Crash course in slide valve timing and reverse

This forum is dedicated to the Live Steam Hobbyist Community.

Moderators: Harold_V, WJH, cbrew

Post Reply
rkcarguy
Posts: 588
Joined: Tue Aug 22, 2017 10:33 am

Crash course in slide valve timing and reverse

Post by rkcarguy » Wed Jan 03, 2018 8:34 pm

It's going to be a long ways off before I can start anything, but I've started drawing up a 1/6th scale version of Northern Pacific's #1070 0-6-0 L9 Switcher. This will be a "fabricators special" using laser cut plate and pipe to make as much of the locomotive as possible.
This locomotive is a bit odd for the 0-6-0, as it has slide valve type cylinders and almost all of them found online have piston valves.
I see how the valve works, it's pretty simple. I've got a few questions.
For the reversing linkage, it appears that the "tie-rod" for the valve will slide up and down in a slotted bracket, such that it's slid from one end to the other(for modeling purposes-no "company notch") to reverse the engine?

Secondly, the valve should just start to open when the piston hits the end of it's throw on each end? Is any "advance" used like a gas engine or is it just timed straight up 0 both ways?

Lastly, I've roughed together a linkage and cylinder drawing at around a 3" bore and a 5-1/2" stroke, as the wheels are 8-1/2" in diameter. Any sweet spot for these as far as bore and stroke?
steamengine.jpg

User avatar
Dick_Morris
Posts: 2106
Joined: Sat Jan 04, 2003 2:09 pm
Location: Anchorage, AK

Re: Crash course in slide valve timing and reverse

Post by Dick_Morris » Thu Jan 04, 2018 2:40 am

Since you are following the stroke would typically be determined in by the stroke of the prototype. When I was doing calculations for my Consolidation, I found that just scaling the bore would result in a locomotive that used too much steam and was slippery due to too much torque. (A friend started with scaled prototype dimensions on his 2-8-2. He later installed a sleeve to decrease the bore. Afterwards, the locomotive performed better.) I also found that many of the commercial designs had a larger bore than they needed or could effectively use. A couple of other variables that you need to use to determine the bore are the boiler pressure and the weight on drivers. The maximum draw bar pull that a locomotive can use without slipping in in the neighborhood of 20% of the weight on the drivers. From this, you can see that extra weight on drivers is usually a good thing, so design accordingly.

This thread gives the formula that shows how the factors interact - http://www.chaski.org/homemachinist/vie ... rt#p317338. There have been several discussions here on the design considerations and some searching should find them.

I don't have the web address, but do a Google search for "Dockstader" and "valve gear." Charlie Dockstader created a whole suite of valve gear simulations which includes Stephenson as your image above shows. They allow the user to change the dimensions of all the links to see how a particular design of valve gear would work.

User avatar
BAdams
Posts: 136
Joined: Wed Feb 23, 2011 7:15 pm
Location: Moreno Valley, CA

Re: Crash course in slide valve timing and reverse

Post by BAdams » Thu Jan 04, 2018 4:44 am

The link Dick was speaking of:
http://www.billp.org/Dockstader/ValveGear.html

-Brook

User avatar
Marty_Knox
Posts: 1287
Joined: Thu Jan 09, 2003 6:50 pm
Location: Michigan, USA

Re: Crash course in slide valve timing and reverse

Post by Marty_Knox » Thu Jan 04, 2018 8:31 am

That is way too long a stroke for an 8 1/2" wheel. Stroke is usually about 1/2 the wheel diameter, or less.

User avatar
Fred_V
Posts: 4170
Joined: Sun Jan 12, 2003 3:26 pm

Re: Crash course in slide valve timing and reverse

Post by Fred_V » Thu Jan 04, 2018 8:48 am

rkcarguy wrote: I see how the valve works, it's pretty simple. I've got a few questions.
For the reversing linkage, it appears that the "tie-rod" for the valve will slide up and down in a slotted bracket, such that it's slid from one end to the other(for modeling purposes-no "company notch") to reverse the engine?

Secondly, the valve should just start to open when the piston hits the end of it's throw on each end? Is any "advance" used like a gas engine or is it just timed straight up 0 both ways?
steamengine.jpg
The notching is all done with the Johnson bar (reversing bar) in the loco cab. The demo model you show doesn't have a complete Johnson bar with notched quadrant.

With some valve gear the valve opens to steam a bit before dead center. There is a lot to learn about valve gear so here is a good starting point.
http://www.donashton.co.uk/
Fred V
Pensacola, Fl.

rkcarguy
Posts: 588
Joined: Tue Aug 22, 2017 10:33 am

Re: Crash course in slide valve timing and reverse

Post by rkcarguy » Thu Jan 04, 2018 11:24 am

Thanks for all the input. I watched a youtube video that showed the valve box in action, looks like the linkage was double nutted so it could be adjusted. I understand that I would need a reversing lever back in the cab, I just posed that picture because it explains to me how the reversing part is done.

