Topic: inside/outside admission

[locodan]

Hi Tim,
One of the problems with the naming system for steam engine parts and functions is that it is really engineering shorthand.

The simple way I think to understand the difference is to look at the plain D Slide valve. The most common arrangement is three ports. The center port is always exhaust. The other ports that go to the top and bottom of the cylinder are called the steam ports but they only admit steam when the slide valve is not covering them. The steam line is connected to the steam chest which is on the outside of the valve. The steam port is actually the exhaust passage when the slide valve covers both the center exhaust port and the steam port. Full boiler pressure is on the valve stem packing. This is clasic outside admission.

You mentioned Peabody so Plate IX shows a typical piston valve with inside admission. The outside center of the spool valve is at full boiler pressure. The spool valve or piston valve is hollow and one end of the piston valve bore is connected to the exhaust line. The other end is the valve stem and packing gland. The ports in and out of the cylinder are still as always called the steam ports. In this case the central port is subject to steam pressure so It is known as inside admission.

The Docksteader valve gear files can be used to study the difference between inside and outside admission. Most of the common types of valve gear have a file that shows D slide valves and piston valves. If one of these are put in motion and the graphics and sound turned on the whole cycle can be viewed. The program changes color to indicate steam or exhaust. The exhaust chuff can be heard when the "steam port" is connected to the exhaust.

Piston valves can be outside admission. Check Plate IX in Peapody "Valve Gears for Steam Engines" simply change the "S" for steam and the "E" for exhaust and that is now outside admission and full boiler pressure is on the valve stem packing gland. The center port or passage is now connected to the exhaust line, and the center of the valve and the bore of the valve is connected to the steam line. As this just reversed the valve events by 180 degrees the eccentrics also have to be flipped 180 degrees for the proper valve timing.

Professor Cecil H. Peabody was the head of the Marine Engineering Dept. at M.I.T. He started the department and is one of the best authors in my opinion on steam engine valve gears.

I hope I am clearing things up and not confusing the issue more than nessary.
Cheers Dan

[BRMW]

Thanks Dan,
That helps me to understand why they are different.  But don't put away that bar napkin.
Having never seen one, I'm struggling to visualize the mechanical difference.  Does outside admission admit steam through a port at the top and then exhaust it through the bottom port?  And does inside admission admit steam from a port in the middle and exhaust out the top or bottom?

Are there any close-up photos of an inside admission system or an outside system available?  Peabody's "Valve Gears for Steam Engines" includes an illustration but a picture showing the real deal would be great.
Tim

[locodan]

Hi Tim,
Inside admission is not an option for a D slide valve the force of the steam wanting to excape the center exhaust port is all that is really needed to hold the valve on the port face. Some valves such as most Shays had a balance piston that helped to reduce the large force that high pressure causes on the sliding surfaces.

As for piston valves yes inside or outside admission is possible. The prefered arrangement was inside admission because live steam is not on the valve stem gland packing with this arrangement.

Several types of replacement after market valves used outside admission so the same valve gear could be used unchanged. Some of the Shays got Baker Marine piston replacement valves and I have to check the photos but this is believed to be outside admission.

The cylinders used for replacment on S/N 3336 must have been the LLW piston version with inside admission because the photos show that the eccentrics are 180 degrees out of phase with the setup for D slide valves with outside admission.
Note that the same steam bracket was most likely used so the enternal steam passages in the replacement cylinder would have to send the steam and exhaust to the proper holes in the steam bracket. For those not familiar with Shay terms the steam bracket is the steam and exhaust manifold which also served as the upper cylinder mount for most of the Shay engines produced. The steam bracket is bolted to the boiler and the back side of the cylinders are pined and bolted to the steam bracket. The steam bracket is hidden behind the engine cylinders so it is rarely seen.

I hope that helps. Does anyone need the bar napkin version?
Cheers Dan

[BRMW]

Hi Ferd and Dan,
I'm gratefull for your encouragement to ask newbie questions.  Pull out one of your cocktail napkins please.
There was discussion about SN 3336 valves on LivesteamShays.  Inside and outside admission was mentioned.  What's the difference?
Is inside admission an option for slide and piston valve gear?
Tim