Stuart 6A Valves Variant?

A special section just for steam engines and boilers, as without these you may as well fit a sail.
Old Steamer
Warming the Engine
Warming the Engine
Posts: 70
Joined: Mon Feb 24, 2014 11:08 am
Boat Name: Nell

Re: Stuart 6A Valves Variant?

Post by Old Steamer » Wed Sep 20, 2017 12:01 pm

So that I can check whether the engine I'm looking at has/has not been modified, will somebody advise the length of the columns (minus the threaded ends) of the extended column version?
Thanks for response.
OS
steamboatjack
Full Steam Ahead
Full Steam Ahead
Posts: 283
Joined: Fri Nov 20, 2009 8:02 am
Boat Name: grayling
Location: Cumbria U.K.

Re: Stuart 6A Valves Variant?

Post by steamboatjack » Fri Sep 22, 2017 7:48 am

The original Stuart 6A front columns are about 11.5 inches face to face.
The original conn rod length being 6.75 inches centres, this is only 2.25 times the engine stroke, Ideally the conn rod length should be a minimum of 2.5 times, this is in fact to improve the errors in the valve events due to obliquity rather than excessive load on the cross head. There have been a number of these engines converted to piston valves on the H.P. I would recommend this rather than balanced slide valves. Balanced slide valves are ok but require careful design and periodic inspection, I have some on a Taylor twin, the amount of pressure and therefore friction loading they relieve is difficult to quantify.
Regards
Jack
Old Steamer
Warming the Engine
Warming the Engine
Posts: 70
Joined: Mon Feb 24, 2014 11:08 am
Boat Name: Nell

Re: Stuart 6A Valves Variant?

Post by Old Steamer » Fri Sep 22, 2017 12:00 pm

Jack,
Thanks for the info which will be very helpful.
Regards,
OS
RGSP
Full Steam Ahead
Full Steam Ahead
Posts: 246
Joined: Mon Dec 14, 2015 2:12 pm
Boat Name: Platypus, Shelduck
Location: Very eastern England

Re: Stuart 6A Valves Variant?

Post by RGSP » Fri Sep 22, 2017 4:00 pm

Thanks from me too on that Jack. I could have taken a steel rule to my 6A2, but that's a lot better. Having thought about it a bit, for a 6A running at 100 - 120 psi normally, and without much if any superheat, I think I'm in favour of simple slide valves. It wouldn't be difficult to put a strain gauges on valve operating rods, and actually measure how much more force the slide valves needed as compared with piston valves, and I suspect the difference will be small in terms of engine power. Furthermore, the square/cube relationship between volume and surface area when scaling suggests to me that there is little need to increase port areas in our small steamboat engines of only a few HP, and running them at 200 or 300 rpm - a different matter when large VTE engines are considered, or indeed most later locomotives required occasionally to run at high speed.

There is a good amount of recorded evidence on locomotive valve gear from around 1900 when pressures were rising, and superheaters were being introduced - really for fuel economy. With boiler pressures of 150 -170 psi or thereabouts, the evidence in favour of superheating was a bit thin: it probably worked quite well, but with stop-start working and small goods engines, any advantage was hidden by other factors. The thing that WAS clear though, was that the simple slide valve engines had a much lower maintenance bill, and indeed often outlived their superheated/piston valve cousins from the same class.
User avatar
barts
Full Steam Ahead
Full Steam Ahead
Posts: 1069
Joined: Wed Nov 18, 2009 6:08 am
Boat Name: Otter, Rainbow
Location: Lopez Island, WA and sometimes Menlo Park, CA
Contact:

Re: Stuart 6A Valves Variant?

Post by barts » Fri Sep 22, 2017 5:44 pm

RGSP wrote:...
There is a good amount of recorded evidence on locomotive valve gear from around 1900 when pressures were rising, and superheaters were being introduced - really for fuel economy. With boiler pressures of 150 -170 psi or thereabouts, the evidence in favour of superheating was a bit thin: it probably worked quite well, but with stop-start working and small goods engines, any advantage was hidden by other factors. The thing that WAS clear though, was that the simple slide valve engines had a much lower maintenance bill, and indeed often outlived their superheated/piston valve cousins from the same class.
Indeed. Piston valves are a more complex design, and early small versions that omitted the rings on the piston valve (such as the ~1893
engine in our Otter) quickly start leaking steam as the valve wears. Slide valves take up automatically for wear, which is a substantial win
if compatible materials (cast iron/bronze on cast iron) are used.

