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332

Tribology OnlineJapanese Society of Tribologists

http://www.tribology.jp/trol/

Vol. 15, No. 5 (2020) 332-342.ISSN 1881-2198

DOI 10.2474/trol.15.332

Article

Application of Collar Seals for Bearings in the Lunar Exploration Rover

Koji Matsumoto1)*, Keiichi Yanagase1), Satoshi Takada1), Takashi Yokoyama1), Shogo Kusabe1),Nao Tsujimura2) and Tomoya Nakamura2)

1) Research and Development Directorate, Japan Aerospace Exploration Agency (JAXA), 2-1-1 Sengen, Tsukuba, Ibaraki 305-8505, Japan2) Aerospace Engineering Department, NTN Corporation, 2454 Tsuchijima, Higashikata, Kuwana, Mie 511-8678, Japan

*Corresponding author: Koji Matsumoto ([email protected])

Manuscript received 10 November 2019; accepted 23 August 2020; published 31 October 2020Presented at the International Tribology Conference Sendai 2019, 17-21 September, 2019

Abstract

A lot of exploration missions have been planned for the moon, and lunar activity is likely to increase. The moon’s surface is covered by a great deal of dust, called regolith, and these particles can seriously damage mechanical components. The design of any mechanism that will operate on the moon must take regolith into account. This includes the wheels and crawlers of lunar rovers, and many other mechanical components. This study considers measures for protecting small bearings in a lunar rover that operate close to the surface, in particular simple collar seals. These seals will require exceptional sealing ability, low torque, light weight, and easy installation. Collar seals made of three polymer composite materials (such as PTFE and PEEK) were evaluated. The seals have a convex portion that slides on the end face of the inner ring of a bearing or that of the outer ring; the sliding between the convex portion and the rings prevents regolith from entering the bearing. The seal performance and torque of each type of collar seal were tested using a regolith simulant in vacuum. The test conditions were decided based on the lunar environment and specification of a lunar rover under development. In order to maintain seal performance and to obtain lower torque, the thicknesses of the collar seals and their axial deflection were varied. Sliding on the end face of the inner ring of the bearing showed good seal performance and lower torque with optimum collar thickness and axial deflection for all composite materials. There was some variation in sealing and torque between the composites. The tribological performance and results of surface analysis by XPS are discussed.

Keywords

seal, bearing, collar, dust, regolith simulant, PTFE, PEEK

Copyright © 2020 Japanese Society of TribologistsThis article is distributed under the terms of the latest version of CC BY-NC-ND defined by the Creative Commons Attribution License.

1 Introduction

Many lunar exploration programs have been planned with international collaboration. The opportunity for activity on the moon will increase, and the scope of activity will widen. Exploring the polar regions in particular has attracted attention because of the potential existence of water [1, 2]. Exploration at the polar region requires moving about in a low-temperature environment where little electric power is available because of the lack of sunlight. Furthermore, the moon’s surface is covered by a large amount of dust, called regolith. These dust particles can damage mechanical components of lunar rovers [3-5]. There is a pressing need to establish better sealing technology to prevent regolith intrusion into mechanical elements in wheels, crawlers of lunar rovers, and many other mechanisms. Much research has been conducted into developing seal systems that seal against dust and can be used in a vacuum, and into methods to evaluate their effectiveness in the environments [6-10]. However, lunar polar exploration involves additional

issues. To be judged successful, sealing technology must show compatibility of reliable seal performance and low torque over a wide range of temperature, even in a cryogenic environment, and far more effectively than in conventional missions such as the Apollo program. Lower torque is especially significant because little electricity can be obtained. Furthermore, a dust environment will be more severe in some missions. Sampling lunar dust in deep regions and moving on soft, sandy hills are needed to get a suitable point and find water. The moving parts in a rover need to have sealing system to prevent dust infiltration, even in the situations that the dust is poured on the rover, or the rover becomes buried in the dust.

