ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
40 Motorization Margin  Analyses  

The ability of a drive motor  to be able to drive a mechanism can be quantified using the Motorization  Margin or Torque Margin (MoS TORQUE ). This margin is defined as MoS TORQUE  = [ T  MIN,MOTOR  / TL ] – 1.  Herein, T  MIN,MOTOR  is the minimal motor torque available and T L is torque of t he load. The latter is defined  as T L = 2*(FoS FRICTION * T FRICTION ). Herein, FoS FRICTION  = 3 which is the Factor of Safety on friction and  TFRICTION is the torque in bearings. According to ECSS [4 ], inertia and spring effects should also be taken  into ac count, but for these mechanisms these budgets are considered negligible. The torque margin is   calculated for the following load cases:   • At 20°C  • At worst case operational range   The following inputs are elaborated on. The friction torque is calculated with CABARET as described in   the previous  section. It concerns here a mean friction torque with a worst case coefficient of friction. Next,  in order to obtain the maximum (0- peak) friction torque, the mean fri ction torque is multiplied by a factor of  three. Thi s factor is based on empirical data as deduced by ESTL. Next, margins according to [4] are  incorporated in the calculations.  The resulting margins for both cases are 3.1 and 2.6 respectively, which  are both > 0. From this it is concluded that the motor is suited for the mechanisms.  

Life Test Model Qualification    The Tropomi Life Test Model Calibration Unit has successfully passed the life test program, consisting of:   • Vibration testing (see Figure 11) with a 9.71 g rms input spectrum, resulting in 28.6 g rms on the  carousel   • Thermal Vacuum Testing (from -50°C to +45°C ) as shown in Figure 12   • On-ground cycles and in- orbit cycles up to 72000 cycles  

No change in performance between start and end of the life test program, determined by:   • Pre and Post Vibration reso nance search  • Minimum start current (torque margin) during the life test program   • Destructive Physical Analysis  

References  

1. ADR, www.adr.fr    2. ESTL, www.esrtechnology.com/centres/estl

 3. Tabak, E. , de Goeij, B., van Riel, L., Meijer, E., van der Knaap, F., Doornink, J., de Graaf, H.  "Design,  building and testing of a Sun Calibration Mechanism for the MSI -VNS Instrument on  EarthCARE."  ESMATS 2013 .   4. ESA- ESTEC, ECSS- E-ST-33-01C Mechanisms, Requirements & Standards Division   Noordwijk, The Netherlands , 6 th Marc h, 2009    5. Phytron, www.phytron -elektronik.de
44 This includes appropriate selection of preload with respect to fastener yield and rupture, minimizing or  controlling preload uncertainty, and quantifying worse case external load conditions .   

A literature search reveals very little published data on this topic . As a result , the data in this paper helps fill  an existing void in the literature.  

Test Specimens    The test fasteners used in this work are aerospace bolts made from Titanium 6Al -4V (i.e., Ti 6 -4) and A286.  Size and thread are ¼ -28 UNJF  (M6x1)  with length of 1.99 in (50.5 mm) for the Ti 6- 4 bolts and 2.03 in  (51.5 mm) for the A286  bolts . The heads are 12 point for the Ti 6- 4 bolts and hex for the A286 bolts. This  difference did not affect the test results in this work since all failures occurred in the threaded  section of the  bolts. The test fasteners were obtained from aerospace fastener suppliers with full certifications [7] . The Ti  6-4 fasteners meet AMS 4928 Rev: R and AMS 4967 Rev: J specifications. The A286 fasteners meet NAS  1004- 24A specifications.    Twelve test specimens for each fastener material are tested. This is found to be a more than adequate  sample size based on the mean difference in ductility ratio found for the two different fastener materials [7] .  Each test fastener is assigned a number as listed in Table 1 . The test order or sequence is randomized as  listed in Table 2.  

Table 1.  Test f astener number    Fastener number   Titanium 6 -4  1    2    3     4    5    6    7     8    9   10  11  12   A286  13  14  15   16  17  18  19   20  21  22  23  24  

Table 2 Randomized test sequence   Test sequence  Fastener number  Material   1 6 Ti 6-4  2 2 Ti 6-4  3 12 Ti 6-4  4 8 Ti 6-4  5 21 A286   6 7 Ti 6-4  7 17 A286   8 20 A286   9 13 A286   10 11 Ti 6-4  11 24 A286   12 5 Ti 6-4  13 9 Ti 6-4  14 15 A286   15 18 A286   16 10 Ti 6-4  17 4 Ti 6-4  18 23 A286   19 14 A286   20 19 A286   21 16 A286   22 1 Ti 6-4  23 22 A286   24 3 Ti 6-4
45 The twenty -four fasteners were visually inspected, coated with spray -on dry -film MoS 2 lubricant, allowed to  dry, and placed in a sectioned container labeled by fastener number. When a test fastener number came  up in the test order, it was picked out of the container, tested to failure, and remains placed back into its  cell.  

Equipment and T est Procedure  

An MTS tensile test machine is used for the testing in this work . The tensile tests in this work consist of a  bolt in a tapped fixture rather than a bolt and nut configuration. Figure 1  illustrates a test fastener in the  tensi le machine . The test fastener head is secured by a high strength steel fastener tensile grip fixture and  puck per ASTM F606 [8] . The other end of this fastener tensile grip fixture is threaded into the MTS crossbar  head.  

Figure 1.  Sample test fastener in fastener tensile grip with puck and lower test fixture in   MTS machine.    The test fastener threads are secured to the lower steel fixture with a ¼ -28 UNJF  (M6x1)  tapped- hole. This  provides  3/8 in (9.5 mm) of thread engagement between the test fastener and t he lower fixture. The external  threads of this lower fixture are threaded into the MTS hydraulic ram.    Once the fixtures are installed on the tensile test machine, the following test setup is performed for each test fastener:  

1. The upper crossbar head is adjusted and locked into a position such that the lower hydraulic ram  has sufficient range of motion to fracture the test fastener specimen in tension.   2. The test fastener is inserted through the puck and placed into the recessed area of the upper tensil e grip fixture.
46 3. The lower ram is raised up allowing the test bolt to be fully threaded into the low er test fixture in  the ram which provides 3/8 in (9.5 mm)  thread engagement between the test fastener and the  lower steel fixture.   4. The ram is slowly lowered until no gap between the washer and head is visible.   5. The ram  is further lowered until a 100- lb (445- N) load is placed on the test fastener.  

The test procedure for each test fastener is as follows:  

1. The ram is lowered at a controlled rate of 2000 lb/min (8.9 kN/min) which  is within specification of  ASTM F606  [8] for ¼ -28 (M6x1)  fasteners .  2. Tensile machine load and displacement are sampled and recorded from onset of test at 4.6 Hz  providing over 600 data points per test . Test duration was 130 seconds or greater.   3. Test is run until test fastener failure.  

Test Data and Analysis    A compilation of all twelve A286 test fastener tensile test load versus displacement curves are presented in Figure 2. This reveals minimal scatter in test data across the sample set . The corresponding compilation  of all twelve Ti 6 -4 test fastener tensile test load versus displacement curves are shown in Figure 3. More  scatter is present in this sample set.    Since the displacement scale is different for eac h fastener material in Figures 2 and 3, sample curves from  each fastener material are plotted in Figure 4. This more clearly illustrates the relative displacement to  failure of the different fastener materials . The data point at the largest displacement value of each curve  corresponds to the test specimen fracture. Since the A286 and Ti 6- 4 test fasteners are both ¼ -28 UNFJ  (M6x1) and have comparable maximum tensile load, the tensile strength for the test fasteners is also  comparable.  

Figure 2 .  Load versus displacement d ata for  all twelve A286 test fasteners
48   Although the tensile strength for the test fasteners is comparable, the displacement from yield to fracture is notably different . The ductility index is calculated to assess this characteristic.  

The ductility index is quantified by the ratio of plastic deformation at rupture to elastic deformation at rupture  [4-6]. Figure 5 illustrates how the ductility index is determined from a sample load versus displacement  curve . De represents elasti c deformation and Dp represents plastic deformation . The ratio of Dp/De is  defined as the ductility index. This index is used to quantify the relative ductility in these two test fastener  materials in this work . The ductility index provides a means to quantify ductility of product such as  full scale  fasteners that are not ideal machined tensile test spec imens . The test curves with annotation for ductility  index parameters for all 24 test fasteners are provided elsewhere [7] . 

