Moonshots, Mine Carts, and Million Mile Motion Stages

Long-distance motor racing has been established as a battleground where engineers fight for the best balance of performance and longevity and brands can build trust in their operations and products. And while R&D and manufacturing engineers work hard to blend art, science, and engineering that fuel the fascination of millions of performance automobile enthusiasts around the world with the latest technologies – including hybrid and fully-electric drive trains – their colleagues in industrial precision motion control work just as relentlessly to push the limits of the possible. Their success is usually not crowned with trophies, despite the incredible performance of modern precision motion systems, such as the ones employed in the industrial production of cutting-edge chip technology. 

Here, tolerances are often measured in nanometers as Moore’s Law keeps asking for smaller and smaller feature sizes with every new chip generation. Tolerances on modern high-performance engine parts are also continuously shrinking and moving towards the micron realm. Yet, despite output and efficiency are reaching new heights, durability concerns often persist. For many performance car enthusiasts, the portent of 100,000 miles on the odometer signals a point of departure and concern for future reliability.

Imagine though, if we re-calibrated our expectations to 500,000 or even 1,000,000 Miles.

Precision automation systems share an uncanny similarity to high-performance sports cars. The parallels of dynamic performance, cornering, stiffness, precision stopping (step-settle), and weight are combined with micron and sub-micron scale tolerances for repeatability and trajectory following. In both cases and in similar fashion at scale, the operating life of car engines and motion systems is similar.  It is common knowledge that higher RPM operation of a reciprocating engine stresses materials and increases the load on the bearings in a non-linear manner that significantly affects lifetime. As a consequence, high revving race engines, despite employing the highest quality materials, the most sophisticated design and production techniques, and highest standards of quality control, have a significantly shorter service life between rebuilds compared to their low RPM siblings, employed in everyday utility or transportation applications.

Before we look at the relation of load and lifetime in precision linear motion stages such as the ones used in high performance automation applications, it’s worth taking a journey to Germany in the 1820s.

To understand how bearing life is quantified in a “million-mile” stage design, we’ll use the V-817 Linear Motor Stage Series as an example. To start, it is important for the type of bearings used in the V-817 to understand the equation that governs bearing life of this type:

L=50*(C/(f_w*P))³

Where:

L= Bearing Rated Life

C= Basic Dynamic Load Rating

P= Dynamic Equivalent Load

F_w=Load Factor

*Courtesy of IKO Bearing

C is an empirical constant specific to the bearing, statistical in nature that defines under what load the bearing can reproducibly achieve an operating life of 50,000 km. Naturally, the larger this value, the longer the rated life under a lower specific applied load condition.

P is a value intended to be representative of a single applied load operated continuously such that it is representative of the various loads that may be acting over time on the bearing that would produce the same estimated lifetime. 

F_w is the load factor that is applied based on the operating environment. In high vibration or shock inducing environments, this value is greater than 1 (and as high as 3). In quiet environments, this value is typically 1-1.2 (optimal).

The main takeaway of the above equation is the cubic relation of lifetime and load – reduce your load by 50% and bearing life increases by 800%. This knowledge can be decisive when selecting the right stage for the job. A stage with twice the required load capacity will last a lot longer.