Figure 1 shows a typical lunar rover, now being developed in Japan. The rover has many moving parts in the excavation device, a crawler system to move over dust hills, and so on [11]. In the moving component, some mechanical elements such as gears and bearings will be directly exposed to regolith dust particles. In particular, protecting bearings from lunar regolith is one of the technological issues that should be resolved for

Tribology Online, Vol. 15, No. 5 (2020) /333Japanese Society of Tribologists (http://www.tribology.jp/)


lunar exploration. However, it is impractical to use heavy, sturdy seals, such as mechanical seal, for every moving part, so simple, lightweight seals with good seal performance and low torque should be found and researched.

As dust-proof technology for bearings, shield plate is generally applied to bearings used near the ground. The clearance between a shield plate and the inner race of a bearing is kept to a minimum, usually on the order of a few hundred micrometers, but this clearance is inadequate for protecting a bearing from lunar regolith. Contact seals are also used on earth for bearings – various rubbers (e.g., nitrile rubber, acrylic rubber, silicon rubber, and fluorine-contained rubber) are generally used for these sealed bearings, but these materials are not suited to use in space where outgassing and contamination are real problems. In addition, as the usable temperature range of the rubber materials is about -50 to 200 degrees C, they are not suited to space applications where the operating temperature is often higher or lower. Therefore, the seal material is required that can withstand a wide range of temperatures. Furthermore, in order to protect a bearing against dust infiltration, a seal must maintain a sealed condition such as elastic deformation while the inner and outer rings of a bearing have some clearance and they move relatively.

This study investigates the simple collar seals, which are made of engineering plastic, as dust seal for bearings. These seals have a convex portion to enable elastic deformation and

maintain seal performance while relative position between the inner and outer rings is changed. Collar seals can be easily installed outside of bearings and are also more cost effective than bearings with built-in seals. The characteristics of each tested material and the results of seal performance and torque are reported, including optimum shapes and axial deflection. The study will also discuss whether the new type of collar seal can be used with bearings in lunar rovers, and mechanism of seal performance.

2 Tested specimens and method of evaluation

2.1 Shapes and materials of tested collars sealsThis study investigates simple collar seals made of

engineering plastics to protect bearings from regolith dust. Two types of new designed collar seals with different shapes were evaluated. Figure 2 shows the detailed shapes and sizes of the collar seals. Both types of collar seal can be installed at the end face of a bearing, and the convex portion contacts and slides on the end of the inner or outer rings of the bearing to prevent dust infiltration. In the case of sliding on the end face of the inner ring, the outside of the collar seal is fixed to the outer ring of the bearing. The thrust load from the attachment causes the collar to deflect, and it makes contact between the bulge of the lip of the collar and the inner ring of the bearing with contact pressure by elastic force. Similarly, by fixing the inside of the collar seal to the inner ring of the bearing, the bulge of the lip slides against the end face of the outer ring with contact pressure.

Three kinds of commercially available polymer composites were selected to be evaluated for use in the collar seals [12-14]. The compositions of the tested composites are listed in Table 1. Polytetrafluoroethylene (PTFE) has been used in space applications because it has excellent tribological properties both in a vacuum and in air. Composites A and B, whose main material was PTFE, were selected for this study. The difference between composites A and B is mainly the reinforcement material. Composite A is reinforced by glass fiber, composite B by carbon fiber. Composite A has been used for bearing retainers in space environment. Composite C mainly uses polyether ether ketone (PEEK) as based material, and also reinforced by carbon fiber as well as composite B. Recently, PEEK is increasingly used in general industry because of its hardness. To compensate for the higher friction of PEEK, composite C also contains PTFE in it.

