Linear guides, cam followers, and crossed-roller bearings can satisfy the demanding requirements for space-flight hardware.
By IKO engineering staff | www.ikont.com
As space activity shifts from governmental agencies to a growing industry driven by commercial entities and even startups, the types of spacecrafts and the missions they support are also expanding. This diversifying industry is delivering innovative concepts such as autonomous exploration vehicles, solar sails, internet satellite constellations, space junk removal, and reusable spacecraft, to name a few.
Like many other innovative technologies, all these designs depend on precision motion components that must reliably work as planned once in space.
High-quality products get designs off the ground
Although mechanical systems typically behave differently in space than on earth, it still makes sense to specify standard products as a starting point for a given design before committing to customized motion components. After all, reliability is paramount in space applications … and component-supplier applications engineers have years of experience and application data on all their company’s standard components. Aerospace machine builders can leverage this knowledge to get an optimized suite of stock components, modified components, and fully custom components that will perform reliably in space.
Here are some points for design engineers to consider when specifying three common motion components for space applications.
Aerospace-grade linear guides: Applications for linear guides and rails abound on autonomous space-exploration vehicles. Linear guides on such vehicles must have exceptional metallurgical properties to deliver smooth and accurate motion with good rigidity — and long life. Those linear guides with interchangeable parts let design engineers modify their guides for the unique conditions of space.
Aerospace-grade crossed roller bearings: With modifications to withstand vacuums and low temperatures, these crossed roller rotary bearings can reliably operate aboard spacecraft. For example, crossed-roller bearings support the function of radar dishes on miniature Wifi satellites that send signals to ground-based systems. Their small size also makes these bearings suitable for swivel mechanisms in military surveillance cameras.
Aerospace-grade cam followers: These are widely used to convert rotary motion to linear motion for twisting, turning, and pulling actions. For example, cam followers are well suited for opening or closing antennae or solar panels such as aboard space stations and satellites. Some cam followers on the market today also feature a small coefficient of friction for excellent rotational performance of various spacecrafts’ outer rings — even those bearing exceptionally high loads.
For all these motion components, aerospace engineers can often start their design work with standard product. This approach is both practical and often the most economical option. With a few modifications for the environment of space, these otherwise stock components can be significantly less expensive than exotic fully custom components that may ultimately prove to be cost prohibitive for the mission.
Tailoring motion components for space
Following are just some factors to consider when specifying a motion component for space deployment. Many of these considerations include those made when modifying miniature rotary and linear motion components for NASA’s Jet Propulsion Laboratory (JPL) and the Mast Cameras (Mastcams) of the Mars Curiosity and Perseverance rovers. These onboard camera systems create panoramic images for navigation and transmission back to Earth for analysis.
Design requirement one — Operating in a vacuum: While some standard linear guides are made for controlled vacuum environments on Earth, even these require customization to perform as intended in space. Contamination caused by outgassing can present many opportunities for component failure — so components destined for space should not exhibit this behavior. What’s more, traditional greases for lubricating would fail in space due to the extremely cold temperature there (of 2.7 Kelvin or -455° F). For this reason, component suppliers seasoned in aerospace design typically recommend specialty greases or dry-film lubricants for equipment going into space.
The linear guide used aboard the Curiosity rover was given specialized vacuum and cleanroom-style packaging, free of dust, lint, oil, and rust … so the linear guide satisfies the above requirements.
Design requirement two — surface coatings: Lubricating greases and traditional surface protection such as chrome plating may not perform well in space. Fortunately, motion-component application specialists can suggest suitable alternatives. One option for aerospace designs is precision thin dense chromium deposition. Otherwise, if a device will need to interact with lasers or other light sources (and light-reflective coatings are unsuitable) some component suppliers offer dark and flat coatings on their motion components.
Design requirement three — withstanding extreme temperatures: Excessive heat can cause critical parts within a linear guide or rotary bearing to expand or contract and degrade reliability. So wherever an axis must withstand high temperatures, the design engineer should consult component-supplier specialists about the material thermodynamics of components under consideration. Different materials may expand and shrink at different rates. If that behavior might cause a potential problem — say, by closing clearances needed for proper operation or taking a subassembly of its design preload value — the component supplier should be consulted for alternatives.
For the Mars Curiosity rover, one linear guide was customized to operate when exposed to high heat … and provide continuously smooth motion down to -130° C. For the latter, the linear guide underwent rigorous low-temperature bench tests prior to launch to ensure it operated with proper clearances. The guide’s ball recirculation and ball retaining features were also optimized to ensure consistent motion even in the face of wildly fluctuating temperatures.
Design requirement four — preventing corrosion: Space applications face a significant threat of corrosion and its detrimental effects. So, it often makes sense to specify motion components made of stainless steel. Some standard stainless-steel linear guides employing rolling bearing elements are suitable for applications in which corrosion-preventing oils can’t be used. For such situations, some manufacturers can even recommend a nonstandard rust-preventing lubricant to complement compact and lightweight (and therefore space suitable) linear guides.
Design requirement five — reducing weight: Sometimes trimming a few ounces from a spacecraft can mean the difference between a design being declared unfeasible, and it being successfully built and launched. Lightweight components can also save thousands of dollars in launching fuel costs. Component suppliers capable of customization can often work with designers to reach suitable space performance-to-weight ratios — and can suggest lightening strategies. These approaches might include the selection of a smaller more power-dense product; lightweighting the rolling elements in linear and rotary bearings; and drilling extra holes (between the mounting holes) in linear rails.
Design requirement six — preventing contamination: One lesson learned by engineers of the lens systems for Curiosity’s cameras is that small mechanisms tend to easily jam. So to prevent contaminants from impeding a device’s accuracy or causing it to prematurely fail, aerospace engineers should consider accessories such as seals, wipers, or cover sheets and end plates that are specially engineered to withstand hostile or corrosive environments.
Consider how the JPL requires systems and components to operate on their own via remote control for their entire mission. After launching in 2011, Curiosity’s two-year mission was indefinitely extended … and it’s still operational today. Linear guides on the Curiosity (as well as miniature linear guides on Curiosity’s successor Perseverance) continue to deliver reliable operation — a testament to the value of choosing a motion-component supplier that will help engineering teams plan, test, and tailor components to the unique eventualities of space applications.
Suppliers can be design support for space applications
When selecting motion components for a space application, engineers should look for suppliers having a deep understanding of motion goals; a large portfolio of products; and the capability to satisfy unique design objectives. Many suppliers focus on one given specialty (rotary or linear motion, for example) and lack a full range of standard products to serve as starting points for honing the ultimate solution. Others offer extensive motion expertise and a wide range of design options to satisfy stringent requirements; they should be engaged early in the design process.
Through extensive collaboration, suppliers can modify and custom engineer components to make them meet expectations for the operating environment of space. Whether the motion component is intended for flying craft, an exploration vehicle, robotic satellite mechanisms, or ground support equipment, designers of space systems can then be confident that their design will reliably operate for the life of its mission and beyond.
IKO Intl. | www.ikont.com
Filed Under: Linear Motion Tips