# Steam Engine Rotation



## Brian Rupnow (Sep 17, 2011)

I'm curious---On a slide valve engine like the Overcrank single I am currently building, and on Elmers #33 mill engine, what determines which direction the engine will rotate. I think I know the answer, I just need confirmation. In theory, when the piston is exactly at mid travel in the cylinder, the slide valve should be centered in its travel. If this was truly the case, the engine should run equally well in either direction. I assume that by offseting the valve a bit from center, either way, when the piston is at mid travel, provides a bias to rotate in one direction. Have I got that right?


----------



## kcmillin (Sep 17, 2011)

I think maybe, perhaps, the eccentric has to be flipped 180 degrees on the crankshaft in order to reverse. But thats about all I "Think" I know. :shrug:

Kel


----------



## doc1955 (Sep 17, 2011)

kcmillin  said:
			
		

> I think maybe, perhaps, the eccentric has to be flipped 180 degrees on the crankshaft in order to reverse. But thats about all I "Think" I know. :shrug:
> 
> Kel


Correct Kel.


----------



## rleete (Sep 17, 2011)

On my Coke bottle engine from PM Research it says it's just a matter of timing with the cam.


----------



## Mainer (Sep 17, 2011)

180 degrees, or is it 90 degrees? Seems to me it's 90, but what do I know.... That's not worrying about cutoff or any of that stuff.

A related detail: do you know the correct direction for the engine to run? Answer: it should run "over" rather than "under," that is, when the piston rod is moving out of the steam chest the connecting rod should be pushing the crankshaft "over" the top rather than pushing it "under." The reason has to do with how pressure is being applied to the crosshead bearings.

My old mentor, Dan Fay, shamed me into changing the rotation of my Stuart Victoria so it was proper.


----------



## Captain Jerry (Sep 17, 2011)

When the valve is at mid position, one of the steam ports has just closed and one is about to open and the piston is at TDC on the end that is about to open and BDC on the one that has just closed. If you attempt to rotate the flywheel in the forward direction, the correct port will open and the engine will run. If you attempt to rotate it in the wrong direction, the port that has just closed will be reopened and you will feel air pressure resist rotation. A few degrees mistiming of the valve will affect the efficiency but will not cause it to run in the opposite direction. To get it to run the other way, you have to rotate the eccentric 180 degrees or use something like a Stephenson Linkage to pick up the motion from another eccentric. The Eccentric throw PRECEEDS the crank throw in the direction of rotation.

However if you are using supercooled steam it may not rotate at all due to thermal inversion and hyper indirection.

Jerry


----------



## Richard1 (Sep 18, 2011)

With outside admission valves (most slide valves and outside admission piston valves) that are driven directly instead of through a rocker the eccentric precedes the crank. Some mill engines have the valve on top of the cylinder and are driven through a rocker which reverses the eccentric motion. In this case the eccentric trails the crank.

Reverse all of this for inside admission valves.

Richard


----------



## Brian Rupnow (Sep 18, 2011)

I may have made a serious mis statement in my very first post here. Looking more closely at my model, it appears that when the piston is fully retracted into the cylinder, that is when the slide valve is centered in its travel. Now I have to back up a bit and do some research to find the truth of this matter.


----------



## Brian Rupnow (Sep 18, 2011)

This is what Elmer has to say about timing the #33 mill engine
_At the final assembly turn the crank to one dead center position (full stroke either in or out of the cylinder) and tighten the eccentric with its axis 90 degrees from the centerline through the crank. Temporarily hold the steam chest in place while adjusting the valve to equally expose the valve holes at each end of stroke._
 And I suppose that depending wheter you choose top dead center or bottom dead center to do this set-up is what determines the direction your engine will turn in.


----------



## Jasonb (Sep 18, 2011)

See my reply in the other thread

_"If its at 90 deg it will run in either direction but not that well, better to give it 20 to 30 deg or so of lead in the direction you want it to run.

By admitting steam/air into the end of the cyl before the piston has reached the end you get a cushioning effect and a much smoother running engine."_


J


----------



## Tin Falcon (Sep 18, 2011)

From what I have seen most engines the valve precedes the piston by 90 degrees. 
so


> I think maybe, perhaps, the eccentric has to be flipped 180 degrees on the crankshaft in order to reverse. But thats about all I "Think" I know.


 would be a correct statement. 
Tin


----------



## Ken I (Sep 18, 2011)

See prior post on Gerry's Beam Engine

http://www.homemodelenginemachinist.com/index.php?topic=11452.30

Valve cycles 90° from crank - flip 180° to reverse.

