How to start Walschaerts Design as a beginner?

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Old Bill
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Re: How to start Walschaerts Design as a beginner?

Post by Old Bill »

Hi VGC.

Thank you for taking an interest. Here is the reverse gear oval diagram for equalised lap.

Image

The exhaust ports are still biased by 5% of stroke away from the mid point and I am getting more valve stroke at one end than the other. That, I think, is due to reverse rather than the lap modification. Thinking about the 5% bias some more, I suspect that this might cause the 'galloping horse' effect described by Moron. We shall see in due course!

I have been trying to increase the cut-off as suggested. I thought of just pushing it further into forward gear but the yoke is already at 26.5°against a recommended maximum of 25° (KN Harris in Model Engineer). I have therefore increased the stroke at the return crank from 2 3/8" to 2 5/8" and achieved this chart (with the unequal lap again).

Image

The exhaust ports are opening at mid stoke in mid gear and I have even lead. In the steam chart below, for forward gear, I have achieved another 4% cut-off.

Image

The same chart for reverse has also increased the cut-off by 2% but is significantly less even.

Image
I feel inclined to live with it and 'have a go' to see what happens. These are my final charts with the even lap and increased eccentric stroke in forward gear:

Image

Image

If anyone can offer any thoughts, comments or observations, then I would very much appreciate them!

Steve
Moron
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Re: How to start Walschaerts Design as a beginner?

Post by Moron »

Steve, to increase cut-off (percentage), you have to reduce the lap. It’s mostly a relationship between the valve and the ports in the cylinder moreso than it is increasing valve travel. There’s 2 ways to do this, 1: reduce the lap (equally on both sides) and leave the rest as is. The downside to this is that the lead will increase. The other way is the reduce the lap, then redesign the combination lever points so that it includes the new lap figure as well as your desired lead (which is controlled by the combo lever). When I get home from work I will try and find these equations for you. They are simple and will get you where you need to be. Another thing to note, at least with walschaerts (baker may be different) is that the locomotives in America were usually designed for full port opening in full gear. Actually they were designed for a given slight overtravel to compensate for wear, as well as to make the gear design a little more standard (everything came out in a fraction more or less) perhaps something else to consider if possible. Baker is different so don’t pull your hair out if that isn’t achievable haha.
Old Bill
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Re: How to start Walschaerts Design as a beginner?

Post by Old Bill »

Thank you, Moron. That is exactly the sort of understanding I have been looking for! I shall look forward to seeing the equations.

Steve
Moron
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Re: How to start Walschaerts Design as a beginner?

Post by Moron »

Old Bill wrote: Tue May 18, 2021 4:34 pm Thank you, Moron. That is exactly the sort of understanding I have been looking for! I shall look forward to seeing the equations.

Steve

Happy to help! Here are some of the equations that I will write out. First one is a simple one for determining cut-off in full gear. Keep in mind this equation will only get you in the ballpark as it doesn’t include lead. But being able to look at the simulator screen, you should be able to get a good idea how things will turn out. Full gear cut off does not have to be (and often isn’t) a perfect percentage.

P(squared) + 2PL
________________ X 100%= cut off in full gear
(P+L) squared

P=Port Opening
L=Lap(you can try +lead but I can’t garuntee it would be accurate but I don’t see why it wouldn’t)
Keep in mind this is port opening and Lap (plus lead) for one side only

Next equation is for the combo lever. To keep it simple I will give you inside admission. All you would need to do afterwards is set the gear to run in center and edit the Union link length .001” at a time until your leads (or cut-off’s) in center are equal to whatever lead you entered in (keep it a decimal point)

L=Lap plus lead (one side)

L= VS
___
2R

V=distance between top 2 pins
S=Stroke
R=Distance between top pin and bottom pin connecting to the Union link.

Given this equation we can now find the top 2 pin locations to be

V= 2RL
_____
S
Old Bill
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Re: How to start Walschaerts Design as a beginner?

