Timothy L. Krantz

Glenn Research Center, Cleveland, Ohio

Iqbal Shareef

SFF Bradley University, Peoria, Illinois

Wear of Steel and Ti6Al4V Rollers in Vacuum

NASA/TM—2012-217619

June 2012
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Timothy L. Krantz

Glenn Research Center, Cleveland, Ohio

Iqbal Shareef

SFF Bradley University, Peoria, Illinois

Wear of Steel and Ti6Al4V Rollers in Vacuum

NASA/TM—2012-217619

June 2012

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

41st Aerospace Mechanisms Symposium

cosponsored by the NASA Jet Propulsion Laboratory and Lockeed Martin Space Systems Company

Pasadena, California, May 16–18, 2012
Acknowledgments

This research was supported by the NASA Engineering Safety Center. The experimental work was done with assistance from 

Mr. Richard Manco, Sierra Lobo Inc. Dr. Iqbal Shareef’s contributions were supported by the NASA GRC Summer Faculty 

Fellowship Program.

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Level of Review: This material has been technically reviewed by technical management. 

This report contains preliminary fi ndings, 

subject to revision as analysis proceeds.
NASA/TM—2012-217619 1 

Wear of Steel and Ti6Al4V Rollers in Vacuum 

Timothy L. Krantz 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Iqbal Shareef 

SFF Bradley University 

Peoria, Illinois 61625 

Abstract 

This investigation was prompted by results of a qualification test of a mechanism to be used for the 

James Webb Space Telescope. Post-test inspections of the qualification test article revealed some loose 

wear debris and wear of the steel rollers and the mating Ti6Al4V surfaces. An engineering assessment of 

the design and observations from the tested qualification unit suggested that roller misalignment was a 

controlling factor. The wear phenomena were investigated using dedicated laboratory experiments. Tests 

were done using a vacuum roller rig for a range of roller misalignment angles. The wear in these tests was 

mainly adhesive wear. The measured wear rates were highly correlated to the misalignment angle. For all 

tests with some roller misalignment, the steel rollers lost mass while the titanium rollers gained mass 

indicating strong adhesion of the steel with the titanium alloy. Inspection of the rollers revealed that the 

adhesive wear was a two-way process as titanium alloy was found on the steel rollers and vice versa. The 

qualification test unit made use of 440F steel rollers in the annealed condition. Both annealed 440F steel 

rollers and hardened 440C rollers were tested in the vacuum roller rig to investigate possibility to reduce 

wear rates and the risk of loose debris formation. The 440F and 440C rollers had differing wear behaviors 

with significantly lesser wear rates for the 440C. For the test condition of zero roller misalignment, the 

adhesive wear rates were very low, but still some loose debris was formed. 

Introduction 

This investigation was prompted by results of a qualification test of a mechanism to be used for the 

James Webb Space Telescope. The mechanism is used to move a magnet for certain operations of the 

telescope’s near infrared spectrometer (NIRSpec) instrument. The motion of the magnet is guided by a set 

of preloaded steel rollers in contact with anodized Ti6Al4V. The qualification testing was accomplished 

in a cold vacuum chamber to match as closely as possible the extreme deep space environment. Post-test 

inspections of the qualification test article revealed some wear of the steel rollers and mating Ti6Al4V 

surfaces, and some loose debris was found. The NASA Engineering Safety Center (NESC) is 

investigating the potential risk of the wear and debris toward hindering the full capability of the NIRSpec 

instrument. This article will discuss the NESC investigation of the wear phenomena including dedicated 

laboratory experiments. 

Qualification Testing Article and Testing Results 

The mechanism of interest is a translator assembly that features a set of 11 rollers to guide the motion 

of a magnet (Fig. 1). The translation of the magnet is used to affect the positions of micro-shutters. A 

motor and linkages provide the motive force. The translator assembly undergoes oscillatory motion in a 

straight line. The translating distance is approximately 200 mm on each stroke (total of 400 mm of travel 

during back and forth motion) and translation time in one direction during qualification testing was 

10.5 sec. The qualification test included 96,000 of such forward and return excursions. 
NASA/TM—2012-217619 2 

             

The translator assembly motion is guided by a set of eleven rollers. The in-plane guide is provided by 

a set of four rollers (two of these rollers are visible to the far right side of Fig. 1). These four rollers 

contact both sides of a guide rail defining the direction of motion. The out-of-plane position and motion is 

guided by a set of seven rollers. Three rollers are in contact with a base plate defining one plane and four 

rollers are in contact with a cover defining a second parallel plane. The roller preloads are set by a 

shimming procedure to a nominal normal load of about 18 N. The roller is a cylinder with diameter of 

21.2 mm. The roller profile is a crown radius with a flat feature in the center of the crown. Assuming line 

contact across the flat feature, the Hertz contact maximum pressure is about 240 MPa. The rollers are 

made from 440F steel (a free machining variant of 440C). The roller material was in the annealed 

condition and passivated. A pair of deep groove ball bearings supports each roller. The fastener securing 

the roller bearings to the axle is a locking style bolt assembled in a manner allowing for a small amount of 

axial play. The rollers are in contact with Ti6Al4V that had been stress relieved and anodized. 

