ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
144   Figure 10. Test setup for characterization of the hermetic seal activation at off -nominal angular alignment.  

Figure 11. Data for hermetic seal activation at off -nominal alignment.  The tests at higher level of lateral  load and cross -moment were performed at twice the expected misalignment during flight operations. (left  top) measured force along PRS axis, (left middle) measured lateral force at ram tip, orthogonal to PRS  axis, (left bottom) calculated net cross -moment reacted by the PRS, (right) histogram of the number of  tests performed at various cross -moment measurements. Indicated in red is the expected max cross - moment for expected misalignment  during flight and ground test operations.
145   Figure 12. Test setup for reproducing flight -like loading conditions on the PRS in the Ram Mechanism.  Load levels, misalignment levels, and ram tip lubricant were varied across tests. Push plates were single  use components due to the fact that each of the 45 hermetic seals contact the ram only once.  

Figure 13. Data for tests performed on the test setup shown in Figure 12. (left top) measured force along  PRS axis, (left middle) measured lateral force at ram tip, orthogonal to PRS axis, (left bottom) measured  net cross -moment reacted by the PRS, (right) histogram of the number of tests performed at various  cross- moment measurements. Indicated in red is the expected max cross -moment for expected  misalignment. 1.25X life was performed and no degradation in performance was observed, nor any  detrimental wear on the components.
146   Figure 14. PRS Nut. Axial key for torque transmission used in place of commercial key.  

Design of Preload for Tapered Roller Bearings   With the functional load case characterized through the testing described in the previous section , the  bearing preload could be chosen and analysis could be completed. Results show that s ince t he design load  cases are high enough in axial force to counter balance the cross -moment load, these  bearings are not  required to support the cross -moment  through prel oad, allowing the preload to be low, between 44- 88 N  (10-20 lb) . Preload is therefore set to maintain ram t ip alignment throughout the stroke of the mechanism,  which is critical for insertion into the seal c up during seal activation motion, in addition to keeping drag  torque low, below 0.5 Nm at - 70°C.  

COBRA AHS bearing  analysis software, [ 6], was used to ana lyze the contact stress in the tapered roller  bearings , and was  paired with MATLAB to run multiple analysis cases  that vary input parameters such as:  load definition, temperature, roller crown profile,  and ID & OD & axial interference/clearance*. This M ATLAB   code al lowed for thousands of analysis cases to be computed overnight with the COBRA software , that  otherwise strictly requires manual input of analysis parameters.  Analysis results show that , at the limit load,   the contact stress in the upper bearing can be as high as 3310 MPa ( 480 ksi), and a more typical high load  of 2070- 2410 MPa ( 300-350 ksi). This contact stress is high but the overall intended life of the mechanism  is very low . Most of  the total revs of the mechanism are unloaded, retracting the ram from stow to allow the  Sample Tube and Hermetic Seal into the Sealing Station. The high force motion is only 7%, 175 revs,  of  the total life of  2500 revs of the output bearings. Life testing of this mechanism to a level 200% greater than  expected lif e was recently completed at the time of this publication, with no sign of degradation in  performance in the mechanism telemetry.  

Lesson Learned on Load Dependent  Drag   A parasitic drag source, which was unintentionally omitted from the early detailed design phase efficiency  estimate for the Ram Mechanism,  was later found to be a significant source of drag. This drag source is  the load dependent  drag within the rolling element components, specifically the Tapered Roller Bearings  and the PRS†. Analysis of dyno test data showed that the peak output force measured was significantly  lower than predicted. At a force level of 16,400 N, a drag source of about 1 Nm was calculated to be the  discrepant drag magnitude, as observed at the mechanism pinion gear. An estimate for the omitted drag  source was computed by,   𝜏𝜏𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 =𝐹𝐹𝑑𝑑𝑎𝑎𝑎𝑎 𝑏𝑏𝑏𝑏𝑑𝑑𝑑𝑑𝑏𝑏𝑏𝑏𝑑𝑑 𝑑𝑑𝑟𝑟𝑎𝑎𝑎𝑎𝑏𝑏𝑑𝑑𝑟𝑟⋅𝑟𝑟𝑏𝑏𝑏𝑏𝑑𝑑𝑑𝑑𝑏𝑏𝑏𝑏𝑑𝑑 𝑝𝑝𝑏𝑏𝑝𝑝𝑝𝑝ℎ 𝑑𝑑𝑏𝑏𝑑𝑑𝑑𝑑𝑏𝑏𝑝𝑝𝑏𝑏𝑑𝑑⋅𝜇𝜇 

