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An arrangement of concentric, independently rotating shafts for a rotating gear train system is disclosed. The arrangement may include an inner shaft, the inner shaft operatively couplet to a rotating element, and an outer shaft, the outer shaft concentric with the inner shaft and arranged radially

An arrangement of concentric, independently rotating shafts for a rotating gear train system is disclosed. The arrangement may include an inner shaft, the inner shaft operatively couplet to a rotating element, and an outer shaft, the outer shaft concentric with the inner shaft and arranged radially outward from the inner shaft. The arrangement may further include a first bearing stack, the first bearing stack arranged radially outward from the inner shaft and including at least one preloaded ball bearing and a second bearing stack, the second bearing stack arranged radially outward from the outer shaft and comprising at least one ball bearing. The arrangement may include a bearing stack retainer, the bearing stack retainer mating with the first and second bearing stacks to hold the first and second bearing stacks in position with respect to the inner and outer shafts and the bearing stack retainer coupled with and rotating with the outer shaft.

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1. An arrangement of concentric, independently rotating shafts for a rotating gear train system, the arrangement comprising: an inner shaft, the inner shaft operatively coupled to a rotating element;an outer shaft, the outer shaft concentric with the inner shaft and arranged radially outward from th

1. An arrangement of concentric, independently rotating shafts for a rotating gear train system, the arrangement comprising: an inner shaft, the inner shaft operatively coupled to a rotating element;an outer shaft, the outer shaft concentric with the inner shaft and arranged radially outward from the inner shaft;a first bearing stack, the first bearing stack arranged radially outward from the inner shaft and comprising at least one preloaded ball bearing;a second bearing stack, the second bearing stack arranged radially outward from the outer shaft and comprising at least one ball bearing; anda bearing stack retainer, mating with the first and second bearing stacks to hold the first and second bearing stacks in position with respect to the inner and outer shafts. 2. The arrangement of claim 1, wherein the bearing stack retainer comprises a retaining ring for holding the stack retainer in position with respect to the outer shaft. 3. The arrangement of claim 1, wherein the bearing stack retainer defines at least one castellation, the castellation designed to hold the stack retainer in position with respect to the outer shaft. 4. The arrangement of claim 3, wherein the outer shaft defines at least one matching castellation, the at least one matching castellation designed to mate with the at least one castellation of the bearing stack retainer. 5. The arrangement of claim 1, wherein the bearing stack retainer comprises an oil retaining dam. 6. The arrangement of claim 1, further comprising a preload disc, the preload disc applying pressure to a race of the preloaded ball bearing of the first bearing stack. 7. The arrangement of claim 1, wherein the bearing stack retainer applies pressure to a race of the ball bearing of the first bearing stack to provide pre-load for the first bearing stack. 8. A method for arranging concentric, independently rotating shafts for a rotating gear train system, the method comprising: providing an inner shaft;operatively coupling the inner shaft to a rotating element;arranging an outer shaft radially outward from the inner shaft, the outer shaft concentric with the inner shaft;arranging a first bearing stack radially outward from the inner shaft, the first bearing stack comprising at least one preloaded ball bearing;arranging a second bearing stack radially outward from the outer shaft, the second bearing stack comprising at least one ball bearing; andmating a bearing stack retainer with the first and second bearing stacks to hold the first and second bearing stacks in position with respect to the inner and outer shafts, the bearing stack retainer coupled with the outer shaft and rotating with the outer shaft. 9. The method of claim 8, wherein the bearing stack retainer defines at least one castellation, the castellation designed to hold the stack retainer in position with respect to the outer shaft. 10. The method of claim 9, wherein the outer shaft defines at least one matching castellation, the at least one matching castellation designed to mate with the at least one castellation of the bearing stack retainer. 11. The method of claim 8, the method further comprising providing an oil retaining dam using the bearing stack retainer. 12. The method of claim 8, further comprising applying pressure for preload of the first bearing stack using a preload disc, the preload disc applying pressure to a race of the preloaded ball bearing of the first bearing stack. 13. The method of claim 8, further comprising applying pressure for preload of the first bearing stack using the bearing stack retainer, the bearing stack retainer applying pressure to a race of the ball bearing of the first bearing stack. 14. A gas turbine engine comprising: a fan section, the fan section including a rotating element;a compressor section downstream of the fan section;a combustor section downstream of the compressor section;a turbine section downstream of the combustor section;an inner shaft, the inner shaft operatively coupled to the rotating element of the fan section;an outer shaft, the outer shaft concentric with the inner shaft and arranged radially outward from the inner shaft;a first bearing stack, the first bearing stack arranged radially outward from the inner shaft and comprising at least one preloaded ball bearing;a second bearing stack, the second bearing stack arranged radially outward from the outer shaft and comprising at least one ball bearing; anda bearing stack retainer, the bearing stack retainer mating with the first and second bearing stacks to hold the first and second bearing stacks in position with respect to the inner and outer shafts. 15. The gas turbine engine of claim 14, wherein the compressor section comprises a high pressure compressor and a low pressure compressor and the turbine section comprises a low pressure turbine and a high pressure turbine. 16. The gas turbine engine of claim 15, wherein the inner shaft interconnects to the low pressure compressor and the low pressure turbine and the outer shaft interconnects to the high pressure compressor and the high pressure turbine. 17. The gas turbine engine of claim 14, wherein the bearing stack retainer defines at least one castellation, the castellation designed to hold the stack retainer in position with respect to the outer shaft. 18. The arrangement of claim 17, wherein the outer shaft defines at least one matching castellation, the at least one matching castellation designed to mate with the at least one castellation of the bearing stack retainer. 19. The gas turbine engine of claim 14, further comprising a preload disc, the preload disc applying pressure to a race of the preloaded ball bearing of the first bearing stack. 20. The gas turbine engine of claim 14, wherein the bearing stack retainer applies pressure to a race of the ball bearing of the first bearing stack to provide pre-load for the first bearing stack.