Not All Slide Valves Are Equal

[Dan Rowe]

Trout,
I checked the drawings on John-Tom's site and the design has a 1/64" NEGATIVE lap on the HP end. This is a short hand way of saying the ports are 1/16" wide and the length of the HP end of the valve is 3/64" or 1/64" less than the port width so negative lap. This will defiantly give 100% admission for the HP end.

The last paragraph says to time the valve 90 degrees with the crank or zero angle of advance. He also says try flipping the valve 1/2 a turn or 180 degrees to see which way it works best.

Dan

 

[Troutsqueezer]

Thanks for looking into it Dan. I just came back from the shed. I did the modification as I described above. I figured out I have to remake the valve with a new hollow location and different size to fit the double acting scenario.

As for timing, I spent a few hours fine tuning the system, to no avail. The engine never would do a complete cycle, wouldn't even pretend. I think I'd better stick to the simpler designs until I get more experience.

I'll see how the piston works out. I'd say it's a better fit than my last two engines and they work OK but you may be right.

-Trout

 

[mklotz]

Dennis,

Do you have a boiler? Or perhaps a friend with a boiler?

It would be interesting to try the engine with steam. If it runs under steam, that will be a sure hint that the problem, as George et al have suggested, is the fact that compounds just don't work well with non-expansive air.

 

[Dan Rowe]

A compound steam engine if properly designed will have the same amount of work done on each expansion. As this is set up for the expansion of steam there is no way the cylinder diameters will be optimal for air expansion.

This does not mean that compound air engines will not work they just need to be designed for the properties of air not steam.

For those with an Engineering/Scientific interest see this paper that compares a single stage air expansion with a four stage expansion.
http://www.efcf.com/reports/E14.pdf

Dan

 

[steamer]

You guys have hit it...compounds don't behave well on air...mine included...I end up leaving the drain valves open on the LP as I end up "dragging" a vacuum around .....

Try it on steam....you may be very suprised....

Dave

 

[Troutsqueezer]

Marv, I wish I did know someone with a boiler but I get the impression sometimes that I'm the only model engine enthusiast in the entire Central California area. I'm thinking at this point that I have ruled out all fundamental errors with the possible exception of the piston fit. But like I said, the fit does not look too bad to my untrained eye and if that's the problem here, this design is too fussy for my taste.

Anyhow, it's too late now, I've done gone and modified the cylinder porting to a double acting setup. Tomorrow I'll make a new valve to fit it. Fingers crossed.

Dan - when my brain comes back this way I'll see if I can absorb the theory here. Tomorrow A.M. likely.

Dave, steam is in my future. I'm waiting for Zee to finish his loco as I am learning quite a bit following along.

-dennis

 

[lordedmond]

I apologise in advance if you know how Oil groves , or wire drawing groves work


They are not for oil at all they are steam seals

the theory is that they are intended for steam use only, the steam passes the piston into the first grove and expands , loosing energy in the process, then on to the next grove same thing happens after three groves or so the steam has no energy left thus no leak past

In fact in some turbine designs that use wire drawing grooves for end seals you gan see though the seal at rest but no steam escapes in use , and they do use HP steam

Stuart

 

[zeeprogrammer]

Thanks for the plug Trout.

My fingers are crossed for you as well.

 

[Dan Rowe]

In fact in some turbine designs that use wire drawing grooves for end seals you can see though the seal at rest but no steam escapes in use , and they do use HP steam

Stuart

Stuart,
Thanks for mentioning that. I know about that type of turbine gland called over here a labyrinth seal I just did not spot it on the piston.

Dan

 

[Maryak]

Labyrinth Glands on a steam turbine set are usually supplied with their own gland steam via a gland steam controller, on older designs it was done manually by the watchkeeper. As more load is applied, less gland steam is required and vice versa. At low loads the LP end glands are operating in a vacuum and air would be drawn in causing a loss of vacuum without the input of gland steam.

Hope this helps.

Best Regards
Bob