Post by Old Bill »

Many thanks for those. I shall study and digest! In the mean time, I have tried doubling the lead from 0.015" to 0.030" by shortening the valve at both ends reducing the lap as you suggested. This is what I get in forward gear:

Image

This has increased the cut-off by a further 1% at both ends so I think I may be there. I am sure it will work but the question is how well. I will find out in due course!

Very many thanks for all of your assistance. I am really learning somethng here. I just need to sort out the actual mechaincs of it now!
Steve :D
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Bill Shields
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Re: How to start Walschaerts Design as a beginner?

Post by Bill Shields »

Is a fantastic learning experience and you have done an exemplary job of sorting out parameters

Now...suggest that you chart with reasonable tolerances for manufacturing and wear....and consider what the events will look like in forward and reverse.

What happens when the drive wheels rise or fall?

Do you know the exact centerline distance between the valve gear and the drive axle with the boiler 1/2 full of water?

Theory is great but in the real world we have to deal with tolerances and make adjustments -> sometimes in the design stage and sometimes after construction.

Plan ahead..
Too many things going on to bother listing them.
Old Bill
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Re: How to start Walschaerts Design as a beginner?

Post by Old Bill »

Thanks for your thoughts Bill. I am not quite sure how to allow for wear. I guess there will be lost motion with the effect that the events will be delayed so I will lose lead and get reduced port opening. That is the sort of thing that will creep up on me with usage and I won't notice until I give it an overhaul.

Build errors will need a bit of care though. I shall have to check what I have actually made as I go along and tweak it as necessary. All good fun!

There is nothing like making something to get a better understanding of it. The trick is to learn from the experience and do better next time!

Steve :)
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Bill Shields
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Re: How to start Walschaerts Design as a beginner?

Post by Bill Shields »

you are correct n assuming that you are not going to get the full intended valve travel from the 'system'...even if you allow for 0.0005" on every pin bearing involved in the motion, you will be surprised at how much valve motion you are going to lose.

the whole world is a compromise...

if you just ignore wear and allow for reasonable manufacturing clearances you will be on the right path.
Too many things going on to bother listing them.
Moron
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Re: How to start Walschaerts Design as a beginner?

Post by Moron »

Bill Shields wrote: Mon May 24, 2021 3:47 pm you are correct n assuming that you are not going to get the full intended valve travel from the 'system'...even if you allow for 0.0005" on every pin bearing involved in the motion, you will be surprised at how much valve motion you are going to lose.

the whole world is a compromise...

if you just ignore wear and allow for reasonable manufacturing clearances you will be on the right path.
Indeed one of the reasons I like the idea of a slight overtravel of the ports, as well as higher percentage of cut-off in full gear, allows for future lost motion without being as noticeable. Design it as well as you can, build it as well as you can, but many would be surprised at how shot some of the full sized engine’s valve gears got while still sounding decently square. The Pennsy comes to mind Ha!! It’s a steam engine. They’re meant to be run into the ground and then rebuilt
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Bill Shields
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Re: How to start Walschaerts Design as a beginner?

Post by Bill Shields »

Giggle ..no point in rebuilding if running...
Too many things going on to bother listing them.
Berkman
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Re: How to start Walschaerts Design as a beginner?

Post by Berkman »

Neat discussion.

I can't imagine the original designers of the LE locomotives or RRSC mikado etc went to quite this length of charting valve movements when designing the gear on their locomotives.
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Bill Shields
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Re: How to start Walschaerts Design as a beginner?

Post by Bill Shields »

the bigger the loco the less finicky the design can be.

If you have 3/16" wide ports, you can lose .030" of travel and the loco will still run

consider a Gauge 1 steamer and the intended valve travel (or lack thereof) and the % of intended motion is lost on every .0001" of wear.

Charlie Purinton once told me that he spent the majority of the precision time building his 3/4" locos on getting the valve gear correct.

Ever see his little 0-4-0 that had 5/64" diameter round ports and 2" wheels?

A wide open throttle was a truly death defying experience...
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