Since force on the friction plate is inversely proportional to the square of the plate’s distance from the  solenoid, the shim thickness has a large effect on the force balance in the brake assembly. This effect is  larger when the friction plate is closer to the solenoi d, which is why the addition of a second shim in the  brake assembly has a significant effect on the drop- out voltage while having a minor effect on the pull-in  voltage. This sensitivity allows the shim  modification rework to be effective. 

From Ohm’s law, current in the solenoid varies  with the applied voltage through a constant coil  resistance. Using this relationship in equation 11 re sults in the observation that the drop-out voltage  varies linearly with the shim thickness. The shim  thickness in the LPAs can vary from 0.0459 mm to  0.0561 mm because of the part tolerances. The expec ted variation in drop-out voltage from shim  thickness variation is a factor of  0.0561/0.0459 = 1.22. This does not acc ount for much of the variation in  drop-out voltage seen over the range of LPAs, which is a factor of 6 to 10. At the drop-out position, the  unit-to-unit force variation of the 6 springs is 7. 65 N to 10.45 N due to the tolerances of the spring  parameters and the geometry of the br ake assembly. Since drop-out voltage is related to the square root  of force, spring force variation coul d account for a factor of (10.45/7.65)1/2 = 1.17, also not sufficient to  explain the observed variation.  

Equation 6 as written contains t he assumption that the only source  of magnetomotive force in the  solenoid is the brake coil. However the evidence of the residual magnetism in the failed brakes indicates  differently. The flux flowing through the solenoid when the brake coil is energized causes the friction plate  to magnetize. When power is removed, the fricti on plate continues to behave as a weak permanent  magnet, making it an additional source of magnetomotiv e force in the solenoid. This behavior is caused  by the large hysteresis loop in the B-H curve of the 15-5 PH stainless steel. The core material has a B-H  curve with an extremely small hyster esis loop therefore it retains no fi eld. When the additional source of  magnetomotive force from the friction plate is added in  series with the other elements in the magnet  circuit model, equation 6 becomes  (GAP PLATE SHIM CORE PLATE R R R R mmfNi )+++Φ= +  (12) 

Applying the same assumptions as before to equat ion 12 yields a new version of equation 7.   AlR mmfNiPLATE 00 022µΦ=Φ= +  (13) 

Substituting equation 8 into equation 13, and combini ng with equation 10 gives a new expression relating  the magnetic force on the friction plate to the shim thickness.  (2 2 00 8Ni mmf lAFPLATE M + = )µ (14) 

Table 5 shows the magnetomotive force of the fricti on plate in the spare brake, which was calculated  using equation 14, the data in Table 3, and the geometry of the brake.  Table 6 lists the magnetomotive  force of the solenoid coil for the typical range of brake drop-out voltage, values which are of the same  magnitude as the magnetomotive force of the friction plate. The effect  of changes in the magnetomotive  force of the friction plate on the drop-out volt age is derived from equation 14. Keeping all other  parameters unchanged, decreasing  causes an equivalent increase in . The drop-out  voltage increase corresponding to this increase in  is determined from PLATEmmf Ni Ni ()( N NiCOIL) R V= , where   is the coil resistance. A decrease in  of 13.4 causes an increase in drop-out voltage of  1.7 volts since COILRPLATEmmf ()()()()127.0Ni= 3040385Ni=V . The exact range of variation of  over all  the LPAs is not known but varying this parameter clearly can have a significant effect on the LPA dropout voltage. The rest of the magnetic circuit and t he geometric parameters of the brake are well known,  and their variations are insufficient to cause the observed range of drop-out voltage. Therefore, the  conclusion is that the majority of the large unit-to- unit variation in drop-out voltage is due to unit-to-unit  variation in the magnetic properties of the 15-5 PH stainless steel brake plates. This conclusion is  consistent with the experiences of a JPL m agnetics expert when dealing with this material. PLATEmmf 234