Stephenson valve system
Stephenson valve system
All valve surfaces ground clean and flat. Dialing in the linkage I discovered that there is only 1/2" total travel of the valve.
The way I see it 1/2" total travel can never adjusted to operate properly. 1.6 scale 7 1/2" gauge.
Please let me know what you think.
The way I see it 1/2" total travel can never adjusted to operate properly. 1.6 scale 7 1/2" gauge.
Please let me know what you think.
- Steamer Al
- Posts: 109
- Joined: Tue May 26, 2020 3:17 am
- Location: Comox, BC
Re: Stephenson valve system
Very dependant on how big your ports are there is a lot of information missing with your question.... a suggested read is Don Ashtons book, and website.Emfinger wrote: ↑Sat May 29, 2021 2:37 pm All valve surfaces ground clean and flat. Dialing in the linkage I discovered that there is only 1/2" total travel of the valve.Scan0068.jpg
The way I see it 1/2" total travel can never adjusted to operate properly. 1.6 scale 7 1/2" gauge.
Please let me know what you think.
http://www.donashton.co.uk/html/downloads.html
Also, Stephen Viktovitz (sorry if I butchered the spelling) has an excellent series, so far my favorite valve gear reference material, in Live Steam magazine in the 1980's. I could find the exact dates if it was intetest.
Alex
Re: Stephenson valve system
Emfinger wrote: ↑Sat May 29, 2021 2:42 pmEmfinger wrote: ↑Sat May 29, 2021 2:37 pm All valve surfaces ground clean and flat. Dialing in the linkage I discovered that there is only 1/2" total travel of the valve.Scan0068.jpg
The way I see it 1/2" total travel can never adjusted to operate properly. 1.6 scale 7 1/2" gauge.
Please let me know what you think.
I'm trying to post a drawing of the valve and the ports.
Re: Stephenson valve system
Steamer Al wrote: ↑Sat May 29, 2021 2:46 pmVery dependant on how big your ports are there is a lot of information missing with your question.... a suggested read is Don Ashtons book, and website.Emfinger wrote: ↑Sat May 29, 2021 2:37 pm All valve surfaces ground clean and flat. Dialing in the linkage I discovered that there is only 1/2" total travel of the valve.Scan0068.jpg
The way I see it 1/2" total travel can never adjusted to operate properly. 1.6 scale 7 1/2" gauge.
Please let me know what you think.
http://www.donashton.co.uk/html/downloads.html
Also, Stephen Viktovitz (sorry if I butchered the spelling) has an excellent series, so far my favorite valve gear reference material, in Live Steam magazine in the 1980's. I could find the exact dates if it was intetest.
Alex
I hope you get this.
Tom
download/file.php?mode=view&id=84261
- Steamer Al
- Posts: 109
- Joined: Tue May 26, 2020 3:17 am
- Location: Comox, BC
Re: Stephenson valve system
If nobody beats me too it I can give you a bit of a breakdown once I get home from work tonight. At first glance, everything seems fine, is there a particular adjustment you are worried about?
Re: Stephenson valve system
I'm sure all of the adjustment are correct save valve position on the valve stem.Steamer Al wrote: ↑Sat May 29, 2021 3:05 pm If nobody beats me too it I can give you a bit of a breakdown once I get home from work tonight. At first glance, everything seems fine, is there a particular adjustment you are worried about?
If I set the lead so that there is @ .015" of the port uncovered at TDF The forward stroke won't see any steam.
Top Dead Forward (perhaps my term)
- Bill Shields
- Posts: 10460
- Joined: Fri Dec 21, 2007 4:57 am
- Location: 39.367, -75.765
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Re: Stephenson valve system
What is the eccentricity of the eccentrics?
Too many things going on to bother listing them.
- Steamer Al
- Posts: 109
- Joined: Tue May 26, 2020 3:17 am
- Location: Comox, BC
Re: Stephenson valve system
The valve should be opening as the piston nears the respective dead centre. Almost sounds like your eccentrics are 180⁰ out?
Re: Stephenson valve system
If the dimensions you give on your drawing are correct (and I'm interpreting them correctly) then the lap of the valves is 1/8". The valve travel to fully open the ports will be:
2 x (lap + port width) = 2 x (1/8 + 1/8) = 1/2"
so your 1/2" travel of the valve rod is fine and both ports should open fully if the valve is set correctly. If not, then the eccentric setting could be suspect.
However, the valve exhaust cavity width of 11/16" seems far too wide. I would expect it to be 9/16" to match the distance between the inside edges of the steam ports. The width of the 'bridges' between the steam ports and the exhaust port will be 5/32" so the width between the inside edges of the steam ports will be 5/32 + 1/4 + 5/32 = 9/16"
If the width of the exhaust cavity in the valve is indeed 11/16" then the valve will have massive 'exhaust lap' of 1/16" and the ports will open to exhaust much too early. Exhaust lap was sometimes used on full sized locos as it supposedly makes the loco freer running by allowing the exhaust steam to exit the cylinder earlier and reducing the compression in the cylinder at the end of the stroke but it's normally only a very small amount. LBSC sometimes specified exhaust lap on his loco designs but it was only a tiny amount e.g. 0.008"
John
2 x (lap + port width) = 2 x (1/8 + 1/8) = 1/2"
so your 1/2" travel of the valve rod is fine and both ports should open fully if the valve is set correctly. If not, then the eccentric setting could be suspect.
However, the valve exhaust cavity width of 11/16" seems far too wide. I would expect it to be 9/16" to match the distance between the inside edges of the steam ports. The width of the 'bridges' between the steam ports and the exhaust port will be 5/32" so the width between the inside edges of the steam ports will be 5/32 + 1/4 + 5/32 = 9/16"
If the width of the exhaust cavity in the valve is indeed 11/16" then the valve will have massive 'exhaust lap' of 1/16" and the ports will open to exhaust much too early. Exhaust lap was sometimes used on full sized locos as it supposedly makes the loco freer running by allowing the exhaust steam to exit the cylinder earlier and reducing the compression in the cylinder at the end of the stroke but it's normally only a very small amount. LBSC sometimes specified exhaust lap on his loco designs but it was only a tiny amount e.g. 0.008"
John
Secretary of The National 2½" Gauge Association
Member of North West Liecestershire SME
http://www.modeng.johnbaguley.info
Member of North West Liecestershire SME
http://www.modeng.johnbaguley.info
Re: Stephenson valve system
I think you left out the space between the ports....so did I. No there is nothing wrong with the eccentrics Trust me!
You don't have the measurements of the port spacing. But I'll get them to you soon, you seen to know what is going on...
Thank you
Tom
You don't have the measurements of the port spacing. But I'll get them to you soon, you seen to know what is going on...
Thank you
Tom
- Bill Shields
- Posts: 10460
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Re: Stephenson valve system
The space between the ports can be determined by basic math...assuming the shown dimensions are correct.
Too many things going on to bother listing them.
Re: Stephenson valve system
Not Basic Math !
But I'll sent the measurements Tomorrow. Please stand buy.
Thanks
Tom
But I'll sent the measurements Tomorrow. Please stand buy.
Thanks
Tom