Christopher DellaCorte and Timothy L. Krantz

Glenn Research Center, Cleveland, Ohio

Michael J. Dube

Goddard Space Flight Center, Greenbelt, Maryland

ISS Solar Array Alpha Rotary Joint (SARJ)

Bearing Failure and Recovery: Technical and

Project Management Lessons Learned

NASA/TP—2011-217116

August 2011
NASA STI Program . . . in Profi le

Since its founding, NASA has been dedicated to the 

advancement of aeronautics and space science. The 

NASA Scientifi c and Technical Information (STI) 

program plays a key part in helping NASA maintain 

this important role.

The NASA STI Program operates under the auspices 

of the Agency Chief Information Offi cer. It collects, 

organizes, provides for archiving, and disseminates 

NASA’s STI. The NASA STI program provides access 

to the NASA Aeronautics and Space Database and 

its public interface, the NASA Technical Reports 

Server, thus providing one of the largest collections 

of aeronautical and space science STI in the world. 

Results are published in both non-NASA channels 

and by NASA in the NASA STI Report Series, which 

includes the following report types:

• TECHNICAL PUBLICATION. Reports of 

completed research or a major signifi cant phase 

of research that present the results of NASA 

programs and include extensive data or theoretical 

analysis. Includes compilations of signifi cant 

scientifi c and technical data and information 

deemed to be of continuing reference value. 

NASA counterpart of peer-reviewed formal 

professional papers but has less stringent 

limitations on manuscript length and extent of 

graphic presentations.

• TECHNICAL MEMORANDUM. Scientifi c 

and technical fi ndings that are preliminary or 

of specialized interest, e.g., quick release 

reports, working papers, and bibliographies that 

contain minimal annotation. Does not contain 

extensive analysis.

• CONTRACTOR REPORT. Scientifi c and 

technical fi ndings by NASA-sponsored 

contractors and grantees.

• CONFERENCE PUBLICATION. Collected 

papers from scientifi c and technical 

conferences, symposia, seminars, or other 

meetings sponsored or cosponsored by NASA.

• SPECIAL PUBLICATION. Scientifi c, 

technical, or historical information from 

NASA programs, projects, and missions, often 

concerned with subjects having substantial 

public interest.

• TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientifi c and 

technical material pertinent to NASA’s mission.

Specialized services also include creating custom 

thesauri, building customized databases, organizing 

and publishing research results.

For more information about the NASA STI 

program, see the following:

• Access the NASA STI program home page at 

http://www.sti.nasa.gov

• E-mail your question via the Internet to help@

sti.nasa.gov

• Fax your question to the NASA STI Help Desk 

at 443–757–5803

• Telephone the NASA STI Help Desk at

 443–757–5802

• Write to:

 NASA Center for AeroSpace Information (CASI)

 7115 Standard Drive

 Hanover, MD 21076–1320
Christopher DellaCorte and Timothy L. Krantz

Glenn Research Center, Cleveland, Ohio

Michael J. Dube

Goddard Space Flight Center, Greenbelt, Maryland

ISS Solar Array Alpha Rotary Joint (SARJ)

Bearing Failure and Recovery: Technical and

Project Management Lessons Learned

NASA/TP—2011-217116

August 2011

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identifi cation 

only. Their usage does not constitute an offi cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by a committee of peers. 
NASA/TP—2011-217116 1 

ISS Solar Array Alpha Rotary Joint (SARJ) Bearing Failure and Recovery: 

Technical and Project Management Lessons Learned 

Christopher DellaCorte and Timothy L. Krantz 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Michael J. Dube 

National Aeronautics and Space Administration 

Goddard Space Flight Center 

Greenbelt, Maryland 20771 

Summary 

The photovoltaic solar panels on the International Space 

Station (ISS) track the Sun through continuous rotating motion 

enabled by large bearings on the main truss called solar array 

alpha rotary joints (SARJs). In late 2007, shortly after 

installation, the starboard SARJ had become hard to turn and 

had to be shut down after exceeding drive current safety 

limits. The port SARJ, of the same design, had been working 

well for over 2 years. An exhaustive failure investigation 

ensued that included multiple extravehicular activities to 

collect information and samples for engineering forensics, 

detailed structural and thermal analyses, and a careful review 

of the build records. The ultimate root cause was determined 

to be kinematic design vulnerability coupled with inadequate 

lubrication, and manufacturing flaws; this was corroborated 

through ground tests, metallurgical studies, and modeling. A 

highly successful recovery plan was developed and 

implemented that included replacing worn and damaged 

components in orbit and applying space-compatible grease to 

improve lubrication. Beyond the technical aspects, however, 

lie several key programmatic lessons learned. These lessons, 

such as running ground tests to intentional failure to 

experimentally verify failure modes, are reviewed and 

discussed so they can be applied to future projects to avoid 

such problems. 

Introduction 

The International Space Station (ISS), shown in Figure 1, 

bears a slight resemblance to a dragonfly such that it consists 

of a long, slender but strong truss structure from which large 

wings and other key components are attached. Among the 

most critical of these are two solar array wings (port and 

starboard sides) made up of numerous photovoltaic (PV) 

panels that convert sunlight directly into electricity to power 

the Station. 

To maximize generated power, the PV panels slowly rotate 

through 360 as the ISS orbits the Earth to keep the panels 

pointed towards the Sun during the ISS “day.” Specially 

designed bearings and drive mechanisms, aptly named “solar 

array alpha rotary joints,” or SARJs, are built into the ISS 

backbone truss adjacent to each PV wing to allow the panels 

to track the sunlight while the rest of the Station remains 

facing the surface of the Earth as seen in Figure 2. 

The SARJs are shown schematically in Figure 3 and loosely 

resemble a pair of steel hula hoops, nearly 10 ft in diameter, 

that are arranged side by side and linked by a dozen arms, 

called trundle arms, placed at regular 30 intervals around the 

circumference. Each trundle arm has a clamp (i.e., gorilla 

mount, or gorilla clamp) on one end to grasp one hula hoop 

(race ring) and a fixture on the other end containing three 

opposing rollers that surround the cross section of the other 

hula hoop, allowing it to roll freely. 

Unlike hula hoops, which are hollow, made from flimsy 

plastic, and have a round cross section, the SARJ race rings 

have a triangular cross section and are made from hard-coated 

stainless steel. Figures 4, 5, and 6 show more closeup details of 

the SARJ race arrangement, the trundle arms, and the rollers. 

SARJ Operational Requirements 

Whereas this over-simplified description of the SARJ does 

allow a macroscopic understanding of its operation and 

purpose, the engineering implementation is anything but 

simple. The SARJs must accomplish a number of unique tasks 

and exhibit characteristics that challenge many facets of 

engineering design and operation. These requirements are 

reviewed below. The SARJs are unusual mechanisms and are 

essentially the first of their kind to have been designed and 

implemented. This aspect must be kept in mind when reading 

the remainder of this paper to temper the tendency to question 

why design flaws were overlooked or the inevitable failure 

was not foreseen. 

