Rebuilding the Birch 3/4" NYC Pacific

Where users can chronicle their builds. Start one thread and continue to add on to it.

Moderators: Harold_V, WJH, hwboivin3

Post Reply
AnthonyDuarte
Posts: 268
Joined: Tue Jan 19, 2010 10:46 am
Location: Mountain View, CA
Contact:

Rebuilding the Birch 3/4" NYC Pacific

Post by AnthonyDuarte » Tue Jan 08, 2019 12:01 pm

Greetings everyone,

Some of you may recall this engine was rescued back in 2013, but if you don't here's the thread regarding this engine and its history: viewtopic.php?f=8&t=97358

I started taking it apart to rebuild it after an unfortunate boiler incident rendered it inoperable 4-5 years ago. It's been staring at me from the project table ever since I got it down to the bare frame, but I've started to dig into it again.
The plan is to completely overhaul the chassis, obviously replace the old boiler, and add more detail to make it more closely resemble a NYC K-3 pacific. I have hundreds of drawings from the NYCHS that I'll reference to make new valve gear and other detail parts.

I'm hoping starting this thread will make me feel some sense of obligation to actually work on the engine.

Does anyone have or know of Birch drawings that might exist out there??? They would be tremendously helpful, but it would be no surprise to me if they're all lost.

Anthony

AnthonyDuarte
Posts: 268
Joined: Tue Jan 19, 2010 10:46 am
Location: Mountain View, CA
Contact:

Re: Rebuilding the Birch 3/4" NYC Pacific

Post by AnthonyDuarte » Tue Jan 08, 2019 2:09 pm

All new rods and walschaerts valve gear will be made out of stainless. With the original NYC drawings as a reference, they’ll be styled more accurately after the real thing while fitting within the fixed proportions of the model.

To make new valve gear that will actually work, I had to accurately measure some existing locations on the frame. Luckily the 3/4” chassis fits nicely on the old knee mill. Sitting on 123 blocks and indicated in straight, I was able to use an edge finder to locate the cylinder centerline, and from there measure the distances to the main driver center, the link fulcrum, and distances between the link centers as well as a few other necessary measurements. With an indicator I was able to measure the height of the link fulcrum and piston valve centerline.

My manual mill is a combo ProtoTrak CNC conversion, so the DRO has two separate screens for absolute or incremental. This was incredibly helpful, as I was able to established an XYZ centerline off the cylinder block in absolute mode, and do all my edgefinding in incremental. After finding the center of the link fulcrum or main driving pedestals, a simple switch back to the absolute screen showed me the coordinates relative to my cylinder centerline.


Edge finding the front of the cylinder face using a 123 block. Knowing the width of the block, I moved in half the distance. I found a difference of .002" from measuring the pocket in the frames without the cylinders. Not bad.
CD718704-B4E4-4BCE-936B-AB1F20F845EA.jpeg

Measuring the center line to the link fulcrum.
585EA901-4CA8-4D80-BC9F-9EA705748861.jpeg

Measuring the height of the link fulcrum relative to the piston center line. 123 block and .01" shim stock worked great knowing that the distance between the guides is 1.01"
19E6DB6D-A195-40AB-BE43-AA5A48C9DE00.jpeg

flyguy
Posts: 27
Joined: Sat Dec 20, 2014 4:28 pm
Location: Santa Clarita, CA

Re: Rebuilding the Birch 3/4" NYC Pacific

Post by flyguy » Wed Jan 09, 2019 9:15 pm

Very cool setup. I really like how you used the 123 block and shim stock to help figure out your centerlines. Do you have any detailed pictures of the old rods?

Since you are in the middle of valve gear design, I'd like to know the best method of determining the length of the combination lever and union link.

Thanks for the info,

James

AnthonyDuarte
Posts: 268
Joined: Tue Jan 19, 2010 10:46 am
Location: Mountain View, CA
Contact:

Re: Rebuilding the Birch 3/4" NYC Pacific

Post by AnthonyDuarte » Thu Jan 10, 2019 8:34 pm

flyguy wrote:
Wed Jan 09, 2019 9:15 pm

Since you are in the middle of valve gear design, I'd like to know the best method of determining the length of the combination lever and union link.

