Lubricants and Heat Transfer in Space

In space, there is seldom a second chance. Precision motion-control devices must function reliably upon demand. Even after long-term exposure to hard vacuum, radiation environments, and extreme temperatures.

Lubricated bearings are at the heart of many of these systems:

  • Helping solar panels unfold
  • Deploying safety covers
  • Pointing mission-critical cameras and other sensors at targets of interest.

Precision heating can mean the difference between smooth operation and space junk.

Lubrication Selection in the Vacuum of Space

Typical lubrication selection criteria include the loads imposed on the rolling elements. As well as their speeds and the chemical compatibility of the lubricant and adjacent elements. The loads, speeds, and sizes of the components and interpart clearances limit the acceptable viscosity of a lubricant. The design must also consider lubricant migration under load.

Selection for space application adds to these criteria the suitability of the lubricant in radiation. In a vacuum, it is no use sending up oil that will evaporate or disintegrate before the mission concludes. Moreover, the viscosity of many lubricants is strongly influenced by temperature.

Consider the case of Castrol Braycote 601EF, a widely-used aerospace grease. The low-temperature starting torque was tested per ASTM D 1478. The results show a variation of nearly a factor of three across eleven degrees Celsius.

Variation of this magnitude suggests that thermal control of lubricants in general can set the stage for a smooth swing or a stalled-out system. This particularly true for aerospace lubricants.

Boundary Layer

The boundary layer of lubricant between rolling elements is critical for dynamic performance. It is where the extreme pressures of the Hertz stresses are supported on minutely thin layers of the lubricant.

In a lubricated system, a boundary layer is attached both to the free and fixed elements. That is, the bearing and the race.

Heat generates in this layer by the shearing action of the combined slip and rotation of the elements, and fortuitous feedback exists. The regions of highest shear experience the greatest viscous heating. As the local temperature rises, the viscosity is consequently reduced.

A cold system typically experiences much lower running torques than starting torques. By more than a factor of two when considering the Braycote 601EF. 

If the system must tolerate cold storage or a cold start, as most spaceborne systems must. Then the bearing and race may find themselves encased in a chilled lubricant. The lubricant can range in hardness from soap-like to epoxy-like.

Application of Heat

Precision heating of the bearing race is the ideal solution for continuous survival heating. As well as on-demand preheating of a lubricated mechanism.

Minimal applied heat is almost always advantageous in space applications. There a single finite power supply must generate adequate wattage for all spacecraft systems.

By applying heat to the bearing race, usually the only accessible lubricant-loaded surface, the viscosity of the lubricant in the boundary layer adjacent to the raceway can be preferentially lowered. This allows initial sliding motion between the bearing elements and the race.

Typical press-in or pillow-block bearing mounts may not allow heat application directly to the bearing race, but such mounting mechanisms may have adequate material to allow embedded heaters, such as the cartridge-type, to deposit the needed warmup heat into the support structure.

Lubricant Longevity

The longevity of a lubricant is an important design consideration. Particularly in space-going applications. Two aspects of longevity are equally vital:

  1. The breakdown of the lubricant itself
  2. The migration of the lubricant away from the bearing interface.

Lubricant Breakdown

The first concern, breakdown, is addressed primarily through material selection and operating conditions.

Ordinary engine oil breaks down largely due to high temperatures. As well as combustion-product blow-by in the engine. Whereas space systems are seldom as hot as an internal combustion engine and thermal breakdown is not a concern.

Radiation breakdown can be of interest. Particularly in mission environments where penetrating and ionizing radiation are present. For example, Jupiter, the Sun, or the Van Allen belts.

Depending on the type of lubricant, radiation damage can tend to increase viscosity due to undesired cross-linking of the molecules. Or it can scission primary bonds, breaking down the lubricant and rendering it more fluid.

The first type of damage is more typical in oils. Meaning that the desired lower viscosity of an oil is compromised.

The second type is found more often in greases. Meaning that the desired higher viscosity of the grease is lost.

Both damage mechanisms can result in off-design thermal performance of the lubricant. They change the desired operating temperature of the system to compensate for the radiation-induced viscosity variation.

Whatever the application, understanding lubricant reactivity with the contacting materials is essential. This includes the primary reaction of the base lubricant with the metals, ceramics, or polymers it will encounter. As well as the reaction of secondary products through ordinary use or radiation breakdown. The wide range of potential lubricants, additives, and substrate materials virtually requires life testing of new configurations.

Controlling Lubricant Migration

The other important component of lubricant longevity, especially in space applications, is control of lubricant migration. As a rolling mechanism moves, the lubricant is naturally squeezed out of the contact area. So supplying suitable replenishment is critical. This allows the load-bearing layer of lubricant to persist and perform its task.

This means keeping the lubricant soft enough to wick or spread, and firm enough not to run and leak.

Thermal control of the lubricated system is critical to maintaining this balance. Mechanical methods such as barrier films can be necessary to entrain the lubricant.

Exposure to the hard vacuum of space will evaporate out any molecules with nontrivial vapor pressure. These shorter, lighter molecules tend to lower the viscosity of a lubricant. Therefore, many manufacturers will vacuum-degas products prior to characterizing properties.

Even after degassing, lubricants may still be a source of volatile polymers. Even trace amounts can compromise the components of optical systems or other sensors.

High-temperature excursions can prompt undesired outgassing events. As well as risking bulk lubricant migration, and must be avoided. A reliable, properly-sized heating system provides the needed heat and avoid thermal runaways. A heater with redundant thermostatic control is optimal.

Advances in Lubrication

Lubrication advances in the last two decades have been considerable. Multiply-alkated cyclopentane (MAC) greases, such as Nye Rheolube 2000, are an important addition to the design options. In many applications, they may be a direct competitor to heritage polyalphaolefin (PAO) greases. For instance, the Castrol Braycote 601 discussed above.

These MAC formulations typically exhibit much longer wear life than an equivalent PAO system. But at the cost of a narrower functional temperature range.

Space applications push lubrication systems into challenging territory, but successful designs are achievable. Lubricant behavior can be substantially improved, and mission success enabled, by a well-designed heat management strategy using precision applied heating.

Lubricant Selection for Martian Atmosphere

During the development of a scanning infrared spectrometer destined to study the Martian atmosphere, a long-life actuator mechanism came under scrutiny due to lubricant life issues.

Heritage work had employed the PAO-based Braycote 601 lubricant. Due to the high reduction ratio required between the stepper motor actuator and the output, a gold-coated infrared mirror, the bearings supporting the stepper motor shaft were called into question, and a longer-life MAC lubricant, Rheolube 2000, was evaluated.

The longevity study disqualified Braycote, and the Rheolube was the remaining design option. The Martian orbital environment has minimal radiation risk. Heating the stepper motor mounting structure mitigates the higher Rheolube solidification temperature. With temperature sensors deployed on the motor itself.

Extensive life testing was performed at extreme temperatures. Including startup at both cold and hot extrema at operational voltage extrema. Followed by disassembly of the mechanism to determine the lubricant behavior.

Typical migration was observed. Lubricant starvation was not encountered and the barrier films in the bearings performed satisfactorily. Even though the mechanism contained both Rheolube 2000 and Braycote 601EF in neighboring bearings, cross-contamination was not observed.

This effort is characteristic of an aerospace philosophy of minimal operational risk ensured by design and test to stacked-worst-case assumptions, followed by a mitigation strategy to ensure such conditions cannot be achieved in the mission. In this case, non-operational heating ensures suitable performance of the bearings.

In this example, thermal design became the enabler of mechanical design. So thermal control hardware became the key to mission success.

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