Friction and wear pose incredible challenges to 21st Century mechanical and electromechanical systems, particularly in the fields of nanotechnology, aerospace and biotechnology. By one estimate, 6% of the gross national product is lost or wasted due to damaging friction and wear. Yet, despite tribology’s obvious importance, most engineers don’t have effective ways of including it in their design processes.
The reason so many engineers lack good methods of including tribology in design is at least partly due to its complexity. In some applications, bearings and gears have contact points that must carry enormous loads at low and high speeds, and the stresses are unbelievably high. Motions are also extreme and contact areas are small—the size of a pinhead, in some cases. There’s also heat generated, which brings chemistry and hydrodynamics into the equation. All of this happens on a micro-scale and is controlling performance. Factor in new and sometimes unfamiliar tribology interface materials and manufacturing requirements and things get even more complicated.
How do you design for life and durability under such extraordinary circumstances?
The Moonshot Challenge
The sad reality is that all too often, engineers don’t. As a result, some projects don’t go as well as hoped. There are delays, mistakes and missteps because of the complex tribology challenges that come into play.
Take, for instance, the Apollo 17 mission. NASA officials agreed that lunar dust was one of the greatest hurdles to nominal operation on the moon because of the mechanical problems it causes. In a previous mission, lunar grit clogged radiators and even wore a hole in the knee of an astronaut’s spacesuit. NASA is now planning an experiment for next year, the Regolith Adherence Characterization mission. Its goal is to determine how and why dust sticks to materials during lunar landing and other operations. The findings will help determine how to design equipment that repels dust and how to make spacesuits that don’t break under the wear-and-tear of contact with the moon’s sandpaper-like grit.
This is a good example of the problems that can arise in the world of tribology and how it can take a long time to solve those problems. If we could solve tribology problems faster, we could not only promote faster innovations but also greatly reduce the costs and risks of mistakes and unforeseen events.
Tribological failure can also be dangerous or even deadly, as demonstrated when a wind-turbine gearbox caught fire in Scotland in 2011 or when Alaska Airlines flight 261 crashed in 2000 due to excessive wear on a jackscrew in the flight control system. And in the early years of the space shuttle, NASA engineers had to replace the ship’s main engine turbopump bearings after every flight. It took more than a decade to solve this risky and expensive problem, which involved harmonizing tribology materials with complex design and manufacturing.
Tribology by Design: A Formula for Success
Whether you work in aerospace, automotive or aviation, where advanced tribology started, there is now an approach to reduce the risk by helping engineers better understand tribology challenges and more competently design for them. It’s called Tribology-by-Design (T/D).
T/D combines a theory, a set of test and analysis tools, and a methodology. It was developed to get powerful tribology mechanisms into engineering design to let engineers design and develop component contact interfaces that can carry loads and transmit power under extreme conditions.
This year T/D is being taught to engineers around the globe as part of the Massachusetts Institute of Technology’s Professional Education course, “Tribology: Friction, Wear and Lubrication.” I will be an instructor for a session that explores how T/D connects and differs from axiomatic design (AxD), a widely adopted design methodology developed by the course’s lead instructor, Dr. Nam Pyo Suh, Cross Professor Emeritus at MIT.
AxD uses matrix methods to systematically analyze the transformation of customer needs into functional requirements, design parameters and process variables. It’s used to design the best possible solution for planned features and functions.
T/D methodology is complimentary. It differs in that it targets the operating conditions or duty cycle of a critical component within its operating system, and predicts and solves tribology problems to save time on testing and redesign. The theory characterizes critical tribology interfaces in terms of motion, stress and temperature, and how these parameters activate the tribology interface materials and mechanisms during operation (MST-Tm).The T/D process extracts and delivers the targeted MST-Tm to a virtual TRL 4 (Technology Readiness Level 4), where innovative tribology R&D can be done using T/D test and analysis tools.
The three T/D tools represent component analysis, single contact models (SCM) and single contact simulation testing. These tools provide, respectively, a component digital twin, an interface digital twin and an interface simulation twin.
A Missing Link: Motion
Design and life theories for bearings and gears are based on stress and contact-fatigue stress-cycle criteria, along with a “material life factor.” In contrast, T/D theory is based on motion-driven mechanisms for lubrication and performance which is more consistent with physical reality. “Tribology” by definition is the science and technology associated with contacting bodies in relative motion. It is all about motion.
Consider a violin. A bow resting on a string does nothing but add motion to the bow, and you get a frictional mechanism or a motion-driven mechanism to make harmony. We need motion to engineer the interface materials and achieve functional harmony.
Tribology is enormous, and it is everywhere. Increasing demands for more performance and energy savings continue to drive innovation in mechanical systems. Introducing new materials, designs and test methods will play an important role in future progress.
But relevant tribology design parameters are urgently needed to support new bearing and gear materials, lubricants and designs for transportation, energy and industrial components. Using T/D theory, test and analysis tools, and methods to discover and apply new technologies will open the door to a much more rapid response to tribology challenges, faster innovation, reduced costs and mitigating risk.
Engineers have much to benefit from bringing T/D into design. It will let them keep pace by providing a systematic way to cover all the bases.
Vern Wedeven is the founder and president of Wedeven Associates, Inc. He has written more than 90 technical publications and is a co-instructor of the MIT Professional Education course.