Curvic Couplings
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  1. #1
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    The jet engine industry needed an ultra precise locating arrangement to assure the exact alignment of components along the main engine shaft. The arrangement needed to be easy to work with, so that during maintenance, the alignment could be easily reestablished. And, the arrangement needed to be extremely robust, so that the continual engagement of engine components could be assured.

    The solution was found in the curvic coupling. This type of coupling, in which mating teeth are configured with convex "curves" on their sides on the top or bottom bottom half of the coupling, and concave "curves" on the mating half of the coupling, guarantees proper alignments when the face of the top and bottom halves of the coupling are pressed together. Further, because of the mating of the convex and concave teeth, the distance from coupling centerline to the OD of the coupling will always be the same, and so will be the face of whatever the coupling is fixed onto.

    W/S was the first to use the curvic coupling on a turning machine, I believe. It was considered by others to be expensive. W/S saw it as a way to assure the repetitive index accuracy and rigidity it wanted to promise its customers. Further, the curvic coupling has the unique property of getting even better with time! As the teeth wear in to each other over many years, accuracy of location gets even more precise. Probably better then ever needed on a turning machine.

    The only cautionary aspect of curvic coupling application is that good guarding must be provided around the base of the turret, top of saddle, to prevent the entry of chips into the coupling. Our design, through several revisions, was successful in this.

    Other builders were using what they called "face gears" or "face couplings", not possessing the superior characterisitcs of our curvic couplings.

    These coupling teeth, of relatively low projection (maybe only around 3/16" above the center flat portion of the gear blank) were ground on expensive Gleason Curvic Coupling Grinders. The dressing of the grinding wheels for this process was complex by itself, but the results paid off. This was another example of the fine, solid engineering which went into the W/S NC and CNC turning machines from their birth!

  2. #2
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    It never ceases to amaze me how a pioneering giant like W&S went under!

  3. #3
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    Hi just though I would throw in here... Ingersoll Milling Machine used these on the swivel attachments for the big rail mills. Gave them enough small movement but a very solid lock-up for positions for milling odd angles. We rated the spindles at about 90 hp if my memory hasn't gone completely

  4. #4
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    I worked at The American Tool Works in 1963 - 1968 before I went to W/S. Sometime before 1963 they bought a war surplus Gleason curvic coupling grinder for $5000, brand new in the crate. American sold several quick change turrets with their lathes each with its own curvic coupling on the bottom all of which mated with the one botton coupling in the turret.
    I have seen several cases where coolant essentially disolved the teeth on the couplings and they just bottomed out and were not at all accurate.
    Before ATW I was a jet engine mechanic in the Navy working on J-47's and later worked at GE Evendale building J-79 & J-93 engines. In the GE engines there was no continuous shaft through the compressor. Each compressor disk had a curvic coupling on each side and the disks were stacked coupling to coupling and the whole stack was held together with 6 or 8 through bolts. Only the front and rear disk had a stub shaft. No two couplings were the same size as the compressor rotor was tapered from small in the front to large in the rear. The whole thing was designed to save the weight of a shaft all the way through. The couplings continuously transmitted torque from the turbine in the rear to the front stages of the compressor. They also had to maintain the alignment and concentricity of the stub shafts front and rear. I guess this is just another case of defense developments being converted to civilian use. Our G.A. Gray division sold GE a lot of VFR machines to make these disks.

  5. #5
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    How does the european hirth tooth coupling fit into this mix?

  6. #6
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    One more comment on the jet engine conection - many W/S people may remember John Martanovic, formerly of Sales Service, Houston Sales Office and Dallas Tech Center - John is now a plant engineer or facilities engineer at GE/SNECMA in San Marcos Texas. The product at this plant is first stage jet engine compressor blades made of some kind of exotic plastic. They were historically made of titanium. These are probably the very long first stage blades used on fan-jet or bypass engines.

