Stevenson valve gear.

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Stevenson valve gear.

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  • #121575
    malcolm hearne
    Participant
      @malcolmhearne84128

      I am seeking assistance in understanding how to work out the angle of advance when using Stevenson valve gear when using a loco link. I have been studying Don Ashton's booklet on the gear ,but I am lost in working out the equivalent eccentric (EE.). I understand the principle of the EE. but actually calculating mine is the problem as I do not have all the data, basically because I do not have a figure for the half valve travel (R), which I need the EE. to work out, but because I don't know the valve travel, I can't work out the EE. I know the eccentric throw , but because its a loco link ,the valve travel is not the same as the throw, but modified by the EE . Chicken and egg situation.

      The valve gear is for an Isle of Wight O2 loco. The eccentics are split and keyed to the crank axle so I need to know the angle of advance to cut the eccentric keyway in the correct position.

      I would be most grateful for any assistance fron onyone who can unravel my rambling request.

      Malcolm Hearne

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      #1189
      malcolm hearne
      Participant
        @malcolmhearne84128

        equivalent eccentric

        #121586
        John Baguley
        Participant
          @johnbaguley78655

          Hi Malcolm,

          Have you tried putting your data into one of the valve gear simulators? The easiest one to use for Stephenson's is Bill Hall's. There's a link on Don Ashton's website to where you can download it from. The angle of advance determines the amount of lead given to the valve and the simulator allows you to play with the angle until you get the amount of lead required. I normally go for 0.010" at 50% cut off.

          John

          Edited By John Baguley on 05/06/2013 10:32:45

          #121587
          MICHAEL WILLIAMS
          Participant
            @michaelwilliams41215

            Hi Malcolm ,

            The equivalent eccentric concept is not always the most helpful way of designing simple valve gears .

            It's an enormous subject but I'll make a few suggestions :

            (1) Just use the old fashioned layout on a drawing board method – move things around on paper until it all works out . CAD helps because of the accuracy and rapidity of making changes .

            (2) Use the equivalent eccentric idea with a postulated set of starting conditions – eg guess a plausible valve travel and / or an initial setting of eccentrics . Adjust set up progressively until again it all works out .

            (3) Set up a computer model either using one of the available programs or just using BASIC or spreadsheet ..Either use multiple iterations to optimise or drive the design yourself using progressive changes .

            (4) There is a math method but we'll leave that one for now .

            Get back to me if you want any more specific help .

            Regards ,

            Michael Williams .

            nb: It is actually a common problem in engineering to be faced with large set of variables and only a few things positively known . Inspired / intuitive guesswork , experience and simple calcs are often used to have a first stab at a design which is then progressivley refined .

            nb : Solutions to design problems are not usually unique . A quite simple problem can have multiple valid solutions – and this is true of valve gears .

            #121631
            julian atkins
            Participant
              @julianatkins58923

              hi malcolm,

              what scale are you building your 02? ive driven quite a few 02s to don young's 1" scale design FISHBOURNE and all perform ok though there are a few errors on the valvegear design that can be easily corrected. i am very familiar with don's design and the fullsize CALBOURNE. ive a set of frames and a machined set of cast iron cylinders out of meehanite if you are interested plus some of the fullsize eastleigh drawings.

              in order to work out R you need to know what you want the valvegear to do within the confines of your cylinder/steamchest design/size. don went for quite a short valve travel and relatively small lap and small ports (in width). don ashton gives the figure to work out the expansion link dimensions to avoid excessive eccentric throw and if you use this figure for c/u then you can work out the increase in lead to arrive at M. X = 2 times x, so R is easy to work out.

              of crucial importance is the suspension offset if you want to 'notch up'. a computer simulator will provide vastly superior results to working this out with templates. in don young's FISHBOURNE the expansion link radius and length of the eccentric rods is wrong as is the suspension offset. however with the shoprt travel you arent going to get much benefit from expansive working by 'notching up' anyway.

              it is possible using don ashton's excellent book to work backwards if you have a pre-determined eccentric throw.

