How to start Walschaerts Design as a beginner?

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VO4454
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Re: How to start Walschaerts Design as a beginner?

Post by VO4454 » Tue Apr 08, 2014 2:29 am

Image
There is a pitfall in oversimplification! A locomotive on an 1/8 scale may have its volume reduced to 1/512 but not its weight! For instance a 1:1 boiler plate thickness of say 3/4 inch is not scaled to 3/32 but more, adding weight.
Regarding the rods in a locomotive, the above reasoning disregards the fact that rods are pulled and pushed 50% of their working time. Since you do not want your eccentric rod to deform it should be stiffer to avoid buckling. (for an explanation of buckling see the Wikipedia page). Since the buckling force formula has the rods Moment of inertia divided by the square of the rods lengths, this goes wrong if the diameter and length are scaled to 1/8 of the original. The inertia moment of a round rod is pi*d^4/64 and since 1/8^4 is only 0.000244 it should be clear that a scaled rod cannot withstand a scaled pushing force! So there is very much physical evidence for a heavier model valve motion.
Kind regards
Jos Koopmans
I had better call Ed Yungling tomorrow, first thing, and let him know that the Cab Forward is in imminent danger of valve gear failure, and will have to be immediately rebuilt, as it was built to scale and must be at least 30 years old. I will also let Don know that he should consider going thru the scale valve gear on his GS-1 while he is doing all that beautiful detail work. Now he will be able to easily make the valve crossheads and guides bigger while he has the cylinder block off the chassis. His valve gear is close to 20 years old. And thank god I read this before I run my GS-4 again and have the big end of the eccentric rod let go while running at speed. It was scaled directly from the Lima blueprints. I will let Ed know that he had better stop this practice of scaling valve gear from railroad blueprints. And anyone else out there with scale gear should immediately bad order their locomotives, have them towed dead to the backshop and inspect for repair before any more damage occurs.

Vic

P.S. Dorian, here is a link to an outfit that does quite a bit of gauge 1 work and mods. I have not had any dealings with them, but it may be worth a phone call.

http://www.realsteamservices.com/shop-work/
For Duty and Humanity

JJG Koopmans
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Re: How to start Walschaerts Design as a beginner?

Post by JJG Koopmans » Tue Apr 08, 2014 9:34 am

Well, a good try to ridicule my arguments! However, it also shows that you have zero understanding of buckling, which
has nothing to do with the parts you name but only with the rod itself, its diameter in particular.
Kind regards
Jos Koopmans

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VGC
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Re: How to start Walschaerts Design as a beginner?

Post by VGC » Wed Apr 16, 2014 4:34 pm

After all that I had learned, also in a lesson series called "similarity theory" (that by the way teaches you that many dependencies are of a more common and less complex nature than one may have thought before), the effect of buckling should not show other dependencies from model scale than other effects, so it should not become a problem.

I just see that the corresponding formula is to be found in Wikipedia, so look at http://en.wikipedia.org/wiki/Buckling
The buckling force F is growing proportionally with I and reciprocal with L ^2.
I is growing proportionally with a length dimension ^4, L with a lengt dimension ^1.
So F is growing proportionally with a length dimension ^2.
This is the same dependency as it is with other forces, as area sizes grow with a length dimension ^2 and forces grow with area sizes ^1.

So buckling is no special effect to consider when scaling an object without changing its material.

This kind of looking at parameters or units may seem dilettantic or stupid to someone who has no insight into the fundamental basics of similarity theory (or who perhaps has failed trying something that seemed similar to this), but it is not at all, it is pure application of clean similarity theory.
Online program for valve gear design:

ValveGear Constructor [EN]

Image

(New: Website also in English language, now much more user-friendly!)

JJG Koopmans
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Re: How to start Walschaerts Design as a beginner?

Post by JJG Koopmans » Thu Apr 17, 2014 10:33 am

You are right of course, but strictly speaking this is valid only for a piston rod which is not under discussion. I have severe doubts whether the forces in a scaled valve gear motion follow similarity laws since pressure and friction forces are quite different. The packing on the valve rod for instance has to withstand half the pressure of the original but must also be steamtight, so it is certainly not an example of scaled similarity.
Kind regards
Jos Koopmans
Last edited by JJG Koopmans on Thu Apr 17, 2014 11:52 am, edited 1 time in total.

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VGC
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Re: How to start Walschaerts Design as a beginner?

Post by VGC » Thu Apr 17, 2014 11:11 am

I only wanted to proove that the main forces or the tendency of buckling does not grow when scaling down a model.
Especially the material tension from main forces of drive gears are even smaller at the piston and driving rod because a usual piston layout is based on friction parameters, and these are based on locomotive weight that changes with the length dimensions ^3, so e.g. a locomotive that is scaled down by 1/10 has 1/1000 weight of the original, whereas the area sizes are 1/100 of the original. So the allowed forces are 1/100, the existing forces on grounds of limited friction are about 1/1000. This means that the material tension is 1/10 of the tension of the original.

Of course there can be other factors that rise the real model tensions a little bit, e.g. a bigger model weight, greater machining tolerances and so on, but the tendency remains that model parts do not break as early as original parts.

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Re: How to start Walschaerts Design as a beginner?

