ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
Figure 5.  RANCOR breakoff tube It is expected that the life of the breakoff tube will be able to outlive the life of the auger. In future designs,  the following items should be considered to improve the design of the mechanism: /g120Minimize the sliding friction between the inside surface of the auger and the outside surface of the  breakoff tube. This could be done by using bronze bushings or even a concentric turning of the middle section of the breakoff tube. /g120A better system of constraining the breakoff tube should be considered. Primarily, this concerns the  replacement of the set screws. From test results, it is evident that the set screws have the potential of backing out if not installed correctly. Because of this, gouging can occur between the auger and  breakoff tube, potentially causing them both to become jammed. The primary reason for using set  screws here was to enable frequent disassembly of the drill bit assembly to monitor component wear and dust migration. The final component that comprises the RANCOR coring drill bit is the JPL sample tube (Figure 6). This  component is manufactured by JPL and is made of stainless steel. From an operational perspective, once  the drill bit is docked with the JPL IMSAH system, the drill head separates from the drill bit and the JPL sample tube is extracted through the back end of the bit using an internal IMSAH mechanism. It is then  cached within IMSAH for sample collection and analysis. Figure 6.  JPL Sample tube RANCOR Chuck Bridging the coring drill bit to the rest of the drill body is the chuck (Figure 7). Studied extensively during  the concept and breadboarding phase of the project, the architecture of the chuck did not change dramatically from its initial concept. One of the driving requirements on the RANCOR drill was the ability  to sustain the loads induced on the drill body and bit should the rover be drilling on a sloped surface and lose traction, or slip. The chuck serves as the primary interface for surviving and handling the load from the theoretical rover slip condition under Mars gravity. Slip conditions of a 180-kg rover on a 20-degree  slope were needed to be survivable with the final chuck design. Under this loading condition the  RANCOR chuck and drill bit would be expected to survive side loads as high as 173 N, as well as be able to safely eject the bit. Both conditions were met. 220
allows for utilizing one actuator instead of two. Three Maxon motor combinations were tested for the drill  unit. This was done to allow a high RPM as well as a high torque option to be evaluated during testing. Outputs from these various combinations were 272 RPM at 3.1 N-m continuous, 188 RPM at 4.3 N-m continuous, and 103 RPM at 6.7 N-m continuous. The high speed option worked well at speeds above 20 RPM. However, lower speeds were required for initializing and aligning the auger and breakoff tube when the system starts. The easiest solution to this was to simply exchange the planetary gearhead on the motor. The next size that fit the existing pinion gear on the motor produced the 103-RPM drill rate. This solution was not ideal from a drilling perspective, but met the system requirements and functioned well. Later the 188-RPM solution was implemented (this required a longer lead time) and also fit the requirements, but improved the performance of the drill. Ultimately, this option was selected as the final version. RANCOR Overrunning Clutch (Sprag) As previously mentioned, the rotation, percussion, and breakoff axes are driven by a single actuator. During a drilling operation, the actuator drives the auger and breakoff shaft simultaneously while the percussion axis rotates at 5 times the rate of the auger / breakoff axes. After drilling, the rock core is  sheared at the base to capture it within the bit. To accomplish this, a clutch was implemented to separate  the percussion cam and auger from the breakoff axis while driven in the opposite drilling direction. This  allows the breakoff axis to rotate while the percussion and auger remain stationary. The drive train path is highlighted in Figure 9 with the blue path representing the breakoff axis and the red path representing the  percussion and auger axes.  Figure 9.  Ratchet used to c ouple / decouple the percussion &   auger rotation from the breakoff axis Initially an over running clutch, which utilizes sprags, was implemented. Though this mechanism  performed as it should have, there was an operational behavior that was overlooked. Since the CAM must compress a spring and release the load over a short duration, there is a highly cyclical load on the CAM shaft. When the load is released there seems to be a combination of inertial effects and physical effects with the interface between the follower and the CAM that causes the CAM, and therefore the  Auger axis to advance ahead of the breakoff axis. This effect caused poor quality cores to be produced,  as shown in the bottom left image of Figure 10. Given the fine resolution of the sprag, these small advancements are captured, which causes relative motion between the auger and breakoff axes during a 222