Bearing

© JAXA

Fig. 1 Crawler and bearings in a lunar rover being developed in Japan

Sliding on outer ringSliding on inner ring

tt

t: collar thickness

x

x: axial deflection

x

Fig. 2 Shapes of new designed collar seals


Koji Matsumoto, Keiichi Yanagase, Satoshi Takada, Takashi Yokoyama, Shogo Kusabe, Nao Tsujimura and Tomoya Nakamura

2.2 Test method and conditionsThe collar seal was evaluated in a vacuum chamber with an

ambient pressure of 10-5 Pa. Figure 3 shows the configurations of the test equipment and setups of the tested collar seals and bearings, distinguishing between the cases of sliding on the inner ring and on the outer ring. Table 2 lists the test conditions and details of the tested bearing and shapes of the collar seals [6]. In both cases, the collar seal and bearing to be tested

were placed in a cup then covered by dust particles (regolith simulant). In order to emulate the parts in the crawler of a lunar rover, the outer ring of the bearing rotated with the cup. In the case of sliding on the inner ring, the outside of the collar was attached to the cup and rotated with the outer ring of the bearing. In the case of sliding on the outer ring, the inside of the collar is attached to a shaft. The collar did not rotate and slide with the rotating outer ring of the bearing. The force of the inner

Table 1 Tested Seal Materials

No. Main materials Reinforcement, additives

Composite A PTFE Glass fiber, Mo

Composite B PTFE Carbon fiber, MoS2, Cu alloy

Composite C PEEK Carbon fiber, PTFE

Sliding on outer ringSliding on inner ring

Regolith simulant

Tested collar seals

Applied load Applied load

Fig. 3 Test apparatus for collar seals.

Shaft Shaft

Cup Cup

Fig. 3 Test apparatus for collar seals

Load 0 50 N

Rotation speed 10 rpm

Total rotation cycles

•Evaluating shape of collar 1,000(w/o dust) + 1,000(w/ dust)

•Evaluating dimension and material of collar 1,000(w/o dust) +14,000(w/ dust)

Ambient pressure 10 ‐4 – 10 ‐5 Pa

Temperature Room temperature

Amount of regolith simulant 2 g

Specification of bearing Deep groove ball bearing，SUS440C ID: 8 mm, OD: 22 mm, width: 7 mm

Shapes and dimensions

of collars

•Evaluating shape of collar Collar thickness: 2 mm Axial deflection: 0.5 mm

•Evaluating dimension and material of collar Collar thickness: 1 or 2 mm Axial deflection: 0.1, 0.3, or 0.5 mm

Table 2 Seal test conditions



ring, turned by rotation of the outer ring of the bearing and the cup, was translated into torque. Radial load was applied in steps to 50 N to demonstrate load under moon gravity. It was maintained until just before the end of the test. The load was set according to the design weight of the rover, the number of wheels and bearings, and gravity. The rotation speed was set to 10 rpm for the expected speed of the rover and the size of the wheel.

As the procedure for evaluating collar seals, firstly the characteristics of each case of sliding on the inner ring and outer ring, and their relative merits were evaluated. The duration of the test without dust was 1,000 rotations. Then, the test was performed again for 1,000 cycles after covering the collar seal with regolith simulant and evacuating the air from the test chamber. This test was run for 2,000 cycles in total using a test collar specimen 2.0 mm thick with an axial deflection of 0.5 mm. Axial deflection is defined here as the displacement of the flat part of the collar seal from the free state. Next, in order to obtain lower torque under optimum conditions and to confirm durability for better sliding position to seal, a long-duration test of 15,000 cycles in total cycles was carried out using different collar thicknesses of 1 and 2 mm and axial deflections of 0.1 to 0.5 mm. Differences in seal performance and torque between composite materials were also evaluated. The mechanisms of dust infiltration and suitable seal performance, and the mechanisms of tribological performance were investigated by visual inspection, surface profile measurement, and XPS analysis for the specimens.

