The following is from the Valve Gear series of articles I wrote for Steam in the Garden magazine. I don't do this every day so I have to keep the articles around for every time someone asks the question.
The following table lists the relation of which end of the link the radius rod is in for “Forward” direction, the relation of the Eccentric to the Drive pin (for forward), where the Valve Stem connects to the Combination Lever, and the Admission type.
Stem to Lever:
The second column of this chart shows that for "Forward" direction: the radius rod should be in the "Bottom" of the (Walshaerts valve gear) Link, if the Eccectric "Leads" the drive pin and that the valve stem is connected to the top "End" of the combination lever for "Outside" admission.
Or, is in the "Bottom" of the Link and the Eccentric "Lags" the drive pin and the Valve Stem connects to the Combination Lever between ("Mid") the outer ends if it is "Inside" Admission.
(The last two columns are actually kind of useless since no self-respecting valve gear designer would have the radius rod in the Top of the link for forward gear; they are there just for completelness.)
And the caveat... if the "toy" valve gear is simulating a Piston gear with a "D" (or slide valve) gear then you have to know that it is really outside admission (and use the 2nd column data instead of the 3rd column data). But this makes for a visual error in the toy (when compared to the real thing) in that the eccentric position and the connection to the Combination Lever are "wrong" for the prototype... but then... some toys have a method of reversing the ports so that when the valve is to the rear, the rear of the cylinder is getting the steam and that in turn re-reverses the eccentric and combination lever connections to make them look right and you have to know if the design has that little trick installed.
Based on the connections to the Combination Lever (that verticle stick that connects from the valve stem to the cross head (via the short horizontal link))... the stem from the valve is connected BETWEEN the outer ends of the Lever and the Radius Rod from the (Baker) valve gear is connected to the Upper End of the Lever... I am GUESSING that it actually has piston valves and thus the eccentric should TRAIL or LAG the drive pin when moving forward. Thus I am GUESSING you have it backwards (from the photo it is hard to tell which way you have it, but I THINK you have it leading the drive pin).
Here is a silly compromise... there are two engines, set one to lead and the other to lag. Then apply some air pressure and see which one runs right. Sure, you are then guaranteed to have to fix one of them, but if you guess both the same way then you have a possiblilty of having to correct both of them.
The angle formed by lines from the center of the drive pin to the center of the axle and from the center of the eccentric pin (eccentric rod connection) to the center of the axle MUST be 90-degrees.
The drawing below shows the angle... but the eccentric is on the wrong side for your application... mirror image it!
Posted By JEFF RUNGE on 11/23/2008 6:30 PM
What I gave him is my set up on the Aster Berkshire which has Baker valve gear, which is what the engine in the picture has. IT WORKS, The figure 12 you is for the other side, if you roll the driver so the main pin is forward, 3 o clock the and the eccentric pin will be at about 10 o clock, with the center of the clock being the center of the axle We are in agreement but describing it two different ways You are looking at it with the Main Pin at BDC or 6 o clock I am describing it with the main pin full forward. as we say "six of one, half a dozen of the other"
I have to admit to being befuddled by your statement.
90 degrees on a clock face would be 9 and 12, or 12 and 3, or 3 and 6, and 6 and 3, for just the quadrature points. Any two times separated by 3 hours (e.g.: 10:41 and 1:41, 2:22 and 5:22, etc.)... 90 degrees is 1/4th of 360 and 1/4th of 12 hours is 3.
9 and 2 would be 5 hours difference which is 5/12ths or 150 degrees.
Any valve gear that uses one eccentric (per cylinder/valve pair) must be at 90 degrees if it is to run well in both forward and reverse. Stephenson's has 2 eccentrics (per cylinder/valve pair) and so valve lead can be accomplished by setting each eccentric for what ever is needed. But Walschaerts or Baker or Southern or any of a dozen other single eccentric valve gear (that is designed to reverse) must use 90 degrees and the Valve Lead is accomplished in the linkages, usually via a Combination Lever.
My Fig 12 is for the front of the engine to the right and forward motion is clockwise of the wheel so the eccentric is Trailing the drive pin. If it were to turn so the drive pin is at 9 o'clock then the eccentric pin would be at 6 o'clock, but it is for outside admission and thus the Malley would be (assuming inside admission) the opposite and Lead the drive pin in forward.
edit, I told you I was befuddled... nuttin' new about that... I had to edit the above to correct the "(assuming inside admission)" to make it "inside" instead of "outside".
AH HAH! Jeff, you are looking at the drive pin as the center of the clock! That can lead to error for others as the angle that is formed by the drive rod and the eccentric rod will depend on the stroke of the valve which is controlled by the length of the eccentric arm. The angle should always be based on the axle as the center of the clock. That is the apex of the angle and the angle is always 90 degrees.
Well, since it is a sparky, most of my blathering was useless... I was thinking Live Steam! I should have noticed this was not in the Live Steam forum!
Still, for "correctness" the 30 degree angle of the eccentric arm is meaningless and should not be "remembered" because that is not the place to measure the angle. It is the angle at the axel from the end points of the arm. The eccentric arm is the hypotenuse of the right triangle and it would not necessarily be a 30-60 right triangle.
Yes some model mfgs get it wrong on a sparky because they are not paying enough attention.
But in live steam it has to actually work! and it sometimes "appears" wrong when compared to the prototype. This is because they substituted a "D" slide valve (cheaper and easier to manufacture) for the piston valve the real engine used and thus the model has to be opposite the real one. Few people will notice that error because it is so obscure what it oughta be... I have studied it a lot and still have to refer to charts to remember what it is supposed to be.
You can usually tell the difference between Piston valves and "D" slide valves by the shape of the steam chest (the volume above the Power cylinder)... if it is rounded it is a Piston Valve and if it is squarish it is a "D" slide valve... USUALLY!
From the photo of the loco in question, it looks like both engines are piston valves, but some Mallets had piston valves on one engine (the High Pressure one, usually the rear one) and slide valves on the other (the Low pressure, usually the front, articulated one) and the angle of the eccentrics would be opposite each other and that confused a few folk seeing them not in the same position.
Oh dear, I'm blathering again! Sorry.
1 - 9 of 9 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.