By Chuck Yung
EASA Technical Support Specialist
Most EASAns are familiar with the basics of rotor rebarring. We know that the endrings and bars are not always the same material, and understand the importance of maintaining the cage resistance. For those of us who only occasionally rebar rotors, here are a few tips to assure a quality repair.
After the new bars have been inserted, machine the ends in a lathe so that all bars are of equal length and in the same plane. If bars of uneven lengths are brazed, the longer bars will be subject to compressive stress. At the same time, stresses on shorter bars are trying to stretch them. The probable result: Premature failure at a brazed joint. Clue: The resulting break looks as if the bar was physically pried away from the endring.
If the bars are loose, they should be swaged1 to tighten the fit in the slot (before machining to length, so they don’t push away from the tool). Loose bars will cause increased vibration levels — especially in 2-pole machines. Bar “chatter” during starting is more than a noisy nuisance — the movement accelerates metal fatigue, leading to broken bars.
Once the bars are tight in their slots and trimmed to uniform length, the next step is to position the endring and rotor for brazing. The endring should be placed flat, with the rotor positioned vertically and the bars resting firmly on the endring.
Because the endring has a large mass, a lot of heat is required. Anything in contact with the endring acts as a heat sink, but the endring must be supported for brazing. To minimize this problem, use a steel table with a hole in the middle for shaft clearance (see figure). Weld several pieces of steel plate — on edge — positioned radially around the hole. The plates should be placed to support the endring. The plates minimize the surface area in contact with the endring, so the heat stays concentrated in the endring. Once the endring is positioned, with the bar ends resting on the endring and centered2, brush on the appropriate flux for silver-solder. Use large torches to preheat the endring from underneath. Depending on endring size, one or two “rosebud” acetylene torches work well for this. When the endring is almost hot enough to melt silphos, an additional torch will be needed. A large brazing tip works best for this last torch, as the operator will be doing the actual brazing of the bars to the endring. Use silphos of at least 15% silver content; 1/8” diameter sticks work well. Tip: 15% silver bridges gaps better than silphos with a higher silver content.
It is important that the endring be kept hot, at a uniform temperature, during the entire brazing operation. Keeping the entire endring at a uniform temperature minimizes the mechanical stresses induced in the cage assembly. Bar life and vibration levels depend on this. Tip: If you have to braze a bar that has broken loose from the endring, heat the entire endring to avoid causing stresses in the adjacent bars. Failure to do this will increase the mechanical stresses in the remaining bars, and can lead to more bar fractures.
As the bars are brazed, observe the flow of silver solder from endring to bars. If the parts are within the correct temperature range, the silphos will form a meniscus indicating a good wetting action, demonstrating a solid bond to both surfaces. (This meniscus can be demonstrated by pouring water into a clear glass. The visible curve where the water surface contacts the glass container is the meniscus.) An inverted meniscus is proof that the bar was not hot enough, and the braze joint is inadequate.
Once all the bars are brazed, heat the entire endring to make sure the temperature is uniform. Visually inspect both sides of each bar to confirm the integrity of the braze joint.
Allow the rotor to cool a bit before flipping it to do the other end. Repeat the entire process for the second endring. After brazing is completed, permit the rotor to cool to room temperature. The rotor should be steam-cleaned (or sandblasted) to remove flux residue. Visually inspect the rotor again to make certain that all the bars are properly brazed.
Tips for a better job:
- When swaging is required, try an air-hammer with a modified chisel. It saves time and gives a consistent swage.
- Machine a slight lip in the endring, to center the endring and bars. This simplifies the job of positioning the endring on the bars. It reduces the amount of final machining and helps with balance.
- If the endring is made slightly oversized, it can be faced and the OD “trued up” concentric to the shaft. The cross-sectional area of the finished endring should duplicate the original.
- Those doing a lot of rotor work should consider setting up a table with a ring-torch arrangement to minimize the labor required for brazing.
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