              PM me if i can be of further help.

              cheers,

              julian

              #121640
              julian atkins
              Participant
                @julianatkins58923

                i should add that 10 thou lead means absolutely nothing on its own, with all due respect to john.

                lead is much better expressed as the number of degrees in advance of FDC and BDC. the late E A Langridge expressed this as 4 degrees in advance. this is far more accurate than expressing it as a dimension in thous, and of course depends on whether you have walshaerts with fixed lead or stephensons with variable lead. with stephensons anymore than 4 or 5% when 'notched up' will cause compression problems and too early an admission, though the first loco i built to don young's design had what might be termed by some as excessive lead with walshaerts and performed very well and is still going well!

                cheers,

                julian

                #121647
                Francois Meunier
                Participant
                  @francoismeunier96697


                  You will find the responses in the late Charlie Dockstader Valve gears simulator programs (still available on the net), you enter your data, yes, the complete valve gear set up of your own loco from the main driver to the valve and piston, and run the program and you will see your loco running before being finished ! there are no approximations or "equivalent eccentric" in it, only exact geometric valve positions during run.

                  you would be able to change dynamically the data (ie link length, pivot postions or angular setting etc) see their effects on valve events and the diagrams, then tune your dimensions to get the steam distribution you wish as regards advance, cut-off, symetry…

                  **LINK**

                  #121675
                  MICHAEL WILLIAMS
                  Participant
                    @michaelwilliams41215

                    None of you will believe this but the difference in performance between a valve gear thrown together anyhow but which basically manages to open and close the valves now and then and the performance of the most sophisticated valve gear that could ever be devised is only a few percent and this in the overall performance of the engine amounts to almost nothing .

                    #121701
                    Sub Mandrel
                    Participant
                      @submandrel

                      I was going to suggest the LBSC method of using a tram ticket to set the lead, but having read Michael's post I suppose a bus ticket would work just as well.

                      Neil

                      #121713
                      Anonymous

                        A train ticket might be appropriate; question is, should it be a single or return?

                        Andrew

                        #121717
                        DMB
                        Participant
                          @dmb

                          When LBSC said "Tram Ticket," I expect it was an Edmunsen card, like the old type railway tickets. As far as I know, the preserved railways like Bluebell are still using them, so if anyone wants one well you know what to do – support your local pres. rly.

                          #122635
                          malcolm hearne
                          Participant
                            @malcolmhearne84128

                            Thanks to all who replyed to my request for assistance.I am trying The Prof. Hall's simulator at the moment , but am not certain what is meant by certain termanoligy ie.

                            1 circ. disp. of lifting pin and radial disp. of lifting pin ,

                            2 I assume that the lenght of the eccentric rod is from the pin centre to the eccentric centre..

                            3 .Is the Ecc. rod pin to link slot the distance from the centre of the rod pin to the centre of the die block when it is closest to the rod pin.

                            Also there is no mention as to whether the gear is loco or launch link. I presume that the simulator works this out for itself.

                            I have tried Dockstader before, but again I was not familiar with the information inputs that were .required . I t would be very helpful if somebody , who is familiar with these programs, could produce a set of definitions describing the inputs required.

                            #122636
                            Weary
                            Participant
                              @weary

                              Malcolm, have sent you a PM.

                              Regards, Phil.

                              #122642
                              John Baguley
                              Participant
                                @johnbaguley78655

                                Hi Malcolm,

                                To answer your questions:

                                1) Circ. displacement of lifting pin is the vertical distance of the link suspension pin above the horizontal centre line of the link. This will normally be zero unless the link is suspended by the top or bottom pin attaching the eccentric rods. In this case, the figure is the vertical distance of the eccentric rod pin from the horizontal centreline of the link. (terrible way to do it though but much loved by LBSC!)

                                Radial displacement of the suspension point is the horizontal distance of the link suspension pin in front of or behind the centre of the curved slot in the link. This is usually more than zero to equalise the cut offs. If it is zero then the gear probably hasn't been designed properly and unlikely to give equal cut offs.

                                2) length of eccentric rod is as you say, from the connecting pin to the centre of the eccentric.

                                3) The eccentric rod pin to link slot is the distance of the eccentric rod pin to the centre of the link slot. For loco links this will be zero as the pins are on the centreline of the slot but for launch links it will have a value as the pins are behind the slot.

                                All distances are to the centre of the pins.

                                Hope that helps,

                                John

                                Edited By John Baguley on 19/06/2013 11:57:36

                                Edited By John Baguley on 19/06/2013 12:02:13

                                #122813
                                malcolm hearne
                                Participant
                                  @malcolmhearne84128

                                  I have managed to enter the figure for the O2 valve gear only to find when I press results button all I get is two error messages which mean nothing to me. ie.

                                  1 asin: DOMAIN error

                                  2 Invalid floating point operation.

                                  Can somebody explain what these mean? as I cannot see anything wrong with the figures that I entered.

                                  Thank you

                                  #122819
                                  John Baguley
                                  Participant
                                    @johnbaguley78655

                                    Hi Malcolm,

                                    It's because one of your dimensions is incorrect and doesn't make sense to the software. Happens to me quite often!

                                    If you like, pm me the dimensions and I'll have a look (or contact me via my website)

                                    John

                                    Edited By John Baguley on 21/06/2013 10:30:30

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