Figure 5.  Calculation of ductility index parameters from s ample load vs displacement data    The ductility index is calculated for  all test fasteners . Table 3  lists the ductility index statistics for both the  A286 and Ti 6- 4 test fasteners . This data reveals that the ductility index for the A286 test fasteners is about  ten times larger than the ductility index for the Ti 6- 4 test fas teners . In addition, the spread or scatter in t he  ductility index is modest  for each material as indicated by the minimum, maximum and quartile values.  

Table 3.  Test fastener ductility index statistics    A286  Ti 6-4  Average  5.40 0.53  Minimum  5.05 0.43  25th Quartile  5.12 0.51  75th Quartile  5.65 0.56  Maximum  5.90 0.59
49 Figures 6 and 7 show fracture surfaces from sample Ti 6- 4 and A286 test specimens, respectively . The  fracture occurs in the threaded region for all test fasteners as expected. However, the Ti 6- 4 test fasteners  fracture across only one thread (i.e., nearly perpendicular to the axis of tensile loading which is typical for  brittle materials ), whereas the A286 test fasteners fracture across three or four threads (i.e., approximately   at a 45 degree angle to the axis of tensile loading which is typical for ductile materials ). 

Figure 6 .  Sample Ti 6-4 test fastener failure            Figure 7 .  Sample A286 test fastener failure  

Conclusions    This work assessed the relative ductility of titanium alloy Ti 6- 4 aerospace fasteners to stainless steel A286  aerospace fasteners . A test procedure was developed for this assessment . Tests were conducted using an  MTS tensile test machine and load versus displacement curves were obtained for 24 test fasteners . The  test curves were found to be repeatable for each fastener material, with substantial differences plastic deformation between materials . The tensile strength for the Ti 6- 4 and A286 test fasteners were found to  be comparable.    Relative ductility was assessed using the ductility index which is the ratio of plastic deformation to elastic deformation from a tensile test . In addition, fracture surfaces of the test specimens were examined for  characteris tics inherent of  ductile and brittle failure.    The results of this work show:  

1. The ductility index of the titanium alloy Ti 6- 4 aerospace fasteners are about one- tenth of the  ductility index of the A286 aerospace fasteners.   2. The average ductility index of t he titanium alloy Ti 6 -4 aerospace fasteners was found to be 0.53.   3. The average ductility index of the A286 aerospace fasteners was found to be 5.40.   4. The fracture surfaces of the titanium alloy Ti 6- 4 aerospace fasteners from tensile testing are  perpendicul ar to the axis of the tensile loading which is characteristic of brittle fracture.
50 5. The fracture surfaces of the A286 aerospace fasteners from tensile testing are at about 45  degrees to the axis of the tensile loading which is characteristic of ductile frac ture.   These results are not int ended to discourage the use of titanium alloy fasteners, but rather to provide  additional data for use in proper joint design. In cases were weight savings or temperature requirements  demand such fasteners, titanium alloy fasteners may be the best option. Proper joint design with titanium  alloy fasteners include keeping preloads at or below 65% of yield, minimizing or controlling torque- tension  scatter, and accurately quantifyi ng worse case external loads . 

Acknowledgement 

The authors gratefully acknowledge the funding and support of the NASA Engineering and Safety Center  (NESC) and Dr. Michael Dube for this work.  

