ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
39 Harmonic Drive™ Gear Material  Selection and Life Testing 

Jeffrey Mobley* and Jonathan Parker*    

Abstract 

Sierra Nevada Corporation (SNC), along with NASA Goddard Space Flight Center (GSFC), tested several  Harmonic Drive gears for their longevity and performance to determine the best material combinations for a mission, requiring an actuator life nearly ten times previously qualified. Accelerated life testing in both ambient and vacuum environments revealed two suitable candidates for the flight build. The combination of a Nitronic 60 circular spline and 15-5PH H1075 flexible spline was ultimately chosen. It was surprising though, that 15-5PH H1075 on both circular and flexible splines outperformed the heritage material combination (Melonited Flex Spline against a 15-5PH Circular Spline) and also could have been used in the flight build.  Introduction  The NASA Global Precipitation Measurement (GPM) mission required actuators for its High Gain Antenna System (HGAS) and Solar Array Drive Assembly (SADA) with a life significantly higher than the required life of heritage actuators previously qualified by SNC for the Lunar Reconnaissance Orbiter (LRO) mission and the Solar Dynamics Observatory (SDO) mission as shown in Table 1. One of the life-limiting components in the actuator design is the Harmonic Drive gear. Wear was observed in the circular spline teeth during the post-life test teardown inspection on the SDO and LRO actuators. This wear was determined to be acceptable for those missions, but when extrapolated out to the life requirements of the GPM mission, the effect of tooth wear on performance became uncertain. The heritage LRO and SDO actuators used T-Cup Harmonic Drive gears manufactured by Harmonic Drive LLC in Peabody, MA in order to provide a large through hole in the actuator. The drives had a 200:1 gear ratio and utilized a combination of a Melonite TM treated Flex Spline and 15-5PH Circular Spline. This particular material  combination had not been life tested by either SNC or Harmonic Drive LLC for the number of cycles required for the GPM mission.   Table 1: Required Life Comparison  Application Cycle # of Cycles (for 1X Life) Total Degrees  SDO 360° (continuous) 2,500 900,000°  LRO 180° CW, 180° CCW 5,700 2,052,000°  GPM SADA 250°CW, 250° CCW 17,500 8,750,000°  GPM HGAS 190° CW, 190° CCW 48,600 18,468,000° 

Harmonic Drive™ is a registered trade mark of Harmonic Drive LLC                                                      * Sierra Nevada Corporation, Durham, NC 

Proceedings of the 41st Aerospace Mechanisms Symposium, Jet Propulsion Laboratory, May 16-18, 2012
40 Hardware Configurations  Based on discussions between Harmonic Drive LLC and SNC, the following material combinations were  selected for comparative life testing, with the goal of extending the life of the Harmonic Drive gear in order to meet the GPM HGAS requirement:    15-5PH H1075 Circular Spline against Melonited 15-5PH H1100 Flex Spline  o Most similar to heritage  o Removed weight reduction cuts for more repeatable performance  o Reduced the size of the pre-melonite grit blast media for better surface finish  o Increased the circular spline hardness from H1150 to H1075 for better wear resistance   Nitronic 60 Circular Spline against Melonited Flex Spline  o Same as above except for Circular Spline material  o Nitronic 60 recommended by Harmonic Drive LLC based upon testing reported in Reference  1   Nitronic 60 Circular Spline against 15-5PH H1075 Flex Spline  o Same as above except for flex spline material  o Melonite removed to result in less abrasive flex spline  o Flex Spline hardness increased to H1075 for better wear resistance   15-5PH H1075 Circular Spline against 15-5PH H1075 Flex Spline  o Chosen for test because the components were available from the other combinations  o Low expectations due to potential for galling/cold welding from identical materials in contact  All Harmonic Drive gear configurations were customized HDT-25, 200:1, T-cup component sets  manufactured in Peabody MA by Harmonic Drive LLC and all were of the same dimensional design. All combinations used the same wave-generator bearing material and design, and each was identically lubricated with Pennzane 2001-3PbNp oil and Rheolube 2004 grease per standard procedures. Table 2 summarizes the tested configurations with assigned part numbers.  Table 2:  Material Combinations  Part # Circular Spline Material Flexible Spline Material  36638-1 15-5PH H1075 15-5PH H1100 w/ Melonite  36638-2 15-5PH H1075 15-5PH H1075  36638-3 Nitronic 60 Annealed 15-5PH H1100 w/ Melonite  36638-4 Nitronic 60 Annealed 15-5PH H1075
41 Ambient Life Test Plan  The Life Test plan is shown in Figure 1. A standard run-in was performed by Harmonic Drive LLC on each  Harmonic Drive component prior to the start of any testing. Following run-in, each Harmonic Drive gear was characterized by measuring starting torque and torsional stiffness. The starting torque of the Wave Generator input was measured in six equidistant positions with the Circular Spline fixed and no load applied to the Flex Spline output. Torsional stiffness was measured in six equidistant locations at the Flex Spline output with the Circular Spline and Wave Generator input held stationary. Once each Harmonic Drive gear was assembled into its life test fixture, the torsional stiffness test was repeated at 25 equidistant locations to characterize the baseline assembled condition. The ambient life test was conducted to simulate 2X the required mission life and was conducted in air at room temperature. The Harmonic Drive gear input was driven at a nominal input speed of 52.36 rad/s (500 rpm). The Harmonic  Drive gear output was loaded to 2.93 N-m (26 in-lb) with a friction brake (see Figure 2) to simulate the application frictional load plus the load required to accelerate the specified inertia. The life test was run alternating between clockwise and counter-clockwise directions for approximately 24 hours at a time. At the conclusion of the life test, the Harmonic Drive gear had completed over 51,300 output revolutions in each direction (39,936,000° of total travel). Following the ambient life test, the units were again tested for torsional stiffness while assembled in the test fixture. The units were then disassembled, inspected, cleaned, and inspected further with findings documented below. 

