IMPROVEMENT OF LUBRICATION LIFE OF GREASE-LUBRICATED STRAIN WAVE GEARING BY CARBURIZING AND TWO-STAGE SHOT PEENING

Kazuaki Maniwa (1), Yoshitomo Mizoguchi (2), Akihiro Orita (3) and Shingo Obara (1)

(1) Japan Aerospace Exploration Agency, 2-1-1 Sengen, Tsukuba-shi, Ibaraki-ken, Japan, Email: [email protected] (2) Harmonic Drive Systems Inc., 1856-1 Hotakamaki, Azumino-shi, Nagano-ken, Japan, Email: [email protected] (3) Asahi Heat Treatment Co., Ltd, 2-9-1 Kuzuhara, Neyagawa-shi, Osaka, Japan, Email: [email protected]

ABSTRACT improved to prolong the lifetime of grease-lubricated SWG under vacuum operation. There are some well- This study investigated a technique to extend the life of known techniques to improve wear resistance under strain wave gearing (SWG) under a vacuum. We boundary lubrication, such as hard coatings and the use applied carburizing and two-stage shot peening to the of new lubricants. In this study, based on fundamental flexspline (FS) inner surface and the FS and circular friction tests and an evaluation of surface properties, we spline (CS) teeth surfaces to reduce wear, particularly at selected a combined surface modification technique— the wave generator (WG)/FS interface. With these carburizing and two-stage shot peening. This technique surface modifications, the FS inner surface was finished was applied to the FS inner surface and the FS/CS teeth to achieve hardness 1.3 to 1.4 times higher than that of surfaces. Surface-modified SWGs were manufactured the WG outer surface, with average roughness of 0.05 to and subjected to a vacuum life test, followed by an µ 0.1 m and surface skewness of -2 to -0.3. initial performance test and vibration test for clarifying the effect of surface modification. The surface-modified SWG lubricated with grease was subjected to a vacuum life test. As a result, the SWG reached about 516,000 output shaft revolutions without significantly increasing the input torque. After the life test, there was no severe wear at the FS inner surface. Flexspline (FS) The WG outer surface and the FS/CS teeth surfaces also exhibited much less wear. Circular spline (CS) INTRODUCTION Wave generator A grease-lubricated SWG has been widely used in space (WG) rotary devices that require lightweight and compact mechanical components. We have developed SWG with grease lubrication to achieve a longer life and lighter weight of components [1]. Fig. 1 shows a photograph of the developed SWG with a reduction ratio of 1/160 and tooth pitch diameter of approximately 50 mm. During the development, however, we found that the non- surface-modified SWG resulted in a significantly short life under a vacuum due to severe wear at the WG/FS interface and the FS/CS teeth interface as shown in Fig. 2. Operating the WG/FS interface under boundary Figure 1. SHF type strain wave gearing lubrication due to lubricant starvation in a vacuum was concluded to be the cause of severe wear at the WG/FS interface. Also, the state of high friction at the WG/FS interface under boundary lubrication could adversely affect the lubricating condition at the FS/CS teeth interface.

According to the lubricating conditions in the grease- lubricated SWG mentioned above, wear-resistant performance at the WG/FS interface must be mainly

______Proc. 18. European Space Mechanisms and Tribology Symposium 2019, Munich, Germany, 18.-20. September 2019 Table 1. Specifications of test SWG No. I II III Model SHF-20-160-2A-GR-SP number φ82 mm × 30.4 mm (a pitch circle Size diameter of approximately 50 mm) Reduction 1/160 (a) FS inside surface (b) WG outer race ratio WG bearing: 440C stainless steel WG retainer: phenolic resin Materials FS: 15-5PH stainless steel CS: 630 stainless steel Modification contents: 1. Carburizing (for surface hardening) (c) CS teeth (d) FS teeth 2. Fine particle shot-peening

(for improving fatigue property) Figure 2. Wear conditions of rubbing surface of non- Surface 3. Super fine particle shot-peening surface-modified strain wave gearing after 34,996 modification (for surface polishing) output shaft revs (the surfaces were cleaned out) [1] Application parts:

