6 sample to be augured up the hole into the sample ch amber. For robustness purposes, the wall thickness  of the DBA was maximized while still en suring effective sample  collection. There are four recesses in the  bit tube that are used to retain the fr esh bits in their bit box (Figure 4a). 

The rotating bit is supported by a back-to-back duplex bea ring pair within a housing that is connected to  the outer DBA housing by two titanium diaphragms. These bearings, the only ones on the Drill in the sample flow, are protected by a spring energized seal and an integrated shield that diverts the ingested  powdered sample from the moving interface (Figure 4b).   The DBA diaphragms provide radial constraint of the rotating bit and form the sample chambers. The axial compliance of the diaphragms (combined with anvil springs in the Drill Percussion Mechanism)  reacts the weight on bit generated by the Drill Trans lation Mechanism. Between the diaphragms there is a  sample exit tube from which the sample is transfer red to the CHIMRA. To ensure that all sample is  retained no matter the orientation of the Drill with respect to gravity during sampling, the pass-through  from the forward to the aft chamber resides opposite to the exit tube (Figure 4b).    

Figure 4.  (a) EM Drill Bit Assembly (b) Sample paths and interfaces of the DBA 

The DBA interfaces to the rest of the Drill via eight ci rcumferential, conical rece sses in its housing and a  torque coupling affixed to the end of the bit shank. There are through holes at the bottom of each recess so that any Martian dust accumulated on spare bits stored on the rover can be pushed outward when a  fresh bit is acquired. The DBA interface to the Drill has a small prescribed amount of free play. This both allows the DBA to find its natural path into the rock and decouples most of the cyclic radial load due to the  bit-rock interaction from  being transmitted to the rest of the Drill.    Lesson learned: 

Mechanism components that operate in a dirty env ironment require some or all of the following  features: dedicated pathways for the flow of part icles, cavities that can receive debris, and  compliant elements to accommodate the presence of junk. In addition to the DBA, these features were implemented in the Drill Chuck Mechanism a nd the switch assembly of the Drill Contact  Sensor / Stabilizer.   Drill Spindle Mechanism

The Drill Spindle Mechanism (DSM), nested within the Chuck / Spindle Sub-assembly (Figure 5) provides  the torque to rotate the bit for drilling and unlocking the fresh bit assemblies from the bit box. The mechanism is actuated by an electrically-commutated gea rmotor that drives the spindle shaft via a spur  geartrain. The output shaft is support by a bearing pair at the nose (near the torque coupling to the bit)  and a single deep groove bearing at the rear (under t he spindle gear). The maximum mean contact stress  in the bearings is kept low to prevent lube degrada tion. Mounted to the shaft is a dirt-tolerant torque  coupling that transmits torque to the bit. The coupli ng accommodates axial, radial and angular motion  Forward  Sample  Chambe r  Sample Exit Tube Bit (box) retention feature Aft Sample Chambe rChuck interface (1 of 8)   Spindle interface  DBA Bearing Seal and Shield NASA/CP-2010-216272