The ISS is an orbiting research laboratory designed to 

house astronauts, laboratory and living space, experiments, 

and life support systems. Its purpose is to enable experiments 

in a zero-gravity (microgravity) environment and to conduct 

observational studies of the Earth and space (Ref. 1). Also, the 

ISS is an ideal engineering testbed for life support and other 

systems that are needed for future long-term space 

exploration. To ensure a safe, vibration free, and robust 
NASA/TP—2011-217116 2 
NASA/TP—2011-217116 3 

 

environment, the ISS structure must be strong and stiff while 

simultaneously allowing for physical expansion and rearrangement, in-space component replacement, and 

operational redundancy on key and critical elements such as 

power generation (e.g., replaceable battery packs) and cooling. 

In addition, all systems on the ISS must be long lived (a 30-

year maintenance-free design life is the goal), require minimal 

power, and not create contamination (i.e., out-gassing and 

debris generation) and be tolerant of degradation by radiation, 

micrometeorite impact, and atomic oxygen. All of these 

requirements are also constrained by the payload size and 

mass limitations of the space shuttle cargo bays in which ISS 

components were launched into orbit. 

With respect to the SARJs, the ISS operational 

requirements led to many of the resulting design features. For 

instance, the SARJ construction materials consist of hardened 

stainless steels, vacuum-compatible greases and bearings, and 

the use of bolts and fittings with capture features to prevent 

the loss of parts during extravehicular activity (EVA) 

assembly and replacement. To yield a long rolling-contact 

fatigue life, the race rings are nitride-treated to present a hard 

ceramic surface to the hardened stainless steel bearing rollers. 

Each component that makes up the SARJs is carefully 

engineered to carry the loads with adequate margins at 

minimum overall mass while maintaining extreme levels of 

precision alignment and accuracy. The SARJ trundle arms are 
NASA/TP—2011-217116 4 

fully replaceable by a suited astronaut, one arm at a time, and 

can be configured to allow either race ring to be stationary or 

free to rotate. These clever design features ensure the 

redundancy of the race ring should one wear out or fail 

without necessitating the breaking of the ISS backbone truss—

a very risky endeavor. Lastly, care had been taken at every 

step to minimize friction. In fact, rotating each SARJ on the 

ISS required less electricity than a standard reading light 

(~30 W). In short, the ISS SARJs are a true engineering 

marvel. 

The SARJ Design, Build, and Test 

Sequence 

The SARJ conceptual design began in the mid-1980s when 

NASA began to solicit ideas that might guide the overall 

layout and operation of an operating space laboratory intended 

for a 30-year lifespan in near-Earth orbit. With respect to 

mechanical elements of the ISS, two highly relevant reference 

papers written at the time provide insight into this period and 

the eventual SARJ design (Refs. 2 and 3). While these papers 

are helpful in understanding the genesis of the SARJ, it is 

important to point out that no single comprehensive document 

exists that explains the overall SARJ design, how it functions, 

and how it was built. This lack of a single source of 

information available to all who contributed to the SARJ is 

possibly the single most likely culprit for the failure. 

Nonetheless, when taken together the following two papers do 

help one understand the SARJ. 

The first paper discusses the challenges of selecting 

lubricants and materials for bearings, gears, and mechanisms 

that must cope with the aggressive low-Earth-orbit (LEO) 

space environment and yet permit long life with no chance of 

contamination of sensitive ISS systems (Ref. 2). For example, 

hydrocarbon-based greases and Teflon, silver, and lead solid 

lubricants are often used in spacecraft, but these were deemed 

unacceptable on the ISS. Hydrocarbon greases vaporize over 

long times and tend to become too thick and stiff at the 

subzero temperatures anticipated for the mechanisms external 

to the ISS during the orbital night. Traditional solid lubricants 

are ravaged by atomic oxygen. The paper does suggest that if 

the engineering need arises to use such materials, then a fullduration life test should be done before launch. 

With regards to friction and lubrication of mechanisms, the 

following generalization can be safely made: 

Rolling is better than sliding, oils and 

greases are better than solid lubricants, and 

traditional solid lubricants (e.g., molybdenum disulfide, polytetrafluoro-ethylene 

(PTFE), and engineered and intercalated 

graphite) are better at reducing friction than 

metal films. 

As an example of this generalization, one can consider the 

simple case of a wheel or roller moving along a plane. If the 

wheel or roller moves along a flat surface in rolling contact the 

friction is very small. If one divides the rolling component of 

the friction force by the contact load, the rolling friction 

coefficient is determined. This condition is termed “pure 

rolling,” and rolling friction coefficients for hard contacting 

surfaces are very low, typically less than 0.01. However, if the 

same roller is dragged along a surface, high friction results, 

and the value depends upon the materials present at the sliding 

interface. When two hard surfaces rub, friction coefficients 

between 0.5 and possibly over 1.0 are common because 

sliding is accomplished by physical shearing of materials and 

atoms at the moving interface. If a soft lubricant material with 

low shear strength is intentionally placed between two hard 

sliding bodies, friction is greatly reduced. Lubricants such as 

oil, grease, and molybdenum disulfide and graphite 

composites, when properly engineered for vacuum, yield 

sliding friction coefficients typically around 0.1, whereas more 

chemically stable solid lubricants like lead, silver, and gold 

coatings exhibit coefficients between 0.2 and 0.3. This type of 

general understanding drives the design of new space 

mechanisms like the SARJ. As a design approach it is sound, 

but there remain many uncertainties that must always be 

proven through rigorous testing. 

Unfortunately, during the initial phase of the ISS program, 

sufficient time was not available nor were there sufficient 

financial resources for life (30-yr) testing. Further, no widely 

accepted accelerated life test existed then (or even today) to 

confirm a selected lubrication approach. The paper does 

conclude that vacuum-compatible greases based upon 

fluorocarbons may be viable inside sealed bearings and noble 

soft metals such as thin gold films might be suitable for 

contacting surfaces exposed to the space environment. 

Interestingly, these suggestions were adopted in the SARJ 

design. 

The second highly relevant publication, written by NASA 

Lewis Research Center (former name of the NASA Glenn 

Research Center) mechanical components division personnel, 

describes conceptual design approaches for large rotating 

joints used for pointing solar arrays on orbiting spacecraft 

(Ref. 3). This report presents the idea of using side-by-side 

rings joined by triangularly opposing pinch rollers is 

presented. The virtues of this design approach such as built-in 

redundancy, structural rigidity, and low mass are highlighted. 

Interestingly, the design approach included using the trundle 

rollers as traction drives to further reduce complexity and 

mass by eliminating large, exposed drive gears and dedicated 

drive-lock assemblies as was ultimately engineered for the ISS 

SARJs. One engineering aspect the reader takes from this and 

the lubricant materials paper addressed previously is that there 

were many possible designs and material choices available to 

achieve the overall mechanism performance goals. 
NASA/TP—2011-217116 5 

In the absence of deep previous experience with space 

mechanisms similar to the SARJs, 

The best possible design could only have 

been determined through an extensive 

research and development program since 

such a mechanism—the SARJ—was a first 

of its kind. 

The SARJ design appears to have been loosely based upon 

these two key reports and evolved over a 3-yr period between 

1987 and 1990, at which time subcomponent hardware was 

built and tested by a number of different prime contractors, 

subcontractors, and vendors. Based upon a 30-yr design life 

criterion, the failure potential for the SARJ was broken down 

into categories based upon presumed failure modes such as 

thermal-mechanical effects including thermally induced 

pinching and interference, load-induced stress-type failure, 

unintentional foreign object damage (FOD) contamination, 

and micrometeorite damage. For the moving assemblies such 

as the internal roller bearings and the contacting roller-race 

ring surfaces, classical rolling-contact fatigue was considered 

the dominant potential failure mode. The vital internal roller 

bearings were sealed and well lubricated with vacuumcompatible grease and were fit with backup bearing surfaces 

and health-monitoring indicators. With such well understood 

and efficient lubrication, their life expectancy far outpaced 

that of the station. Since the critical contact between the rollers 

and the race ring surfaces were carefully designed for pure 

rolling contact, the need for lubrication appears to have not 

been deemed critical and was not specifically considered as a 

test variable. 