The lunar soil simulant used in this study was FJS-1, and it was supplied by a Japanese construction company [15, 16]. The simulant is made from basaltic lava and simulates the hardness and particle size distributions of the lunar dust samples retrieved in the Apollo programs. The particle size varied from a few µm to a few hundred µm, with a median size of 70 µm. Nearly all the constituents of actual regolith are present in FJS-1, and as with actual regolith, its major constituent is silicon oxide.

2.3 Numerical analysis for contact conditionsA numerical analysis was carried out to determine

contact load and contact pressure applied by deflection of the collar. The finite element method was utilized for analyzing contact load and pressure, and the analysis models were two-dimensionally axisymmetric. The collar was elastic-plastic

body, and the surfaces of the bearing were rigid due to their very large elastic modulus. The analysis was done according to the mechanical characteristics of different composites for collar thicknesses of 1 and 2 mm and axial deflections of 0.1 to 0.5 mm.

3 Results and discussions about comparison of sliding locations

New designs of collar seal with two kinds of shapes that are cases of sliding on the inner ring or the outer ring of the bearing were evaluated using composite A with a collar thickness of 2 mm and an axial deflection of 0.5 mm. Figures 4 shows the torque behaviors for both sliding locations. Regardless of the presence of dust and the sliding locations, torques were unstable when no radial load was applied at the beginning or just before the end of the test. As the applied radial load and number of sliding cycles were increased, the torques became stable or gradually increased. Then the torques became stable after an applied load of 30 N and 400 cycles.

For sliding on the inner ring, the torque for composite A was about 0.03 Nm without dust and it increased slightly to about 0.04 Nm in the dust test. The presence of the dust in the space between the shaft and the seal might affect the torque increase, but the influence was not significant. For sliding on the outer ring, except at the beginning of the test, the torque remained stable at about 0.11 Nm. In both sliding locations, no big fluctuations or changes in torque occurred, which indicated that there was no infiltration of dust between the collar seal and the bearing, or into the bearing. No infiltration was visible after the test either. On the other hand, the torque in the case of sliding on the inner ring was lower than half of that of siding on the outer ring. This may be due to the smaller distance between the shaft center and the sliding point. The collar seal with the shape to slide on inner ring is more suitable as a dust seal for bearings in a lunar rover.

4 Results and discussions about the differences between composite materials

The seal performance and tribological characteristics for various seal materials were evaluated and the mechanisms of the characteristics were investigated. The torque behaviors

(b) Slide on outer ring(a) Slide on inner ring

0

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50

60

0

0.05

0.1

0.15

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0 200 400 600 800 1000

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ial l

oad,

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Torque

, Nm

Number of cycles

Torque w/o dustTorque w/ dustLoad

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Composite ACollar thickness: 2 mmAxial deflection: 0.5 mm

Fig. 4 Torque behaviors in the cases of sliding on inner ring and outer ring



of composites B and C during sliding on inner ring with a collar thickness of 2 mm and an axial deflection of 0.5 mm are shown in Fig. 5. Test conditions were the same as for composite A, mentioned in Section 3. For composite B, there was no difference in torque regardless of whether the test used dust, and the torque was stable at 0.05 Nm. For composite C, the torque increased considerably at the beginning of the test without dust, and then suddenly decreased after a few hundred rotations and became stable. This phenomenon might be due to a build-up of transfer film on the counterpart surface. There are cases when a running-in process is required for the composite. After the process, the torque stayed stable at 0.05 Nm also with dust. For all composite materials, including composite A, no infiltration of dust into the sliding surface and the bearing was observed, and the values of torque just before the end of the test were almost the same at 0.05 Nm.

