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Schmidt Coupling

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  • Schmidt Coupling

    I'm a long-time lurker who is continually amazed and amused at the posts here. Time to contribute one of my own.

    In a book titled "Mechanisms & Mechanical Devices Sourcebook" (which has been mentioned here before), I ran across an intriguing mechanism I had never seen before, nor has anyone I have shown it to since. It was designed to transmit torque between two parallel but misaligned shafts. The shafts could even be repositioned while they were turning, an interesting feature. The illustration made it hard to envision the thing actually turning without locking up. I had to make one.

    The text in the book read:
    "An unorthodox yet remarkably simple arrangement of links and disks forms the basis of a versatile parallel-shaft coupling. This coupling – essentially three disks rotating in unison and interconnected in series by six links – can adapt to wide variations in axial displacement while it is running under load.
    Changes in radial displacement do not affect the constant-velocity relationship between the input and output shafts, nor do they affect initial radial reaction forces that might cause imbalance in the system. Those features open up unusual applications for it in automotive, marine, machine tool, and rolling mill machinery.
    HOW IT WORKS: The inventor of the coupling, Richard Schmidt of Madison AL, said that a similar link arrangement had been known to some German engineers for years. But those engineers were discouraged from applying the theory because they erroneously assumed that the center disk had to be retained by its own bearing. Actually, Schmidt found that the center disk is free to assume its own center of rotation. In operation, all three disks will rotate with equal velocity.
    The bearing-mounted connections of the links to disks are equally spaced at 120° on pitch circles of the same diameter. The distance between the shafts can be varied steplessly between zero (when the shafts are inline) and a maximum that is twice the length of the links. There is no phase shift between shafts while the coupling is undulating."

    The mechanism consists of an input shaft, an output shaft, 6 (or more!) driving links, and 3 disks. The disks don't really have to be round: they could be triangles or spiders or whatever, but disks are easy to build and make the mechanism easier to understand.

    You can see a short movie of my model at In my model, the driving axle is made up of a shaft and bearing out of an old Toyota truck water pump. Pins in the links are made from 3/8 hex bolts. Aluminum disks and links from the scrap bin. Wood base is from old oak pallets. I did have to buy the little Teflon buttons, though, to keep the disks from skidding on the base when people didn't keep the wheels aloft. You can see that while the axes are turning, the driving and driven shafts can be moved anywhere in their mechanical limits. You might notice that the second or middle disk is not supported by anything other than the links. This is why you must have at least 3 links supporting the disks. It becomes somewhat unstable if the input and output shafts are collinear, as there are then an infinite number of solutions as to where the middle disk can be. I made the center disk with 6 link connection points instead of three like the first and third disks so that I could use the same connection pins throughout.

    The range of offset between the input and output shafts depends on the length of the links, and the effective disk sizes (all must be the same) must fall within a certain percentage range of the link length so that the links don't interfere with each other. The load and speed rating of the mechanism is based on the mechanical design (bearing type, link and pin strength, etc). I easily made up a SolidWorks model just to visualize how it could actually rotate - it was simple as there are only two parts, with multiple instances. Mate them up and watch it turn.

    I also found this mechanism in Volume IV of the "Ingenious Mechanisms" book set (which has also been mentioned here before). Since my original book said that this was originally invented by a Richard Schmidt of Madison, Alabama, I've been calling it a Schmidt Coupling, and was kind of surprised to find a company selling stock items ( using that name. These are ready-to-go items with a variety of speed and load capacities.

    My model is quite a conversation piece, although those who are most interested are the geeky types who like messing with machines and such. Kids generally grab the handle and spin it as fast as they can without lifting the other disks.
    If ignorance is bliss, why aren't there more happy people?

  • #2
    Schmidt coupling

    Really neat. Shel


    • #3
      Makes you wonder what you could build if the center disk was a pulley. Couple that with some of those clever 90 elbow rod/piston engines as couplings to tie the two outer shafts back to each other and you would have a fun mechanical demo machine.

      Edit: found Marv's elbow engine link:

      The engine can also serve as a 90 degree coupling just to create interest.
      Last edited by dp; 09-22-2007, 04:40 PM.


      • #4
        That is really cool!!! I love seeing mechanisms like that, something that looks like it can’t work but does.


        • #5
          That is absolutely fascinating. Thanks!


          • #6
            sb, great post and very nice work


            • #7
              Thanks for the post. A friend of mine talks about a coupling like that. Now I understand whats going on. JIM


              • #8
                That's Amazing!Something for my to-do list.Thanks for the link.


                • #9
                  Didn't know that it had a special name for that coupling, so thank you, I learned something new today.

                  This coupling is in use in industry every day of the week, 24 x 7, the application is for where you have two feed rollers on a machine and the product being pushed thru these feed rollers varies in thickness.

                  Think chipboard, that has that thin plastic coating glued to it, same machine can be used for varying thickness boards, be it 10 mm to 30 mm thick.
                  Bottom roller is fixed, top roller is fitted with this device and now you can adjust for the different thickness boards.

                  Once again, thanks for letting me know what it's called.

                  regards radish


                  • #10
                    What an amazing coupling. I can't find it in my earlier Chironis volumes, Mechanisms, Linkages, and Mechanical Controls (1965), or Machine Devices and Instrumentation (1966), so I guess it is later than that.

                    What about vehicular applications? Perhaps it could be used to transmit power to an independently suspended wheel, front or rear, on an ORV, or to a dual-track snowmobile.

                    [edit] On second thought, too much unsprung weight for that.
                    Last edited by aostling; 09-23-2007, 12:28 AM.
                    Allan Ostling


                    • #11
                      that is just the coolest thing. i have to build one.

                      andy b.
                      The danger is not that computers will come to think like men - but that men will come to think like computers. - some guy on another forum not dedicated to machining


                      • #12
                        I just installed a Schmidt coupling on a piece of machinery about a year ago. Think of it as an offset ujoint of sorts. We installed it between two separate pieces of machinery where the centerlines of the shafts (one driving the other) due to design, were unable to be inline. This allows (at least on this application) about 6 inches of misalignment. Amazing, but works. They also make a unit that allows centerline offset with axial misalignment of 5 degrees.



                        • #13
                          Very nice...and excellent work on the model! Good post.


                          • #14
                            Thank you for that, most interesting.

                            There is a web site somewhere, well two actually, ones gives working movements like this and the Geneva wheel in animated Java shots and the other one I'm thinking of is some guy who has built a load of these models from the mechanism's books.

                            No doubt someone can Google them up, where's the Google Child when we need him ?

                            It makes me wonder where this mechanism can't be altered to do something different that we all use every day.
                            I'll get Small Son to fire Solid edge up as he's more proficient than I am with it and see if it does.


                            Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.


                            • #15
                              Here's one link to linkage animation...but don't call me "Google Child"! I've had this one on my favorites list for over 2 years.


                              "Accuracy is the sum total of your compensating mistakes."

                              "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton