Stephenson's Valve Gear

[Runner]

Hi all,

I am building a twin cylinder launch engine with Stephenson's reverse valve gear. It is of my own design, I haven't produced drawings, just made it up as I go. No castings are used, utilising stock material that I have to hand. The preconceived technical requirements that I have established for the launch engine is 1.375" stroke, 1.25" bore. In designing the valve gear I have decided upon a 0.1875 " offset on the eccentrics producing a 0.375" throw. The expansion link has a 0.8125" distance between the points where the eccentric rods connect and the curvature in the expansion link is 2.5" radius, to suit the length of the eccentric rods and straps that are also 2.5". I have completed it all except designing and producing the cylinders and steam chests, which will be made out of gunmetal which is going to be an expensive buy (relatively) so I am concerned that my version of Stephenson's valve gear is going to be a goer, before committing to the purchase of gunmetal for the cylinders and steam chests. My concern is that the expansion link rotates rather a lot, producing a steep angle to valve crosshead, making the valve cross head sit almost on the corners of expansion link, however it's the die that transmits the motion from the eccentrics to valve crosshead. This angle is controlled by the eccentric offset of 0.1875" and the distance between the the eccentric rods of 0.8125". This is rather difficult to explain and probably difficult to understand.

I expect that some leeway is available in designing versions of Stephenson's reverse valve gear, because I have seen pictures of many and they appear to be variations on a theme. However, there is probably an optimum solution. What is this? Will my design be suitable as is?

Thanks in advance.

Brian

 

[abby]

Hi Brian , by co-incidence the design criteria for Stevenson's valve gear are being discussed here.
http://www.unionsteammodels.co.uk/cgi-bin/yabb/YaBB.pl?num=1336946752

 

[Maryak]

Basically the eccentric rod length is some 2.5 x stroke and the radius of the links equals the rod length.

That should overcome your............

My concern is that the expansion link rotates rather a lot, producing a steep angle to valve crosshead, making the valve cross head sit almost on the corners of expansion link.

Brian

Best Regards
Bob

 

[steamer]

Haven't looked at the attached link, but wot Bob said , and the length of the link should be 2.5 times the valve travel.

 

[Dan Rowe]

Brian,
I am assuming that your launch engine is a vertical engine. It is not simple to get Dockstader's vale gear program to work for direct Stephension as it is set to work with rocker as a locomotive uses.
http://www.billp.org/Dockstader/ValveGear.html

I can explain how to do it if you need the information.

There is another program for launch engines with marine links written by Rainer Radow, which is a bit simpler to use.
http://www.steamboating.de/valve/valve-maine.html

Dan

 

[Runner]

Thanks Abby and Dan for your links, there is a lot to digest so it will take some time to know what if any modifications are necessary.

Thanks Steamer and Bob for the snippet of information. So that I understand correctly the valve travel in my situation is 0.375" so the expansion link length should be 0.9375". This is the length of the curved slot where the die sits and not the distance between two holes that are used to connect the eccentric rods/straps to the expansion link? In my situation the length of the curved slot is 1.375", which is 3.7 times the valve travel. Is this too much?

Brian

 

[steamer]

That would be total travel of the link...from full ahead to full astern...if it's a boat.... ;D

That would be a minimum to avoid serious angularity issues......

I gotta think there is very little bad that would happen if it's longer than that.

I used that geometry on my launch engine and it runs great.....and links up nice

Dave

 

[Maryak]

Bobs Crap O Cad

Best Regards
Bob

 

[Runner]

Thanks Dave for the assurance.

Bob thanks for the Cof C Sketch.

In retrospect I may have been premature in my concern because part of the valve setup, ie the valve rod, valve and steam chest are missing, so one can't gauge the alignment of the expansion link without that part of the valve gear. Also I have not set the full forward and reverse positions of the valve gear, which can only be done with the remaining valve parts in position. I am able to go beyond these set positions which probably caused the valve cross head to be seen at the extreme ends of the expansion link. I may have to modify the expansion link.

Brian

 

[Runner]

Postscript.

Abby in reading the information listed at http://www.unionsteammodels.co.uk/cgi-bin/yabb/YaBB.pl?num=1336946752 it states that the angle of the expansion link should not increase beyond 25 degs, this can be controlled by the pin centre distance. The reason given is for mechanical exigency?? Another good criterion to easily remember, same as the length of the expansion link being 2.5 times the valve travel.

Dan I have downloaded both programs and running them, it will take some time for me to understand all the parameters that are required to be inputted and see how changes to them will alter the performance of the engine. BTW it is a vertical twin steam engine.

Brian.