After testing, the surface profile was measured by a surface roughness tester and its surface observed by a microscope for the sliding surfaces of collar seals. Figure 6 shows micrographs

and surface profiles of the sliding surfaces of three composites sliding on the inner ring. No remarkable wear was observed for any composites, but the widths of the sliding track and surface conditions were different between composites. The width of the sliding track of composite A was the narrowest, followed by that of composite B. Composite C widely contacted to the inner ring of bearing as a counterpart. The result seemed to come from the difference in mechanical characteristics between the composite materials. Moreover, the contact point for composite C was located at a higher position on the convex portion of the sealing surface than for composites A and B. PEEK material, the main component of composite C, has high hardness and low flexibility. The lower flexibility of composite C might be the cause. For usage of PEEK composite, the shapes should be designed considering such characteristics.

Figure 7 shows photographs of the bearing and sliding tracks on the end of the bearing after test sliding on the inner ring. The width of sliding tracks increased in the order of composite A, B, and C, and it was the same as the observations

(a) Composite B (b) Composite C

0

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Sliding on inner ringsCollar thickness: 2 mmAxial deflection: 0.5 mm

Fig. 5 Torque behaviors of composites B and C in the case of sliding on inner ring

(a) Composite A (b) Composite B (c) Composite C

-100.00

0.00

100.00

200.00

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400.00

500.00

600.00

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mm -100.00

0.00

100.00

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μm

mm

Sliding on inner ring Collar thickness: 2 mmAxial deflection: 0.5 mm

Fig. 6 Contact surfaces of collar specimen after 2,000 cycles in the case of sliding on inner ring



for the collar seal surface. For the center of the sliding tracks on the end of the bearing, XPS depth analysis was conducted. Table 3 lists the conditions of the XPS analysis. A cluster ion gun was used for the depth analysis in order to analyze details of transferred polymer materials for depth direction. As the results of XPS analysis for transfer film, Figs. 8 and 9 show the quantitative values (mol %) of each element against etching time, indicating depth, and the peaks of some elements against the binding energy, respectively. High ratios of fluorine which was transferred from the collar seal material were detected for all composites. It was close to 50% at the surface and decreased to a stable value of 10-20%. The main component of both composites A and B was the same, PTFE. Although

composite A kept a high ratio of fluorine to the deep region, that of composite B decreased to less than 10%. The difference in ratios of iron between composites A and B was also found – composite A: 20%, composite B: 40%. Comparing peaks of fluorine and iron of composite A and those of composite B in Fig. 9 showed similar peak positions between composite A and B and the transferred materials were almost the same. On the other hand, a peak of 689 eV by -CF2-CF2- from PTFE remained for a long etching time for composite A, however, the PTFE’s peak decreased soon and a peak of 685 eV for a metal fluoride was easily detected and kept whole etching time for composite B. From peaks of iron, a peak of about 707 eV from simple iron appeared at high counts for composite B from an early etching time, although that for composite A also gradually appeared. These results mean formation of tribofilm on the counterpart was different in area and/or depth between composites A and B. It is seemed to be due to a difference in the build-up process of tribofilm by a difference in reinforcement and/or additives.

Fluorine was detected also for composite C and the peak of the binding energy indicated presence of PTFE. Tribofilm contained PTFE from composite C was built up, though PTFE was not the main material in composite C. This tribofilm must contribute the low torque obtained for the PEEK composite, as well as for composites A and B. In addition, the carbon ratio versus fluorine ratio was high for composite C compared to other composites, as shown in Fig. 8. It seemed that PEEK itself also transferred to the counterpart. The build-up of these transferred materials might be affected by high contact pressure due to the mechanical characteristics of composite C.


250 µm

Sliding on inner ringCollar thickness: 2 mmAxial deflection: 0.5 mm

Fig. 7 Appearance of inner rings of test bearings after the test

X‐ray gun Al monochromater

Analyzed size ø110 µm

Etching conditions (Depth analysis)

Ion gun voltage 10 keV

Cluster size Ar 1,000+

Raster size 1.5 mm

Etching time 20 sec × 40 times

Table 3 Conditions of XPS depth analysis for counterpart bearing




0

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60

0 200 400 600

Con

cent

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n, %

Etch time, s

Fe 2p Cr 2p C 1s F 1s O 1s Si 2p Mo 3d

0

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Con

cent

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Fe 2p Cr 2p C 1s F 1s O 1s Mo 3d