drilling operation. At 100-RPM auger rotation, it takes about 500 percussion impacts to create a full 360°  relative displacement between the auger and breakoff axes. Tests have shown that lower velocities generate higher displacements per blow and that when drilling into harder materials, this relative motion can be contained. However, this motion is unacceptable from a performance perspective. To solve this problem, a custom ratchet mechanism was designed to replace the sprag clutch.  The ratchet design shown in Figure 10 required more components and was significantly more expensive  than the off the shelf sprag clutch. However, it offers two key features that make it an ideal solution to the  problem of the auger advancing ahead of the breakoff axis. The first feature is the more coarse resolution. In this case, the cam would have to advance 18° ahead of the input shaft upon release to cause a shift in one tooth of the ratchet. This is far more than the average 4° shift that was observed with  the sprag. The second key feature offered by the ratchet is that a more controllable force is required to  cause the ratchet to advance. In the ratchet designed for this application, it takes approximately 0.5 N-m to advance the outer housing for the ratchet (well within the capabilities of the drive motor). This excludes frictional forces which are difficult to characterize and vary with temperature, pressure, vibration, and a number of other factors. In the case of the sprag, there were only rolling contact frictional forces to  overcome to advance the outer race of the sprag. After implementing the ratchet mechanism into the final RANCOR drill assembly, the auger no longer advanced ahead of the breakoff shaft during drilling, resulting in a significant improvement in core quality, as shown in the bottom right image of Figure 10.  Figure 10.  Sprag (left) [7] a nd Ratchet (right) mechanisms and corresponding core qualities RANCOR Percussive Mechanism Following the heritage of previous Honeybee drills, the RANCOR percussion mechanism was built upon  existing successful designs. Mechanically, the percussion depends on employing a rotating cam to lift the follower fixed on the hammer, which then compresses a spring. Once the follower reaches the end of the cam, the spring potential energy is released and the hammer is free to impact the rear end of the drill bit, delivering the 0.6 joules per blow of energy. A unique feature of the Honeybee CAM follower mechanism is the canted follower concept (Figure 11). This concept utilizes a follower mounted on bearings and tilted to match the slope of the CAM. By using  this approach, the interaction between the CAM and follower is primarily rolling contact. However, there is  sliding contact at the point where the CAM releases the follower. This approach also increases the operating efficiency of the mechanism. In the IceBreaker drill for instance, the CAM / follower mechanism operated at about 70% efficient [9, 10]. In alternative approaches where the follower is perpendicular to  the vertical axis, efficiencies were typically in the 30% to 40% range.  223
Figure 11. Canted Follower matches  slope of CAMFigure 12. Wear on SASSI CAM Another advantage to the canted follower design is an increased life of the CAM / follower mechanism.  Two RANCOR units were assembled and tested; a Honeybee and a JPL unit. After over 50,000  percussive cycles with the Honeybee RANCOR unit and over 30,000 percussive cycles with the JPL  RANCOR unit, there is little wear on the mechanism (Figure 13). The only obvious wear location is the end of the ramp on the CAM where the follower is released. The follower also had a lubricious coating (Magnaplate Nedox FM-5) that has been worn off on the contact area which was anticipated. At the end  of the CAM ramp the Magnaplate Nedox SF-2 plating has been worn off entirely. In designs where the  follower is not canted (Figure 12) this plating is removed over essentially the entire contact area of the  CAM after only a few thousand cycles. Figure 13.  Images of wear in CAM and Follower for Honeybee unit after 50,000+ cycles (top 3  images) and the JPL unit after 30,000+ cycles (bottom 3 images) The method for constraining the percussive hammer had a large impact on the overall mass of the drill. Originally a ball spline was considered to constrain the motion of the hammer mechanism (left of Figure 14). However, this required a large amount of volume and added a large amount of mass. For the  percussive cam system to work, a fixed amount of vertical travel of the hammer is required. The volume  224
due to this travel is fixed; however, the height of the linear slide mechanism can be reduced to only  accommodate the required stroke plus some margin. Figure 14. Image of evolution of the ham mer assembly. Initial concept on the left  and final design on the right To reduce the mass of the hammer assembly, an alternative for the ball spline was sought. The first  alternative approach considered, took inspiration from the chuck roller mechanism. This concept utilized 4  rollers to constrain a custom shaft to axial motion only (center of Figure 14). Due to the complexity of the  mechanism and concerns over holding tolerances another alternative mechanism was designed. The final  iteration of the design involved two linear bearings (right of Figure 14), which were later replaced by SAE 841 Bronze journal bearings. This is because the linear bearings did not prove durable enough to handle the shock loads of the percussing hammer. This design ended up being the most compact and had the  greatest impact in mass reduction of all the designs considered for constraining the hammer motion. Figure 15.  Hammer Sub Assembly Another unexpected failure was the spring responsible for generating percussive energy (Figure 15). The original spring selected for the mechanism was a 303 Stainless Steel compression spring. Failure of the spring occurred towards the end of an extended test of 2 hours (typical tests would run for 10 min to 15  min). Testing conditions were earth ambient temperature and pressure. The spring selected had a free  length of 2.54 cm, 2.78-cm outside diameter, a wire diameter of 0.285 cm and 4.1 coils and the designated part number LC112M00S. In the RANCOR application, the spring was constrained by its inside diameter against the fixed, steel backed aluminum housing and the reciprocating follower on top of  the drill’s hammer. The spring carried an initial preload of 13.9 N due to its compression of 0.081 cm. At  maximum compression, caused by the cam lifting the follower, the spring exerted 147.5 N of force under  225