Figure 1:  Test Flow 

Figure 2:  Ambient Life Test Setup
47 Ambient Life Test Results Summary  The two Melonite Harmonic Drive gears (36638-1 and 36638-3) were eliminated from contention due to  the reduction in torsional stiffness at the end of life and the presence of relatively large particles of metallic debris in the gear mesh. The two non-Melonite Harmonic Drive gears (36638-2 & 36638-4) were relatively comparable. 36638-2 visually looked slightly better, (primarily with regard to the circular spline teeth and gear lubricant) but the 36638-4 was nonetheless acceptable. The concern over how 15-5PH against 15-5PH would perform during vibration and in a vacuum environment, resulted in the 36638-4 configuration (15-5/Nitronic) being chosen as the least risky path forward for the flight build. A qualitative assessment of the key parameters of the post- life inspection is shown in Table 3. In parallel with building flight hardware with the chosen configuration, the program decided to further this study by performing a vacuum life test comparison between the non-Melonite drive configurations (36638-2 & 36638-4). 

Table 3:  Qualitative Summary of Ambient Life Test Results   36638-1   15-5/Melonite  36638-2   15-5/15-5  36638-3   Nitronic/Melonite  36638-4   Nitronic/15-5   Stiffness Change Poor Good Poor Good  Circular Spline Tooth Wear Poor Average Poor Average  Flex Spline Tooth Wear Good Average Good Average  Flex Spline Cup Good Average Good Average  Wave Generator OD Average Average Average Average  Gear Teeth Lube Poor Average Poor Average  Wave Generator Lube Good Average Good Average 

Vacuum Life Test Plan  Based on the ambient life test results and concerns regarding the validity of a life test in ambient air, a second round of testing was conducted in a vacuum. The testing was repeated on new Harmonic Drive gears with the best performing configurations from the ambient test, 36638-2 and 36638-4, in a vacuum environment combined with thermal cycling. 

