Equivalent eccentric

[bambuko]

I am reading H.S.Gowan's design document on Stephenson's valve gear, and have a question about his example of equivalent eccentric.
He says:



as per Fig2:



and yet when I try to repeat this exercise, I get different numbers for valve displacement and quadrature (highlighted yellow in original text)



My results:
valve displacement=2.795" (Gowan=2.5")
quadrature=2.5" (Gowan=2.165")

I have checked it using other methods and it seems my numbers are fine, but...
Am I doing something wrong?
If any of you fine gentlemen would help me with sanity check, I would be grateful.

[don9f]

Hi, I don't see a number "5" on your drawing, which should be the centre of the circle. This centre should be at where perpendiculars from half way 0 to 3 & from half way 0 to 4 meet and it looks like it's not on a perpendicular from half way 0 to 4, thus the circle is too big a diameter? (Point 4 should lie on the circle).

I'm no expert on this at all but can see you have bisected the angle between lines 0 to 3 & 0 to 4 which has put the centre of the circle in the wrong place....i.e. too low down.

Hopefully this helps....

Regards Don

[bambuko]

Thank you Don,

The instructions say "... perpendiculars from 0-3 and 0-4 bisected..." so I have bisected the angle betwee 0-3 and 0-4, rather than bisecting each of this lines... before dropping perpendiculars - silly me

Now sorted:

[don9f]

That's great....

Don

[jma1009]

Hi Chris,

I have H.S. Gowan's original article as I have the SMEE journal.

I have learnt a lot from same, as it contains a few nuggets of gold, but I think only a masochist would attempt what you are doing vis Gowan these days.

Modern computer valve gear simulation does this all for us.

Do you know what an equivalent eccentric is? It is the loco link as a by product of the launch type link, but depends on the c/u function for proper interpretation using Don Ashton's parlance. X and R verses x and r. Those who have gone down this route will know to what I am referring.

All good fun, and I used Don's original book pre computers to work out many a valve gear mathematically and prove same, but these days I can run it on the computer and get a result in an hour instead of doing the maths and struggling for a day or two to get a result.

Cheers,
Julian

[bambuko]

Yes Julian, you are probably right about it verging on masochism
As for modern computers doing all this for us... many years ago when CAD was introduced, young apprentices thought that becoming skilled CAD jockey made them into "designers".
Same here (in my opinion) with valve gear simulators - being able to feed it with numbers and getting something out, doesn't mean that I have a clue how good the output is.
Hence my desire to learn a bit more.
Yes, I am well familiar (by now) with c, u, and X ve x as well as R ve r
I have every intention of using gear simulators, but I also like to design things on CAD and have a bit of a clue "why?" I am doing something.
Plus, as you say - it's fun (and what other reason do we need to do something?)

[jma1009]

Hi Chris,

I would agree with you that going back to basics and doing it the hard way is probably the only way to learn what's what, plus playing around on a chassis to see that what Don Ashton and H.S. Gowan wrote does actually make sense and makes a difference.

For example, if you do the maths you can see that for a given eccentric throw and eccentric rod length and type of link you can work out the increase of lead. Then you can see this as you notch up on a working chassis by watching the valves or testing the events via the draincocks. You can see the point of admission and work this out as an angle in degrees of point before or after FDC or BDC, and then test the chassis.

With both the mathematical old way, and the computer simulator, you are still left in miniature with a large number of variables to determine when designing a loco with Stephensons gear. How much lead when notched up? What is the expectation of cut off when notched up to achieve efficient expansive working? How much lap on the valves? What steam port width? What should be the full gear cut off (and how many cylinders and how are the cranks set?)?

Then other factors come into play. Do we increase lap and valve travel on old Victorian fullsize examples? If you do, will everything still fit? Will the top ends of the links and lifting arms clout the boiler? Will there be enough room for larger eccentrics? Will the expansion links swing either side too much making reversing difficult?

I played around with expansion link templates as per Gowan to determine the suspension offset, but it is a tricky process. These days the computer allows you to change the suspension offset and get an instantaneous result (which usually requires a lot of playing around with to get it right) but I would prefer to do this than go back to using templates as it is dead accurate.

There are also other factors involved. There is no point having a super duper valve gear that can be notched up to 15-20% cut off if the loco is unable to take advantage of expansive working - because it doesnt have superheaters fitted or the passageways are restricted or there is too much back pressure caused by a poor smokebox draughting design. Or because the cylinders are dustbin size, and if you opened up the regulator to run on the reverser the loco would slip wildly.

Cheers,
Julian

[bambuko]

Julian, all what you say makes perfect sense - thank you.
I am at a stage where, working through DA and HG I am beginning to see what is what and how it is calculated.
All the other things that you mention (i.e. proper design question), I will enjoy once I have mastered the basics.
I haven't got yet to suspension offset in HG yet - look forward to playing with it!
No doubt, "I'l be back" with more questions...

Chris