References  

1. Gross, F,  “Concerns with the Use of Titanium Fasteners,” NASA Materials Engineering Branch  Technic al Information Paper No. 133, 2003.   2. Stevenson,  M, McDougall, J, Cline, K,  “Metallurgical Failure Analysis of Titanium Wing Attachment  Bolts,” ASM International -  Practical Failure Analysis , 3 (2003),  75-80.  3. Jha, A, Singh, S, Kiranmayee,  M, Sreekumar, K, Si nha, P , “Failure Analysis of Titanium Alloy Ti6Al4V  Fastener Used in Aerospace Application,” Engineering Failure Analysis , 17 (2010), 1457- 1465.   4. NASA- STD-5020,  Requirements for Threaded Fastening Systems in Spaceflight Hardware, .2012   5. Gerard, G,  “Structural Significance of Ductility in Aerospace Pressure Vessels,” ARS Journal , 32  (1962),  1216- 1221.   6. Samuel, K , “Dependence of Strength and Ductility Ratios in Austenitic Stainless Steels,” International  Journal of Pressure Vessels and Piping, 83 (2006),  474-476.  7. Whittaker, J , Ductility and Use of Titanium Alloy and Stainless Steel Aerospace Fasteners , Univer sity  of South Florida MS Thesis, 2015.   8. ASTM F606 -13, Standard Test Methods for Determining the Mechanical Properties of Externally and  Internally Threaded Fasteners, Washers, and Rivets , 2013 .
52 • MicrOmega, a visible and infrared imaging spectrometer   • Raman Laser Spectrometer   • Mars O rganic Molecule Analyzer consisting of a Laser Desorption Mass Spectrometer and a Gas  Chrom atography Mass Spectrometer , includ ing a mechanism to seal O vens (the so- called  Tapping Station)  

In order to supply samples to the instruments in a condition allowing an accurate analysis , the R over is  equipped with the Sample Preparation and Distribution System (SPDS ), which  is also part of the ALD and  represents one of the key components of the 2018 mission [1]. It is developed by OHB System AG as  subcontractor to the mission prime contractor  Thales Alenia Space Ital ia. To ensure the required  cleanliness for the highly sensi tive instruments, the ALD and the SPDS form an enclosed volume, the socalled Ultra -Clean Zone (UCZ), which remains pressurized until first opening on Mars.  

Figure 1.  SPDS (left) and PS DDS  (right) Q ualification CAD models.    The SPDS (see  Figure 1, left side, integrated into the ALD ) consists of four separate mechanisms that  interact with each other to transport the sample within the UCZ. The Core Sample Handling System (CSHS) transfers the sample to the Crushing Station (CS) where it is crushed to a certain grain size    (50 µm-500 µm). The PSDDS ( see Figure 1, right side)  receives the powder ed samp le and doses it in  defined quantities to different sample receptacles, which are subsequently brought to the instruments for  analysis by the Powdered Sample Handling System (PSHS ) [2].   This sample handling mechanism has three unique design solutions originated by the mission specific  challenges :  • Prevent cementation of powdered sample during storage time on M ars  • It is designed to be virtually “backlash- free” to achieve compliance to the positioning requirements  originated by sample handover demands of the instruments   • The design is gas -tight to preserve the UCZ  pressurized until operation on Mars and to ensure  sealing barrier s to the Rover ’s internal non ultra- cleaned volume, to preventing false  measurements of the instruments  due to sample contamination originated from the Rover itself.  

The paper is structured as follows:  In the first section,  the main design drivers of the PSDDS are   described . This is followed by a detailed description of the  PSDDS design. In three sub-sections , special
55 Motor, which is inside of the housing. T he sealing is guaranteed through a dynamic seal to ensure that  the UCZ maintains its cleanliness level . To make sure that  the dosing works correctly and to avoid  cementation of the sample in the inlet funnel,  a Piezo vibrator, which is placed inside the PSDDS hub,  shakes the complete Dosing Station. To transmit the vibration to the inlet funnels within the UCZ , the  Piezo is coupled to a membrane,  which guides the vibration to the Dosing Station  funnel s. 

Figure 2.  Cross Section of the PSDDS (left) and Dosing Station  (right).  