After completion of the qualification test, wear was visible on the rollers of the translator assembly 

and on the mating surfaces. Figure 2 shows the condition of a qualification test roller. Some loose 

particulate debris was also found, which was collected and analyzed (Refs. 1 and 2). The worn surfaces 

and wear debris provide evidence that the wear process was primarily adhesive wear. In total it appears 

that the steel has transferred to the mating Ti6Al4V surfaces. The loose wear debris included both the 

steel and titanium alloy materials. The severity of wear was not the same on all rollers. 

From observations of the wear patterns and wear debris of the qualification unit, study of the 

literature, and engineering assessment of the kinematic configuration, manufacturing tolerances, and 

roller mounting details, the roller alignment seemed likely to be a key variable influencing the wear rate. 

To better understand the wear phenomena and to explore possible mitigation strategies, a set of laboratory 

roller tests were conducted in vacuum. The next sections will discuss the testing apparatus, specimens, 

procedures, and the corresponding results. 

Laboratory Test Apparatus, Specimens, and Procedure 

Test Apparatus for Roller Pairs 

Testing was done using the NASA Glenn Research Center Vacuum Roller Rig (Fig. 3). The rig 

allows for application and measurement of a load pressing the rollers together while having a purposely 

misaligned and adjustable shaft angle. The rig is depicted in schematic form in Figure 4. A drive motor 

provides motion to the driving roller. A magnetic-particle brake attached to the output shaft imposes 

torque on the driven roller. The rig can be operated with the brake not energized. For such a condition the 

torque transmitted through the roller pair is the drag torque of the output shaft (drag of the seals and 

support bearings). The normal load pressing the rollers together is provided by an air cylinder. 

Figure 1.—Translator assembly (Ref. 1). Figure 2.—Condition of roller after 

completion of qualification test 

(Ref. 2). 
NASA/TM—2012-217619 3 

Figure 3.—Vacuum roller rig. 

 (a) 

 (b) 

Figure 4.—Schematic views of the vacuum roller rig. (a) Schematic, side view. (b) Schematic, 

overhead view with shaft misalignment depicted and exaggerated. 

ferrofluid seals

and bearings

torque/force sensor

brake

pivot point for drive motor 

plate relative to bed plate

drive motor

bedplate

hydraulic cylinder

“normal load” 

force sensor

vac chamber

turntable

bedplate

bellows

ferrofluid seal and bearing

torque/force sensor

brake

drive motor

bedplate

turntable on bearing

load cell for “tangential” force,

mounted to turntable
NASA/TM—2012-217619 4 

Figure 5.—Simplified schematic view including some of the important sensed data. (a) Schematic, 

front view. (b) Schematic, side view. 

The cylinder acts through a gimbal point to rotate the plate that mounts the driving shaft and drive motor. 

The rotation of the drive motor plate displaces the driving roller toward the driven roller shaft in an arc 

motion. The pressure to the cylinder, and thereby the load between the contacting rollers, is adjusted by a 

hand-operated valve (open-loop control). A turbomolecular pump assisted by a scroll pump provides 

vacuum in the test chamber. The typical condition in the test chamber is a pressure of about 310–7 torr. 

The most prevalent remaining constituent in the chamber during testing is water vapor as determined by 

residual gas analyzer (Ref. 3). Figure 5 provides a simplified schematic of the test rollers labeled with 

some of the nomenclature used herein. 

A set of sensors on the test apparatus monitors the test conditions. The outputs of the analog sensors 

were digitized and stored via a data acquisition system at a rate of up to 0.66 Hz. Each of the sensors will 

be described in turn. 

The misalignment of the driving roller shaft and driven roller shaft is depicted in an exaggerated 

manner in Figure 4(b). The misalignment is measured via a linear variable differential transformer 

(LVDT). The transducer housing is attached to the bedplate, and the translating, spring-loaded transducer 

tip contacts against a mechanical stop on the turntable. To establish the aligned condition, special tooling 

blocks were machined to locate the roller-mounting surfaces of the two shafts as parallel. With the shafts 

aligned by the tooling blocks, the transducer circuit balance was adjusted to provide an output of zero. 

