I posted some time ago about cracked sliprings on the rotor of my Lincoln 250 Weldanpower. The machine was bought new by me in 1987, and had very low hours on it. Despite this, both sliprings on the generator rotor cracked through and came loose on the rotor. Both sliprings had cracked in the same manner, a sharp and straight crack running longitudinally.
My suspicions were that Lincoln had used brass tubing with a welded seam to make the sliprings. The tubing had a fairly thin wall. with the stress of a shrink fit onto the insulating bushing and the centrifugal force (and additional stresses coming on and off idle), the tubing wall had failed at the welded seam. I called Lincoln and discussed this with one of their engineers. He said he beleived that Lincoln had generally used seamless drawn brass tube for the sliprings on their welding generator rotors. From time to time, they'd get a batch of brass tube witha seam weld. When this type tube was used, the factory would get calls about broken sliprings on welding generators.
Since my machine was 22 years old, Lincoln wasn't going to do anything for me. They offered to sell me new sliprings. Seeing the thin wall and poor design of the original parts, I decided to design and make new parts on my own. I contacted a local generator reapir shop and they agreed to work with me. They routinely repair all sorts of motors and generators including welding generators. To break loose the tapered fit of the rotor ont he engine cranksahft, we wound up destroying the original plastic insulating bushing. The pigtails wires fromt he field were already torn loose when the sliprings had broken.
I designed new parts, and machined them myself. I machined (2) new sliprings from a piece of solid bronze barstock. I made them with a heavier wall and slightly wider than original. I left about 0.030' for a truing cut after they were in place on the rotor. The finished wall is about 1/16' thicker than the original. I then machined a new insulating bushing from 'Micarta' (phenolic resin reinforced with fabric). This bushing was bored for a 0.003' interference on the rotor stub shaft. It was turned to give a very light interference fit on the sliprings. I mileld slots for the field pigtails to pass under the sliprings. The generator repair shop gave me some magnet wire and Kevlar sleeve insulation. I silbrazed the pigtails to the inner wall of the sliprings.
The sliprings were warmed on top of my basement barrel stove (wood stove for supplemental house heat). They dropped onto the Micarta bushing and were placed in position.
The rotor was chilled outdoors in about 10 degree weather. The Micarta bushing + sliprings, as an assembly was warmed in the kitchen oven to about 350 degrees F. I had made up a piece of allthread rod and machined collars to draw the Micarta bushing onto the rotor stub. the temperature difference made things go on fairly easily, and the all thread held things until temperatures normalized.
The result was the Micrata's outer diameter 'swelled' about 0.002', but was 'hooped' by the sliprings. The sliprings acted like oldtime wagon tires around the Micarta. By my calculations, I had 12,000 psi developed tensile stress in the sliprings, so plenty to clamp them fast to the rotor bushing but not enough stress to crack them.
The generator shop finished the job. They soldered the pigtails to the field windings, then taped the pigtails with fiberglass taping. The completed rotor was then dipped in a tank of hot epoxy armature varnish, then baked. The finished rotor was meggered, skim cut true in the generator shop;s lathe, and then put back into my welder.
The resulting job made a fine repair. I had been noticing the welder had not had the most stable arc and difficulty in strikign an arc for some time prior to the failure of the sliprings. After this repair, the welder is a pleasure to use.
The original repair estimate from the generator shop was so high that it would not have made any sense to repair this welder. By doing my own machine work, the bill was about 1/3 of what it would have been.
My own take on this is Lincoln used a minimal design for the original sliprings. Lincoln 'cut things to the bone' to build a compact and low-priced welding power supply in the original 'Weldanpower' machines. The use of seamless brass tube for the sliprings was critical due to the high tensile stress and stress cycling the walls of the sliprings see, even when the machine is not in use and sitting dead.
The generator shop worked with me, and that is a rarity in today's world. They are an oldtime family run shop and work on generators up to about 40 megawatts. They told me they had repaired an older Hobart engine-driven welder for another customer. The machine was so old that Hobart did not support it with parts. A printed circuit board with some diodes had failed. This same shop reverse engineered things and repaired that guy's machine for about 400 dollars.
The shop which did the repairs and worked with me is: H.A. Schreck, Inc.
32 Van Wgner Road
Poughkeepsie, NY
845-454-3560
I am sure, as is the case for that guy as well as myself, there are those of us who keep smaller engine driven welders at home. We use them for standby power as well as for welding. Out in the country, power outages occur more often then in cities or suburbs. Our welders are critical equipment to our homes, aside from use as welding power supplies. These machines see low hours. We are generally not in the welding or fabrication business, so cannot justify buying a new machine at current pricing. There is little or no salvage value to a machne such as mine if I were to have decided not to repair it. While it is not the most up to date welding power supply, it is more than adequate for SMAW and occasional scratch start TIG. SMAW is what I grew up with, and SMAW is what I use for most fabrication and repair jobs around home. Hopefully, the repair will last another 20+ years. If nothing else, there is something satisfying about doing this kind of repair for myself, using scrap materials in my own shop, and knowing I improved on Lincoln's design.
Joe Michaels