The Alternative Transport Container (ATC) is a pas sive mechanism to by -pass the Dosing Station in case  both of them have lost their  functionalities  due to cementation effects . It can only deliver the entire sample  at once to the receptacles of the PSHS. Specifically d efined dosing  quantities as required by the O vens  are not possible. The design of the ATC can be seen in Figure 3. The ATC consists of two hatches that  are opened simultaneously by a passive coulisse when the ATC is driven to the PSHS ATC port. The  coulisse is located on the PSDDS housing, The ATC is fix ed on the moving hub of the PSDDS -Positioner.  The position of the ATC was chosen in such a way that an undesired actuation is avoided during nominal  PSDDS operation to eliminate this potential single point failure. The ATC is only  actuated when  commanded to that position and not during other operations. The ATC input is large enough to collect the  entire sample that exits the Crushing Station. The outer hatch is pressed against the inner hatch via  springs when the ATC is closed during sample acquisition and transport. When the ATC is moved to its  interface port with the PSHS, the ball bearing wheel  that is connected to the outer hatch is pushed down  by the coulisse, so that the hatch rotates around its hinge against the spring pressure. They are coupled  via a spur gear pair  to ensure both hatches open simultaneously. To protect the mechanism from dust , a  dust shield is screwed onto the top of the out er hatch.
56   Figure 3.  ATC CAD Modell  (left) and ATC integrated (right).  

The last functionality  of the PSDDS provides  the Cleaning Device, which  is placed on the PSDDS hub. Its  design and position can be seen in Figure 1. The Cleaning Device consists of  a series of four cleaning  blades with spacers in between that are all cut into stripes providing enough flexibility to have the effect of  a brush. The cuts ar e shifted from blade to blade to enable the complete  cleaning of  the entire powdered  sample on the Refillable  Container  of the PSHS  and leaving it visibly clean for the next sample. The  blades are made of Gylon  (a PTFE -based plastic)  which  provides  enough stiffness  for the cleaning  function without being  too stiff  to have powder flicked off the Refillable Container through the ALD.  Gylon  is one of the few plastics that fulfills the requirements for planetary protection and thus avoids   contamination risks .    In the next three subsections  three particularly noteworthy design solutions  are be described.  

Pre-torque Device  Due to the required high accuracy of the mechanism and the limitation with regards to space,  mass  and  ultra-cleaning,  a high-resolution encoder could not be imple mented on the last gear stage, a s it is usually  done if such high accuracy is  needed. Instead,  a low -resolution encoder (8 quad- counts) has been place d  on the rear side of the motor , which provides the advantage of a simpler  electronic  unit and greater  robustness . To achieve the required resolution on the end- shaft,  a gear with a high resolution  (27228:1)   has been installed in between the motor and the end- effector shaft . With this, it is possible to measure a  high resolution at the end of the drive train but the resulting problem is the high play  in the system . The  minimization of this  play has been achieved through preloading the gears. This means that the actuator is  equipped with two spur gears on its output shaft that are counter -preload ed via a leg spring with a  constant torque to minimize the backlash in the following gear stages. The torque is divided  by these spur  gears into two identical gear -boxes and combined again in the  internal ring gear  on the positioner end  shaft. On each gearbox output shaft , a spur gear is mounted that interfaces with this wheel. Due to the  preload,  both wheels are in contact with the opposite flanks of  the internal ring gear  teeth,  leading to a  large reduction of backlash. When torque is introduced by the actuator , the torque at one spur gear  increases in the same way the other reduces  to provide the resulting torque to perform the motion.  However , the torque is never reduced below zero on any one of the spur gears  to maintain the preload to  always ensure a reduced backlash.    The moving hub is connected to the fixed positioner by a preloaded wire race bearing.
57   Figure 4.  Pre-torque Device CAD model (left) and hardware (right).  

Bearing Concept   The only bearing on the positioner to take the axial and radial loads as well as to provide the rotational  DOF is a wire race bearing (see Figure 5). This bearing connects the movable part with the fixed structure  of the PSDDS.  The balls are guided in a PTFE cage and run on four wire races. The bearing is preloaded  to sustain the launch loads and to provide the demanded accuracy . It is dry lubricated via the PTFE cage.  The balls and the wire races are made of steel.  

It is preload ed via the preload frame that is mounted with screws  on the structure . The bearing and the  outer two wire races are integrated into  the bearing housing and the lower inner wire race in the fixed part  of the housing. When the bearing housing is placed on to the fixed part of the housing, the last wire race is  installed and pushed down by the preload frame, which is shimmed to adjust the preload. This force  eliminates the play in the bearing and induces a preload.    The accuracy of the bearing strongly depends on the surrounding structure that  has been optimized  according to the recommendations  provided by  the bear ing supplier  (Franke) .