The precision of this method for aligning the shafts was limited by the dimensions of the roller mounting 

surfaces used as the reference planes. From analysis of the test rig drawing tolerances and geometry, the 

alignment procedure using the tooling blocks to define the zero-degree position has a precision within 

0.08. Rotation of the turntable from the aligned position moves the LVDT sensor. The angular 

displacement of the turntable was determined by mounting a laser light source on the moving shaft at the 

roller mounting location and directing the light onto a paper placed at a known radial distance from the 

center of the turntable. The movement of the laser light was marked on the paper and distance between 

the points measured and used to relate the sensor output to the angular motion of the turntable. 

The torque on the output shaft is monitored by a strain-gage type torquemeter of 22 N-m (200 in.-lb) 

torque capacity. Calibration was done in place using deadweights acting on a torque arm of known length 

attached at the test roller position and reacting the output shaft to ground. 

The load pressing the rollers together is termed herein the “normal load” (Fig. 5). The normal load is 

applied via an air-pressure actuated piston. The air piston acts through a load cell against the drive motor 

plate that is gimbal-mounted relative to the test chamber (Fig. 4(a)). In this way the air piston moves the 

roller on the input shaft in an arc motion toward the test roller. The arc motion of the input shaft table is 

measured by an LVDT. Once the rollers are in contact, additional force commanded to the air piston 

increases the normal load between the test rollers. Careful calibration processes allow calculating the 

normal load on the test roller based on the sensor outputs from the load cell and the LVDT that measures 

the input shaft position. 

normal load

braking torque

tangential force

normal load

normal load

axial force

normal load

input shaft

output shaft output shaft

(a) (b)

normal load

braking torque

tangential force

normal load

normal load

axial force

normal load

input shaft

output shaft output shaft

(a) (b)
NASA/TM—2012-217619 5 

When rollers operate in a misaligned condition a force will develop in the direction of the shaft axis 

(Refs. 4 to 7). In such a condition points on the two rollers in intimate contact and within a “stick” zone of 

the contact patch are constrained to move in unison. If the points were not in contact the kinematic 

constraints would provide a slightly different path of motion. The difference in the actual path of motion 

and that defined by the motion if the points were not in contact gives rise to surface strains and a resultant 

axial force. A sensor to measure this force is labeled as the “axial force” sensor in Figure 4. The axial 

force sensor is co-located on the output shaft with the torquemeter sensor. The configuration of the rig did 

not allow for direct deadweight calibration in place. To calibrate the sensor in place, the following 

procedure was used. First, a load cell was calibrated via deadweights and then was placed on the free end 

of the output shaft to act as a reference load cell. A threaded jackscrew acted against the reference load 

cell and a hard stop in the vacuum chamber. Adjusting the jackscrew length allowed for changing the 

force imparted on both the reference load cell and the rig’s axial load cell and to the machine frame. In 

this manner the same force was applied to both load cells, and the reference cell output was used to 

calibrate the axial load cell sensor in place. 

The preceding two paragraphs describe the sensors (and sensor calibrations) to determine two 

mutually perpendicular forces acting on the driven test roller. A force also acts along a third axis. This is 

the force directed tangential to the roller diameter and is termed here as the “tangential” force. The 

tangential force on the input shaft roller acts through a gimbal point (Fig. 4(b)). The rotational motion 

about the gimbal point is restrained by a mechanical link to the turntable structure. A load cell is used to 

sense the force in said mechanical link to the turntable structure. This sensor was calibrated in place by 

using a pulley-cable system and dead weights to relate the tangential force applied at the test roller 

position to the sensor output. During testing, this sensor is also affected by spin moments (Refs. 4 and 5) 

that can develop in roller contacts. The data from the tangential force sensor was recorded for possible 

future use, but such data were not of immediate interest and are not reported herein. 

Shaft speeds and total number of shaft revolutions were measured using encoders on each shaft. The 

encoder pulses were counted and recorded via a digital pulse counter. The encoder pulses were also 

monitored by a frequency converter to provide a convenient shaft speed display to the test operator. The 

encoders provide 6,000 pulses for each shaft revolution. 

Test roller conditions were photographed at regular intervals through a viewport. The images were 

captured digitally using a single-lens reflex camera with a 150 mm micro lens and a 12 million effective 

pixel image sensor. A debris pan was used to capture debris created from the roller pairs. A video camera 

recorded the condition of a portion of the debris pan. In spite of several attempts to adjust video camera 

setting and lighting, the video images failed to capture all that could be observed by eye through the 

viewport. 

Test Specimens 

The test specimens used for this research had a nominal geometry of 35.6 mm outer diameter and a 

12.7 mm width. The apparatus requires that at least one roller has a crowned profile to avoid edge loading. 

The roller on the drive motor (input) shaft was provided a crown radius of 200 mm and was made from 

steel. The roller on the lower (output) shaft had a flat profile and was made from Ti6Al4V alloy. 

