Another failure mode is that if the roller completely fails and the metal follower come s in contact with the  pre-valve visor, an ignition source would be created in the highly volatile LO 2 environment.   After the second failure was discovered, two likely causes were identified:  roller material LO 2 ompatibility testing marks, and detent groove leading edge sharpness. Before a soft good or nonwith a dimple has reduced fracture toughness. However,  uring this testing, the loads required to crack a roller were less than the estimated loads imparted by the  r ould be taken. It was determined that the radius of the lip of the leading edge of the detent groove was   of the leading  dge was slightly higher than the other. This resulted in an unparallelism between the roller and visor c metallic material is used in Space Shuttle LO 2 systems, each batch of the material must be compatibility  tested. The Vespel SP21 used for the roller requires this testing. To ensure that rollers that aren’t batch tested are not installed in the vehicle, a small dimple is machined into the end of the roller to indicate that  the material successfully completed the required te sting. The depth and diameter of the dimple are  drawing controlled. The mark is to be no more than 0.76-mm (0.030-inch) deep and 1.57 mm (0.062 inch) in diameter. It was noted that the crack observed on the second and third cracked rollers initiated at the  dimple and propagated through the thickness of the roller. The dimple was identified as a stress riser and  a contributor to the cracked rollers. However, it was also noted that the first cracked roller did not have a  dimple, so there is likely an additional cause.    Testing was performed to verify that a roller  d Belleville spring stack in a nominal assembly. This information prompted further investigation after the third roller was found cracked. The original intent of the testing was only to prove that the dimple did  affect the fracture toughness of the roller. The testing was performed at ambi ent conditions. The load  required to crack a dimple-less roller was twice that of the load required to crack a roller with a dimple.    Molds of the visor detent groove were made so that measurements of the key dimensions of the viso csharper for this particular valve visor than drawings for the part allowed. The edge radius underwent a  grinding and polishing process with the valve installed on the vehicle. Prior to machining, the radius was  found to vary between 0.13 to 0.51 mm (0.005 to 0.020 inch) at various points along the length of the detent groove. The drawing requirement for this radius was 0.38 to 0.76 mm (0.015 to 0.030 inch). After  the procedure, the sharpest dimension found along the length of the detent leading edge was 0.48 mm  (0.019 inch). The polishing process was performed between the second and third rollers.    It was also noted from the mold impressions that the one end (labeled as “5” in Figure 9) e interface. This unparallelism resulted in one side of the leading edge being approximately 0.64 mm (0.025  inch) higher than the other. This meant that one end, the horseshoe shaped end, compressed the detent roller more than the open end. All three cracks initiated on the end corresponding to the higher  compression.  Figure 9:  Pre-valve Visor Detent Groove  103