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40

50

60

0 200 400 600

Con

cent

ratio

n, %

Etch time, s

Fe 2p Cr 2p C 1s F 1s O 1s

Fig. 8 Quantitative value by XPS depth analysis on bearing ring

F 1s Fe 2p

(a) Composite A

(b) Composite B

(c) Composite C

300

400

500

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700

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900

1000

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702712722732

Cou

nts

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Fe oxideFe fluoride

Fe

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675680685690695

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-CF2CF2-

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702712722732

Cou

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Binding energy, eV

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FeF3CrF3

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702712722732

Cou

nts

per s

econ

ds

Binding energy, eV

100

500

900

1300

1700

2100

675680685690695

Cou

nts

per s

econ

ds

Binding energy, eV

Etching time:400 s

Surface

F 1s Fe 2p

F 1s Fe 2p

Fig. 9 Peaks of each element by XPS depth analysis for tribofilm



5 Results and discussions about the collars’ dimensions and contact pressures

As mentioned above, the contact pressures of composites were different, as were collar thicknesses and axial deflections. These should affect torque and seal performance, so collars of different materials and thicknesses were made and evaluated by the seal test in a vacuum with different axial deflections. Tested collar thickness was 1 or 2 mm, and axial deflection was 0.1, 0.3, or 0.5 mm, except a collar thickness of 2 mm for composite C because of much high friction due to high contact load. In the test, the collar slid on the inner ring of the bearing and lasted for 14,000 cycles with dust (a total of 15,000 cycles) to confirm the durability of the seal. Table 4 shows the results of the seal performance for each composite material, thickness, and axial deflection. “OK” indicates that the collar seal could prevent intrusion, and “×” means there was a failure in sealing out dust particles, which were found on or in the bearing after the test. For composites A and B, only a 2 mm-thick collar succeeded in sealing out dust particles, and a maximum axial deflection of 0.5 mm was needed for composite A. Composite C could protect bearings from dust with a collar 1 mm thick.

When seal performance was good, no remarkable wear was found. Sufficient seal performance, torque, wear resistance, and durability were obtained by optimizing collar thickness and axial deflection for all composites.

Numerical analysis was carried out on the data for the mechanical properties of each material, and the same conditions as experiment for the case of sliding on the inner ring. Figure 10 shows (a) contact load and (b) contact pressure against axial deflection for each composite, calculated by elastic-plastic FEM analysis. Contact pressure means pressure on contact area calculated by the analysis. The experimental results mentioned above are also expressed in the graph. Filled circles and diamonds indicate success in sealing dust, and open circles and diamonds mean failure. In the Fig. 10(a), the results indicated that dust entered inside the seal surface when the contact load between the seal composite and the end of the bearing was less than 300 N, although in the case of an axial deflection of 0.1 mm, a few exceptions were observed. Figure 10(b) shows that contact pressures did not simply increase with axial deflection – they stayed at a constant or low value from an axial deflection of 0.3 mm in some cases. This means plastic deformation actually occurred at a high axial deflection. When the contact pressure is expressed by a vertical axis, dust was kept out at more than 20 MPa, except for composite C with a thickness of 1 mm at an axial deflection of 0.1 mm. In both cases of considering criteria from contact load and contact pressure, exceptions are found for an axial deflection of 0.1 mm. Movable range of the inner ring against the outer ring in axial direction and dimensional accuracy of collars might affect them in the case of short axial deflection. In addition, even if high contact load or pressure is applied to the contact surface, when there is a high spring constant and low displacement, trackability for axial and angular motion between seal surfaces is inferior, and the surfaces contact disproportionally. That is also reason why good seal performance was not obtained for composite C with a thickness of 1 mm. For fluid seals, seal performance usually depends on contact pressure. However, for a dust seal to prevent infiltration of regolith with heterogenous sizes and