The material condition, manufacturing details, and surface roughness can all influence the wear 

behavior. The details of the roller conditions are given in Table I and additional details follow. The 

qualification test unit has rollers made from 440F steel in the annealed condition and the working surface 

specified to have a roughness of 0.4 m maximum. The qualification test rollers were passivated per 

ASTM A-380-06. The 440F alloy is similar to 440C but includes the introduction of either selenium 

and/or additional sulfur to alter the machining characteristics relative to 440C. From x-ray photoelectron 

spectroscopy both the qualification test unit rollers and the 440F laboratory test rollers were found to have 

selenium as a constituent. The surface hardness of a 440F laboratory test roller was measured as 24 RC. 

Laboratory test rollers were made intending to enhance resistance to adhesive wear and to reduce debris 

generation. These rollers were made from 440C, hardened to RC 58, and provided a fine ground surface. 
NASA/TM—2012-217619 6 

Figure 6.—Typical surface roughness of test rollers, profile traces obtained using a 

2-m radius tip conisphere stylus tracing in the direction of rolling. (a) Ti6Al4V roller. 

(b) 440F roller. (c) 440C rollers. 

Roughness of the test rollers was documented by stylus profilometer inspections. Typical roughness 

profiles in the rolling direction are provided in Figure 6. The roughness data of Figure 6 were prepared 

filtering the data using 0.8 mm cutoff and 300:1 bandwidth ISO standard filter. The roughness of the 

440F rollers (0.15 m Ra) was significantly larger than the roughness of the 440C rollers (0.07 m Ra). 

The test rollers that mated with the 440 series rollers were made of Ti6Al4V. The Ti6Al4V rolling 

surfaces of the qualification unit were observed to have distinctive machining marks, and so the 

laboratory Ti6Al4V rollers were made using a turning operation as the resulting surface texture of the test 

rollers were judged similar to the surfaces of the qualification unit. The rollers were stress relieved per 

AMS 2801 after machining. The roller hardness was RC 35. Some of the Ti6Al4V rollers were anodized 

per Tifin 200 matching the qualification unit treatment. A typical roughness profile for the Ti6Al4V 

rollers after anodizing is provided in Figure 6. The typical roughness average value was 0.44 m Ra. 

Some testing was done with Ti6Al4V rollers that were not anodized to make project progress previous to 

the completion of the anodize process. 

Procedure to Install Test Rollers 

Test specimens were cleaned and installed using careful procedures to provide clean test surfaces. 

The test rollers were cleaned just prior to installation into the rig using de-ionized water and 0.05 m 

alumina powder. After appropriate hand scrubbing, the cleaning powder was rinsed with deionizer water 

making sure that the entire roller surface wetted uniformly to confirm complete cleaning of surface oils. 

The water was removed from the roller using dried pressurized nitrogen. Test rollers and mounting 

hardware were handled only with gloved hands and clean tools to complete installation into the test 

apparatus. 

-3

-2

-1

0

1

2

3

(a)

height (micrometer)

-3

-2

-1

0

1

2

3

(b)

profile position (mm)

0 2 4 6 8 10 12

-3

-2

-1

0

1

2

3

(c)
NASA/TM—2012-217619 7 

Procedure for Testing Rollers 

The first step for testing after installation of the test rollers was to immediately isolate the testing 

chamber and provide a vacuum, using the scroll roughing pump, to approximately 5010–3 torr chamber 

pressure. This isolation step was done even if test scheduling required some delay between the time of 

installation of rollers and the time for testing to minimize exposure of the cleaned surfaces to any 

contaminants that might be present in the atmosphere. Prior to testing the turbomolecular pump was used 

to bring the testing chamber to approximately 310–7 torr. 

Contact analyses were completed to select roller normal loads, speeds, and alignment angle and to 

relate those choices to the operating conditions of the qualification test unit rollers. The contact conditions 

for the qualification test were studied assuming that the contact pressure distribution was affected by the 

moment produced by the axial load caused by misalignment. To estimate this effect, it was assumed that 

the ratio of axial load to normal load was 0.6 and such load produced an overturning moment by acting 

through the ball bearing center. It was also assumed that the supporting structure provided 90 percent of 

the reaction to the overturning moment while the pressure distribution provided 10 percent of the reaction. 

The pressure distribution for such a loading condition while accounting for the roller profile having a 

crown but modified with a flat section was calculated using the method of Vijayakar (Refs. 9 and 10). The 

contact condition for a perfectly aligned roller having zero axial force was also solved. The predicted 

contact pressure distributions are provided in Figure 7. The misalignment of the roller axis will cause a 

shift of the pressure from the central flat section onto the crowned region, and a maximum contact 

pressure of about 450 MPa occurs near the transition from flat to crown geometry. Because of test rig 

limitations, the laboratory test conditions could not match these conditions exactly. The laboratory testing 

was done at maximum contact pressure of about 640 MPa. The laboratory tests were used to study trends 

and fundamental qualitative wear behavior. 

