contained in these oils tends to chemically react with the iron of the steel in the contacting moving parts (for example bearing balls and races). This leads to a breakdown of the oil which is detected by a huge increase in the contact loads or torques. This contact loads increase is due to a polymerisation of the oil (creation of solid particles). Some oils like MAC oils (for example Nye2001a) do not contain PFPE and are therefore less sensitive to lubricant breakdown.

Greases: Greases are widely used in space mechanisms because they tend to generate relatively low noise and provide good lubrication even at low speed, and they are less sensitive to evaporation and surface creep compared to oils. Practically a grease for space application is oil (which is known as the base oil of the grease) in which solid substances helping in the lubrication process are added (for example PTFE particles). The same problem with the degradation of the lubricant exists for the greases since grease contains its base oil. For example Braycote601EF which contains as base oil the Brayco-815Z is subject to lubricant breakdown by polymerisation. The MaplubSH50a which contains as base oil the Nye2001a is much less sensitive to this lubricant breakdown. As an example, while the typical lubricant life expectation in a steel bearing lubricated with Braycote601EF is about 10 millions revolutions, a factor 10 or more is expected on the typical lubricant life expectation in a bearing lubricated with MaplubSH50a.

Solid lubricants: These lubricants tend to generate more noise in the resistive forces and torques but do not evaporate. They are therefore suitable for extreme temperatures (cryogenics or very hot applications) or in applications where contamination by condensation could be an issue (e.g. in optical systems). These lubricants are used for example for Bepi-Colombo where temperatures of about 250 deg C are expected. The mostly used solid lubricants for space mechanisms are the sputtered MoS2 and the Ion Plated Lead. The sputtered MoS2 exhibits a lower friction than the Ion Plated Lead (and it is typically less noisy) but has a lower life expectancy. Both lubricants are not suitable to be used in air since the friction coefficient significantly increases and the life expectancy is considerably reduced. To be able to use solid lubricants during ground testing in air (which has the advantage of leading to reduced cost), the solid lubricant can be located in a kind of reservoir which is, in case of a bearing, the cage. In this case, the lubricant is transferred from the cage to the balls and from the balls to the races. Thanks to the reservoir, even if the solid lubricant is degraded in air, new fresh lubricant is provided by the reservoir. For sputtered MoS2, the reservoir (cage in case of a bearing) can be made of special compound materials, e.g. PGM-HT. For Ion Plated Lead, the reservoir (cage in case of a bearing) can be made of leaded bronze. To help in the lubrication before the lubricant transfer is installed, sputtered MoS2 will be also placed on the contacting surfaces (balls and races in case of a ball bearing using a PGM-HT cage). When using a leaded bronze cage, Ion Plated Lead will be placed as well on races for the same reason. It is important to perform the run-in of the mechanism (or the bearing) in vacuum because of the degradation of the sputtered MoS2 and Ion Plated Lead in air. A typical run-in will consist in about 50000 revolutions (case of a bearing). It is to be noted that, while after run-in, PGM-HT can withstand several million revolutions in air; leaded bronze is limited to not more than 100000 revolutions. Another important point to mention is that PGM-HT needs to be heat treated because it tends to shrink when going from high to ambient temperature. Both PGM-HT and leaded bronze can provide life expectancies of several hundreds of million revolutions in vacuum (case of a bearing).

Examples

The Meteosat Second Generation (MSG) spacecraft to be launched allows Europe to maintain its leading role in gathering global weather data until at least the year 2012. The main payload, the SEVIRI optical imaging radiometer and GERB, an instrument that monitors the Earth's radiation budget at the top of the atmosphere provided challenging tribological issues. Bearings supporting the scan mirrors on both payloads were required for contamination reasons to be lubricated by specialised solid-lubricants. However as MSG is spin-stabilised, bearings in both instruments are subjected to a continuous acceleration (g-forces) throughout life. In the case of GERB this is a radial acceleration of 16g and to demonstrate the lubricant it was necessary to carry out a lifetest for 230million revs, compressing the whole operating lifetime and safety margins into approximately 1 year of testing.

MIDAS Wheel

Evaluation of different lubricant options for these challenging applications commenced 5 years prior to the first flight. To simulate the g-forces generated in these instruments whilst in orbit, bearing lifetests were conducted in dedicated centrifuge facilities at ESTL that imparted the required accelerations to various test bearings including the most likely candidate lubricants. The finally selected lubricant option, lead-coated bearings using a hybrid leaded-bronze cage, were shown to operate for the required period.

Translation Stage Sub-Assemblies after Test at ESTL

Rosetta is an interplanetary mission whose main objective is to rendezvous with and make in-situ measurements of a comet. The MIDAS Instrument onboard the Rosetta spacecraft will investigate the physical parameters of cometary dust with an atomic force microscope. MIDAS designers at ESA were closely supported by experts at ESTL in the design and selection of the numerous tribological items (rotary and linear bearings, launch locks, piezomotors etc.) within the instrument and in deciding on the most appropriate lubricants and surface treatments.

One interesting challenge for the lubrication was the requirement for 9 years of instrument dormancy during the long cruise-phase of the mission prior to operation for a period of 2 years once rendezvous with the comet occurs.

Following its consultancy advice, ESTL procured, lubricated and assembled, critical tribological sub-assemblies and then carried out the necessary thermal vacuum tests to qualify them for flight.

Last update: 7 May 2014

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