  7. #7
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    On British jet engines the Hirth coupling fullfilled a similar roll to the curvic coupling , though all the ones I worked on had a supporting compressor shaft on which the compressor discs were mounted (sliding fit) with the hirth couplings transmitting the drive from disc to disc, and maintaining the correct spacing between the individual compressor discs which carried the aerofoil section compressor blades on the periphery.

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    I am not familiar with a hirth coupling. Curvic couplings mated up on a pitch line in the middle of the angled face on the teeth. If the wore to the point that they bottomed out they were worthless.
    Remembering a little more, I saw the ATE coupling grinder in the crate in 1963. I think before that time they were buying the couplings from Gleason. I heard it said that you had to have a master set to ber able to successfull;y make a set. Apparently the masters were used to gage the coupling being ground. I don't believe some of the couplings used on our lathes were as deep as those in the jet engines. In one machine I know that 0.1 inch of disengagement made them open enough to rotate the turret.

  9. #9
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    Thank you John. I'm remembering when a grinder was installed in plant one, and not being climate controlled they had a dickens of a time with accuracy. Bob Balser was the tool technician assigned to that installation and the time frame was the late 50's I believe.

  10. #10
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    Mr. J michaels... Those first stage blades are composite and being cut on a small Ingersoll gantry router with a 20k Setco spindle. I worked with J Martanovic there at CFAN as they called the place back in about 1991. Great guy.
    No curvic on that machine, just a live swivel head. using all diamond crusted tools

  11. #11
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    There are two types of Curvic Couplings, movable and fixed. The geometry is different, the movable or indexing couplings ars shallower and their coupling teeth are at a greater angle to the axis.

    Face gears and Hirth couplings do the same thing. They all form a rigid assembly that repeats both concentricity and axial location of the two components when disassembled and reassembled. They all provide for very accurate angular indexing. They all wear to a more accurate angular index when used.

    The beauty of the Curvic Coupling is the process of manufacture. The coupling teeth are ground or cut by a cylindrical cup wheel or cutter with the desired shape dressed on the front edge. This is fed along the blank's axis but off center from it.

    This cuts the gash between four teeth at a time, one gash on each side of the blank. Like all Gleason machines the curvic coupling machines are fully automatic. The coupling is complete in half the number if indexes. Production rates are very high.

    So, while there are other couplings that do the same as a Curvic, the Curvics are easier to make while having the same (maybe greater) strength and accuracy of alignment.

    The Gleason Works are the bevel gear people. It isn't surprising that they would be the inventors of such a coupling.

    They are easier than a bevel gear to make and the machines to make them are simpler. The idea is to make a precision coupling that can be mass produced, after all, the mass production of difficult curcular parts is Gleason's specialty.

  12. #12
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    J. Henricksen,
    I saw the Ingersol, It was next to the Lucas I was there to service. In 1991 what they were making was a tough plastic shroud that went around a whole jet engine to capture anything thrown off the rotor. It was supposed to stop a broken turbine bucket. They don't make the shrouds anymore, just blades as far as I know. I was there last in the late 1990's.
    The Ingersol was interesting in that a large part of the machine was two concrete walls. I don't know if they need that machine for blades. Maybe I can find out in April when I go to John's son's wedding.
    John

  13. #13
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    J MIchaels.in 91 I may well have been there if anyone was working on the Ingersoll other than the operators. I think the Lucas is gone. I saw John back in about 2001 when they were doing a retrofit on the Ingersoll. He came to Rockford.
    The shroud being cut on the Lucas was for F 14 or F15 I don't remember which. The ingersoll was originally going to be used to make payload nose cones for rockets to launch satelites. Then SNECMA took over and they started making the blades for I think GE 90's for the 777 project.
    Thre cement walls were the cheap and stiffest way to give that machine the height it needed.They used a slightly modified runway structure used on the tapelayers to save engineering and fab costs
    Tapelayers had those runways on top of 12 foot legs.
    I've been out of Ingersoll since they changed hands so I don't get the latest gossip on the machines. Say Hi to John for me.


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