The test rig speed was selected using the idea that the “contact time” of the test unit and laboratory 

rollers should be matched. Here the “contact time” is the time required for a point on the roller to pass 

through the Hertz contact region. The contact time on the qualification test unit for condition of 

misaligned rollers was 0.015 sec. To match this condition the test rig was operated at 1.6 rad/s (15 rpm) 

for the majority of laboratory testing. 

Figure 7.—Contact pressures on qualification test unit roller for cases of misaligned and aligned rollers. 

The pressures shown are at the midpoint of the Hertz contact in the direction of rolling. 

 

location along roller (mm)

-1.0 -0.5 0.0 0.5 1.0

 pressure (MPa)

0

100

200

300

400

500 misaligned

aligned
NASA/TM—2012-217619 8 

The lab apparatus can be operated with the misalignment angle at a maximum value of about 1.5. 

The influence of the misalignment angle on the contact conditions depends on overall system stiffness 

and the contact dimensions. The sliding distance in such a condition can be approximately quantified as 

proportional to the product of the misalignment angle and the length of the contact patch in the rolling 

direction. Using this concept the misalignment angle of the lab test apparatus times a factor of 1.7 

provides the approximate simulated misalignment angle of the qualification test unit. The lab test was 

conducted for misalignment angles up to 1.5 that would simulate a misalignment angle of about 2.5 for 

the qualification unit. 

A summary of the test conditions is provided in Table 1. The tests are listed sequentially in the order 

of testing. In general one would prefer to have a randomly chosen testing order, but in this work roller 

availability dictated the testing sequence. In some cases the same roller pair was tested at more than one 

misalignment angle. In those cases the angles were tested sequencing from smallest to largest. 

Post-test documentation of the rollers included recording of the mass of each roller, profilometry, and 

photographs. Some rollers were inspected via scanning electron microscope. Debris was collected. In 

some cases the debris was swept from the debris tray, collected to a glass vial, and total mass of debris 

determined. In some cases debris was collected using a square piece of tacky material that could be placed 

onto the tray and lifted to collect the debris. The collected debris could then be subjected to automated 

analysis of debris particle counts and sizes. At completion of one test there was no debris readily visible 

on the brass-hued debris tray, but a swiping of the tray with a cotton-gloved finger revealed debris. The 

glove fingertip was saved to retain the debris. For later tests a dark grey anodized plate was used for the 

debris tray. The darker and matte-finished plate allowed to more easily see the small particles. 

Test Results 

Roller Wear 

Photographs of the test rollers were recorded through a view port during test operations at regular 

intervals. Figure 8 provides a set of photographs documenting the progression of wear during tests with 

test ID 3, 4 and 8 (test ID per Table 1). These photos show the difference in wear resulting from material 

condition. The first two rows of Figure 8 can be used to compare and contrast the wear when using 

annealed 440F rollers (first row) to the wear when using hardened 440C (second row). In these photos the 

upper roller is the one made from steel alloy and the lower roller is the titanium alloy. The steel rollers 

took on a relatively uniform appearance of wear while the titanium rollers had patches of irregular 

appearance. The wear rate can be qualitatively judged by the width of the wear track of the upper roller 

that, when new, had a circular crown. As this roller loses material the wear track widens. The worn 440C 

roller appears slightly smoother than the 440F roller suggesting that steel wear debris would in general be 

smaller for the case of 440C compared to 440F. Recall that the 440F is alloyed to be “free machining” 

and to thereby produce chips more easily and of greater length during machining. The last row of Figure 8 

shows dramatically reduced wear when the rollers are exactly aligned (final row of Figure 8). Even after 

significantly longer running time the aligned rollers appear almost like the running-in condition (first few 

hundred cycles) of the misaligned rollers. Still, for the aligned condition one can notice in the very center 

of the wear track a narrow band that exhibits adhesive wear. 
NASA/TM—2012-217619 9 

(a) (b) (c) (d) 

 588 cycles 5,965 cycles 25,158 cycles 77,357 cycles 

(e) (f) (g) (h) 

 402 cycles 6,140 cycles 24,468 cycles 96,193 cycles 

(i) (j) (k) (l) 

 500 cycles 93,854 cycles 250,196 cycles 347,925 cycles 

Figure 8.—Progression of wear during 3 tests. The cycle count is denoted below each photo. 