Testing was performed per the same flow shown in Figure 1 except as noted below. Baseline torsional stiffness testing was performed on the vacuum test Harmonic Drive gears prior to life testing as previously done with the ambient test to characterize the pre-life performance. The vacuum life test consisted of 100,000 cycles of reversing 180° output revolutions in a <5.0x10 -5 Torr vacuum environment, cycling  between 0 and 40°C. The input to the Harmonic Drive gear  was set to 52.36 rad/s (500 rpm). A magnetic particle brake was used to apply a constant 2.93 N-m (26 in-lb) friction load to the output for the duration of the test. Following the ambient life test, the units were again tested for torsional stiffness while assembled in the test fixture. The units were then disassembled, inspected, cleaned, and inspected further with findings documented below.
48 Vacuum Life Test Results  Torsional Stiffness  As shown in Figure 11, there was very little difference between the post- life torsional stiffness of the two  configurations. The 36638-4 saw a 5% reduction in torsional stiffness while the 36638-4 saw a 9%  reduction in torsional stiffness, but based on the stiffness plots seen in Figure 12 , neither drive saw a  significant reduction in low torque torsional stiffness that was seen on the Melonited drives during the  ambient life test.   Teardown Inspection  Each Harmonic Drive gear was disassembled to examine its components for wear and debris generation.  Circular Splines  Figure 13 illustrates the differences between the Circular Splines at the end of the life test. The Nitronic  60 Circular Spline (36638-4) only showed a change in surface finish at the edge of flex spline contact, whereas the 15-5PH Circular Spline (36638-2) showed a slight wear step. Neither drive displayed significant generation of metallic debris or any buildup of debris in the roots. The gear mesh lubricant was lighter than seen after the ambient life test most likely due to reduced oxidation in vacuum.   Flex Splines  Figure 14 and Figure 15 show the teeth and ID of the Flex Splines at the end of the life test. The Flex  Spline teeth of both drives were minimally worn in the area of highest tooth contact. There was no sign of galling or cold welding in the 36638-2 configuration (15-5PH against 15-5PH). The ID of the Flex Splines were visually worn in the area of contact with the wave generator OD, but there were no signs of galling and the amount of wear was acceptable for a 2X life condition and comparable to the ambient life test.   Wave Generator  Figure 16 and Figure 17 show the OD and balls of the Wave Generators at the end of the life test. Both  Wave Generators showed some polishing on the OD from contact with the Flex Splines, but results were comparable to the ambient life test and to be expected for 2X life. Again, the wave generator lubricant was slightly lighter than seen in the ambient life test (less oxidation) and the 36638-4 configuration was slightly darker than the 36638-2 configuration.  GSFC Material Analysis  Very slight wear was observed on the flex spline teeth of both assemblies which appeared to be slight  burnishing of the tooth surface. No pitting or scoring was visible. Grease from both assemblies exhibited evidence of metallic wear. Infrared spectroscopic analysis of the grease was inconclusive in determining a ‘winner’. In all the samples, a weak intensity carbonyl band was detected, indicating very little grease/oil degradation. The greater amount of metallic wear debris in the flex spline-circular spline contact of the 15-5PH & 15-5PH combination suggests the 15-5PH & Nitronic 60 is a slightly better configuration
52 Vacuum Life Test Results Summary  The vacuum environment had little to no effect on the wear rate of the two Harmonic Drive gears tested.  The slight differences in wear between the two configurations are not significant, and changes in wear between vacuum and ambient tests can be attributed to normal variation from unit to unit. The results of the vacuum life test supported the decision by the program to use the 15-5PH against Nitronic 60 Harmonic Drive gear configuration (36638-4) for the GPM program. Surprisingly, the 15-5PH against 15-5PH configuration (36638-2) would have also been an acceptable choice. A qualitative assessment of the key parameters of the post life inspection is shown in Table 4. 

Table 4:  Qualitative Summary of Vacuum Life Test Results   36638-2   15-5PH / 15-5PH  36638-4   Nitronic / 15-5PH   Stiffness Change Good+ Good-  Circular Spline Tooth Wear Good- Good+  Flex Spline Tooth Wear Good Good  Flex Spline Cup Average Average  Wave Generator OD Average Average  Gear Teeth Lube Average Average  Wave Generator Lube Average Average 

Conclusion  Accelerated Harmonic Drive gear life testing validated the concern that the heritage material combination of a Melonite Flex Spline running against a 15-5PH Circular Spline was not optimal for an extended life requirement of nearly ten times previously qualified. An alternate combination of a Melonite Flex Spline against a Nitronic 60 Circular Spline was also ruled out based upon test results. After completion of both Ambient and Vacuum accelerated life tests, two material combinations displayed test results indicating a high level of confidence in passing the increased life requirement. After joint deliberation between SNC and NASA GSFC, the combination of a 15-5PH H1075 Flex Spline and a Nitronic 60 Circular Spline was chosen for incorporation into the GPM HGAS and SADA actuators. Despite industry concerns over identical stainless steels operating in contact, the material combination of 15-5PH H1075 against 15-5PH 1075 performed nearly identically to the combination selected and would have been a high confidence selection as well.  