0

20

40

60

80

100

0 0.1 0.2 0.3 0.4 0.5 0.6

Con

tact

pre

ssur

e, M

Pa

Axial deflection, mm

Composite A (t:1.0mm)Composite A (t:2.0mm)Composite B (t:1.0mm)

Composite B (t:2.0mm)Composite C (t:1.0mm)

(b) Contact pressure

0

200

400

600

800

1000

1200

1400

1600

0 0.1 0.2 0.3 0.4 0.5 0.6

Con

tact

Loa

d, N


Composite A (t:1.0mm)Composite A (t:2.0mm)Composite B (t:1.0mm)Composite B (t:2.0mm)Composite C (t:1.0mm)

(a) Contact loadFig. 10 Results of numerical elastic-plastic analysis. Filled circle and diamond plots mean the case of success in sealing dust

No. Collar thickness, mm


0.1 0.3 0.5

Composite A 1 × × ×

2 × × OK

Composite B 1 × × ×

2 OK OK OK

Composite C 1 × OK OK

Table 4 Results of seal performance at each condition



shapes, a gap between seal surfaces may be opened by shaft runout and trapped particles, then small dust can enter from the gap. In this case, criteria can be considered from contact load, because spring force by elasticity of whole collar seal greatly affect seal performance. On the other hand, convex portion also has elasticity and can trap dust particles. In this consideration, contact pressure has much effect on seal performance. So, the experimental test results and numerical analysis result indicate that seal performance is determined by not only contact pressure and wear resistance but also contact load and trackability of sealing contact surface.

Figure 11 shows torque behaviors for the lowest torque successful in sealing for each composite. For all composites, low and stable torques were obtained. With dust, composites A and B showed a higher torque at the beginning of the test than for the test without dust. Composite C showed the lowest torque of 0.02 Nm, although the results indicate that a running-in process

is needed to get this low value. Figures 12 shows photographs and surface profiles of the rubbing track of collar seals after the test. For composites A and B, PTFE composites, contact pressure from the test condition in this figure were almost the same, and sealing was maintained. Although a little wear and plastic deformation were observed for composite A, composite B showed slight wear by dust particles and no deformation due to low load. Almost no wear or deformation was observed for composite C. From the results of torque and wear resistance, the PEEK composite had the best performance in this study.

6 Conclusions

This study seeks a simple, light-weight sealing system for bearings to be used in lunar rovers and come into direct contact with lunar regolith, a few types of collar seals made of different polymer composite were tested in vacuum, with dust particles.

(a) Composite A

(b) Composite B

(c) Composite CFig. 11 Torque behaviors in the case of the lowest torque and success in sealing for each composite.

without dust with dust

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0 2000 4000 6000 8000 10000 12000 14000

Rad

ial l

oad,

N

Torq

ue, N

m

Number of cycles

Torque Load

Collar thickness: 2 mmAxial deflection: 0.5 mm



Fig. 11 Torque behaviors in the case of the lowest torque and success in sealing for each composite



The feasibility of using a new collar seal, made of three polymer composites, was evaluated for seal performance and torque. The results were:• Collar seals were tested by sliding them on the end surfaces

of a bearing’s outer ring and inner ring. Both sliding locations showed their effectiveness in preventing the infiltration of dust particles. The collar seal sliding on the inner ring showed a lower torque than the collar seal sliding on the outer ring.

• It was recognized that seal performance depended on contact pressure. In addition, contact load and trackability of sealing contact surface were also significant for dust seals.

• Optimum collar thickness and axial deflection were found for each collar seal sliding on the inner ring for lower torque, and durability of the seal was verified for application to lunar rovers. A low, stable torque was obtained from collars made of PTFE composites. The PEEK composite also showed low torque and great wear resistance.