(a-d) 440F at 1.4° misalignment. (e-h) 440C at 1.5° misalignment. (i-l) 440F at 0.0° misalignment. 
NASA/TM—2012-217619 10 

TABLE 1.—SUMMARY OF TEST CONDITIONS 

To study the wear, rollers were inspected using a scanning electron microscope (SEM). By using 

x-ray photoelectron spectroscopy the constituents of particular regions of the rollers could be investigated. 

In all cases, two-way transfer of material was observed. That is, titanium alloy substrate was found on the 

440 steel rollers and steel alloy was found adhered to the titanium alloy rollers. An example from 

inspection of the titanium alloy roller from test ID 3 of Table 1 is provided in Figure 9. An overall view 

of a region near the center of the wear track is given in Figure 9(a), and also noted are regions where 

spectroscopy was assessed. The bright regions noted as #1 and #2 in Figure 9(a) produced the respective 

spectra of Figure 9(b) and (c). Here in the spectra of Figure 9(b) and (c) we note the clear presence of 

iron, chromium, and alloying elements of the 440 steel. Region #2 had a brighter appearance suggesting a 

more complete coverage of the substrate, and the spectrum for region #2, Figure 9(c), has a very strong 

peak correlating to iron. The darker region noted as numeral 3 in Figure 9(a) produced the spectrum of 

Figure 9(d). Here the titanium is still exposed, we observe peaks correlating to titanium and aluminum, 

and peaks correlating to iron and chromium are not present. The spectrum for the region marked with the 

numeral 4 in Figure 9(a) is not shown here but was consistent with that of Figure 9(d). These trends were 

true for all of many samples inspected. In all cases the worn regions included regions of the base material 

still exposed and also contained adhered material from the mating roller. As will be noted, in net, mass 

was lost for the 440 steel rollers. However, in all cases Ti6Al4V material was found on the tested steel 

rollers via SEM spectroscopy. 

The roller masses were determined using a scale with digital readout to 0.0001 g. Masses of the 

rollers were measured after cleaning and just before installation in the test rig. The roller masses were also 

measured just after removal from the test chamber. The change in mass at the end of the test quantifies the 

net transfer of material by adhesive wear. The sum of the mass change for the two rollers provides a 

calculated value of debris lost from the roller pair. Because of practical test considerations, the test 

durations were not the same for each test. To provide a method for direct quantitative comparison, the 

change in mass was divided by the total number of revolutions of the input shaft (that is the total number 

of contact stress passes) to quantify the wear rate. The mass change data and calculated wear rates are 

summarized in Table 2.

test # roller pair test ID

material outer surface 

condition material 

outer 

surface 

condition

speed 

(rpm)

misalignment 

angle (deg)

1 1 1 440C bare Ti6Al4V bare 15 -1.4

2 2 2 440F bare Ti6Al4V bare 15 -1.4

3 3 3 440F bare Ti6Al4V anodized 15 -1.4

4 4 4 440F bare Ti6Al4V anodized 15 0.0

5 5 5 440F passivated Ti6Al4V anodized 9 -1.4

6 6 6 440F passivated Ti6Al4V anodized 15 -0.4

7 7 7 440F passivated Ti6Al4V anodized 15 -0.9

8 8 8(a) 440F passivated Ti6Al4V anodized 15 0.0

9 8 8(b) 440F passivated Ti6Al4V anodized 15 -0.1

10 8 8(c) 440F passivated Ti6Al4V anodized 15 -0.2

11 8 8(d) 440F passivated Ti6Al4V anodized 15 -0.3

12 8 8(e) 440F passivated Ti6Al4V anodized 15 -0.7

13 9 9 440C bare Ti6Al4V anodized 15 -1.5

14 10 10 440C bare Ti6Al4V anodized 15 -1.4

15 11 11 440C passivated Ti6Al4V anodized 15 -0.9

upper roller lower roller test conditions
NASA/TM—2012-217619 11 

(a) (b) 

(c) (d) 

Figure 9.—Scanning electron inspections of the wear track on the titanium test roller for test #3. (a) SEM image. 

(b) Spectrum for region noted as 1 in 9(a). (c) Spectrum for region noted as 2 in Figure 9(a). (d) Spectrum for 

region noted as 3 in Figure 9(a). 