Overall, the SARJ design is impressive. Great care was 

taken to ensure nearly perfect rolling contact between the 

trundle rollers and the race ring surfaces. Precision-tapered 

roller bearings with specifically measured grease volumes and 

carefully set and recorded preloads were used inside each 

trundle roller. The drag torque for each roller was repeatedly 

measured and recorded, as was the drag of assembled trundles. 

Every effort was made to ensure that all the trundles were 

manufactured employing the same techniques and yielded 

reproducibly low drag. Detailed lab notes, data records, and 

photographs were made of the entire SARJ mechanism at 

every step (Ref. 4). 

The final design included a special nitride treatment for the 

race rings that deposited a near-diamond-hard iron nitride 

coating in all locations that would undergo trundle roller 

contact. With this approach, the anticipated rolling-contact 

fatigue life was estimated to be literally hundreds of years of 

ISS operation. Additionally, thin films of gold were deposited 

onto the trundle roller surfaces for solid lubrication, though it 

was not clear that this function was widely understood. 

Figure 7 shows a photograph of a finished trundle assembly 

(port SARJ) in which the gold-plated rollers are clearly visible. 

Although Reference 1 clearly indicates that gold is used for 

lubrication, Reference 2 suggests that gold films serve a 

slightly different function: helping to maintain a smooth 

contact surface, needed to ensure a low-vibration bearing. One 

contacting surface that was left unlubricated, despite 

significant sliding, was the drive gear for the race ring. 

Concerns regarding the potential for gear wear led to a special 

full system-level, accelerated test to ensure adequate gear life. 

This system test, aptly named the pinion gear life test, was 

conducted on the structural test article (STA), one of the three 

SARJs built. 

The STA was the first SARJ built and was used to validate 

the general design. It was also used to measure joint stiffness 

and dynamic characteristics needed by the ISS structural team 

tasked with providing a low-gravity research environment for 

the ISS. Following the initial characterization tests, the 

accelerated pinion gear life test was performed. This test was 

conducted in open air, not vacuum, and it was run at a rotation 

rate about 40 times faster than nominal speed to reduce test 

time. No real-speed life test, in vacuum, was run for the SARJ; 

many may have felt at the time that the pinion test of the STA 

represented an adequate life test. For the SARJ, a combination 

of redundant design features and calculations were used to 

ensure that adequate design margins existed for anticipated 

failure modes. For the critical trundle roller-race surface 

contact, the expected failure mode was rolling-contact fatigue. 

Because of this, when questions related to build anomalies and 

their affect on performance and life arose, the expected failure 

modes were used as the relevant yardstick. 

Subsequently, the two flight joints were built: first the port 

SARJ and then the starboard SARJ. The build records show 

that the sequence of events encountered during the builds was 

similar in many respects yet different in others. This is 

unsurprising for projects involving major undertakings of 
NASA/TP—2011-217116 6 

precision hardware. Following assembly of the port SARJ, it 

was functionally tested and found to give performance, with 

respect to drive torque, akin to the STA. The starboard SARJ 

was built and assembled about 6 months after the port and 

then functionally tested in the same manner as the port SARJ. 

Although both joints exhibited similar stiffness characteristics, 

the starboard SARJ differed in one key aspect: Its drive 

current, an indicator of drag torque, was about 30 percent 

higher than both the STA and port SARJs. 

The higher-than-expected starboard drive current caused 

considerable concern amongst the project team. All three 

joints had been carefully designed and built, and each 

subassembly had been tested for drag and performance. A 

review of the build records at that time showed that the 

components used in the starboard SARJ had nominally the 

same drag and misalignment as the STA and port SARJs’ 

components. Since the overall torque had been calculated as 

the sum of the drag torque of the components, the discrepancy 

was unexpected. Despite considerable investigations, no 

specific reason for the higher drag torque on the starboard 

SARJ was determined. Since the starboard SARJ drive current 

levels were still well within the limits initially set for the 

project, the matter was dropped and attributed to simply a 

normal variation for a SARJ. 

The SARJ Launch and Operation 

The port SARJ was launched early in 2006 and began regular 

operation in September of that year. Its behavior was closely 

monitored and found to be nominal, largely mirroring the 

behavior observed during ground tests. The starboard SARJ was 

launched in the spring of 2007 and began rotation in June of 

2007. By September, after just 3 months of operation, engineers 

detected changes in the starboard SARJ performance. After 5 

weeks of investigation, engineers had been able to eliminate 

many of the probable electronic and control contributors, thus 

zeroing in on mechanical problems. During this period of 

investigation the drive current fluctuations grew to unacceptable 

levels, prompting the shutting down of the starboard SARJ 

rotation in October. Although the drive system had adequate 

capacity for rotation, even at these increased torque levels, there 

was serious concern that the SARJ might become stalled. 

Several short-duration joint movement tests were conducted to 

study the problem, and it was deduced that increased drag 

torque was directly causing the observed current draw 

fluctuations. With this information as a backdrop, a full-scale 

investigation was begun. 

SARJ Investigation 

The investigative team was far reaching and included 

representatives from space operations, the prime contractors 

and subcontractors who built the SARJ, research engineers, 

analysts, and specialists in root-cause investigation processes. 

Information regarding the hardware as well as fabrication and 

testing history along with the on-orbit performance was 

uploaded onto a shared documents site and made available to 

all of the root-cause team members. Selected team personnel 

served as liaisons to mission operations personnel to offer 

input for space walks (EVAs) that were planned to collect 

more information about the failed joint. 

The first EVA, conducted in late November 2007, was 

initiated to seek obvious causes for SARJ distress. The 

astronauts were instructed to examine the SARJ multilayer 

insulation and cover panels for signs of micrometeorite 

damage and to seek any mechanical interference from nearby 

hardware such as hold-down brackets and deployment 

mechanisms. In short, the EVA goal was to look for a 

“smoking gun” that would explain the anomalous behavior of 

the starboard SARJ (Ref. 5). The astronauts found nothing offnominal during the external examination. When they removed 

one cover to inspect the internal mechanisms they 

immediately noticed damage to one surface of the rotating 

race ring and copious amounts of wear debris near and around 

the damaged race as well as adhered to the trundle bearings 

(see Fig. 8). 

Samples of the debris were collected and returned to Earth 

for analysis. While the root cause for the race damage was not 

yet known, it was apparent that the roughened surface was 

likely the reason for the increased drag torque and the rough 

nature of the joint operation. A followup inspection of the port 

SARJ showed no such damage. The port SARJ appeared in 

pristine condition (see Fig. 9). 

