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Repairing a small screw compressor

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  • Repairing a small screw compressor

    A friend has a couple compressors, he had bought a small 3HP kobelco screw compressor years ago with a dead motor, being Japanese it has a weird frame motor so he ended up just getting the motor rewound. All that just to find the thing leaked like a sieve though the shaft seal on the pump. He then got a good deal on a 5hp scroll compressor like I have which was working fine until it ate itself when a bearing went bad, so we are back to looking at fixing the screw compressor. The oil coming of the shaft housing was all glittery but the oil in the reservoir was perfectly clean, so it looked like the mechanical seal was shot. We managed to get a manual with a cutaway from Japan but no parts are available.

    So I start yanking the thing apart. I start getting into it and its nothing like the drawing, the stationary seal is the same but there is a brass sleeve in there. Surrounding that is a whole bunch of shredded brass, obviously the source of the sparkles in the oil. That sleeve did not want to come off, managed to get it all pulled apart with a big puller grabbing onto the pump housing. I grabbed all the parts and brought them home to make some replacement parts

    The seal and the old sleeve and new.

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    Looking at the drawing there were obviously part missing and parts there that are not supposed to be. Eventually it dawned on me that someone had tried to repair this before. The sleeve was to increase the shaft size to 1.25" which you can get standard mechanical seals for. But in doing so they seem not to understood how these seal work and the rotating seal that sealed agains the sleeve had it's spring resting against the housing of the compressor, so it just sat there and the sleeve just spun in the rubber gland of the inner seal. It lasted a while until it started shredding itself.

    So I am going to repair it the right way this time, another seal is on order from McMaster and I make an adapter to go from the shaft to the new seal and probably make a holder to mount the stationary seal in the place of the old one, but those parts won't show up till next week.

    So while I am waiting for those parts I tackled the next problem, the gasket was destroyed getting this thing apart. I took the housing and set it on a flatbed scanner and scanned it. Imported that into mastercam and there is a art feature that allows you to do a raster to vector conversion. The auto mode did not work so well so I just used the spline tool to trace it out. Pop in some circles for bolt holes too. I printed it out on regular paper and it looks like a exact match size wise.

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    From that Image some tool paths and dumped it into the cnc mill and used my fancy laser attachment to cut out the new gaskets, recycled brown paper bags.



    Came out pretty good!

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  • #2
    Wow. That's impressive!

    Never tried a brown paper bag as a gasket. Going to remember that one. Depending on size, I usually I make them from a new manila file folder.

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    • #3
      Works well. It's something I learned from my dad. I still use something like permatex with it though. I could have just used some form a gasket but I am not sure if there are supposed to be specific clearances. This is the bad thing about buying anything Japanese, Japanese companies do not support their stuff more than maybe 10 years out. They just expect people to buy new.

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      • #4
        Macona that's incredible --- I just trace stuff out with a pen or sharpie and cut and then own circle punches and use some UHMW as a base behind it - but that gasket looks factory bought very nice,,,

        something that thin i would consider a thin layer of spray on gasket sealant to "seal the deal" nice work...

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        • #5
          Originally posted by macona View Post
          Works well. It's something I learned from my dad. I still use something like permatex with it though. I could have just used some form a gasket but I am not sure if there are supposed to be specific clearances. This is the bad thing about buying anything Japanese, Japanese companies do not support their stuff more than maybe 10 years out. They just expect people to buy new.
          The recipe I learned was the manila folder used with Indian Head Gasket Shellac. You can still get it, and it's a Permatex product. Just a very faint whiff of that stuff brings back a flood of memories.

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          • #6
            Originally posted by A.K. Boomer View Post
            Macona that's incredible --- I just trace stuff out with a pen or sharpie and cut and then own circle punches and use some UHMW as a base behind it - but that gasket looks factory bought very nice,,,

            something that thin i would consider a thin layer of spray on gasket sealant to "seal the deal" nice work...
            I honestly didn’t think it would be this easy. I didn’t expect the scan to be accurate dimension wise. But it all worked out.

            Going to put the housing back on tomorrow. I need to take some measurements with the cover off to make the parts to adapt the new seal. I’m planning on making a single piece that will have an internal o-ring to seal to the shaft and a surface to drive the seal and for it to seal against.

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            • #7
              On a screw compressor that size what do you or they use for an oil separator? We used big 1000cfm compressors in my sand blasting business and the oil separators were a big and expensive unit.
              Location: The Black Forest in Germany

              How to become a millionaire: Start out with 10 million and take up machining as a hobby!

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              • #8
                Originally posted by macona View Post

                [snip]
                The seal and the old sleeve and new.

                Click image for larger version Name:	IMG_1519.jpeg Views:	0 Size:	1.83 MB ID:	1937141

                [snip]
                Which one is the old & which is new?

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                • #9
                  The paper bag gasket is an old technique. My father-in-law uses a small aluminum hammer to cut it out against the casting.... put the piece of paper over the casting, and go around it tapping against the edge inside and out, and each hole. The edge will cut the paper and you end up with a perfect gasket.
                  CNC machines only go through the motions

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                  • #10
                    Originally posted by J Tiers View Post
                    The paper bag gasket is an old technique. My father-in-law uses a small aluminum hammer to cut it out against the casting.... put the piece of paper over the casting, and go around it tapping against the edge inside and out, and each hole. The edge will cut the paper and you end up with a perfect gasket.
                    Yeah, that's how we normally use it, then use a ball peen to knock out the holes. But if I don't have to whack against a aluminum casting, the better. Plus I can makes as many as I want after.

                    On a screw compressor that size what do you or they use for an oil separator? We used big 1000cfm compressors in my sand blasting business and the oil separators were a big and expensive unit.
                    Im not sure what he has planned yet, it dumps into a couple hundred gallon tank and there is a regenerative zeolite dryer after that. But he will need something.

                    Went and put the case back together yesterday, I needed some measurements with the housing to to locate the end of the inner needlebearing race to make the part I need. Gasket fit perfect.

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                    • #11
                      super neat work, must have been very satisfying to have the laser cut those out so cleanly and accurately.

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                      • #12
                        Originally posted by mattthemuppet View Post
                        super neat work, must have been very satisfying to have the laser cut those out so cleanly and accurately.
                        Sure beats the hammer method Jerry mentioned which is how I would have done it. Mechanical seal should be here tomorrow so I should be able to get this thing running this weekend, sooner deepening on what else happens this week.

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                        • #13
                          also gives you more options if the mating surface doesn't have a suitable edge nearby to use to cut the gasket.

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                          • #14
                            If I had needed to make that gasket I would have had to use the "dirty" finger method. You place your paper on the housing and then with a dirty, greasy finger you rub around all the edges. It leaves a mark you can then cut out. As long as you make sure your paper doesn't move it works quite good. It also works for other uses as well.
                            Location: The Black Forest in Germany

                            How to become a millionaire: Start out with 10 million and take up machining as a hobby!

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                            • #15
                              Originally posted by Black Forest View Post
                              If I had needed to make that gasket I would have had to use the "dirty" finger method. You place your paper on the housing and then with a dirty, greasy finger you rub around all the edges. It leaves a mark you can then cut out. As long as you make sure your paper doesn't move it works quite good. It also works for other uses as well.
                              Yeah, there is that and also using something like paint, dye, or even grease to transfer the shape to the gasket material.

                              I am mostly still amazed that everything matched up, I really didnt expect the scan to be that accurate. Printing I have found to be accurate to thousandths very often with laser printers and use that a lot for verification. When I worked at the diabetes research place we did some measurements to verify the accuracy of laser printers because we were using them to make shadow masks for etching.

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