Understanding Limitations of Standard Catalog Hardware, COTS

4 June 2024
The correct selection of standard catalog hardware, also known as Commercial Off The Shelf (COTS) parts, is frequently overlooked especially when it comes to their suitability for use.  In practical engineering, where cost and schedule constraints pace the final product design, a majority of effort is observed to be expended conceptualizing and iterating on new and original designs for parts and details.  To bring these definitions together, standard catalog hardware selections are frequently made, typically occurring towards the latter part of the product design phase, prior to the authorization to proceed in the construction of assemblies and installations.  The convenience of standard catalog hardware being defined in easily accessible engineering drawings regulated by industry standards and procurement specifications (ie AN, NASM, AS, MIL, MS Standards), makes them appear to be quick, simple, and straightforward solutions toward completing the design, mitigating the need for smaller design activities by building off of those already established.  But if the engineer does not pay close attention nor have a thorough understanding of their selection’s capabilities and limitations, serious consequences are likely to be encountered during a qualification activity or in service.  

COTS Example:  Rod End Bearings

One case study worth reviewing are Rod End Bearings per AS81935 “Bearings, Plain, Rod End, Self-Aligning, Self-Lubricating, General Specification for (-65 to +325F)” when being considered for cryogenic environments like those experienced by rocket spacefaring vehicle applications.  Comprising an MS14101 spherical bearing swaged into a banjo shaped housing body with a threaded shank, these parts are commonly used in airframe and hydraulic applications offering advantages in the design and manufacturing phases of the engineering product lifecycle.  The utility of these parts at joint connections, where two rotational degrees of freedom are afforded, relieves moment loads on structural members.  Doing so simplifies structural analysis by reducing loading considerations to axial compressive or tensile stresses on those same members and the Rod Ends Bearings themselves, but only when the Rod End Bearing is functioning correctly (ie slipping and not seized). 

AS81935 Rod End Bearings constructed from PH 13-8Mo stainless steel, specifically, are the most attractive selections afforded by these specifications due to their good corrosion resistance in comparison to other material options in addition to their higher strength room temperature performance achieved through a heat treatment process called tempering.  Simple dash numbers appended to a parent character string are used to designate different sizes for call out with conveniently defined load limits per size which can be readily consumed in analysis activities.  Since these parts are industry defined and controlled by procurement specifications, they may be easily procured from numerous sources of supply and, therefore, subject to market competition to lower and control cost.  

By inspection, these parts certainly convey some kind of low temperature performance down to as low as -65F, and certainly suitable for airframe and hydraulic applications as previously mentioned where conditions in service are not expected to go beyond.  However, consider our engineer in search of this kind of part for a spacefaring vehicle applications where cryogenic service environments are much colder, typically modeled at -320F.   After a literature search of available standard catalog hardware for selection, our engineer is unable to find any others except those per AS81935 with PH 13-8Mo Rod End Bearings being available.  The next natural question to ask would concern whether or not such performance can be expected from these same parts at much lower temperatures than what they are rated for.  

Key Decisions

The decision to assume or not to assume extrapolated performance at cryogenic temperatures for example discriminates engineers from each other and is based on many factors including risk tolerance, project delivery time constraints, consequences of failure, and lessons gathered from experiences (whether accurate or not).  The correct decision can be argued either way and is dependent on the surrounding environment of the decider, and we are reminded that in the typical practice, time and schedule become equally important when it comes to the decisions engineers make.  There is no right decision, rather a decision is made right.  Higher risk taking engineers may be interested in moving quickly, and that may be okay.  However, for the conservative engineer focused on correct first time delivery of a functional product, a deeper understanding of the part definition is essential to better assess its abilities and limitations in addition to the implications of such a decision.  

Performance Provisions and Data

Performance capabilities for standard catalog hardware may be evidenced through data sets collected during qualification activities defined in governing procurement specifications.  A review of qualification testing requirements per AS81935 reveals that any low temperature qualification test is only carried out at -65F and is never fully loaded to specified and defined load limits.  Instead, only fractional loads are used for qualification, never testing at full load.  Therefore, statements concerning whether or not such parts can fully support the defined load limits at -65F cannot be made, nor could one readily conclude that assumptions for two rotational degrees of freedom are maintained at these low temperatures, too.  Perhaps this would warrant additional testing campaigns at relevant service conditions at lower temperatures so as to better understand and demonstrate that the needed performance is there.

Material Insight

While one could do that, an understanding of the materials used in construction provides further insight.  It was mentioned previously that PH 13-8Mo stainless steel's higher strength room temperature performance is attained through a heat treatment process called tempering. Tempering of PH 13-8Mo stainless steels results in a tempered martensite microstructure with variable phase fraction of untransformed martensite dependent on the choice tempering temperature.  The martensite crystal structure is known to be Body-Centered-Tetragonal (BCT) and is similar to Body-Centered-Cubic (BCC) with the exception of an elongated length lattice parameter different from the other two.  Metallic materials with BCC crystal structures are notoriously known for showcasing brittle behavior at lower temperature due to a limited reduced number of active slip systems in those environments, unlike that seen in Face-Centered-Cubic (FCC) and Hexagonal-Close-Pack (HCP) crystal structures which maintain their ductile behavior where slip occurs on close packed planes.  Since BCT may be considered a distorted BCC,  with neither having close packed planes, it follows that martensite would also show similar brittle behavior at such low temperatures, too.  This is consistent with observations in authoritative resources such as the Metallic Materials Properties Development and Standardization (MMPDS) Handbook revealing no reporting of properties of PH 13-8Mo steels at temperatures below -100F (most likely for good reason, in the opinion of this author).   Materials that become brittle in service due to lower temperature exposure are not damage tolerant, fail quickly without warning, are fracture analysis intensive, and are best avoided where possible especially when expected loads are/could be tensile, like in the shank-banjo transition region of a Rod End Bearing.

Risk Assessment

From this simple exercise concerning PH 13-8Mo stainless steel AS81935 Rod End Bearings, their fitness for service in rocket vehicle cryogenic space environments is concluded to be low due to the lack of performance data sets intrinsic to the part definition at lower temperatures, with risk of brittle failure modes due to brittle material behavior and tensile loads at cryogenic temperatures.  These statements are not easily interpreted nor inferred without an understanding of AS81935 and awareness of PH 13-8Mo stainless steel metallurgy and are garnered with a close read of definitions coupled with experience.  While the example here only covers Rod End Bearings for cryogenic environments, this approach to understand and assess risk around standard catalog hardware fitness for service is the same, from the simplest bolt to the more complex valve. 

High Judgement

High judgment and attention to detail during the selection of standard catalog hardware are essential in the practice of engineering design.  Parallel to the process of producing original designs where an understanding of requirement sets is balanced by trades concerning configuration and dimensions, structural margins of safety, materials, manufacturing, and risk tolerance, this same methodology is just as valuable in the selection of standard catalog hardware in defining assemblies and installations.  A review of NASA’s Parts, Materials, and Process Plan NASA-STD-6016 clearly includes provisions for the conformance of standard catalog hardware to materials and process requirements just like any other custom design.  With decades of aerospace history behind them, let guidance such as NASA-STD-6016 be the lessons learned from NASA’s mission successes and failures to help better inform engineering designs on what they should be, down to the smallest standard catalog hardware that is often taken for granted.

Thx, m