Closer examination of the damaged starboard SARJ 

components was accomplished through additional spacewalks 

and extensive photographic surveys. The wear debris analysis 

showed that the debris was made up predominantly of 

fractured pieces of the hardened nitride surface of the race 

ring. Little debris from the 440C stainless steel rollers was 

present. The lack of any circumferential scratches, grooves, or 

wear marks on the race surface suggested that all of the rollers 

were indeed rolling. Further, rolling sensors integrated into 
NASA/TP—2011-217116 7 

each roller verified that none of the Timken-style (tapered 

roller) internal bearings used inside each roller had failed. The 

surface appeared to have been damaged by the intended 

rolling action of the SARJ design. To get a better picture of 

what may have occurred, a careful review of the build records 

ensued. 

The build records, amassed over nearly a decade, were 

immense (Ref. 4). The SARJs (port and starboard) are 

carefully described by over 3000 pages containing detailed 

build records, photographs, and performance graphs. Many of 

these records had been scanned and digitized with the 

exception of some fax communications amongst contractors 

and photographs taken with 35-mm cameras of subassembly 

components. 

The build record review was intriguing. In one respect, the 

review revealed that the SARJ was built with extreme 

precision and care. Each and every part, subassembly and 

assembly, was identified with unique serial numbers that were 

tracked and recorded. Every tapered roller bearing installed 

inside every roller was characterized for preload and drag 

torque. Even the amount of grease used for each bearing was 

metered, weighed, and recorded to assure uniform drag. The 

review also definitively proved that no parts were installed 

incorrectly, out of place, or out of sequence. The review also 

highlighted that in some instances key parts were made to 

less-than-optimum precision. When this happened, documentation supported either acceptance as built, reworking to meet 

specifications, or rejection and replacement. During the 

investigation, the build records concerning the failed SARJ 

were scrutinized, especially those documents related to the 

damaged contact surfaces, the race ring, and the trundle 

bearing roller assemblies. 

It was discovered, for instance, that the heat treatment and 

nitriding of the race rings differed slightly between the 

functioning port SARJ and failed starboard SARJ. The 

differences, however, were judged to be of secondary 

importance at the time because the nitride treatment was 

believed to be an enhancement of the base metal. The primary 

requirement for the nitride layer was good adhesion and 

proper thickness, and the casual metallography conducted 

during the build showed that both races were acceptable. More 

recent and in-depth metallurgical analysis of those witness 

coupons showed that the failed SARJ race ring contained more 

subsurface flaws than the port ring. Under severe loads, well 

beyond design limits, it could be expected that the increased 

incidence of subsurface flaws could lead to surface failure. 

Ultimately, the higher level of flaws found in the starboard 

SARJ race ring were deemed a contributing factor to its failure 

but were not the primary root cause. 

The other components that received scrutiny during the 

record review were the trundle rollers, particularly the smaller 

upper rollers that operate against the canted race surfaces of 

the race ring. These rollers, pointed out in Figure 4, are 

specified to be made from hardened 440C stainless bearing 

steel, precision ground and then plated with a thin layer of 

gold. The build records indicate that the batch of rollers used 

for the starboard side were initially defective, in that the 

contacting surface (roller outside diameter) did not meet 

straightness requirements. To address this shortcoming, the 

rollers were hand-polished until the profiles were within 

specifications. 

Communications and documentation reports were located 

that suggest that there were discussions amongst the 

subcontractors regarding the suitability of reworking the parts. 

Technical input from engineering suggests that the impact of 

less-than-optimum rolling surface geometry was assessed 

based upon the prevailing SARJ operational understanding 

that classical, long-term rolling-contact fatigue was the 

mechanical failure mode of concern. Since modeling 

suggested at least an order-of-magnitude fatigue life margins 

existed for the SARJ, a slightly wavy profile, as long as it was 

within specifications, would not impact mission life. 

Following roller finishing, they were plated with a thin 

(1- to 2-m-thick) gold film. Like the nitride treatment, it 

appears that the lubrication function of the gold film was for 

enhancement. The contact between the rollers and the race 

ring is designed to be as near to pure rolling contact as 

possible. The build records do not describe the purpose of gold 

as a lubricant. The aforementioned joint design paper (Ref. 3) 

suggests that such films provide a smooth running surface and 

prevent undesirable low friction levels that can cause slippage 

when rollers are used to provide driving or braking torque, a 

scenario significantly different from the SARJ. Whatever the 

design intent of the gold film, its condition offered important 
NASA/TP—2011-217116 8 

clues to the SARJ failure. Figure 10 shows a photograph of an 

assembled roller from the port SARJ buildup in which the 

gold film is in its intended pristine state. 

In operation, the gold gradually wears out and transfers to 

the nitrided race ring surface where it helps to lower friction. 

Figure 11 shows an assembled roller from the failed starboard 

SARJ in which the gold film is clearly deficient. 

This image, similar to those of a majority of the starboard 

SARJ rollers, shows that the gold layer did not properly 

adhere to the roller surface. As with the out-of-specification 

roller profiles condition, the peeling gold was the subject of 

several fax communications between subcontractors. Because 

the role of the gold was not understood, the impact of its poor 

condition was not known. Prevailing thinking was that it 

would have little impact on the fatigue life of the SARJ and 

that if it affected performance in a negative fashion, this would 

reveal itself during prelaunch ground testing. From a scientific 

standpoint, it is well understood that the presence or absence 

of the gold film would have little effect on rolling friction but 

a significant effect on sliding friction at the roller-raceway 

contacts. 

These effects are best described and quantified by the 

traction tests conducted at NASA Glenn in the vacuum roller 

rig (VRR). The VRR test loads two cylindrical rollers 

(Fig. 12) on one another and simulates the contact between the 

SARJ rollers and the race ring (Ref. 6). As shown in 

Figure 13, the motor turns the upper roller. The lower roller is 

driven by the rotation of the upper roller through the small 

contact that forms between the rollers under load. 

With this configuration, one can mimic the SARJ contact, 

in which small amounts of roller misalignment (less than 1) 

are inevitable because of tolerance stackups and 

manufacturing errors (Ref. 5). In essence, the contact is mostly 

pure rolling with a small sliding component. A key feature of 

this test is that the rotational axes of the rollers can be parallel 

(pure rolling) or intentionally misaligned resulting in small but 

controlled amounts of sliding. The roller supports are well 

instrumented, and all of the forces and torques that arise from 

the contact (axial thrust forces and radial side forces) are 

measured. 
NASA/TP—2011-217116 9 

When misaligned rollers contact, the contact conditions can 

be very complex. The contact area resembles a line for light 

loads, and this grows to be an ellipse as load increases. 

Additionally, because the contacting materials are elastic, any 

point away from the center of contact can experience elastic 

deformation, rolling, sliding, or a combination of these effects. 

Such contact mechanics nuances have been modeled and 

studied extensively and can be very complex. However, their 

external affects can be simplified (Refs. 7 and 8). 

When misaligned rollers contact, an axial thrust force (side 

load) arises at the contact interface perpendicular to the 

direction of rolling. The magnitude of this side load depends 

upon the misalignment angle and the rigidity or elasticity of 

the materials in contact. For extreme cases of misalignment 

the side load is approximately equal to the contact load 

multiplied by the coefficient of friction for pure sliding. This 

effect can be described by the so-called traction curve that 

plots the side load as a function of misalignment angle. 

Figure 14 shows typical data produced by the VRR. 

Such contact phenomena are well understood by those well 

versed in contact mechanics, such as bearing and gear 

designers and those involved in tire mechanics. For instance, 

Figure 15 shows the side load, sometimes referred to as the 

steering force, for a car tire rolling at varying slip (steer) 

angles. 

