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An air cycle machine includes a shaft formed from end sections attached to a center section. The center section includes a first diameter that is supported on bearings within the air cycle machine and a second diameter that forms a part of an interface with a corresponding end section. The second di

An air cycle machine includes a shaft formed from end sections attached to a center section. The center section includes a first diameter that is supported on bearings within the air cycle machine and a second diameter that forms a part of an interface with a corresponding end section. The second diameter is less than the first diameter and extends an axial distance from an end of the center section and provides for the weld beam to be directed normal to the interface location such that weld quality and manufacturing efficiencies are improved.

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1. An air bearing shaft comprising: a center shaft section including an axially facing surface; andan end cap including an axially facing end abutted against and welded to the axially facing surface of the center shaft section, wherein the end cap includes an outer diameter and the center shaft sect

1. An air bearing shaft comprising: a center shaft section including an axially facing surface; andan end cap including an axially facing end abutted against and welded to the axially facing surface of the center shaft section, wherein the end cap includes an outer diameter and the center shaft section including a first diameter and a second diameter substantially the same as the outer diameter, the second diameter is disposed at an interface between the axially facing surface of the center shaft section and the axially facing end of the end cap and the first diameter is greater than the second diameter and is spaced axially apart from the second diameter. 2. The air bearing shaft as recited in claim 1, including a transition region between the first diameter and the second diameter. 3. The air bearing shaft as recited in claim 2, wherein the transition region comprises a radius beginning at a point axially spaced away from the interface. 4. The air bearing shaft as recited in claim 1, wherein the end cap comprises first and second end caps attached to opposing ends of the center shaft. 5. The air bearing shaft as recited in claim 1, including a thrust flange attached to an intermediate point between ends of the center shaft section. 6. The air bearing shaft as recited in claim 1, wherein welding of the end cap and the center shaft comprises an electron beam weld. 7. An air cycle machine (ACM) comprising: a shaft formed from a center shaft section and first and second end cap sections attached to each end of the center shaft section, an interface between the center shaft section and each of the first and second end cap sections includes a second diameter at an interface between axially facing surface and the axially facing end and a first diameter greater than the second diameter that is spaced axially apart from the second diameter;a first turbine mounted on said shaft;a second turbine mounted on said shaft; anda compressor mounted on said shaft between said first turbine and said second turbine. 8. The ACM as recited in claim 7, wherein said center shaft section is electron-beam welded to each of the first and second end cap sections. 9. The ACM as recited in claim 7, wherein the center shaft includes a hollow internal cavity. 10. The ACM as recited in claim 9, wherein each of said first and second end cap sections include journal bearing surfaces that support radial loads on said shaft. 11. The ACM as recited in claim 10, wherein said shaft includes a radially thrust flange between the first and second end caps that supports axial loads on said shaft. 12. A method of manufacturing an air bearing shaft for an air cycle machine, the method comprising the steps of: forming a center shaft section including a first outer diameter and a second outer diameter at each end, wherein the second outer diameter is smaller than the first outer diameter;holding an axial surface of a first end cap against a first axial surface at an end of the center shaft section;holding an axial surface of a second end cap against a second axial surface an another end of the center shaft section, wherein each of the first and second end caps include an outer diameter that is substantially the same as the second outer diameter of the center shaft;forming a weld between abutted axial surfaces of the first end cap and the center shaft section;forming a weld between abutted axial surfaces of the second end cap and the center shaft section; andremoving the shaft from a fixture after completion of welding operations on the first end cap and the second end cap. 13. The method as recited in claim 12, including the step of welding a thrust flange to the first outer diameter of the center shaft section. 14. The method as recited in claim 12, including the step of forming the center shaft section to include an inner cavity. 15. The method as recited in claim 12, wherein the second outer diameter on each end of the center shaft section begins at the corresponding axial surface and extends axially a distance away from the center shaft section end. 16. The method as recited in claim 12, including forming a transition surface between the first diameter and the second diameter, wherein the transition surface comprises a radius. 17. The method as recited in claim 12, wherein an electron beam is utilized to form the weld on between the first and second end caps and the center shaft section. 18. A method of installing an air bearing shaft into an air cycle machine (ACM), the method comprising the steps of: removing at least a portion of a housing of the ACM such that a first air bearing shaft is removable from the ACM;removing the first air bearing shaft from the ACM;installing a second air bearing shaft into the space of the ACM in which the first air bearing shaft was removed, the second air bearing shaft including end caps attached to a center section where the center section includes a first diameter that is larger than a second diameter, with the second diameter disposed at ends of the center section to which the end caps are attached; andreinstalling that portion of the housing of the ACM removed to provide for removal of the first air bearing shaft. 19. The method as recited in claim 18, including the step of aligning the end caps to the center section such that an outer diameter of each end cap is aligned with the second diameter of the center section. 20. The method as recited in claim 19, including the step of welding the end caps to the ends of the center section with a weld beam directed at angle substantially normal to an axis of rotation of the shaft. 21. The method as recited in claim 19, wherein the second diameter and the outer diameter of a corresponding one of the end caps comprise diameters that are different by no more than +/−0.010″ of an inch. 22. The method as recited in claim 18, where the second diameter of the center shaft extends an axial length from an end of the center shaft to the first diameter.