Thanks for the info,

James
James,

I’m on a trip at the moment, but when I get back Monday I’ll be happy to post a detailed overview of how I laid out the valve gear. Walschearts is really quite simple, and I think it gets diluted with “rules of thumb” that aren’t always true.

Anthony

User avatar
JBodenmann
Posts: 2895
Joined: Sun Oct 26, 2003 1:37 pm
Location: Tehachapi, California

Re: Rebuilding the Birch 3/4" NYC Pacific

Post by JBodenmann » Thu Jan 10, 2019 11:19 pm

Hello My Friends
I don't know about you, but I can't wait to see what Anthony does with this engine.
Jack

User avatar
Steamchris
Posts: 428
Joined: Fri Jan 27, 2012 3:48 pm
Location: Duisburg/Germany
Contact:

Re: Rebuilding the Birch 3/4" NYC Pacific

Post by Steamchris » Fri Jan 11, 2019 2:30 am

Hey Anthony,

pls tell me, what is it for a tool on Pic 1 and 2?
I'm also very excited about what you will do with the locomotive.

Regards
Chris
The light at the end of the tunnel could be a train ;)

Health and Safety Notice

due to the current economic circumstances , the light at the end of the tunnel
has been switched off !

Harold_V
Posts: 17692
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Rebuilding the Birch 3/4" NYC Pacific

Post by Harold_V » Fri Jan 11, 2019 2:45 am

Chris,
I don't mean to walk on a post intended for Anthony, and I'm sure he'll chime in when he has the opportunity, but the tool you see in those pictures is an edge finder. Likely with a ½" body and a .200" diameter sensing end.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

Steve Bratina
Posts: 975
Joined: Tue Jan 16, 2007 9:39 pm
Location: Cambridge Ontario

Re: Rebuilding the Birch 3/4" NYC Pacific

Post by Steve Bratina » Fri Jan 11, 2019 9:11 am

Was there a need to replace the rods? Sometimes it is a shame to change a "classic" engine to something newer. I do realize that as the owner, you paid the bill so you can do as you wish. Just a thought.

User avatar
LVRR2095
Posts: 1389
Joined: Sun Mar 04, 2007 6:50 pm
Location: Maine, USA

Re: Rebuilding the Birch 3/4" NYC Pacific

Post by LVRR2095 » Fri Jan 11, 2019 10:45 am

Steve Bratina wrote:
Fri Jan 11, 2019 9:11 am
Was there a need to replace the rods? Sometimes it is a shame to change a "classic" engine to something newer. I do realize that as the owner, you paid the bill so you can do as you wish. Just a thought.
I was about to write a similar post when I saw Steve’s. Please bear in mind that you have an historic model from the pioneer days of the live steam hobby. While you most certainly need to replace things like the boiler in order to safely operate the locomotive. I think that looking upon ourselves as caretakers of historic models should make us tend more towards preserving as much of the work of the original builder as is possible.
But again as Steve said, it is yours and you make the decisions. I try to look at the long term picture. I know that those things that I own now.....will someday be owned by another after I am gone.
Keith Taylor

AnthonyDuarte
Posts: 268
Joined: Tue Jan 19, 2010 10:46 am
Location: Mountain View, CA
Contact:

Re: Rebuilding the Birch 3/4" NYC Pacific

Post by AnthonyDuarte » Mon Jan 14, 2019 12:54 pm

I completely understand the sentiment about keeping everything as it was by the original builder. It's a question I've wrestled with myself with respect to this engine, however, the crosshead and side rods at the very least need to be replaced. I'm a huge sucker for prototypical rods and valve gear, so I can't help myself but to just refresh it all. Other than that, very little will be changed. It will most definitely be recognizable as the Kollmar-built Birch NYC pacific.