User avatar
NP317
Posts: 1169
Joined: Tue Jun 24, 2014 2:57 pm
Location: Northern Oregon

Re: Crash course in slide valve timing and reverse

Post by NP317 » Thu Jan 04, 2018 11:40 am

That valve gear model shown in the first post is actually for Walschaerts valve gear, set up for outside valve admission.

Stephensen valve gear has the quadrant motion derived from two eccentrics, and results in built-in lead when "hookng up" (centering) the Johnson Bar.
This has a similar effect to ignition advance with increasing rpm.

Walschaerts valve gear advantages: All the mechanism is outside the drivers and accessible.
Stephensen valve gear is inside the frame and difficult to access.
Your choice, or follow the prototype you wish to copy.
~RN

User avatar
Doug_Edwards
Posts: 560
Joined: Thu Apr 10, 2003 8:06 pm
Location: No. Idaho
Contact:

Re: Crash course in slide valve timing and reverse

Post by Doug_Edwards » Thu Jan 04, 2018 1:00 pm

Just for what it is worth, if you scale the loco to 1/6th size, the track gauge will end up being 9.4" and change. 1/5th size will get you closer to 12" gauge. 2.548" to the foot would put you spot on the correct scale to the 12" gauge. Making the loco proportional to the gauge means that you will have much less redesign to build the engine, you can copy the original design with less changes, and less chance to paint yourself into a corner.

You might consider making the loco 1.6" scale to match the majority of the tracks in your area. There are also more parts and fittings available to use on locos in this scale, plus you will not have the burden of laying track for your engine, if you no longer desire to when you finish the loco. A more standardized gauge allows you to travel to other tracks as well.

Nice choice for a prototype. I have always thought these were nice looking switchers.

Fwiw.

D.
http://www.precisionlocomotivecastings.com/
Building a 70 ton Willamette in 1.6"
Building a 80 ton Climax in 1.6"

"Aim to improve!"
"Mine is not to question why, mine is just to tool and die"

rkcarguy
Posts: 588
Joined: Tue Aug 22, 2017 10:33 am

Re: Crash course in slide valve timing and reverse

Post by rkcarguy » Thu Jan 04, 2018 2:20 pm

NP317, I agree. The video I watched of the valve box adjustment had eccentrics(more like what's on a shay) which the user was able to rotate to fine-tune the timing, then lock them down, along with the adjustment nuts to center and lock the slide valve.
I'm drawing this in AutoCAD, and while I can't automate the drawing, I can copy/move the components to various positions to assure things move right and then grab all the dimensions from this once it's exploded into individual parts.

Doug, if you check out my post in grand scale, I have some peculiar requirements for my railroad, therefore the unique scale and gage. I am already building a Baldwin S-12 in gas/hydraulic. This will be a "working railroad" which will haul firewood from the lower 2 acres to the house, and the trash and recycling up to the road. I have lower back issues, so being able to roll the firewood and trash onto a nice low bulkhead flat car has great appeal, instead of lifting it up into my truck or a trailer. Call it work and fun, in lieu of the farm quad and trailer I was originally planning on:)
I wanted 20" wide rolling stock as my wood stove will take 20" long firewood, and also for riding purposes as most of my family and myself are going to need 18" + width of seat. I also didn't feel like 7-1/2" gage was adequate for the loads I want to carry, and would be too tippy, so I opted for 12" gage. I realize it's about 20% too wide for the scale but that's ok with me. Being to scale is great and all, but real trains don't have 36' tall people riding them either so they can get away with a 56-1/2" gage for 10'+ wide trains. Lastly, I didn't want overall length of any loco's or rolling stock to exceed 8', so I can procure materials and move the trains places for welding, painting, etc, and finally to the property with the tailgate on my truck closed.

I am about 3 hours, *or* a border crossing(which would probably mean a customs bond and all kinds of BS to take a train across), from the nearest 7.5" gage club track, and at this point in my life I don't see any travel happening to other tracks except maybe Glenn's 12" RR one of these days:)

I had discussed with my fiancé the idea of having a X-mas tree farm on the acreage, and having the tree buyers ride on the train, pick their tree, and then bring it back on the bulkhead car as a whole Christmas experience. Having a steamer would obviously be worth some bonus points there, as well as giving me tax deduction options for the trains as well and there is state tax breaks on property taxes for tree farms too. I do realize there is some insurance needs there, I would have to look into some "event insurance" I imagine.

User avatar
Dick_Morris
Posts: 2106
Joined: Sat Jan 04, 2003 2:09 pm
Location: Anchorage, AK

Re: Crash course in slide valve timing and reverse

Post by Dick_Morris » Thu Jan 04, 2018 8:11 pm

That valve gear model shown in the first post is actually for Walschaerts valve gear
Oops.

User avatar
NP317
Posts: 1169
Joined: Tue Jun 24, 2014 2:57 pm
Location: Northern Oregon

Re: Crash course in slide valve timing and reverse

Post by NP317 » Fri Jan 05, 2018 12:58 pm

Dick_Morris wrote:
That valve gear model shown in the first post is actually for Walschaerts valve gear
Oops.
Meh.
No issues.
~RN

Post Reply