Ken


----------



## Captain Jerry (Sep 18, 2011)

Here is an Alibre animation on screencast.com. The red pointer represents the position of the crank throw and the piston. You can drag the slider to control the position. The corect rotation for this setting is clockwise.

Jerry

http://www.screencast.com/t/XkQ8s0m3aD


----------



## Jasonb (Sep 18, 2011)

Have a look at this pdf on Stuart models site, again suggests the eccentric should lead the crank

http://www.stuartmodels.com/pdfs/SM-Timing.pdf

J


----------



## JorgensenSteam (Sep 18, 2011)

In order to have an accurate conversation about valves and valve timing, a few things need to be defined, such as steam and exhaust port layout, valve inside and outside lap, angle of advance, and valve travel as defined by the eccentric throw.

Then the valve and eccentric have to be set correctly.

To discuss steam engine valves/ports as if they were all the same with the same settings really does not do justice to a true understanding of valve gear.

Not to be flippant, but it reminds me of a conversation with the wife.
She said "I saw a great car today", "really, what kind" I said.
"Blue" was the wife's response. Well that clears it up.

Pat J

Edit: There are four main valve events which define how the engine runs, and they are admission, cutoff, release and compression. Assuming a standard D-valve with outside admission, the admission and cutoff are determined by the outside lap, and the release and compression are determined by the inside edges of the valve. Inside lap is often zero. Outside lap and angle of advance for model engines is often zero. Without knowing the port/valve/eccentric throw/valve travel arrangement, any discussion of such has little or no meaning.

Edit02:
The drawings for Jerry's Beam engine above seem to show a typical valve/port arrangement for a model engine, ie: no inside or outside lap on the valve, and no cuttoff (steam is admitted to the cylinder for the entire stroke of the piston).
The angle of advance appears to be zero, ie: the valve is not admitting steam into the cylinder until the piston begins its downward stroke.
Some small models will not run with an angle of advance included, since the mass of the flywheel may be too small, and the piston will just stop before it reaches TDC.
The Jerry's Beam engine drawings also show an exhaust port equal in size to the steam ports, but typically the exhaust port is twice the width of the steam ports.
The Beam engine valve/port design is a safe design, ie: it will always work, but as engine size increases, generally a more accurate valve/port/eccentric design is desired since this will greatly affect engine performance and efficiency.
I think steam engine valve gear design is one of the least well understood topics that is discussed on forums.
If Brian's engine does not have the same valve/port/eccentric characteristics that Jerry's Beam engine does, than any comparison between the two is likely to be misleading at best.

Edit 03:
To answer Brian's original question of which way the engine will run depends on whether the valve is about to admit steam to the top or bottom of the cylinder when the crank begins to rotate the engine away from either TDC or BDC.
I believe large horizontal steam engines ran a certain way so that the forces on the crosshead help lift it against gravity, and reduce friction on the crosshead guides. For a vertical engine, it should not matter which way the engine runs, but there are valve considerations for vertical engines, and often asymetrical valves are used for vertical engines to counteract the forces of gravity, with unequal admission points on the upward and downward strokes.
Since the throw of the eccentric can vary from engine to engine, one way to figure out how much you need to rotate the eccentric in order to reverse the engine is by comparing the angle of rotating the eccentric between that required to begin admitting steam to both the top and bottom of the cylinder.

Edit 04:
Normally, on a full sized steam engine, you rotate the engine to TDC (assume this is a crank position of zero degrees), you then rotate the valve to the midpoint position (typically 90 degrees ahead of the crank zero position, then you continue to rotate the eccentric so that the valve is at the point of admitting steam to the top of the cylinder, which is generally another 55 degrees (thus the term 55 degree eccentrics), and then rotate the eccentric any additional degrees for admitting steam before TDC if desired). Dual eccentrics for a standard steam engine would need to be about 110 degrees apart in order for forward and reverse to work properly.


----------



## Dan Rowe (Sep 18, 2011)

Pat, You are over thinking this a bit. Sure to have a conversation the terms have to be defined by the author but I have well over 50 books on the shelf on valve gears only and not all of the authors agree on the terms which is very frustrating for a reader.

To Brian's original question. I am going to use a set up used by F.A Hasley in _Slide Valve Gears_ for the explanition.

Let us assume zero lap for an air engine. This makes the angle of advance equal to zero. Now we will use a scotch yoke for the piston rod and the eccentric rod so there is no error in the connecting rods. Now when the piston is at top or bottom dead center the D slide valve is in the center or mid-stroke. This means that the crank and the eccentric are 900 out of phase. A simple drawing exercise will show that the eccentric leads the connecting rod in the direction of rotation. So to reverse the engine the eccentric has to be shifted 1800 to be 900 out of phase with the connecting rod the other way.


I hope this helps a bit. If not I will post a drawing.