TABLE 2.—SUMMARY OF ROLLER WEAR TEST RESULTS 

2

1

3

4

1 mm 

Ti

Ti

Fe

Fe

Cr

Al

Fe

Ti Fe

Cr

Al

Ti

Ti

Ti

 test duration mass change; 

Ti6Al4V 

mass change; 

440 steel 

calculated 

mass liberated 

calculated rate of 

mass liberated

test # test ID revolutions of 

input shaft grams grams milligrams micrograms per cycle

1 1 26,080 0.0049 -0.0054 0.500 0.019

2 2 60,228 0.0981 -0.1258 27.700 0.460

3 3 77,326 0.1350 -0.2072 72.200 0.934

4 4 347,925 -0.0006 -0.0052 5.800 0.017

5 5 55,411 0.0817 -0.1052 23.500 0.424

6 6 60,850 0.0151 -0.0210 5.900 0.097

7 7 124,900 0.1314 -0.1664 35.000 0.280

8 8(a) 93,027 -0.0005 -0.0038 4.300 0.046

9 8(b) 24,001 0.0012 -0.0012 0.000 0.000

10 8(c) 24,000 0.0017 -0.0018 0.100 0.004

11 8(d) 24,095 0.0025 -0.0027 0.200 0.008

12 8(e) 93,117 0.0464 -0.0501 3.700 0.040

13 9 96,193 0.0682 -0.0698 1.600 0.017

14 10 41,055 0.0200 -0.0210 1.000 0.024

15 11 90,004 0.0672 -0.0688 1.600 0.018
NASA/TM—2012-217619 12 

 

Figure 10.—Change of mass per cycle (shaft revolution) as a function of misalignment angle. (a) Mass loss of 

440 steel rollers. (b) Mass increase of Ti6Al4V rollers, symbols denoting the mating material. 

Often wear is modeled as proportional to the sliding distance. In these roller tests, to a first order 

effect the sliding distance is proportional to the misalignment angle. The rate of mass change was plotted 

as a function of misalignment angle (Fig. 10). Note that in the net, the steel rollers lost mass while the 

titanium alloy rollers gained mass. We also note that the 440F material had a higher rate of wear 

compared to the 440C in all cases. Rabinowicz (Ref. 8) has noted that the experimental evidence for wear 

rate being proportional to sliding distance (as often assumed true) is mixed. He states that usually the 

relationship is not perfectly obeyed but the proportional relationship represents experimental data 

“reasonably well”. This observation matches the behavior from this investigation in that the misalignment 

angle (and thereby the sliding distance) correlates to the wear rate, but the linear relationship is not exact. 

Not only the wear rate but also the wear behavior differed for tests with 440F steel versus 440C steel. 

Adhesive wear tends to progress toward an “equilibrium surface” (Ref. 8) meaning the surface roughness 

changes with running, and the wear process can either roughen or smoothen the surfaces depending on the 

materials and starting conditions. The equilibrium surfaces for the tests with 440F versus 440C differed as 

depicted in Figure 11. Tests with the 440C material resulted in smoother running surfaces (Fig. 11(b)), 

and the photos suggest that the particle size of wear debris is likely smaller, in general, for the case of 

440C. The Ti6Al4V rollers when mated with the 440F steel tended to gain material by adhesive wear in a 

more uniform fashion, that is, the coverage of the surface was more significant and uniform in 

appearance. It is possible that the hardened 440C behavior differed from that of the annealed 440F not 

only because of the change of the elastic limit but also because of an alteration of the metallurgical 

structure and alteration of the adhesion compatibility with the Ti6Al4V (Ref. 8). 

Wear Debris 

During testing of the qualification test unit, some loose debris was formed. The scope of the NESC 

assessment included consideration of mission risk from the loose debris. The laboratory testing revealed 

aspects of the loose debris as follows. From Table 2, note that loose debris was generated at rates on the 

order of 0.5 g per cycle for large roller misalignment. The wear rates were greatly reduced for the case 

of zero roller misalignment. However, the adhesive wear was not completely eliminated with aligned 

rollers, and some debris was formed. Figure 12(a) and (b) provides images of the largest sized particles 

collected from the two tests operated with zero roller misalignment. Figure 12(c) is a typical example of 

the large number and widely distributed debris that occurred for extensive run time and the largest 

misalignment angles tested. A portion of the debris pan was not within direct line of sight of the rollers, 

but that region still contained significant numbers of particles. 

misalignment angle (degree)

0.0 -0.4 -0.8 -1.2 -1.6

mass change

 (microgram / cycle)

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

440F

440C

(a)

misalignment angle (degree)

0.0 -0.4 -0.8 -1.2 -1.6

mass change

 (microgram / cycle)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

440F

440C

(b)
NASA/TM—2012-217619 13 

(a)    (b)

(c)    (d)

Figure 11.—Condition of rollers at end of test showing differing behavior of 440F vs. 440C. (a) 440F, test ID 5. 

(b) 440C, test ID 9. (c) Ti6Al4V test ID 5. (d) Ti6Al4V, test ID 9. The test ID numbers refer to Table 1. 