This figure resembles closely the VRR traction curve 

except that much higher misalignment angles are required to 

achieve high levels of side load. This is because rubber tires 

are very elastic and lack rigidity. For the materials used in the 

SARJ, small misalignment angles result in high side loads. 

Because of the particular kinematic design of the SARJ 

trundle assemblies, the side load leads to a potentially 

damaging condition, depicted in Figure 16 and reported upon 

previously, namely roller tipping (Ref. 5). The rollers are 

mounted on camber pivot mechanisms with the intent to 

ensure flat contact with the race ring surfaces. Under certain 

conditions the side load is of sufficient magnitude to tip the 

roller towards its edge. 
NASA/TP—2011-217116 10 
NASA/TP—2011-217116 11 

Such tipping results in high, localized stresses that could 

damage the contacting surfaces. Based upon detailed 

modeling, when the sliding friction coefficient between the 

roller and race ring reaches 0.4, tipping can occur (Ref. 5). 

Figure 17 shows the traction curves for SARJ-like materials 

as a function of misalignment angle. When lubricants such as 

gold or grease are present, a maximum friction (traction) 

coefficient (side load divided by normal load) of about 0.2 is 

reached at misalignments of about 1.0. (For the SARJ, 

measurements have confirmed that misalignments were 

typically less than 0.5). Beyond this point, gross slip inside 

the contact region prevents higher coefficients and 

commensurately higher side loads. In other words, good 

lubrication minimizes the effect. For the unlubricated case, 

side loads rapidly build, reaching a maximum at an effective 

friction (traction) coefficient of about 0.6, well above the level 

that causes roller tipping. Roller tipping alone leads to very 

high stresses because the load is concentrated. A second but 

critical aspect of the trundle kinematic design greatly 

exacerbates the problem, and that is unexpected rigidity of the 

trundle preload system. 

As shown in Figure 18, each trundle roller is held by its 

own pivot support. The inner 45 roller is additionally backed 

by a spring preload system that ensures that the load between

the rollers and the race ring are maintained within limits. 

Should mechanical or thermal distortions occur within a 

trundle, for instance a large particle or other foreign object on 
NASA/TP—2011-217116 12 

the race ring being rolled over, the spring preload of the lower 

roller can accommodate race deflection. In effect, all three 

rollers are designed to be spring preloaded by the single spring 

pack located behind the inner 45 roller. 

This same design principle is used in single piston-type 

automotive hydraulic disk brake calipers. In such mechanisms, 

the hydraulic braking pressure needs only to act upon one pad. 

Equal and opposite reaction forces are transferred to the 

opposing pad via the sliding action of the caliper. There are 

situations, however, in which load sharing does not occur. In 

such designs, load sharing is dependent upon low friction in 

the sliding mechanism used to transfer force to the opposing 

pad. Should this sliding contact experience high friction, load 

is not shared. 

In the SARJ, if the sliding friction coefficient between the 

rollers and the race surface exceeds about 0.4 the ring can be 

locked into place much the same way plier jaws lock onto a 

small, unlubricated round piece of pipe. If some lubricant is 

applied (low friction) or the pipe diameter is sufficiently large 

enough to change the jaw angle, the pliers spit the pipe away. 

In the SARJ, the lack of adequate roller-race surface 

lubrication (high friction) allowed the outer 45 roller (top right 

in the figure) to tip against the lower roller (bottom roller in 

figure) effectively pinching the race ring. This race-ring 

pinching action combined with a lack of structural compliance 

caused by high friction, led to greatly increased loads resulting 

in damaging the nitride surface. Based upon this information, 

additional modeling, and experimental verification tests, a 

failure scenario was developed and adopted. It is summarized 

as follows: 

Inadequate lubrication of the roller-race 

contact combined with a kinematic 

mechanism design vulnerable to roller 

tipping and high friction led to damaging 

high roller-race surface forces and stresses. 

This failure scenario provided invaluable guidance to 

recovery efforts. For instance, the scenario helped explain why 

the port SARJ that had better lubrication appeared to function 

nominally while the inadequately lubricated starboard SARJ 

failed so rapidly. The scenario also enabled modeling and 

testing efforts to corroborate various pathways to prevent port 

SARJ failure and to get the starboard side functioning. A clear 

path for better design of future SARJ-like systems is also 

revealed. Design elements could include (1) kinematic 

geometries that will not tip, even in the presence of excessive 

friction and (2) through the profiling of roller edges to avoid 

stress concentrations. 

SARJ On-Orbit Repair and Recovery 

Following the development of the root-cause failure 

scenario, an on-orbit repair and recovery plan was developed. 

This plan included an ambitious series of spacewalks to 

replace damaged trundle arms, the removal of wear debris, and 

the addition of space-compatible grease to reduce race-roller 

friction to levels well below those that lead to roller tipping. 

Design changes to the trundles and race rings could not be 

reasonably accommodated because of cost, schedule, and 

practicality reasons. 
NASA/TP—2011-217116 13 

Figures 19 and 20 show highlights of the SARJ repair and 

some of the specialized tools used to carry out the tasks. As a 

testimony to the design team, the SARJ design did allow safe 

component replacement. In addition, the functioning port 

SARJ was also lubricated with grease, as there was a concern 

that several rollers may have had gold film adhesion problems. 

It was later determined that there is a corrosion-driven 

aging effect for gold films deposited on stainless steel. When 

exposed to room air, gold films will delaminate because of 

galvanic corrosion at the interface. Since the port SARJ rollers 

were assembled immediately after gold deposition, they 

suffered less delamination than the starboard SARJ rollers that 

sat for several months after deposition prior to assembly. 

Interestingly, when the SARJs were restarted after the 

application of grease, their overall drag torque levels 

noticeably dropped (see Fig. 21). 

This drop was somewhat unexpected because the addition 

of an extensive layer of grease was thought to be capable of 

actually increasing drag. Further, a SARJ drag-torque analysis 

conducted by the original design team had concluded that 

roller-race sliding friction had no significant effect on overall 

joint drag-torque (Ref. 9). To better understand this contradiction, the original torque analysis was examined. 
NASA/TP—2011-217116 14 

SARJ Torque Analysis 

One performance aspect that was addressed during the 

design and build phase of the project was the anticipated drive 

torque and power required to rotate the SARJs. Correctly 

assessing such a parameter was vital in terms of ISS power 

management since the electrical system and its capacity have 

direct influence on many vital ISS systems, such as life 

support, cooling, and research. The SARJ prime contractor did 

a SARJ drive torque assessment study (Ref. 9). 

The study was based upon a first-principles free-body force 

analysis that took into account roller preload effects, friction 

between the rollers and the race ring, truss-induced loads, and 

friction coming from the internal greased tapered roller 

bearings found inside each trundle roller that enable their free 

rotation. The in-depth torque study was made available as an 

internal document and is part of the SARJ records. 

The SARJ torque study correctly deduced that the drag 

torque would arise from several sources: the drive motor gears 

and bearings, the rolling contact between the trundle rollers 

and the race rings, and the tapered roller bearings found inside 

each trundle roller. The drive motor assemblies and the 

tapered roller bearing friction torques are well understood by 

engineers. The drag torque of a roller on a race ring is not so 

straightforward. Figure 22(a) depicts a roller loaded against a 

race ring surface with a small amount of misalignment. The 

intentional normal load between the roller and the ring is 

applied by the spring preload system. This normal load 

interacts with the rest of the mechanical system causing 

several other forces to arise. 