Dan


----------



## JorgensenSteam (Sep 18, 2011)

Dan-

I guess the point I am trying to make is that all valves and valve gear are not the same, and so it is important to qualify what type of valve, port layout, eccentric throw you are discussing, since the discussion will vary greatly with the actual layout that you have.

I am all into keeping it simple, but some may want to know a little more than the basic no-lap valve.
I would not want to limit the discussion to simple valve mechanisms just because some only want to use simple valve mechanisms. Some may want to learn more.

The basic terms which can be used for any engine are the port dimensions, the location of the inside and outside edges on the valve, and the valve travel. Universal steam engine events are admission, cutoff, release, and compression. These are the only terms needed for a full analysis of a basic D-valve arrarngement.

Pat J


----------



## JorgensenSteam (Sep 18, 2011)

I think the easiest approach to understanding the reverse of a steam engine is to look at an engine with a Stephenson's link and two eccentrics, and note the position of the two eccentrics with regard to each other, and note the position of the valve when you change the link position from "forward" to "reverse".

You can easily do this with Dockstaders program.

Pat J


----------



## Dan Rowe (Sep 18, 2011)

Pat,
I have only really looked into Stephenson reverse in any depth because it was the most common reverse used on Shay locomotives.

Most of the things you mentioned are the finer points of valve gear design. The thing that will flip the crank and eccentric phase angles around is inside or outside admission. That is why the Dockstader programs have inside and outside admission for several of the most popular arrangements. That is why I stated D silde valve which has outside admission. A piston valve has inside admission and the logic is fliped.

Dan


----------



## JorgensenSteam (Sep 18, 2011)

I really like the Stephenson's link since you can vary the cutoff as the engine is running, as well as use it to reverse the engine.

I like the Shay engine since it is a 3-cylinder, which is quite interesting and different than most steam engines.

Pat J


----------



## Dan Rowe (Sep 18, 2011)

Pat, Stephenson gear is not the only gear with variable lead. The advantages over variable lead and constand lead and other finner points of gear design fill pages and all authors have different slants. I like Peabody as he was the head of the Marine Engineering Dept. at MIT. He states his bias for marine engines and sheds some light on other views.

I am most found of the early 2 cylinder Shays but I like all of them. 

The other thing that will flip the logic with Stephenson valve gear is the use of rocker. Most locomotives in the US with Stephenson gear had rocker. Shays ere like a marine engine and have direct Stephenson gear with no rocker. It took me a while to figure out how to make Dockstader work for slide valves and no rocker.

Here is a pop quiz for a Shay 3 cylinder locomotive with D slide valves. The management has decided to change the valves to piston valves. 

Shay cranks have the eccentrics keyed to the shaft. the cylinders are numbered 1,2, & 3 from the back to the front. Cylinders 2 and 3 are mirror images of cylinder 1. The eccentrics for cylinder 1 and 3 can be removed off the ends of the shaft. The #2 eccentric is a two piece split eccentric keyed to the middle of the shaft.

Now we have to decide if we need new parts for the valve gear or can we get by with a shop fix?

Dan


----------



## Maryak (Sep 18, 2011)

for more see attached pdf.

Best Regards
Bob 

View attachment SVrot.pdf


----------



## Dan Rowe (Sep 21, 2011)

Bob,
Which edition of Verbal notes and Sketches  is that from? I have the 13th edition on my shelf.

Lookong at Brian's single cylinder overcrank engine thread again made me thing of one more thing that can flip the logic of a steam engine. Just add a return connecting rod. That type of engine has the crankpin 180o out of phase with the normal arrangement. I did not take that in account with my valve study in that thread but just add 180o more to the answer I gave in that thread using Bilgram's diagram.

This is exactly what I was getting at with the Shay question. I know that several of them had after market piston valves added so that would flip the valve logic. If the piston valve has port dimensions to match the D slide valve all we have to do for #1 and #3 cylinder is swap the eccentrics as they are mirror images. The #2 cylinder is not quite as easy if there is room on the split eccentric a keyway could be cut 180o from the original other wise the key in the crank has to be moved.

Dan


----------



## Maryak (Sep 21, 2011)

Dan Rowe  said:
			
		

> Bob,
> Which edition of Verbal notes and Sketches  is that from? I have the 13th edition on my shelf.
> Dan



18th in 2 Volumes.

Best Regards
Bob


----------



## blackfoxsteam (Jan 17, 2014)

This thread may no longer be active but I have a question regarding JorgensenSteam's Edit 04 on page 2.  Is the 55 degrees eccentric advance mentioned for a full sized steam engine what is required to clear the slide valve outside lap (to start admitting steam to the top of the cylinder)?


----------