* The author would like to note the helpfulness of the COBRA software creators with aiding our efforts to  extend the functionality of their software in this way.   † The cylinder roller bearing and the cam followers was found to contribute less than 1% of the total omitted  drag and is thus excluded from the discussion.
147 Where the friction factor, 𝜇𝜇, was varied between a typical range of rolling element bearings o f 0.0025 to  0.005. For friction values of 0.0021 to 0.0035,  the increased drag corrects the mechanism efficiency   prediction to match test data for the 3 flight units tested. The TRB  was the significant contributor to this drag  source as the radius of the bearing is much larger than other components  in the mechanism. The TRB  has  an OD of 68 mm and contributed ~87% of the load dependent  drag, while the PRS  has a pitch diameter of  15 mm and contributed ~13%.  

Assembly  

Setting Preload in Tapered Roller Bearings   Preload is set in the Tapered Roller Bearing pair by a two-step procedure , where a drag torque to preload  relationship is established, and then the preload is set and locked based solely by the drag torque  measurement. A load cell is used during t he measurements in the first step , see Figure 15, that is not  included in the final design and is thus removed for the second step. Due to the placement of the gear  between the two opposing bearings, preload was not controlled by precise grinding of the ring widths.  Preload is controlled by the installation of the two-piece  retainer installed after the preload- to-drag torque  relationship has been establish ed, and from this relationship,  a target drag torque is established. The  retainer is adjusted to achieve the desired drag , corresponding to the desired preload range,  and locked  into place. The installation torque of this threaded retainer is very low and prone to backing out if not retained  in some additional way. To lock this retainer in place,  the preload between the retainer and housing threads  is increased by installing a second retainer close to, but not touching the first retainer ; a gap size of 0.25 to  0.5 mm (10 to 20 thousandths of an inch) is used. The two retainers are then pulled toward one another by  installing small screws at a standard preload and with a standard secondary retention method (Arathane  5753 for this mission subsystem). Installat ion of the small screws does affect the final preload in the  bearings and must be accounted for  and corrected in the final installation  step. The fully installed retainer  is capable of resisting at least 56 Nm (500 in- lb) whereas it was installed by hand w ith less than 1 Nm.  

Between the characterization measurements and the retainer installation the drag torque measurements  have a different characteristic shape. The characterization measurements are much smoother than with  the flight retainer. This inconsistency  is due to the change in bearing clamp. The permanent  retainer is  threaded and is thus oriented by those threads , which  imposes an alignment on the bearing race such  that  both may not be perfectly square to the rotational axis. Additionally,  it is much more rigid than the  characterization test setup  where a ball bearing is used to intentionally eliminate this kinematic cons traint,   see Figure 15.  

Figure 15. (left) Test configuration for characterizing preload to drag relationship. (right) retainer and lock.
148 Testing  

Dyno Testing Equipment   Dyno testing of the Ram Mechanism involved custom built equipment to apply up to 28 kN of resistive  force  across the full stroke of the mechanism, while also capable of  being reduced to zero force,  and physically  mount ed to an access port on the side of a thermal chamber. The linear d yno equipment was placed outside  of the thermal chamber both for access of the operator to vary the resistive load and to place the load and  position sensors in a room temperat ure environment. See the hardware pictured in Figure 16. A compound  lever system is used to amplify a force applied by the user via a threaded rod and nut. This force squeezes  on a rectangular cross -section shaft applying a friction force, resisting motion of the coupled mechanism.  The mechanism’s intended high force output applies a compressive load to the dyno equipment requiring  good support of the shaft to ensure good motion and prevent ing binding or buckling of the shaft. Two  bushings are used to support the l inear brake shaft as well as the load cell shaft while couplers eliminate  over constraints  on each shaft.  