References

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[2] Hoshino, T., Wakabayashi, S., Shirasawa, Y., Mizuno, H., Inoue, H., Morimoto, H., Shimada, T., Ohtake, M., Karouji, Y., Shiraishi, H. and Kanamori, H., “Study Status of Lunar Polar Exploration Mission - Spacecraft Systems -,” 62nd Space Sciences and Technology Conference, Kurume, JSASS-2018-4384, 2B07, 2018 (in Japanese).

[3] Heiken, G. and Vaniman, D., “Lunar Sourcebook, 3.4. Dust,” Cambridge University Press, 1991, 34.

[4] Spudis, P. D., “The Once and Future Moon,” Smithsonian Institution Press, 1996, 89-108.

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[6] Matsumoto, K., Takada, S., Kakudo, H., Yokoyama, T., Kasahara, H., Ota, T., Okado, J. and Oka, M., “Development of Mechanical Seal

(a) Composite A

(b) Composite B

(c) Composite CFig. 12 Appearances and surface profiles in the case of the lowest torque and success in sealing dust.

-100

0

100

200

300

400

500

600

-1.00 -0.50 0.00 0.50 1.00 1.50 2.00

Before test After testμm

mm

-100

0

100

200

300

400

500

600

-1.00 -0.50 0.00 0.50 1.00 1.50 2.00

Before test After testμm

mm




Fig. 12 Appearances and surface profiles in the case of the lowest torque and success in sealing dust



against Dust for Lunar Exploration,” 32nd International Symposium on Space Technology and Science, Fukui, 2019.

[7] Matsumoto, K., Yanagase, K. and Takada, S., “Evaluation for Practicability of Collar Seal against Dust on Lunar Rover,” 61st Space Sciences and Technology Conference, Niigata, JSASS-2017-4354, 2F05, 2017 (in Japanese).

[8] Delgado, I. and Handschuh, M., “Preliminary Assessment of Seals for Dust Mitigation of Mechanical Components for Lunar Surface Systems,” NASA/CP-2010-216272, 2010, 309-315.

[9] Kobrick, R. L., Budinski, K. G., Street, K. W. Jr. and Klaus, D. M., “Three-Body Abrasion Testing Using Lunar Dust Simulants to Evaluate Surface System Materials,” 40th International Conference on Environmental Systems, AIAA 2010-6077, Barcelona Spain, 2010. (DOI:10.2514/6.2010-6077)

[10] Delgado, I., Gaier, J., Handschuh, M., Panko, S. and Sechkar, E., “Performance Evaluation of an Actuator Dust Seal for Lunar Operation,” NASA/TM—2013-216587, 2013.

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Evaluation of Tracked Lunar Vehicle,” Journal of Terramechanics, 46, 2009, 105–114.

[12] Matsumoto, K., Suzuki, M. and Takada, S., “Tribological Properties of PTFE Composites in Lunar Extreme Environment,” Proceedings of 63rd International Astronautical Congress, Naples, Italy, IAC-12-C2.6.2, 2012.

[13] Matsumoto, K., Takada, S. and Kawai, N., “Wear Properties of Polymer Materials in Vacuum Dust Environments,” Proceedings of JAST Tribology Conference, Fukuoka, 2013 (in Japanese).

[14] Kawai, N., “Recent Technical Trends in the Special Bearings for Satellites,” NTN Technical Review, 67, 1998, 38-42 (in Japanese).

[15] Wakabayashi, S. and Matsumoto, K., “Development of Slope Mobility Testbed using Simulated Lunar Soil,” JAXA-RM-05-003, 2006 (in Japanese).

[16] Sueyoshi, K., Watanabe, T., Nakano, Y., Kanamori, H., Aoki, S., Miyahara, A. and Matsui, K., “Reaction Mechanism of Various Types of Lunar Soil Simulants by Hydrogen Reduction,” Proceedings of Earth & Space, 2008 Conference, Long Beach, USA, 2008.

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