Keeping the slide valve size reasonable helps with reducing friction and wear, as does a robust valve gear. A spot of lubrication doesn't hurt either, esp. if the steam coming from the boiler is dry.

- Bart
-------
Bart Smaalders http://smaalders.net/barts Lopez Island, WA
User avatar
cyberbadger
Full Steam Ahead
Full Steam Ahead
Posts: 1123
Joined: Thu Nov 07, 2013 9:16 pm
Boat Name: SL Nyitra
Location: Northeast Ohio, USA

Re: Stuart 6A Valves Variant?

Post by cyberbadger » Fri Sep 22, 2017 8:25 pm

Nyitra's 1902 vintage Toledo steam carriage engine has Piston valves and someone confirmed my suspicions that there is some wear that can cause it to sound a little weasy, blowby. I'm going to try to open it up this winter and see if some replacements can be made. For what I am doing, a little steam cylinder oil can help temporarily, but it doesnt 100% fix it I'm thinking what I'd like is a crank or hand pump just when I want to to give it a dollap. What I do right now is over open the valves on the Swift to a high flow setting. Then I forget and the Swift had guzzled much more then I want. I like the mechanical crosshead driven feed lubricators that have a hand crank for manually lubricating before engine starting. traction engine folks use em a lot, don't know if I have seen any small ones for sale.
Those old school double valve pressure lock chamber things with the bowl would be a cool option, or just a tiny hand pump to pressurized steam npt.

- CB
User avatar
Lopez Mike
Full Steam Ahead
Full Steam Ahead
Posts: 1903
Joined: Wed Dec 07, 2011 6:41 am
Boat Name: S.L. Spiffy
Location: Lopez Island, Washington State, USA

Re: Stuart 6A Valves Variant?

Post by Lopez Mike » Sat Sep 23, 2017 5:45 am

Interesting. I've never seen an engine that didn't have rings on the piston valve. I've had several full sized locomotives apart and all of them had rings. My 3" x 4" engine has rings.

It would seem like false economy in construction to omit them. Any wear at all end they would start leaking like crazy. I see condensate pumps that depend on water grooves but they operate in a sea of moisture.

Learned something new today.
If you think you are too small to make a difference, try sleeping with a mosquito.
Dalai Lama
steamboatjack
Full Steam Ahead
Full Steam Ahead
Posts: 283
Joined: Fri Nov 20, 2009 8:02 am
Boat Name: grayling
Location: Cumbria U.K.

Re: Stuart 6A Valves Variant?

Post by steamboatjack » Sat Sep 23, 2017 8:07 am

There is an old saying, that piston valves wear out & slide valves wear in. Although I would not deny this as a general fact, I have built a number of successful engines with piston valves without piston rings. All have the same features, “water grooves” on the piston valve diameter; the valve should “float” on its operating rod which is guided top and bottom. A precision fit in the liner is essential, in the region of 0.001” per inch of diameter (assuming liner and valve are similar materials). Steam oil lubrication is to be used and little or no superheat. If you consider the amount of running hours accumulated by the average hobby steam boat then with care these should last well before needing renewal. Finally I disagree that the power difference to drive a slide valve versus a piston valve is low, it is considerable and leads to wear on the valve operating train.
Regards
Jack
User avatar
DetroiTug
Full Steam Ahead
Full Steam Ahead
Posts: 1863
Joined: Fri Nov 27, 2009 5:56 pm
Boat Name: Iron Chief
Location: Northwest Detroit

Re: Stuart 6A Valves Variant?

Post by DetroiTug » Sat Sep 23, 2017 2:33 pm

As Jack stated, the difference in mechanical power to actuate a slide valve versus a piston valve is great.

Calculate the area of the valve face and simply multiply that times the steam chest pressure or MEP. That is how much load there is holding the valve against the cylinder face.

If the slide valve face measures 2" X 3" which is six square inches of area and the MEP is 100 psi, there is 600 pounds of force holding the valve to the cylinder face, it fluctuates a bit with valve events, but the fact remains. It's like a 2" X 3" iron bar that weighs 600 pounds standing on it's end sliding back and forth against a plate. That is why a good steam cylinder oil is so important.

With the piston valve, the pressure is in all directions thereby balancing itself. Yep, several small piston valve engines don't have rings on the piston valve, just grooves.

The important advantage with a slide valve is they tend to release unintended carry over (water)as the valve can lift off the face and release it, a piston valve cannot. Many of the early slide valve production engines had no cylinder drains at all. My Locomobile with it's twin simple and 250 psi operating pressure of which over 5000 were built and sold has no cylinder or steam chest drains at all.

-Ron
Post Reply