Figure 16. Linear brake equipment used for thermal dyno testing of the Ram Mec hanism.  

Lesson Learned with Linear Dyno  One unforeseen issue with this hardware was that the compliance in the brake pad material used at the  interface of the rectangular cross -section shaft and the compound levers. This COTS high friction brake  and clutc h lining material while appearing somewhat metallic and feeling metallic to the touch is in fact  significantly less stiff compared to steel or aluminum. A compression of 0.76 mm ( 0.030 in) at the brake  pad for a resistive sliding force of nearly 20 kN is amplified at the user input to about 7.6 cm ( 3 in). This  meant that to sweep the resistive load from zero to maximum the nuts must be moved up to 10 cm ( 4 in)  along the threaded rod on the l eft and right side. A test can take be as short  as about 15 seconds of  mechanism motion time, requiring the operator to turn the nuts very quickly,  which proved very difficult and  introduced high irregularity of test conditions. The hardware was re- worked to thin the brake pad to reduce  total deflection,  giving the effect of increas ing the operator ability to apply a changing load throughout the
149 brief tests. But even with this fix the rate of successful tests was less than 100% and required several retests  to achieve a good sweep of resistive force from low to high or stall.  

What c ould be better than  the Linear Dyno?   Given the less than satisfying experience with this dyno equipment we suggest a simpler approach: a spring  box. The spring box would consist  of a spring mounted shaft supported by bushings that is pushed by the  mechanism. Bushings prevent the shaft from binding under the high compressive load. The springs, a stack  of Belleville  washers, and should be sized to provide some stroke, ~4- 8 mm would be enough for this  application and reach full displacement at a force above the predicted peak force of the mechanism. A load  cell would be placed between the mechanism and the spring box shaft to measure for ces. The short stroke  of the spring box , an obvious difference to the linear brake,  can be mounted in various locations within the  mechanism stroke to characterize points of interest. For the Ram Mechanism the stroke of inter est is the  end of travel where ferrule -to-ram interaction happens. The rest of travel is unloaded, thus, not necessitating  high force dyno testing in these locations. The spring box would be a much simpler and quicker to design  and fabrication alternative to the linear dyno as well as s impler to operate. Such a spring box was used in  early development testing successfully;  however,  it was not selected because it lacks bi -directionality and  the ability to apply load across the whole stroke of the mechanism. Requirements that lead to the much  more complex linear dyno. The lesson learned here is that because this application did not require high  force dyno testing across the full stroke length , the spring box alternative should have been selected.  

Figure 17 Fligh t Sealing Station
151 Design and Development of a R obust Chuck Mechanism for the Mars  2020 Coring Drill  

Anthony Barletta* 

Abstract  

The Sampling and Caching Subsystem (SCS) onboard the Mars 2020  rover  includes a coring drill that can  perform coring, abrasion, and regolith collection operations. The coring drill requires a chuck mechanism  to allow for the changing of the bits needed to perform sampling.  The chuck mechanism must be capable  of securely connecting a bit under drilling, rover slip, and pullout loads , but also readily release a bit after  application of these loads . The Mars 2020 chuck consists of a ball lock mechanism that allows for robust  engagement and release of bit assemblies that are preloaded to ensure a secure, stiff connection between  the bit assembly and the rest of the corer . To prevent jamming, the chuck incorporates rollers to preload  the ball lock. The rollers are mounted onto a flexured cam that minimizes changes in i nternal mechanism  loads that result from thermally induced dimensional changes , further mitigating the potential for jamming.   The chuck mechanism is moved between engaged, disengaged, and loose chuck positions by rotating the  cam with a gearmotor that incorporates a magnetic detent brake that prevents inadvertent mechanism  motion under design loads.  Ongoing testing is occurring on the qualification model in a dirty environment  while coring and abrading rocks  in a M ars representative environment.  