No. I: only FS inner surface

Nos. II and III: FS inner surface and TEST STRAIN WAVE GEARING teeth of FS and CS Tab. 1 lists the specifications of the test SWG. The SWG model number (SHF-20-160-2A-GR-SP) is same as that of the non-surface-modified type evaluated in the Table 2. Type of grease, application parts, and quantity previous work [1]. Three test pieces (No. I, II and III) No. I II III were evaluated in this study. Only the FS inner surface Grease type MU R2000 for No. I, and the FS inner surface and FS/CS teeth (thickener) (urea) (sodium soap) surfaces for Nos. II and III were subject to modification. WG bearing 0.1 g After the surface modifications, the FS inner surface WG outer 0.2 g was finished to achieve hardness 1.3 to 1.4 times higher Surface than that of the WG outer surface, with average FS inner surface 0.2 g roughness of 0.05 to 0.1 µm and surface skewness of -2 FS external 0.3 g to -0.3. The high hardness is expected to increase wear Teeth resistance, whereas negative skewness is expected to CS internal 0.3 g form lubricant films on plateaus and to retain a lubricant Teeth in small dimples under vacuum operation.

Multiply alkylated cyclopentane (MAC) grease was used to lubricate the FS/CS teeth surfaces, between the FS inner surface and WG outer surface, and between the races and balls of the WG bearing. Tab. 2 lists the type of grease, application parts, and quantity. The WG retainer is made of cotton-based phenolic resin, and was impregnated with the base oil of grease in a vacuum. Fig. 3 shows an example of the grease spread condition (for test piece No. III). Figure 3. Grease spread condition of WG and FS for test piece No. III

______Proc. 18. European Space Mechanisms and Tribology Symposium 2019, Munich, Germany, 18.-20. September 2019 TEST METHOD Support bearings Fig. 4 shows the flowchart for the series of tests. A life (grease lubrication) test of the surface-modified SWG was conducted in a Output shaft Input shaft high vacuum following a running-in test and a vibration test. Fig. 4 also gives the vibration test and life test conditions. Transmission error and torsional stiffness were measured for test piece Nos. II and III, in order to check the SWG performance.

The test SWG was assembled onto a test jig (Fig. 5) for testing. Fig. 6 shows the vibration test configuration. Test SWG Fig. 7 shows the life test apparatus [1]. In the life test, a Support bearings (grease lubrication) sinusoidal torque was loaded using an arm on the output side. During the life test, the maximum input torque was Figure 5. Test jig recorded using the input torque sensor located inside the vacuum chamber. The end of life was defined as the input torque (i.e., SWG friction) increasing by 50%. Acceleration sensor for vibration level control

Performance test before life test for No. II and III

Running-in Environment: atmosphere, room temperature (+22 °C), less than 50 %RH Test jig Input rotational speed: 100 r/min

Load (output) torque: 0 N-m X Test duration: 128 min (80 revolutions of output shaft) Vibration direction Y Z Vibration test (X, Y, Z axis) Sinusoidal vibration: 25 G, 10 to 100 Hz, 2 oct/min Random vibration: 21 Grms, 5 to 2000 Hz, 3 min (a) Vibration test in X / Y axis (radial direction)

Life test Environment: vacuum (less than 10-4 Pa), room temperature (+22 °C) Input rotational speed: 100 rpm Load (output) torque: ± 14 N-m (sinusoidal) Test jig Acceleration sensors for Performance test after life test vibration level control for No. II and III

Z Visual inspection

Vibration direction X Y Figure 4. Flowchart of the series of tests (b) Vibration test in Z axis (axial direction)

Figure 6. Vibration test configuration

______Proc. 18. European Space Mechanisms and Tribology Symposium 2019, Munich, Germany, 18.-20. September 2019 Vacuum chamber

Input torque Output torque sensor Output side Input side sensor Magnetic fluid seal Servomotor Slip ring Test jig Arm

Slip ring

Table Magnetic fluid seal Weight 1000 mm

Figure 7. Life test apparatus (a) No. II

TEST RESULTS Fig. 8 shows the maximum input torque measured at an arm angle of 90 deg. in the sinusoidal load cycle during each life test. All the tests did not reach the end of life and were terminated as described below. The life tests for test piece Nos. I and II were stopped at about 100,000 output shaft revolutions to confirm wear conditions of the rubbing surfaces. For test piece No. I, the input toque temporarily increased at about 43,000 revolutions, and the torque was subsequently recovered by the end of the life test. Test piece No. III reached about 516,000 output shaft revolutions without (b) No. III significantly increasing the input torque. This result corresponds to life about 14 times longer than the case Figure 9. Change in transmission error under life test without surface modification.