One key force, the lateral or side force, arises from elastic 

effects in the contact between the roller and the race surface. 

For high levels of sliding friction and moderate misalignment 

angles, the lateral force can reach significant levels, which 

places an additional load on the internal tapered roller 

bearings, increasing their drag torque. For the SARJ, this 

means that a poorly lubricated roller-race ring contact 

combined with unavoidable misalignment will result in 

increased overall drag torque. The failure to account for the 

side load effect on bearing drag led to an incorrect conclusion 

that roller-race friction (lubrication condition) had no effect on 

overall SARJ torque, as shown in an excerpt from the original 

torque study (Ref. 9) reproduced in Figure 22(b). 

It is not surprising that a loads engineer or analyst would 

overlook such contact mechanics phenomena. Only those well 

versed in mechanical systems that include rolling contact of 

hard bodies, such as railroad or bearing specialists, would be 

intimately familiar with these forces (Refs. 7, 8, and 10). 

Because of the subtle nature of these contact mechanics, the 

fact that SARJ drag is indeed affected by roller-race 

lubrication was not known. During prelaunch ground tests, 

starboard SARJ drag torque was higher than expected by a 

factor of about 30 percent. Despite investigating the issue and 

careful review of subassembly drag values no definitive 

reason for the high “out-of-family” torque was determined. 

Because sufficient drive torque margins existed the SARJ was 

prepared for launch. 
NASA/TP—2011-217116 15 

Lessons Learned 

Complex projects with ambitious and novel goals such as 

building a SARJ bring challenges in three primary categories: 

engineering, communications, and validation. Engineers are 

often well equipped to handle engineering challenges. Meeting 

challenges in communications and validation can prove more 

difficult because such topics are less concrete. It is with regard 

to these two categories, communications and validation, where 

important lessons learned from the SARJ failure can be gleaned. 

When orchestrating large projects involving many people 

over long periods of time, it can be difficult to maintain 

effective communications. One important risk often 

overlooked by those intimately involved with a project or 

program is ensuring that a clear, consistent, and complete 

description of the project is developed and maintained for all 

participants to use for reference. This description can take 

many forms such as a written document or a briefing package 

or even a hardware mockup. The existence and continuous 

dissemination of an official, updated source of project 

information that enables all project participants, both new and 

experienced, to share a common vision of the hardware, its 

intended operation, performance requirements, and 

terminology makes it less likely that potential problems and 

warning signs will be overlooked or misunderstood. For 

complex systems it is especially important that everyone have 

a clear, correct, and basic understanding of the entire system 

so they can see how their own detailed knowledge fits into the 

whole. For an engineered system like the SARJ, under 

development at that time, a frequently updated project 

description document would have been appropriate. 

Such a document, however, does not seem to have existed. 

An early design paper written by NASA researchers best 

describes the SARJ mechanism (Ref. 3). This paper, which 

was not widely disseminated, outlined differing conceptual 

design approaches to building large rotary joints. The actual 

SARJ design was certainly influenced by this paper, but there 

were key differences. One difference was that the early 

conceptual design used the rollers to drive the race ring. In 

effect, the rollers push the ring in a circle. The final design 

utilized a gear built into the race that enabled the driven ring 

to push the rollers. This subtle difference prevents the rollers 

from self-aligning like a follower-castor wheel and can lead to 

roller tipping and race-ring pinching (Ref. 5). The conceptual 

design also took a different approach to the use of “lubricants” 

for the rollers. In the original design concept, the rollers were 

intended to drive the ring, and sufficiently high sliding friction 

was thought to be necessary to prevent roller-race slippage 

(Ref. 3). Thus, surface traction materials were suggested that 

could ensure high friction and enduring smooth surfaces as 
NASA/TP—2011-217116 16 

opposed to the low-friction surfaces actually required by the 

SARJ. Had a revised design document been written and shared 

with all participants, some of these more subtle issues may 

have been discussed and addressed. 

The absence of an accurate written 

description of the mechanism effectively 

isolated participants working on individual 

elements of the system. Such compartmentalizing of participants can reduce project 

time spent answering questions but also 

increases the risk that problems may be 

missed. 

Few people involved with the root-cause investigation 

appeared to have an accurate system-level understanding of 

how the SARJ worked. Only through arduous study of the 

build documents and long discussions with various 

participants could one begin to understand the SARJ operating 

characteristics. With such a fragmented approach to systemlevel understanding, when questions arose during the failure 

investigation about specific components or subassemblies, the 

team had to rely solely on the input from subject matter 

experts who also could not see the “big picture.” At times, this 

input was incorrect. A prime example was the design intent of 

the gold plating used on the rollers. No written SARJ 

document clearly defined its role as a friction-reducing film, 

and few people involved with the failure investigation were 

even aware that gold could be an effective solid lubricant. This 

was likely true during the SARJ build. Considering the general 

lack of system-level understanding, one can more easily see 

how conditions such as peeling gold films on rollers could 

have been downplayed during the SARJ build period. 

Improving communications so that everyone contributing to 

a project understands their role, the roles of other contributors, 

and how the many pieces of a system work together, leads to 

more robust products. The key step is to make system 

knowledge fully and continuously available to all and provide 

sufficient resources to encourage communications amongst 

personnel at all levels. In today’s more electronic age, this can 

be effectively done with an information-sharing Web site that 

could even include a discussion forum, blogs, the most up-todate project description document, and drawing archives. 

Thorough validation, both analytical and experimental, is 

another important area where more extensive effort would 

have been a good investment. There are two primary reasons 

projects conduct validation tests. One is to verify that 

requirements are met. The other is to develop a better 

understanding of how a system performs. Customers willingly 

support tests to prove a system meets requirements. They are 

less likely to spend resources to develop in-depth 

understanding. Such understanding is supposed to be solely a 

function of design. Since design engineers develop their 

concepts and hardware based upon fixed rules and equations, 

it is an understandable view that hardware will obey physics. 

Therefore testing to achieve deeper understanding may appear 

unnecessary. But when “off-nominal” behavior is observed, 

such as higher drive torque than models predict, efforts to 

understand such discrepancies are resources well spent. Even 

if tested operating parameters are within margins, unexpected 

or unexplainable behavior should trigger the need to revisit the 

models, experiments, and design assumptions until closure is 

reached. Otherwise, the models and experimental data are in 

question, and true risks can be more difficult to assess. 

Today, testing can take two forms: analytical testing and 

experimental testing. For simple systems with mild failure 

consequences, one or the other can be sufficient. For instance, 

a hand tool like a screwdriver intended for home use could be 

designed and built with a reliance on modeling and stress 

analysis. Should it fail, because of poor materials or 

workmanship, it can be replaced easily. A similar tool destined 

for use during an EVA outside the ISS would require both 

analytical and experimental testing because failure 

consequences could be dire. The materials used for the tool 

would be proof-tested to verify their yield and ultimate 

strengths. A flightlike completed tool would be tested to 

failure to make sure no sharp pieces could become liberated if 

it were used beyond its limits, lest such debris do harm 

elsewhere. 

Although it might be tempting to utilize one 

method of testing only, both analytical 

modeling and experimental validation are 

essential for critical systems. 