References  1. Johnson, Michael R., Russ Gehling, and Ray Head  “Life Test Failure of Harmonic Gears in a TwoAxis Gimbal for the Mars Reconnaissance Orbiter Spacecraft.” Proceedings of the 38th Aerospace  Mechanisms Symposium. May, 2006.
55 Initial Concerns  1 - In many new design cases, existing laminations and their characteristics can be referred to  laminations that have already been established and proven in previous designs. In this case the lamination designs needed to be designed from scratch. The task of deriving the exact geometry of the lamination, so important to duplication, was doubtful. The 0.75 slot/pole format would require investigating the magnetic flux details of the laminations using magnetic FEA analysis tools, which unfortunately is not always that precise. How close would we come?  2 – How would the resistance vs. inductance winding parameters balance against each other, both a  function of the motor geometry as well as the copper windings?  3 - What is the BEMF waveform for the specified winding profile? Will it meet the 2% distortion  requirement?  Major Parameters for T Motor  The development of the T motor was moderately painless and is illustrated to show how the T motors development was hoped to have also proceeded with the G and H configurations. It was considered painless because the performance parameters were more easily obtained without major modifications to any of the restrictions outlined above. Also, the T motor being the smallest of the 3 configurations made tooling for test easier to handle than the larger motors. Anyone having the experience of mounting rotors and stator with high energy magnets has sooner or later gotten his fingers bitten during the insertion of the rotor into the stator process if the proper tooling was not available. Special tooling “jacks” needed to be designed to allow the assembly into the test fixtures. The basic performance for one of the T motors is listed below:   * Kt > 35  N-cm/amp (49 in-oz/amp)   * R = 2.6 ohms within 10%, L = 8 mH within 25%   * Friction torque < 1.8 N-cm (2.5 in-oz) at 10 RPM   * Detent torque < 3.5 N-cm (5 in-oz) A view of this motor is shown in Figure 1. 

Figure 1.  T Motor   A significant obstacle to the development and delivery effort was that Carpenter 49 lamination material  was not available in the sheet stock we needed which required a special order with a long delivery schedule. It was decided to make the early prototypes using M-15 lamination material, which was
56 available and whose characteristics were well known to us. The plan was to learn as much as possible  before the Carpenter material was received.    The usual manufacturing procedure to make a lamination stack is to stack and bond a number of 0.18 or  0.36-mm (0.007 or 0.014-inch) thick laminations, previously punched to shape, to make the desired stack height required. Since these laminations were of a new design, the technique used for these unique shaped laminations was to bond square sheets of 0.18-mm (0.007-inch) lamination stock to produce the height needed and cut the proper shape with an electrical discharge wire machine (EDM). The sheets are welded on the edges to make the electrical contact needed for the process. The non-recast EDM process produces a lamination stack with very well defined and smooth edges to the dimensions required. Some follow up machining, is required to obtain the 13 µm (0.0005 inch) tolerances required.      As a conservative strategy the thought was to start with a projected high Kt and a high detent prediction  as a conservative approach to become familiar with the 0.75 slot per pole configuration. Using the measured data from a prototype M-15 lamination motor, we would then modify the air gap and turns to bring both parameters into specification. The FEA Kt and detent predictions as well as their ultimate measured parameters are shown in Figures 2 and 3. 

Figure 2.  T Motor FEA Kt Prediction  Initial T motor Kt analysis and measurement data (Note: the program does not allow for separate primary and secondary windings so the 116  N-cm/amp (164 in-oz/amp) value is for both primary and secondary  energized).   

Figure 3.  Initial T Motor Detent Analysis and Measurement  As can be seen the detent prediction was 28 N-cm (39 in-oz) and the measured detent on the prototype was 25 N-cm (35 in-oz). The Kt prediction and the measure Kt were spot on with 58  N-cm/amp (82 in-oz/amp).  Both of these were very high as expected compared with the required values of 3.5 N-cm (5 in-oz) and  35  N-cm/amp (49 in-oz/amp). With these actual values increasing the air gap from 0.635 to 1.52 mm (0.025 to
57 0.060 inch) was predicted to achieve the detent and Kt parameters needed. The Carpenter material was  now on hand and with the following air gap modification the results in Figures 4 were obtained with no surprises and were very close to the required values. 