Figs. 11-1, 11-2, and 11-3 show the conditions of wear and appearance of grease on the rubbing surfaces after each life test for test piece Nos. I, II, and III, No. II: 100,118 revs respectively. The visual inspection results of each test No. I: 100,622 revs piece are summarized below.

No. I: There was no severe wear at the WG outer surface and

No. III: 516,471 revs FS inner surface as shown in Fig. 11-1 (a). The input side of the FS/CS teeth surfaces were partially worn away, with darkened grease and wear particles being observed in the vicinity of the teeth as shown in Fig. 11- 1 (b).

Figure 8. Maximum input torque during life test No. II: The conditions of wear and appearance of grease at the WG/FS interface were similar to those of No. I. Figs. 9 and 10 show the transmission error and torsional Conversely, the teeth wear was mild compared with that stiffness measured before and after the life test for test of No. I. And there was a slight discoloration of grease piece Nos. II and III, respectively. Performance was near the teeth. measured in the middle of the life test only for test piece No. III. For test piece Nos. II and III, the transmission No. III: error increased slightly and the torsional stiffness There was no remarkable wear at the WG outer surface decreased after the life test. and FS inner surface as shown in Fig. 11-3 (a), although the SWG was operated at over 500,000 output shaft revolutions. The FS/CS teeth surfaces also exhibited

______Proc. 18. European Space Mechanisms and Tribology Symposium 2019, Munich, Germany, 18.-20. September 2019 less wear compared with those of No. I. WG bearing CS All surface-modified SWGs demonstrated much less wear, especially at the WG/FS interface. This means that the modification of the FS inner surface by carburizing and two-stage shot peening clearly reduces wear. Outer surface of WG Inner surface of FS The increase in input toque only observed for test piece (a) WG/FS interface No. I in Fig. 8 might be induced by the progression of Internal teeth of CS wear and change in friction at the FS/CS teeth surfaces. The degraded performance shown in Figs. 9 and 10 might be related to the slight wear of the FS inner surface and the teeth surfaces. External teeth of FS Darkened grease

(b) FS/CS teeth interface

Figure 11-1. Conditions of wear and appearance of grease on rubbing surfaces after 100,622 output shaft revs. in life test for No. I

Inner surface of FS

Outer surface of WG

(a) WG/FS interface (a) No. II

Internal teeth of CS External teeth of FS (b) FS/CS teeth interface

Figure 11-2. Conditions of wear and appearance of grease on rubbing surfaces after 100,118 output shaft revs. in life test for No. II

(b) No. III

Figure 10. Change in torsional stiffness under life test

______Proc. 18. European Space Mechanisms and Tribology Symposium 2019, Munich, Germany, 18.-20. September 2019 The life test of the surface-modified CSF type SWG Outer surface of WG was conducted at an output torque of 4.8 N-m (constant), input rotational speed of 100 rpm, temperature of +22°C, and vacuum pressure of less than 10-3 Pa, followed by the vibration test. It reached the end of life due to the increase in input toque at 237,358 output shaft Inner surface of FS revolutions. Fig. 13 shows that remarkable wear was (a) WG/FS interface visible on each rubbing surface after the life test. The FS inner surface was worn intensely, and with the wear reaching 30 µm at the deepest part. The FS/CS teeth

Internal teeth of CS surfaces were also worn away to the point where teeth External teeth of FS disappear. We estimated the mechanism whereby that (b) FS/CS teeth interface CSF type SWG reached the end of life as follows: the FS inner surface initially began to wear due to lubricant starvation, with higher friction force acting on the WG/FS interface, thereby deforming the FS axially and resulting in abnormal teeth wear due to a mismatched teeth engagement zone. This mechanism is considered to be roughly the same as that of a non-surface-modified SWG. (c) FS inner surface and external teeth (cleaned)