Introduction  and Previ ous Flight Heritage  

Mars 2020 is a Mars surface mission planned for launch in the summer of 2020, with a planned landing  date of February 2021 in Jezero Crater. A major objective of the mission is to collect core and regolith  samples  for potential return to Earth by future missions as part of a Mars sample return campaign.  The  Mars 2020  rover will carry a coring drill, also known simply as the corer, to collect these samples. More  details concerning the corer can be found in [ 1] and [ 2]. The chuck mechanism acts as the interface with  the various bit assemblies needed to perform coring, abrasion, and regolith sample collection operations.   The location of the chuck mechanism within the overall Mars 2020 coring drill as well as views of its internal  components can be seen in Figure 2. The Mars 2020  chuck can be described as a preloaded ball lock  mechanism that interfaces with sleeves on the bit assemblies and is able to accept and engage these bit  assemblies regardless of their relative clocking about the central axis of the chuck and corer.  Design and  development  were carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a  contract with the National Aeronautics and Space Administration (80NM0018D0 004). 

Figure 1: Mars 2020 Coring Drill Chuc k with coring bit installed.  

* Jet Propulsion Laboratory, Pasadena, CA; anthony.j.barletta@jpl.nasa.gov  

Proceedings of the 45th Aerospace Mechanisms Symposium, NASA Johnson Space Center, 2020
152   Figure 2: Location of chuck mechanism within overall Mars 2020 coring drill as well as cross sectional  view of major components.  

Ball lock mechanisms have been used previously in spaceflight applications, most notably in the Mars  Science Laboratory (MSL) chuck mechanism (Figure 3). The c huck on the MSL drill also utilized balls to  grab onto a dr ill bit assembly. A one-piece  cam, which was rotated by the chuck actuator, was used to  move these balls radially outwards into pockets on the inside of the drill bit assembly. This design provided  a connection between the drill and the bit assembly that w as sufficiently stiff enough to allow for the  collection of the crushed regolith samples created during drilling. However, when the MSL chuck is in the  engaged state, there is still a small amount of radial play (0.3 mm) at the balls. This amount of play w as  allowed for in the design in order to make the design tolerant to dust and to avoid the possibility of the balls  jamming inside the pockets in the bit  assembly. The radial play at the chuck balls in MSL was acceptable  since the drill was capable of coll ecting acceptable cutting samples even if the bit had a small amount of  play. However, for Mars 2020, it was found after prototype testing with a brass board unit that a bit with  significant play resulted in core samples of unacceptable quality. Therefore,  a stiffer connection between  bit and chuck was needed.  

The ball lock on the Mars 2020  chuck mechanism is unique in that the balls are preloaded in order to  provide a very stiff connection between the bit and corer assembly. Other flight ball lock designs , including  ones used elsewhere on the Mars 2020 Sampling and Caching Subsystem (SCS), avoid  preloading balls  due to fears of potentially jamming the mechanism.  On the Mars 2020 chuck this jamming potential is  eliminated by using rollers mounted onto a flexured cam to provide a stiff but tolerant and robust interface  with the bit assembly.
153   Figure 3: Overview of chuck ball lock design used on the Mars Science Laboratory (MSL). The MSL  chuck allowed for a small amount of play of the bit assembly even when fully chucked. This play resulted  from ~0.3  mm of radial ball travel that was permitted when the bit was engaged. This play was  intentionally included so as to allow for dust accumulation and to accommodate part dimensional and  positional deviations, and was permissible for MSL since a stiff bit -to-drill connection was not a  requirement.  

Design Requirements  

The Mars 2020  corer chuck mechanism had several requirements that led to a design unique from those  used on previous missions and applications. The chuck needs to allow for the exchange of bit assemblies  for different sampling activities and to increase overall sampling possible during the mission. It also needs  to be able to retain a bit under various high load cases, including rover slip and bit pullout. To ensure high  quality samples, especially during coring, the chuck needs to provide a stiff restraint of bit assemblies to  the corer to minimize excessive play of the bit during sampling. At the same time, some degree of  compliance is needed to accommodate bit exchange operations. While providing a secure, stiff connection  between the bit and coring drill, the chuck also needs to be resistant to jamming, which could prevent  removal of the bit and severely hamper mission operations. Another requirement was the ability to fully  release a bit without the application of an external load from another part or mechanism. This requirement  is needed since the corer will launch with a bit already installed in the chuck. After landing, the chuck needs  to be able to disengage, release, and discard this bit onto the surface. The chuck had to satisfy all these  requirements over a range of temperatures and be tolerant to deviations in part and assembly dimensions.
154 Mechanism  Architecture  