Post by JJG Koopmans » Thu Apr 17, 2014 11:52 am

In an earlier contribution I have already argued that the weight of a scaled locomotive is larger than 1/scale to the 3rd power. If the scale is 1/10 the boiler shell thickness is also 1/10 meaning that scale boiler pressure would also be 10% of the original. Instead we use 33 to 50% of the prototype pressure so there is a lot of parts heavier
Kind regards
Jos Koopmans

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VGC
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Re: How to start Walschaerts Design as a beginner?

Post by VGC » Fri Apr 18, 2014 1:40 am

10% of the boiler pressure would result as a needed value if the piston and everything concerning the weight is scaled down exactly - and if some other effects like steam expansion, environment pressure etc. were scalable.

If the boiler dimensions are scaled down exactly and the different behaviour about heat flow, corrosion etc. is ignored, the boiler is capable of about the original pressure. Pressure has the same units AND the same way of effect as material tension, and the allowed values of material tension do not change (or do not change very much) when scaling down.

The special formula for simplified calculation of boiler wall thickness is to be found here:
http://en.wikipedia.org/wiki/Barlow%27s_formula
Insert the model scale into the dimensional parameters, and you will see that the allowed pressure value remains the same.

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Re: How to start Walschaerts Design as a beginner?

Post by JJG Koopmans » Tue Apr 22, 2014 5:14 am

Yes, you are correct. I should have remembered the Barlow formula which was taught to me over 50 years ago!
The same is valid for the longitudinal stress in the boiler shell.
So, please let the man step forward that has succeeded in keeping the weight of his live steam model down
to that of 1/(scale to the third power) of his 1:1 prototype!
Kind regards
Jos Koopmans

Dor_Crank
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Re: How to start Walschaerts Design as a beginner?

Post by Dor_Crank » Tue Dec 02, 2014 11:31 am

AMatthews wrote:Hi Dorian,

The best thing you can do is get hold of DL Ashtons book and also look at his website

http://www.donashton.co.uk/index.html

his mathematics are far more concise and true than either Evans or Greenly.

the other thing to get hold of is Dr Alan Wallace's Valve gear simulator.

Dockstader's is good also, but I would recommend Wallace's for a beginner.

http://avocetconsulting.com.au/modeleng/

I have used it for the last 15 years and designed 6 sets of Walschaerts and a couple of sets of baker with it.

thoroughly read Ashton's information before you put pencil to paper or click on a screen as you will have a much better understanding of what is required.

Cheers,
Andrew

Dor_Crank
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Re: How to start Walschaerts Design as a beginner?

Post by Dor_Crank » Tue Dec 02, 2014 6:07 pm

For some reason, I couldn't post after my hiatus here. Lets see if this gets posted here. For those familiar with the Dockstader's simulation, please answer below.

I want to feed my Walschaerts valve gear dimensions into Dockstader's simulation.
I do not understand following parameters:

1. Drop Link to Crosshead backset - Is this used when the Drop Link is not connected directly to the
crosshead pin that connects to the main rod? Most loco's drop link connects a bit below the pin but
horizontal offset from the pin = 0*.

2. Combination Lever backset - same problem as #1 above, can't see where this parameter is. The picture
gets eclipsed. I guess I can keep it = 0?

3. Reverse Arm to Center Norm - What does this 'Norm' mean? The inquiry shows a horizontal bar which is
vertically sliced by the road in the middle. Is this 1/2 of the reach road travel? but the numerical value of .8
suggests other wise

4. Reverse Arm to Center Max - Same problem as #3. Same picture. Is this the total swing of the Reach road?

____
*When I inquire where this parameter is geometrically. This is done by pressing the tiny button in front of the
variable. But the entire picture of the valve gear gets enlarged and gets eclipsed by the parameter table.
Per Dockstader's instruction, he claims this eclipsing is caused by the FontSize setting. The size must be set
to 'smaller'. However, for my Window 7, it can't be made smaller. It's defaulted to 'small - 100%'
and can't change that 100% to say 50%. The only option I have is the medium size. Has anyone over came
this problem of eclipse? Actually, it won't matter if I know what those parameters represent.

Dorian

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SteveR
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Re: How to start Walschaerts Design as a beginner?

Post by SteveR » Tue Dec 02, 2014 8:25 pm

Hi Dorian,
I've been playing with CD's program to design the valve gear for my recently acquired LE 1" Pacific "project".

In the program, you can easily change the values from 0.0 to some number, then click on the little button on the left side to have the program show you where it is. You can always reset the value using the "default's" button.

The way I see this program, you have to input values, then the program will show you how well your guesses work. You can iterate from there. Anyone - correct me if I'm wrong.

Nice thread.
SteveR
12x36 Enco Lathe, 9x42 Bridgeport, SMAW, O/A, Miller MIG w/gas
Not enough measuring tools...
1.5" Allen Models Consolidation in progress
1" FEF in progress
1" LE Pacific "Project"
3/4" LE Northern Project
"Guaranteed not to rust, bust, collect dust, chip, crack, peel or tarnish."

JJG Koopmans
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Re: How to start Walschaerts Design as a beginner?

Post by JJG Koopmans » Wed Dec 03, 2014 6:47 am

Hi,
I have been trying to get your info. Last time I used it, it was possible to see all the parts checked
on screen. Now they disappear outside the visible screen. My best guess is that something has been changed in the operating system. I am using Windows Vista.
Kind regards
Jos Koopmans

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