In the present case, it may have been possible to set up an 

inexpensive experiment to validate the SARJ torque model 

that was based upon analysis (Ref. 9). The model concluded 

that lubrication of the roller-race interface had a negligible 

effect on SARJ torque. This could have easily been verified 

using the structural test article (STA) SARJ, by adding grease 

and monitoring torque. Conversely, an STA test with unplated 

rollers could have been conducted to see how the system 

responded. Unfortunately, analysis was too heavily relied 

upon, making it difficult for engineers to interpret the distress 

signals broadcast by the starboard SARJ during prelaunch 

ground tests. 

If project management advocates extensive testing, it can be 

interpreted as their lack of confidence in the design team. 

Testing is also expensive and typically occurs late in a project 

cycle when resources, both time and money, are in short 

supply. Thus it is not surprising that much testing is aimed at 

proving that a system meets minimum performance metrics, 

plus perhaps some design margin. Testing to establish limits 

and capabilities and testing to intentional failure are rarely 

done except in cases where human life is involved. 

Aircraft design is a field with little room for uncertainty. 

When a new airframe is being developed extensive full-scale 

tests are conducted before the first unit ever leaves the ground 

for a flight test. One key ground test involves static loading of 

the wings. This is usually done using weighted bags of sand. 

First the design load is placed upon the wings and their 

deflection is measured and compared to models. Inspectors are 

present to make sure the response reasonably agrees with the 
NASA/TP—2011-217116 17 

models. Then additional weights are added until the design 

margin—typically twice the rated load—is reached. Again, 

deflection data are compared to that of the models. The tests 

up to this point are a good representation of the tests NASA 

conducted on the SARJ and serve to prove that the design 

meets minimum and margin requirements. Testing of aircraft 

goes further, however. 

After the aircraft is shown to successfully handle the design 

and margin loads, additional weights are added and deflection 

data is collected. Eventually, the wings break in a dramatic 

fashion. At this point the ultimate load capacity, the failure 

mode, and the exact location of the failure are verified. The 

result is then compared to the model results, and any 

discrepancies are explained. Such testing has proven 

invaluable in refining models and developing a database that 

can be used in the event that failure occurs in flight. 

Such intentional failure testing has drawbacks, however. One 

is that the test article is consumed. This can be expensive. 

Additionally, there is programmatic risk. If the test displays an 

unexpected result, albeit under conditions outside the design 

envelope, a redesign may be indicated. This can place more 

pressure on schedule and cost. Another drawback is political. 

Dramatic images of broken hardware can be difficult to explain 

to those outside the team. In our risk adverse times, some feel it 

is better to avoid failures, even those that are planned. 

Nonetheless, without a test to intentional 

failure, one cannot verify that the design team 

has accurately predicted the failure mode. 

For the SARJ, this is an important lesson to be learned. The 

failure modes identified by the design team included rolling 

contact fatigue of the race ring, wear of the drive gear and 

pinion, foreign object damage (FOD), and failure of the 

internal tapered roller bearings used in each roller. Backup 

bushings and electronic rotation indicators were used to 

monitor the internal bearings. Great care was taken to preclude 

FOD contamination and ultrahard and strong roller, and race 

materials were chosen to push the predicted rolling contact 

fatigue life orders of magnitude beyond ISS needs. A gear life 

test was run to verify pinion-drive gear wear was acceptable. 

For this test, the structural test article (STA) was run at 

heightened speed for an extended time to mimic 13 or more 

years of SARJ duty. It was then inspected for wear. While it 

did show signs of uneven roller loading and wear marks, these 

were not explored further. In effect, the testing that was done 

was aimed at verifying adequate life and margins for those 

failure modes that were predicted. Exploratory testing was not 

conducted to identify unknown failure modes such as roller 

tipping and pinching of the race ring caused by inadequate 

lubrication. Such a subtle failure path would have been 

difficult to predict. It is possible that less purposeful but more 

inquisitive testing of the STA may have revealed such a 

failure mode. 

Among the tests that could have been performed was an 

accelerated life test of the STA in a more flightlike 

environment. The pinion life test was done in air because 

utilizing a vacuum test facility large enough to house the full 

SARJ would have been prohibitively expensive. It is well 

known that the atmosphere heavily influences tribological 

behavior. Friction, for example, is often higher in vacuum than 

in air for bare metal and ceramic interfaces. 

The STA could have also been used to validate some of the 

flight conditions that arose by manufacturing changes and 

errors. For instance, many of the rollers used for the starboard 

SARJ had wavy rather than flat surfaces. The decision to 

accept these rollers was based upon a prediction of their effect 

upon fatigue life, provided a successful short-term systemlevel checkout ground test of the SARJ occurred prior to 

launch. The same philosophy applied to the peeling gold films. 

Discussions were held between subcontractors over the 

effects of wavy roller profiles on performance, but the 

resolution, again, was led by analysis and engineering 

judgment rather than experimental verification. Other than the 

short-term prelaunch ground test, which was done in air, 

NASA flew hardware in a condition that 

was never fully tested. 

The STA could have been reconfigured for such a test 

independent of the build schedule. Anything learned during 

such independent testing, even after launching the flight 

hardware, could have proved valuable. Had such testing led to 

failure, on-orbit greasing of the SARJs could have been done 

without the extensive cleaning, scraping, and trundle 

replacement. 

Other interesting tests could also be envisioned to address 

other scenarios. For example, in a ground test one could 

remove one or more trundles and examine the effect on joint 

stiffness and operation. Rollers could be intentionally locked 

and dragged to see the effects on torque and vibration. If such 

a condition develops on orbit, such ground data can aid in 

deciphering performance anomalies in orbit to better anticipate 

problems. To reiterate, more extensive hardware testing 

provides additional understanding of complex systems and 

should not be undervalued. 

Summary of Results 

The forensic engineering investigation presented in this 

paper identifies missteps that contributed to the SARJ’s 

failure. Some of these missteps were purely technical while 

others were more programmatic. Based upon the factors that 

contributed to the failure and the actions taken to achieve a 

successful on-orbit repair the following conclusions were 

drawn and are reiterated below: 

(1) Rolling is better than sliding, oils and greases are better 

than solid lubricants, and traditional solid lubricants (e.g., 

molybdenum disulfide, polytetrafluoroethylene (PTFE), and 

engineered and intercalated graphite) are better at reducing 

friction than metal films. 
NASA/TP—2011-217116 18 

(2) The best possible design could only have been 

determined through an extensive experimental and analytical 

research and development program since such a mechanism—

the SARJ—was a first of its kind. 

(3) Inadequate lubrication of the roller-race contact combined 

with a kinematic mechanism design vulnerable to roller tipping 

and high friction led to damaging high roller-race surface forces 

and stresses. 

(4) The absence of an accurate written description of the 

mechanism effectively isolated participants working on 

individual elements of the system. Such compartmentalizing of 

participants can reduce project time spent answering questions 

but also increases the risk that problems may be missed. 

(5) Although it might be tempting to utilize one method of 

testing only, both analytical modeling and experimental 

validation are essential for critical systems. 

(6) NASA flew hardware in a condition that was never fully 

tested. 

(7) Despite extensive system analysis and modeling, without 

a test to intentional failure, one cannot verify that the design 

team has accurately predicted the failure mode. 