0.864 cm of compression. At its release, this spring was calculated to provide 0.63 J of energy. The cam  operated at 1400 RPM; lifting the follower and subsequently the spring once per revolution, the operation frequency of the spring was 23.33 Hz. Critical frequency of the spring was calculated to be 517 Hz [6]. This is more than 15 to 20 times the operating efficiency which is typically recommended for compressive springs. The Gerber method was used for calculating spring fatigue life [6]. Fatigue analyses concludedthe spring should last above 10 7cycles. However, after about 250,000 cycles, less than 1/40ththe  calculated design life of the spring, failure occurred. Since no material certification was requested at the time of purchase of the springs, it is difficult to trace  the pedigree of the failed spring. Contacting the manufacturer, Lee Spring, post failure investigation revealed the springs were manufactured in either China or Mexico from a 300-series stainless steel, out  of cold drawn stock as according to ASTM A313. The spring also underwent stress relief between 315 -  371/g113C after forming and had not been shot peened.  Potential reasons for failure were classified into operation or manufacture. Initial reasons investigated for  the spring failure were thought to relate to low cycle failure since initial calculations indicated the spring should have a much longer cycle life then what was measured in testing. High cycle failure is typically caused by subsurface failure from inclusions and low cycle failure, such as perceived in this case, is a symptom of surface imperfections, or surface scratches [8]. Since the operational environment conditions did not exceed the spring’s specifications, handling and assembly of the spring were looked at. The possibility of additional surface scratches during assembly was not ruled out. Magnified inspection of the spring under stereo microscope did not indicate any noticeable surface scratches, though a defect as small as 40 μm in length (and hence difficult to see without the use of a Scanning Electron Microscope)  could cause fracture initiation. A close up of the fractured surfaces is shown in Figure 16. The granular  area in the image indicates brittle fracture. The fracture was at 45 degree (Figure 17) and hence followed the maximum principal stress plane. It should be noted that the gold material shown in Figure 16 is contamination from a bronze sleeve that had started to wear during this 2 hour test. Figure 16. Enlarged imaging of the fracture. Actual wire diameter is 0.285 cm. Side one of the  fracture (left). Side two of the fracture (right).  226
Testing Two RANCOR drills were built to support this effort.  These units are referred to as the Honeybee Support unit and the JPL unit (Figure 18). During the testing  phase, two distinct changes to the system were made  that impacted the performance of the drill. The first was changing the motor planetary gearhead assembly to provide an increased auger velocity. This change  more than doubled the rate of penetration (ROP) in  most cases which helped to reduce the overall cycles on the actuator. Interestingly, the number of percussion and auger cycles remained relatively constant. The second change was switching from the  sprag overrunning clutch to a ratchet design. This  change improved the core grade from primarily D and F grades (generally meaning several fragments and reassembly of core stratigraphy is not obvious as shown in the left of Figure 10) to mostly A, B, and C  grades (generally cores that are in-tact or in only a few fragments with a stratigraphy that can be reconstructed as shown in the right of Figure 10). In general, the JPL drill had lower quality cores. This is likely because this system was not positioned well with respect to the linear stage. That is, the drill bit axis and linear deployment stage axis had some noticeable angular misalignment in the mounts to the deployment stage. Therefore the drill bit was penetrating at an angle. Selected telemetry from these lab tests are shown in Table 1.  Figure 18.  Lab Testing at Honeybee  with the Honeybee Support unit (left) and the JPL delivered unit (right).  Figure 17. Post failure spring, side 1 on the  left, side 2 on the right. An approximate 45  degree failure to the surface of the spring  corresponds with the maximum principal  stress plane. 227