James,
The combination lever and union link is where I always start when designing walschaerts, since its geometry does not necessarily depend on all the other monkey motion going on. However, everything else needs to be designed appropriately for the combination lever.
Whether you're designing from scratch or fixing erroneous valve gear on an existing model, the combination lever geometry can be calculated without worrying too much about existing fixed variables.

Really all you have to do is draw a few triangles, which I will now probably over complicate.
NYC K3 Walschaerts Layout.JPG
The variable letters I've assigned to this drawing are mostly random, so there's no need to read into it.
EDIT: Typo on the drawing. E = F + B1, not A + B1

The first thing that needs to be known is your Lap + Lead. Many existing designs should say what the lap and lead values are. If you're designing from scratch or following a prototype, it's pretty much up to you what this value should be.
As an example, the Lap + Lead on my 3/4" pacific is .1" This was determined by measuring the opening of the steam port in the liner and subtracting that from the width of one end of the piston valve. Value L in the diagram is 2x Lap + 2x Lead. This is how much travel should be introduced to the valve from the combination lever alone when the gear is in neutral (i.e. not influenced by the motion of the expansion link). On my model, that is .2"

Secondly, the stroke of the piston needs to be decided (or in my case measured to be 1.75").

And lastly, the offset of the crosshead arm (if any), needs to be taken into consideration (On my model B1 = .867, B2 = .28125).

To determine the lower segment of the combination lever (C2), a triangle can be formed between segments D, E, and C2.
On my engine, D is .755 (Stroke - L)/2 = (1.75 - .2)/2
E is 2.5545, my valve centerline height plus vertical crosshead arm offset (1.6875 + .867)
Using the friendly Pythagorean Theorem, C2 is calculated to be 2.6976 (dropped to 4 decimal places)
From this triangle, we can also calculate variable A, which is the angle of the combination lever at the end of the stroke. This is needed to calculate C1. On my little engine, the angle is 16.8807 degrees. Don't get lazy rounding off this number!

Now using this angle and L/2, we can figure out C1.
C1 = .5L/SinA
C1 = .1/Sin16.8807
C1 = .3343"

From here, the union link is a game of connect the dots. The horizontal distance of the topmost hole of the lever relative to the crosshead wristpin (at rear end of stroke) needs to be known. This location is affected primarily by the radius rod. If that is a flexible variable for you (for me it is not), then it's imperative to make sure the union link length is set so that your combination lever will not collide with the cylinders or crosshead. Then (again, if radius rod geometry is not determined) connect the dots from the top hole to wherever you decide to place your link fulcrum to find the length of your radius rod.

On my engine, I can't change the length of the radius rod for a variety of reasons. This puts the horizontal distance of my top hole to wristpin at 1.8540" when the crosshead is at the back of the stroke.
Since my combination lever is designed such that the union link is dead horizontal at either end of the stroke, the math is simple.
1.854 - S/2 - B2 = U
1.854 - 1.75/2 - .28125 = U
Union Link = .6977

It is possible to design the combination lever slightly longer or shorter. This will change how the union link swings. It can be angled slightly up or down at the end of the stroke. That is not a problem. But it needs to be taken into consideration when calculating the length of the union link. If there is a difference in height between the combination lever and crosshead arm at the end of the stroke, the Pythagorean Theorem comes into play to calculate it as a triangle, using U and height difference as the two legs, and union link length as the hypotenuse.

I hope you find these ramblings useful.
Anthony

Moron
Posts: 35
Joined: Wed Mar 16, 2016 5:56 pm

Re: Rebuilding the Birch 3/4" NYC Pacific

Post by Moron » Fri May 17, 2019 10:40 am

Love this explanation on the combination lever design. Would love to see more lessons on designing the whole valve gear. I’ve been reading up on it, mostly from the greenly/Evans book and a few books published back in the working steam days. Each one explains it a little differently, but for me it slowly makes it more clear. Looking forward to watching this rebuild as well!

Post Reply