Concluding Remarks 

NASA’s missions, often the first of their kind, rely upon 

many engineered systems that are unique and complex and 

cannot be fully tested before launch. This was most certainly 

the case for the SARJ mechanisms. This challenges the 

engineering community to develop approaches to incremental 

testing and designs that are inherently robust and redundant. In 

hindsight, one could surmise that the SARJ failure mode could 

have been anticipated, but this is an unrealistic conclusion. 

The SARJ is an unusual mechanism that provides an unusual 

function under unusual conditions. Predicting its failure mode 

without extensive ground testing is unlikely. To enhance the 

chances that such vulnerabilities may be uncovered the 

following recommendations are offered: 

A more comprehensive approach to internal engineering 

communications amongst project participants is essential. 

Teams of people with varying expertise, duties, and 

perspectives undertake large, complex systems such as the 

SARJ. Such projects often span many years and must endure 

changes of leadership and key personnel as well as utilize 

many contractors. To avoid overlooking potential problems, 

each and every contributing participant needs to share a 

common understanding of the engineered system, its function, 

and its design. Once achieved it is more likely that the 

ramifications of a seemingly small manufacturing variance, 

such as gold film adhesion, will be put in the proper context. 

Effective communication could take the form of a living, 

descriptive project overview supplemented by presentations or 

even an electronic clearinghouse or Web site accessible to all. 

Finally, caution must be exercised to avoid reliance upon 

validation through solely analytical modeling or experimental 

testing. To minimize risk, both approaches are needed. Such 

redundant test approaches should yield consistent results. If a 

contradictory answer emerges to a common question, its 

resolution is warranted. In the case of the SARJ, the torque 

analyses suggested that joint drag was independent from the 

lubrication condition of the roller-race interface. When the 

prelaunch ground test yielded higher than expected torque, this 

non-intuitive, analytical conclusion could have been 

experimentally verified. 

Unfortunately, active and comprehensive communications 

requires effort and investment. The insistence upon both 

analytical and experimental testing is also costly. Resource 

constraints often inhibit such investments. Further, the 

redundant approach to testing and intentionally testing beyond 

design conditions to failure can be unpalatable to the very 

parties that have developed the requirements and design 

margins. This makes it even more necessary for project 

leadership to encourage and nurture an open and safe 

environment where everyone questions everything and 

criticisms are welcomed. When the consequences for 

unexpected failure are dire, a thorough approach to testing and 

development is more effective. 

It is hoped that the lessons learned through the SARJ failure 

and recovery will serve as a teaching aid to reduce system 

failures on future projects. 

Glenn Research Center 

National Aeronautics and Space Administration 

Cleveland, Ohio, May 16, 2011 

References 

1. Catchpole, John E.: The International Space Station: Building 

for the Future. Springer, Chichester, UK, 2008. 

2. Leger, Lubert J.; and Dufrane, Keith: Space Station Lubrication 

Considerations. 21st Aerospace Mechanisms Symposium. 

NASA CP–2470, 1987, pp. 285–294. 

3. Loewenthal, Stuart H.; and Schuller, Fredrick T.: Feasibility 

Study of a Discrete Bearing/Roller Drive Rotary Joint for the 

Space Station. NASA TM–88800, 1986. 

4. Branda, S.H.: Acceptance Data Package for the SARJ Bearing 

Assembly Trundles. Document #744D1188, AEC–ABLE Co., 

Goleta, CA, internal report, Sept. 1997. 

5. Harik, Elliot P., et al.: The International Space Station Solar 

Alpha Rotary Joint Anomaly Investigation. Proceedings of the 

40th Aerospace Mechanisms Symposium, NASA/CP—2010-

216272, 2010. 

6. Krantz, Timothy; DellaCorte, Christopher; and Dube, Michael: 

Experimental Investigation of Forces Produced by Misaligned 

Steel Rollers. NASA/TM—2010-216741, 2010. 

7. Johnson, K.L.: Contact Mechanics. Cambridge University Press, 

New York, NY, 1987. 

8. Jacobson, Bo O.; and Kalker, J.J.: Rolling Contact Phenomena. 

CISM Courses and Lectures, no. 411, Springer-Verlag, New 

York, NY, 2000. 

9. Klinker, D.H.: SARJ Torque Margin Update. Lockheed Martin 

Engineering Memorandum (EM) #02–ME–349–MS–571, 

internal report, Feb. 1994. 

10. McGinnes, H.: Lateral Forces Induced by a Misaligned Roller. 

Deep Space Network, 1978, pp. 253–257. 
REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 0704-0188 

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the 

data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this 

burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. 

Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB 

control number. 

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 

1. REPORT DATE (DD-MM-YYYY)

01-08-2011 

2. REPORT TYPE

Technical Paper 

3. DATES COVERED (From - To)

4. TITLE AND SUBTITLE

ISS Solar Array Alpha Rotary Joint (SARJ) Bearing Failure and Recovery: Technical and 

Project Management Lessons Learned 

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

DellaCorte, Christopher; Krantz, Timothy, L.; Dube, Michael, J. 

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

WBS 869021.03.07.01.06 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration 

John H. Glenn Research Center at Lewis Field 

Cleveland, Ohio 44135-3191 

8. PERFORMING ORGANIZATION

 REPORT NUMBER

E-17803 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration 

Washington, DC 20546-0001 

10. SPONSORING/MONITOR'S

 ACRONYM(S)

NASA 

11. SPONSORING/MONITORING

 REPORT NUMBER

NASA/TP-2011-217116 

12. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified-Unlimited 

Subject Categories: 18, 37, and 38 

Available electronically at http://www.sti.nasa.gov 

This publication is available from the NASA Center for AeroSpace Information, 443-757-5802

13. SUPPLEMENTARY NOTES

14. ABSTRACT

The photovoltaic solar panels on the International Space Station (ISS) track the Sun through continuous rotating motion enabled by large 

bearings on the main truss called solar array alpha rotary joints (SARJs). In late 2007, shortly after installation, the starboard SARJ had 

become hard to turn and had to be shut down after exceeding drive current safety limits. The port SARJ, of the same design, had been 

working well for over 2 years. An exhaustive failure investigation ensued that included multiple extravehicular activities to collect 

information and samples for engineering forensics, detailed structural and thermal analyses, and a careful review of the build records. The 

ultimate root cause was determined to be kinematic design vulnerability coupled with inadequate lubrication, and manufacturing flaws; this 

was corroborated through ground tests, metallurgical studies, and modeling. A highly successful recovery plan was developed and

implemented that included replacing worn and damaged components in orbit and applying space-compatible grease to improve lubrication. 

Beyond the technical aspects, however, lie several key programmatic lessons learned. These lessons, such as running ground tests to 

intentional failure to experimentally verify failure modes, are reviewed and discussed so they can be applied to future projects to avoid such 

problems. 

15. SUBJECT TERMS

Bearings; Lubrication; ISS; Mechanisms; Mechanical engineering; Mechanical drives; Failure analysis; Failure modes; Lessons 

learned 

16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF

 ABSTRACT

UU 

18. NUMBER

 OF 

 PAGES

24 

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email:help@sti.nasa.gov) 

a. REPORT

U 

b. ABSTRACT

U 

c. THIS 

PAGE

U 

19b. TELEPHONE NUMBER (include area code)

443-757-5802 

Standard Form 298 (Rev. 8-98)

Prescribed by ANSI Std. Z39-18