 

(a) (b) (c) 

Figure 12.—Examples of loose particle debris. (a) Test ID 4 with zero roller misalignment. (b) Test ID 8(a) with zero 

roller misalignment. (c) Test ID 5, rollers misalignment –1.4. The test ID numbers refer to Table 1. 
NASA/TM—2012-217619 14 

Summary 

Laboratory testing of 440 steel rollers in contact with Ti6Al4V in vacuum was completed to study the 

wear behavior and to quantify wear rates. The tests also assessed the influence of material condition and 

roller misalignment. The wear rate was found to be roughly proportional to the roller misalignment angle. 

The rate of loose particle mass created was on the order of 0.5 g per cycle. The adhesive wear was a 

two-way phenomenon with titanium alloy material found adhered to the steel and vice versa. Loose 

particles with linear dimensions on the order of 200 m were created even for the condition of zero roller 

misalignment. For large misalignment angles, large numbers of loose particles could be created, and 

debris was found in locations that were not within a direct line of sight. The wear behavior differed for the 

case of 440F and 440C steel rollers. With the 440C rollers, the adhesive wear rate was reduced, and the 

final surface textures of the 440C rollers were smoother than the 440F rollers. 

References 

1. McClendon, M., “NIRSpec MSS Magnet Actuator Life Test Unit Wear Particle Evaluation,” 

obtained by Krantz, T., May 16, 2011. 

2. Authors unstated, “Micro Shutter Subsystem (MSS) Qualification Unit Test Report,” JWST-RPT013819, Rev. A, June 2010. 

3. Pepper, S., “Research Note-Characterization of the Test Environment of JWST Roller Wear 

Evaluation at NASA-GRC,” Aug. 1, 2011. 

4. Johnson, K.L., Contact Mechanics, Cambridge University Press, 1985. 

5. Kalker, J.J., “Rolling contact phenomena: linear elasticity,” Rolling Contact Phenomena CISM 

Courses and Lectures, Issue 411, Spinger-Verlag, 2000. 

6. McGinness, H., “Lateral forces induced by a misaligned roller,” DSN Progress Report 42-45, March 

and April 1978, Jet Propulsion Laboratory, Pasadena, Calif., 1978. 

7. Krantz, T., DellaCorte, C., Dube, M., “Experimental Investigation of Forces Produced by 

Misaligned Steel Rollers,” proceedings of the 40th Aerospace Mechanisms Symposium, 

NASA/CP—2010-216272, also NASA/TM—2010-216741, 2010. 

8. Rabinowicz, E., Friction and Wear of Materials, 2nd edition, Wiley-Interscience, 1995. 

9. Vijayakar, S., “A combined surface integral and finite element solution for a three-dimensional 

contact problem,” International J. of Numerical Methods for Engineering, vol. 31, 1991. 

10. Vijayakar, S., “Multi-body Dynamic Contact Analysis Tool for Transmission Design – SBIR 

Phase II Final Report,” Army Research Laboratory ARL-CR-487, 2003. 
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4. TITLE AND SUBTITLE

Wear of Steel and Ti6Al4V Rollers in Vacuum 

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Krantz, Timothy, L.; Shareef, Iqbal 

5d. PROJECT NUMBER

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WBS 869021.05.03.05.12 

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National Aeronautics and Space Administration 

John H. Glenn Research Center at Lewis Field 

Cleveland, Ohio 44135-3191 

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E-18207 

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National Aeronautics and Space Administration 

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NASA 

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NASA/TM-2012-217619 

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

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13. SUPPLEMENTARY NOTES

14. ABSTRACT

This investigation was prompted by results of a qualification test of a mechanism to be used for the James Webb Space Telescope. Post-test 

inspections of the qualification test article revealed some loose wear debris and wear of the steel rollers and the mating Ti6Al4V surfaces. 

An engineering assessment of the design and observations from the tested qualification unit suggested that roller misalignment was a 

controlling factor. The wear phenomena were investigated using dedicated laboratory experiments. Tests were done using a vacuum roller 

rig for a range of roller misalignment angles. The wear in these tests was mainly adhesive wear. The measured wear rates were highly 

correlated to the misalignment angle. For all tests with some roller misalignment, the steel rollers lost mass while the titanium rollers gained 

mass indicating strong adhesion of the steel with the titanium alloy. Inspection of the rollers revealed that the adhesive wear was a two-way 

process as titanium alloy was found on the steel rollers and vice versa. The qualification test unit made use of 440F steel rollers in the 

annealed condition. Both annealed 440F steel rollers and hardened 440C rollers were tested in the vacuum roller rig to investigate possibility 

to reduce wear rates and the risk of loose debris formation. The 440F and 440C rollers had differing wear behaviors with significantly lesser 

wear rates for the 440C. For the test condition of zero roller misalignment, the adhesive wear rates were very low, but still some loose debris 

was formed. 

15. SUBJECT TERMS

Wear; Steel; Titanium; Tribology; Rollers 

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