The lifetime of the CSF type was shorter than that of Figure 11-3. Conditions of wear and appearance of test piece No. III. This might be caused by applying grease on rubbing surfaces after 516,471 output shaft constant output torque to the SWG. A constant output revs. in life test for No. III torque imposes continuous contact between the FS/CS

teeth surfaces, resulting in a larger average contact load COMPARISON WITH OTHER LIFE during one output shaft revolution than in the case of TEST DATA sinusoidal torque. In addition, lubricant is not likely to Fig. 12 plots the relationship between vacuum lifetime be supplied into the clearance from grease accumulated (total input number of revolutions) and output torque, in in the vicinity of the WG/FS interface, because the order to compare the lifetime of the surface-modified distribution of clearance between WG and FS does not SWGs obtained in this study with other SWGs for vary under constant output torque. Due to the two points space-use [2-4]. The output torque is divided by the mentioned above, the CSF type SWG exhibited a rated torque of each SWG. Note that the type, size, and relatively short lifetime. reduction ratio of each SWG, type of lubricant, and life test conditions are different between each item of test data in Fig. 12. The data plotted as “Surface-modified Outer surface of WG CSF type SWG” shows the result of the in-vacuum life test of CSF-17-100-2A-R-SP with the same surface modification as for test piece Nos. II and III in Tab. 1. SWG test piece Nos. I, II, and III have a longer lifetime than the others even in the high output torque region. Inner surface of FS (a) WG/FS interface

Internal teeth of CS (b) FS/CS teeth interface

Figure 13. Conditions of wear and appearance of grease on rubbing surfaces after 237,358 output shaft revs. in life test for CSF type SWG

Figure 12. Relationship between lifetime of SWG and output torque under a vacuum

______Proc. 18. European Space Mechanisms and Tribology Symposium 2019, Munich, Germany, 18.-20. September 2019 CONCLUSIONS A life extension technique for grease-lubricated SWGs was established using the surface-modification technique based on in-vacuum life tests.

1. Carburizing and two-stage shot peening were selected as the surface modification technique and then applied to the FS inner surface and FS/CS teeth surfaces for increased wear resistance.

2. Three surface-modified SWGs were manufactured and then subjected to in-vacuum life tests for verifying the effect of surface modification. The longest life of the three SWGs exceeded 500,000 output shaft revolutions, which corresponds to life 14 times longer than the case without surface modification.

3. There was no remarkable wear at the WG/FS interface of all SWGs after the life test. However, slight teeth wear that could adversely affect performance was observed.

4. The mechanism whereby a surface-modified SWG under vacuum operation reached the end of life was estimated. Lubrication of the WG/FS interface is thus considered most important for realizing a long life SWG.

REFERENCES 1. Ueura, K., Kiyosawa, Y., Kurogi, J., Kanai, S., Miyaba, H., Maniwa, K., Suzuki, M. & Obara, S. (2007). Development of Strain Wave Gearing for Space Applications. In Proc. 12th ESMATS, ESA SP-653, Liverpool, UK. 2. Schafer, I., Bourlier, P., Hantschack, F., Roberts, E. W., Lewis, S. D., Forster, D. J. & John, C. (2005). Space Lubrication and Performance of Harmonic Drive Gears. In Proc. 11th ESMATS, ESA SP-591, Lucerne, Switzerland, pp65-72. 3. Johnson, M. R., Gehling, R. & Head, R. (2006). Failure of Harmonic Gears During Verification of a Two-Axis Gimbal for the Mars Reconnaissance Orbiter Spacecraft. In Proc. 38th Aerospace Mechanisms Symposium, Williamsburg, NASA CP-2006-214290, Virginia, pp37-50. 4. Roberts, E. W., Bridgeman, P., Jansson, M., Schulke, M. & Tvaruzka, A. (2015). The Performance and Life of Fluid-Lubricated Harmonic Drive® Gears, In Proc. 16th ESMATS, ESA SP-737, Bilbao, Spain.

______Proc. 18. European Space Mechanisms and Tribology Symposium 2019, Munich, Germany, 18.-20. September 2019