## 2 Materials and Methods  
COTS synthetic sapphire axial bearings have been subjected to dynamic tests. In detail, the tested bearings are the ES-40 models provided by Swiss Jewel. An endstone, which is a sapphire disc, is mounted on a housing that terminates with an adjusting screw,  
Figure 1: Sketch of the NGLR experiment cardan suspension layout.  
as evidenced in Figure 1. The sapphire has a diameter of 0.187\({}^{\prime\prime}\) and a height of 0.035\({}^{\prime\prime}\). The housing is made of 300 Series stainless steel. The endstone of the bearings is shown in Figure 2, while a picture of the bearings is reported in Figure 3.  
The synthetic sapphire is a single Al\({}_{2}\)O\({}_{3}\) crystal and, therefore, it exhibits alumina's chemical properties combined with mechanical properties typical of monocrystalline ceramic materials. It shows an excellent corrosion resistance and can be produced with accurate surface finishing. A preliminary hardness measurement revealed a Vickers microhardness of 2060 HV. A roughness measurement has also been conducted and the observed profile is reported in Figure 4. The measured surface roughness Ra is 0.061 +/ \(-\) 0.015 \(\upmu\)m.  
Figure 4: Surface roughness profile.  
Figure 3: ES-40 Bearings by Swiss Jewel.  
Figure 2: Endstone assembly.  
In order to allow the fastening of the test item to the shaker armature, and to properly reproduce the loads to which the bearings are subjected, a test hardware was designed. This fixture was composed of:  
* an AA6081 slab used to fix the system to the shaker;
* an AISI 316 mass that reproduces the actual mass that will be aligned by means of the magnetic suspension;
* two sets of spindles, one in AA 6081 and one in AISI 316, in order to test the sapphire's response to the direct contact with two different materials having different Young's modulus;
* a frame, two flanges and a spacer realized in ASA by additive manufacturing.  
The bearings have been placed in pierced thread M8 inserts. The exploded view of the system configuration for the tests conducted on \(z\) axis is evidenced in Figure 5.  
The ES-40 JB has been investigated to determine its response to dynamic loads typical of qualification tests for space missions.  
The vibration environment for space systems during the launch phase can be quite severe. At the end of the Critical Design Review, the integration Authority of the satellite must define the test loads that the satellite, its subsystems and payloads must survive in order for the spacecraft to be qualified for flight. For a payload integrated in a spacecraft, the loads are determined by the coupling analysis between the rocket and the spacecraft [14].  
Depending on the mass, dimensions, shape and location of the item to be tested, the vibration environment can be represented by a subset of different dynamic loads that includes random vibration, sine vibration and transient or shock testing [15]. If the dimensions and the mass of the subsystem are such that the inertial loads are not negligible, the quasi static load test must be included in the list.  
It is quite common that for a payload of small dimensions and mass, the envelope of the random vibration loads does include all of the vibration environment.  
The vibration test system used for this purpose is a standard in the field, manufactured by Sentek Dynamics L1024M-PA115 Vibration Test System. The bearings have been tested with the loads derived by INFN INRRI experiment [16] represented by a random vibration whose test profile is shown in Figure 6. Each random test was preceded and followed by a low level sine swept test for resonance search purpose.  
Figure 5: Layout and dimensions of the HW in the \(z\) axis configuration test.