420     Figure 5.  Theoretic al torque on the gear shaft and (symbols for equation described in the text)  

Mass distribution and energy optimization   The proper mass  ratio of the Outer C asing, Hammer and Suppressor Mass  plays a n important role in the  system’s efficiency and performance. It is particularly important to have the highest possible mass of the  Suppress or Mass assembly and the lowest mass of the Hammer Assembly. This will result i n high energetic  stroke of the H ammer and low kinetic energy of the SM assembly to be compensated by the Brake S pring   and the Wire Helix . At the same time,  the mass of the Hammer Assembly should be balanced with the mass  of the Outer C asing to achieve  the best possible energy transfer between those assemblies. High density   materials were used to accomplish this in the limited volume of the mechanism; in particular tungsten alloys  were used for the Hammer ( Wolfmet HE3925 alloy) and the Motor Housing ( Wolfmet HE395). HE3925  remains more ductile,  which was  particularly important for the H ammer  – the part that experiences  the  highest loads ( over 10000g). Incidents of cracking were encountered in the scope of the project  (described  in the lessons learnt section) , nevertheless the Hammering Mechanism together with the selected materials  proved its durability in thousands of strokes performed (qualification level was 45000 strokes) . The achieved  mass ratios also allowed for the use of more energetic drive springs (c.a. 0.7  J), which is an important factor  to sustain effectiveness  or even allow for penetration of the mole in the regolith.  In the final design,  the  achieved masses of the Hammer and Suppressor Mass assemblies are 0.11 kg and 0.46 kg  correspondingly, so the actual Hammer kinetic energy is about 0.57 J and for the Suppressor Mass is   0.14 J. 

Lubrication plan   All of the sliding bushings were replaced either with ball bearings (for all rolling movements ) or advanced   tribological surfaces  that were provided by Warsaw University of Technology and Lodz University of  Technology . This led to the vast simplification of the mechanism’s design and also increase in its efficiency .  Different coatings and surface treatments were used wherever applicable: lead  (PVD) , MoS 2 (PVD) , TiN.  The general rules were to: 1) use MoS 2 on all sliding surfaces (it was also used on tungsten alloy for the  first time), 2) lead on rolling or stroke surfaces and also springs, 3) and TiN on all titanium surfaces. All  surfaces were hardened (nitride d) before the deposition of coatings , with the exception of Tungsten alloys ,  which reveal relatively larger hardness on their own (24- 26 HRC). The actual main materials and tribological  coatings are shown in  Figure 6.     The applied surface modification processes increase d wear resistance of the used steel, titanium and  tungsten parts of the HP3 device. Appl ying MoS 2 magnetron sputtered coating significantly reduced wear   rates and friction coefficient of these materials . In case of titanium alloy  (Ti6Al4V) , additional plasma  nitriding process was necessary  in order  to improve the lifetime of MoS 2 coating.  Additional tests using „ball  on disc” tribometer with Al 2O3 ball were performed to prove the rightness of MoS 2 application on Tungsten  (HE 395) and titanium alloy (tests in air). As  it can be seen in Figure 7 the coefficient of friction for tungsten  alloy has significantly reduced and stabilized after MoS 2 deposition (from 0.5- 0.7 to below 0.1). For the case  of titanium alloy, the tests revealed t hat the use of nitride titanium surface as a base surface for MoS 2  deposition can increase the wear resistance of the coating over two times (the test ended after 1100  m).