For the Mars 2020 chuck design, it was decided to still use a variation of a ball lock. Ball locks such as that  used in MSL have proven flight heritage, and such a design is compact, s imple, strong, and places few  restrictions on bit orientation during the mating and demating operations.  

Each bit assembly consists of a bit contained within a sleeve (see Figure 4). The chuck mechanism holds  onto the bit vi a this sleeve. B alls in the chuck assembly are preloaded onto a concave surface on the bit  sleeve. Due to the contact angle at the chuck balls, this preloading causes the sleeve to be pulled further  into the chuck until contact occurs between the front of the chuck housing and a spherical surface at the  front of the bit sleeve. Preload exists both between the chuck balls and sleeve and between the chuck  housing and sleeve, stiffly restraining the sleeve and therefore bit within the chuck . The preload torque to  apply onto the cam is chosen based off anticipated side loads during sampling operations, these anticipated  loads being informed by measured loads from prototype testing.  

Figure 4: Fully assembled flight chuck mechanism (left) with major components indicated and a model  illustrating the interface between the chuck mechanism and a bit. The chuck is a ball lock mechanism that  holds onto a bit via preload ing of a sleeve on the bit assembly against balls and the front of the chuck  housing.  

The chuck is unique on Mars 2020 in that it is the only ball lock mechanism on the subsystem where the  balls are purposely preloaded. Elsewhere on the Mars 2020 SCS, ball locks are designed so as not to load  the balls in order to avoid jamming.  Rather than not loading the balls, which would result in an unacceptably  loose bit -to-chuck connection, the corer chuck mitigates jamming potential by using rollers and flexures .  Preloading of the chuck balls and bit sleeve is accomplished via rollers mounted on a rotating cam. When  the cam is rotated in the proper direction, the rollers will make contact wi th the balls, pushing the balls down  towards the chuck centerline and onto the bit sleeve. To release a bit, the cam is rotated in the opposite  direction, separating the rollers from the balls and removing preload from the sleeve. After sufficient cam  rotation, the balls are free to move radially out of their holes in the housing enough to allow the bit sleeve  to be removed from the chuck. Cam internal geometry is such that in the chuck’s disengaged position the  balls are free to move out of their holes eno ugh to allow for bit removal but are still sufficiently captured to  prevent them from falling completely out of their respective holes.  

Rotation of the chuck cam is accomplished via an actuator acting through a segmented gear mesh  (Figure  5 and Figure 6). The chuck actuator incorporates a magnetic detent brake acting through a staged planetary
155 gear train. The magnetic detent brake resists rotation of the actuator output under an externally applied  torque, thereby resisting cam rotation under externally applied loads on the chuck and bit.  More details on  the chuck actuator design and implementation can be found in [ 3]. 

           Figure 5: (Left) CAD view of chuck actuator interfacing with mechanism via segmented gear. (Right) View  of chuck mechanism engaged with bit sleeve. Note the exposed gear mesh between the chuck actuator  output pinion and segmented gear. Segmented gear is fastened to the cam.  

Figure 6: Detail view of mesh between chuck actuator output pinion and segmented gear. Note the  hardstop surfaces on either side of the segmented gear which serve to limit angular motion of the cam  relative to the rest of the corer. A housing cover part (not shown) is installed over the volume occupied by  this gear mesh.  

Cam Rollers   To allow for preloading of the chuck balls while avoiding the possi bility of jamming, the balls are pushed  down onto the bit sleeve using rollers mounted on the chuck cam. The use of cam rollers allows for a rolling  contact that effectively reduces the friction that develops at the cam and ball interface. The use of rolli ng  contacts is common in mechanisms to avoid the potential for jamming or binding which might otherwise  occur for sliding contacts.