walschaerts valve gear issues

[Bill Shields]

Ok...so the crank has not moved on the pin...this eliminates one potential variable.

But is it in the correct location?

This is not something that can be determined from a photo...it requires on site measurements based on the wheels being in forward and reverse dead center.

[lrdg2150]

An easy and quick test to see if the cranks are times correctly is to put one engine in either front dead center or back dead center. While moving the johnson bar back and forth the valve rod should not move
John

[Bill Shields]

That test assumes that the eccentric rods are the correct length and the eccentric crank circle is correct -> which at this point is not a given set of data points.

I would start with checking that the eccentric crank circle is as designed.

If it is, then you can almost assume that the cranks are the correct length (which is easy to measure) and that they are correctly positioned.

Once you know this...take the next step

[lrdg2150]

That is correct. The original post mentioned that the engine is being restored so I am assuming that the engine has run for a number of years. Of course if the engine never ran correctly then...

[Bill Shields]

Do you know what the theoretical crank circle is supposed to be?

[VGC]

Ok...so the crank has not moved on the pin...this eliminates one potential variable.

I already had seen a toy locomotive of 1 metric ton where the pin had rotated in the wheel bore.

[Bill Shields]

As have I..which is why I consider it a possibility

Knowing and measuring the crank circle eliminates that concern

[Fender]

Could you explain the difference to me Bill,this is the first time I'm hearing about it, thanks.

With outside admission, the high-pressure steam is on the outside of the valve, and when the valve moves to uncover the port to the cylinder, the steam begins going into the port, pushing the piston, crosshead and main rod. Slide valves are always outside admission. Most (but not all) piston valves are inside admission, where the high-pressure steam is on the inside of the valve. The valve gear must move the valve in the opposite direction (with respect to the position of the crosshead) on inside vs. outside admission valves, because the port is being uncovered by the inside edge of the valve vs. the outside edge.

[NP317]

Could you explain the difference to me Bill,this is the first time I'm hearing about it, thanks.

IF you were asking about Inside vs. Outside admission steam locomotive valves, Here goes (assuming non-condensing engines):

Slide valves are "outside admission" with the steam from the throttle is surrounding the valve in the chest and moving into the drive cylinder when the valve moves enough to open the port face to the drive cylinder end. At nearly the same time (depending on Johnson Bar adjustment) the other drive cylinder end port becomes connected (by the chamber under the valve face) to the center exhaust passage to atmosphere. CHUG.

Piston valves can be either inside or outside admission, but inside admission is considered more normal and thermally efficient. (explanation later)
Steam from the throttle is admitted to the center chamber of the piston valve, and is allowed to enter the drive cylinder when the piston valve moves and exposes the port to that drive cylinder. At nearly the same time (depending on Johnson Bar adjustment) the other drive cylinder end passage is exposed to the exhaust passage of the piston valve assembly and exhausted to atmosphere. CHUG.

Efficiency:
The slide valve has throttle steam surrounding it and in contact with the entire valve chamber walls, causing greater radiant heat loss due to the exposed surface area. Steam exhausts through a center chamber to atmosphere.
Additionally, the valve control rod must have a high pressure steam seal in the chamber rear wall to prevent the steam from escaping past the rod.

Inside admission piston valves keep the steam from the throttle contained to a smaller volume of the center chamber, limiting heat loss. The exhaust chambers are the outer portions of the valve chamber (with more radiant surface area) where heat loss does not matter because steam is escaping to atmosphere.
Additionally, the piston valve control rod passes through the rear valve cylinder cover, but only has to seal against the lower pressure exhaust steam pressures.

Hopefully I wrote this correctly. (only 1 cup-o-coffee the morning.)
Please add and correct as needed.
RussN

[VGC]

Slide valves are "outside admission" with the steam from the throttle is surrounding the valve in the chest and moving into the drive cylinder when the valve moves enough to open the port face to the drive cylinder end. At nearly the same time (depending on Johnson Bar adjustment) the other drive cylinder end port becomes connected (by the chamber under the valve face) to the center exhaust passage to atmosphere. CHUG.

Piston valves can be either inside or outside admission, but inside admission is considered more normal and thermally efficient. (explanation later)
Steam from the throttle is admitted to the center chamber of the piston valve, and is allowed to enter the drive cylinder when the piston valve moves and exposes the port to that drive cylinder. At nearly the same time (depending on Johnson Bar adjustment) the other drive cylinder end passage is exposed to the exhaust passage of the piston valve assembly and exhausted to atmosphere. CHUG.

Same time??? See valve piston inlet and outlet positions (e.g. red lines for front cylinder chamber) in the diagram below.

[jscarmozza]

Thanks Russ, my project is inside admission. Why would the crank need to be positioned 90 degrees from an outside admission valve as Bill posted. It was that way to start, I repositioned the crank as per the outside timing instructions I posted previously and it wouldn't make a complete turn of the drive wheel when air pressure was applied, it would make a half turn then bounce to a stop. John

[jscarmozza]

Dan I didn't see your post, you answered my question regarding inside and outside valves being 90 degrees out. Thank you.
John