Patent Application
20080011812
This invention relates generally to the repair of rotating equipment and, more specifically, to the welding of a new shaft end or stub shaft onto the end of a damaged rotor shaft.
When performing a stub shaft repair on rotating equipment, the damaged rotor shaft and new stub shaft (both of which comprise hollow cylindrical members or components) need to be positioned accurately and secured in place before welding. The rotor shaft is prepared by having its damaged end cut off and chamfered at the designed weld prep angle. The new shaft end or stub shaft is similarly prepared, forming a substantially V-shaped weld groove at the weld interface. The root or base of the weld groove geometry where the side walls of the groove come together is an area that typically requires a backing ring to prevent weld burn-through into the rotor bore.
Typically, separate rings are inserted into the shaft or rotor bore in order to establish required fit geometry and to prevent burn-through of the weld material. These rings require separate machining operations to establish the required fit geometry. Each ring is made with a loose fit to insert into the existing bore and into the bore of the roughed-in stub shaft. The separate ring (or rings) provides crack initiation points, however, where the three parts, a shaft, the stub shaft and the ring, come together. Crack initiation sites are typically managed by machining the ring and some portion of the root of the weld out of the finished part, thereby eliminating the potential for future crack growth from this area. Nevertheless, there remains a need for a more effective weld prep configuration that enhances the assembly process, requires less machining and eliminates or at least minimizes crack initiation points.
In an exemplary embodiment, the present invention provides an integral backing ring machined into either the rotor or stub shaft end at the root or base of the weld groove.
More specifically, the incorporation of an integral backing ring at the stub shaft ID results in only two pieces coming together, with a single crack initiation point. The outer diameter of the integral backing ring is machined to an interference fit with the bore of the existing rotor shaft. This provides precise location of the stub shaft relative to the rotor shaft. The integral ring is also provided with a recess or groove circumferentially about its peripheral or outer surface, coinciding with the root or base of the weld groove. As a result, the weld material projects radially inwardly of the rotor bore surface. Complete removal of the root pass can now be accomplished without overboring the rotor shaft beyond its original inner diameter.
Accordingly, in one aspect, the present invention relates to a stub shaft section adapted to be welded to a re-used end portion of a rotor shaft, the stub shaft section comprising a hollow cylindrical member having a substantially uniform inner diameter and first end adapted to form one side of a weld groove, and an integral backing ring extending axially beyond the first chamfered end, the backing ring having a circumferential recess in a radially outer surface thereof.
In another aspect, the invention relates to a stub shaft section adapted to be welded to a re-used end portion of a rotor shaft, the stub shaft section comprising a hollow cylindrical member having a substantially uniform inner diameter and first end adapted to form one side of a weld groove, and an integral backing ring extending axially beyond the first chamfered end, the backing ring having a circumferential recess in a radially outer surface thereof; wherein the backing ring has an inner diameter less than the substantially uniform inner diameter of the hollow cylindrical member; and wherein the recess forms a base of the substantially V-shaped weld groove, the recess projecting radially inwardly of the substantially uniform inner diameter of the hollow cylindrical member.
In still another aspect, the invention relates to a method of replacing a stub shaft on an end of a rotor shaft comprising: (a) providing a new stub shaft with a first end and an integral backing ring;
(b) aligning the first end with a second end on the rotor shaft, with the integral backing ring overlapping the second end, and with the first and second ends forming a weld groove, with the integral backing ring overlapping the second end; (c) filling the groove with weld material; (d) welding the stub shaft to the rotor shaft; and (e) machining the integral backing ring away from the stub shaft.
The invention will now be described in detail in connection with the drawing identified below.
With reference to
A new stub shaft section (or simply stub shaft) 16 is provided with a similarly machined chamfer at 18 directly opposed to the chamfer 14 and also extending 360° about the rotor. At the inner diameter of the stub shaft, an integral backing ring 20 extends axially forward of the chamfered surface 18 so as to underlie and form the root or base of a weld groove (or weld prep) 22 formed by the opposed chamfered surfaces 14, 18. The backing ring 20 thus also partially underlies or overlaps the inner diameter of the rotor 12. In this regard, the outer diameter of the integral backing ring is sized to establish an interference fit with the rotor shaft bore, thus providing for accurate axial alignment of the stub shaft and rotor shaft. The integral backing ring 20 lies radially inwardly of the inner diameter surface 24 of the rotor shaft 16 and similarly, radially inward of the finished inner diameter 26 of the stub shaft 16. A shallow recess or groove 28 is formed in the radially outer surface 30 of the integral backing ring 20, forming the root or base of the weld groove 22. For a submerged arc weld process, the weld groove 22 is filled with a metal filler material 32 that extends from the base of the integral backing ring 20 to, and slightly radially outward of, the radially outer surfaces 34, 36 of the rotor shaft and stub shaft, respectively. Note that the relief 28 has a depth that extends radially inward of the inner diameter surface 24, 26, thus insuring a full penetration weld.
After welding, the stub shaft 16 is machined to final dimensions as indicated by the dotted lines in the figure, including machining the inner diameter surface 22 to the finished inner diameter surface 26. Notice that the integral backing ring 20 is machined off in its entirety, and including weld metal present in the recess 28, leaving a smooth and uniform bore diameter across the stub shaft 16 and the rotor shaft 12, particularly advantageous for subsequent bore inspections. Thus, the welding process can now be accomplished without overboring the rotor shaft beyond its original inner diameter. This also eliminates any steps or shoulders in the bore, stress risers and any other requirements for bottle boring, all of which are undesirable and to be avoided. It will be understood, however, that if no bore inspections are planned or required, the backing ring can remain in place. In this case, the backing ring will create a stress riser in the bore.
By utilizing an integral backing ring 20, there is only a single potential crack initiation point. In addition, extra machining operations required for separate backing rings are also eliminated and because the diameter of the integral ring is machined to an interference fit with the bore of the existing rotor, precise alignment between the re-used rotor shaft and the new stub shaft to be welded is assured.
With reference now to
While the rotor and stub shaft are welded in the exemplary embodiment utilizing a submerged arc welding process, it will be understood that this weld prep arrangement may also be employed with a gas tungsten arc welding (GTAW) process (or any other welding process that produces sufficient mechanical properties), utilizing a narrow groove between the opposed rotor shaft end and stub shaft end.
It is also possible to locate the backing ring on the salvaged rotor shaft end such that the hollow stub shaft fits over the backing ring. This arrangement would not provide all of the advantages that having the backing ring on the stub shaft provides, but it would facilitate alignment.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. For example, while described in terms of the repair of an existing rotor shaft, the weld prep configuration described herein could be used in the manufacture of new rotors as well.