Table 1. Selected Drill Telemetry from Controlled Laboratory Tests. (Tests marked with an * were  tests in a brown Gypsum. All other tests were in Indiana Limestone. Test 015 is marked with a **  because the core breakoff was in  the wrong position which resulted  in a very poor core grade.) Test Drill RPMSprag /  RatchetROP  (mm/s)WOB  (N)Energy/co re (Whr)Motor  CyclesPercuss  CyclesCore  Grade 001 HBR 99 Sprag 0.94 39 2.07 168662 3182 D 002 HBR 100 Sprag 0.64 26 2.78 253687 4787 F 003 HBR 100 Sprag 0.56 27 3.19 293248 5533 F 004 HBR 100 Sprag 0.51 27 3.52 331758 6260 C 005 HBR 100 Sprag 0.41 25 4.21 411291 7760 F 006 HBR 100 Sprag 0.49 28 3.91 360050 6793 D 007 HBR 179 Sprag 1.20 37 3.46 146721 5059 F 008 HBR 179 Sprag 1.26 38 3.35 142357 4909 F 009* HBR 179 Sprag 1.38 36 2.82 116164 4006 D 010* HBR 179 Sprag 1.39 36 2.82 124202 4283 D 011* HBR 178 Ratchet 1.24 36 2.37 109073 3761 A 012* HBR 179 Ratchet 1.26 40 3.13 141429 4877 B 013 HBR 179 Ratchet 1.12 40 3.46 155186 5351 C 014 HBR 179 Ratchet 0.89 40 4.02 189509 6535 C 015** HBR 179 Ratchet 0.78 40 4.56 222700 7679 F 016 HBR 179 Ratchet 0.81 41 4.42 208367 7185 A 017 HBR 179 Ratchet 0.90 39 3.85 188187 6489 C 018 JPL 179 Ratchet 1.15 37 3.26 149220 5146 A 019 JPL 179 Ratchet 1.01 39 3.44 167460 5774 D 020 JPL 179 Ratchet 0.88 39 3.78 198538 6846 D 021 JPL 179 Ratchet 1.04 37 3.48 173618 5987 A Lessons Learned A number of useful lessons were learned from the design and testing of the RANCOR drill. As with any  design, there is still room for improvement, but in the end the drill was more than capable of performing  coring tasks in medium to low strength rock targets. Also as requirements for Mars Sample Return (MSR)mature, there may be more mass and volume available to the drill design that can be utilized to increase the reliability and robustness. Lessons learned from the RANCOR drill include the following: 1. In this case, the cost and simplicity of an off-the-shelf sprag clutch versus the design and build of a  custom ratchet and pawl system led to the decision to use the sprag clutch. Although there was nothing functionally wrong with the sprag in this design, it enabled a degree of freedom that should have been locked out during the release of the hammer on the RANCOR. Therefore, the sprag mechanism was replaced with a ratchet and pawl system. The result was a large improvement in core quality (from D through F grades to A and B grades). 2. If a single drill bit is to be used for multiple holes (i.e. 10, 20, 30), care must be taken to design a  proper kinematic constraint for rotation of the inner breakoff tube with the outer auger tube. Also  seals should be used to prevent rock cuttings from migrating between the breakoff and auger tubes.  In the region near the cutting edge of the bit, there should be sufficient clearance between the breakoff tube and auger tube to allow rock cuttings to flow in and out more freely. If there is not sufficient clearance, the cuttings will pack up and seize rotation between the breakoff and auger axes. This can happen after drilling only a few cores if spacing is not sufficient and seals are not in place.  228
3. Using rollers in the drill chuck to constrain the drill bit offers many benefits over traditional spline  joints. These benefits are as follows: a. Rotating joints are much easier to protect from dust than linear spline type joints. b. When docking with a bit, the relatively large rollers on the RANCOR chuck helped account for  some misalignment between the drill chuck axis and the axis of the docked drill bit. c. The rolling interface between the chuck and the bit is a highly efficient joint that enables an  efficient transfer of the percussive energy to the bit. Also, this interface greatly reduces the  force required to remove the drill head from the drill bit if there is a large side load present on  the system. 4. As with all drill bit designs, the grade of carbide makes a big difference in the life of the bit. In  general, at low percussive energies, harder, more brittle carbide can be used to extend the life of  the drill bit by reducing wear. In the case of the RANCOR drill bit design, the harder carbide  performed well in relatively soft rocks (~40-MPa UCS). However, when drilling the ~120-MPa  Saddleback Basalt, these cutters fractured and became less effective. Therefore, for the RANCOR  drill, softer grade carbide is preferred. It is recommended that a number of carbide grades should be tested to determine the optimum combination of hardness and toughness for 0.6 J/blow hammer system in hard rocks. It should be noted that for alternative sample caching architectures, where the bit is used only once and cached together with a core sample, the bit life is less of a concern and hence softer carbide could be selected [11]. Figure 19.  RANCOR drill bit with a harder, more brittle, rotary grade carbide before and after  drilling into Saddleback Basalt. 5. For all spring loaded percussive designs, it’s important to life test the percussive mechanism using  components from a batch process with certification. Spring analyses for the RANCOR concluded that the original stainless steel spring selected for the RANCOR drill should have survived more than 10 7cycles. However, this spring failed on the support unit after only 250,000 cycles. Currently  the music wire spring that replaced the stainless steel spring is at greater than 375,000 cycles.  6. Using a canted or sloped follower for the hammer mechanism (i.e. follower is tilted to match the  slope of the cam) is an ideal solution for the percussive mechanism. This type of solution has now been implemented on its third Honeybee drilling system and has performed efficiently on all systems. When compared to a more traditional follower design where the follower is perpendicular to its axis of travel, the canted follower is about twice as efficient and the wear in the mechanism is significantly reduced.  229