Figure 4.  T motor Kt and detent with 1.52-mm (0.060-inch) mangetic air gap  To get some perspective on the windings, in order to achieve the required 2.6-ohm resistance it was necessary to use a winding of 5 wires of #29AWG and 3 of 28AWG for a total of 8 strands per coil. The inductance was fortunately within the range required (8 mH). It did, however, take several iterations of the winding turns to arrive at the final values of the design. So far so good!  The T motor BEMF curves are shown in Figure 5. The harmonic content  was measured using the FFT  functions of  the scope (not shown). 

Figure 5.  BEMF waveforms with 1.52-mm (0.060-inch) magnetic air gap
58 The spectrum analysis showed the harmonics to be less that the 2% limit. This motor performed as  expected and promised similar expectations that the G and H motors would be equally accommodating…so it was hoped.  Major Motor Parameters for G Motor  The G and H motors were of an inside out design (rotor on the outside). This was not considered as an obstacle for obtaining the same results as was obtained for the T motors. The form factor for the G motors is illustrated in Figure 6.  

Figure 6.  G motor 

Major Specification Parameters  * Kt > 61  N-cm/amp (87 in-oz/amp)   * R = 1.17 ohms within 10%, L = 3.58mH within 30%  * Friction torque < 11  N-cm/amp (15 in-oz/amp) at 105 RPM   * Detent torque < 19 N-cm (27 in-oz)   * % Harmonic Distortion < 2%  The initial lamination stack was already made with M-15 material and was not used since the Carpenter  material had now arrived. (This becomes significant later on.)  The G motor, as with the T motor, began with a known smaller than expected air gap of 0.635 mm   (0.025 inch). The FEA Kt was predicted to be 79.44 N-cm/amp (112.5 in-oz/amp) and the measured was   80.36 N-cm/amp (113.8 in-oz/amp) on this first prototype. The detent was predicted as 65 N-cm (92 in-oz). and  the detent was measured at 64 N-cm (90 in-oz). The wave form, however, was not looking too good as seen in Figure 7:
59   Figure 7.  G motor BEMF waveform  The shape of the BEMF wave looked less like a perfect sine wave and harmonic distortion was viewed as  a potential problem. The gap was then opened to 1.27 mm (0.050 inch) to lower the detent as well as the Kt with the hope this would also improve the distortion issue. The results were as follows:  Kt dropped 80.36  N-cm/amp (113.8 in-oz/amp) to 65.2 N-cm/amp (92.4 in-oz/amp)  Detent dropped from 69 N-cm (98 in-oz) to approximately 18 – 20 N-cm (25 – 28 in-oz) 

Figure 8.  Motor with 1.27-mm (0.050-inch) air gap 

The waveform became a little less peaked but nevertheless was still apparently distorted and a concern.  This was caused by the 5th harmonic and was above the 2% spec and was unacceptable.
60   Figure 9.  G motor Harmonic distortion  It was postulated that the lamination shape was perhaps deficient and that also shaping of the rotor  magnets would perhaps contribute to improving the harmonic distortion. Inspection of the FEA analysis (Figure 10) did not show magnetic saturation in any part of the lamination material.  

Figure 10.  Flux Density Comparison of Lamination Materials  There were several other issues other than the distortion to address such as inductance and detent. It was hoped that correcting these deficiencies would help correct the distortion issues. A brief description of the inductance and detent improvement efforts are summarized in bullet form:   The inductance was too high so the turns were reduced from 45 to 38 (inductance is a function of  the turns squared). To compensate for the loss of Kt due to the turns reduction, decrease the air gap. This worked as intended for the inductance and Kt, but the detent was now 3 times higher than required due to the air gap reduction.   A small skew was introduced into the lamination structure which reduces detent. The detent was  now OK but the Kt dropped below the desired level due to the skew.   Used higher energy product Neodymium magnets instead of samarium cobalt. Kt improved but  still too low   Bring coils to 40 turns to improve Kt with inductance still OK; attempt to go back to Samarium  Cobalt magnets…Kt just 0.7  N-cm/amp (1 in-oz/amp) below the minimum; inductance  marginal….parameters too marginal to atte mpt production. Went back to180 C Neodymium,  ultimately used for production.   Back to distortion issue, solving may resolve the Kt, detent and inductance issues.