Prototypically correct valve gear motion for the SP cab forward

Awesome, you're certainly doing the whole package!

It just bothers me. Even on my expensive brass locos they fail to do this correctly. I don't understand why. So, at least on this one I'm gonna do it. The toughest part was sleeving the steam cylinder to reduce the valve stem slapping around so much.

Some really good work!

OK Ron, I am putting on my stupid hat.  I have no clue what this radius rod looks like and wouldn't know it if it dropped on my foot.

I like the video but perhaps a pencil pointing to what it is would help the great unwashed masses, ie me.

rrman.

Google steam locomotive valve gear and there will be many explanations, drawings and videos. The rod coming directly out of the top cylinder or valve cylinder is the valve stem. It is pulled then pushed back and forth by the radius rod. On most model locomotives it is not pushed and pulled as it is supposed to be.

It's a little thing to carp about, but meaningful to me.

Ron 

Nice!

Larry

Many 2-railers don't pay much attention to this stuff.  I should point those folks over here - the ones that are convinced that 2-railers pay far more attention to detail than the 3-rail scale folks.

I do pin my radius rods at other than neutral - most cab forwards were set up so one engine was down and the other up.  Some modelers have hooked up solenoids or tiny motors to move the radius rod to its correct position.  I shall never do that.

Bob,

I guarantee you before I'll buy a new Kohs product or Key product they better get this right or I'm not a buyer.

Thought about doing the radius rod moving, but NAH.

Ron

An eccentric crank on the main driver outside the main rod moves a rod fastened to the reverse yoke. The radius rod in front of the reverse yoke can be moved up and down to make a locomotive go forward, backward, or stand still (center position). It can also be moved up ("hooked up") to admit less steam to the cylinder as a locomotive picks up speed. This conserves steam and reduces "back pressure" - a buildup of steam in the cylinder.

A radius rod serves the same purpose as gears in a manual transmission and clutches in an automatic transmission. They exert full steam / power to get moving and cut back steam / power as speed increases.

Normally, a radius rod is moved down to go forward and up to go backward.

But on SP Cab-Forwards, the eccentric crank was moved back so the radius rod could be moved down to go forward and up to go backwards. This valve gear was called "Indirect Walschaerts" and became popular in the 1920's.

If Cab-Forwards had direct valve gear, the radius rod would be moved to the upper part of the reverse yoke - ordinarily reverse, but forward for a Cab-Forward. Any failure of the mechanism that moved the rod could let the rod fall down into the lower part of the reverse yoke and force a Cab-Forward into reverse, slamming her into her train.

Thanks Reading Fan.  Very clear now what the radius does. I assume the radius rod on the opposite side is positioned one quarter out of the other side, just as the driving wheels are quartered.

You wrote:

If Cab-Forwards had direct valve gear, the radius rod would be moved to the upper part of the reverse yoke - ordinarily reverse, but forward for a Cab-Forward. Any failure of the mechanism that moved the rod could let the rod fall down into the lower part of the reverse yoke and force a Cab-Forward into reverse, slamming her into her train.

Did this failure actually happen, or just a scenario if failed?

Well, everything on the opposite side is sort of 90 degrees off, but you cannot see it.  If you look at either side, they look the same with the radius rod in exactly the same position, eccentric cranks at exactly the same angle.  The reversing yoke typically has swinging or sliding links, but it moves both radius rods to exactly the same angle with respect to the reversing link.

What Ron is doing is commendable.  It is teaching him most of the things one needs to know to create a model from raw brass.  Once you figure out how to build these things, owning a work of art that somebody else built is less than satisfying in comparison.

Cool 

Bob, I appreciate the kind words. I don't think I'll tackle something from scratch. Even though the work on the valve gear has worked out ok, there is too much slop in the radius rods themselves where the die block goes.... too big a hole.  I may go to Stevenson parts or fabricate my own. Haven't decided yet.

Regards, Ron H.

 25 years ago Key imported PRR H8' H9, and H10 2-8-0  with operating valve motion actuated by a slip coller off one of the driver axels.  The Kohs K4s models go one better with fully working power reverse mechanism driven by a small auxilllary DC motor.

Ed Rappe

All expansion links and valve gear done. On to something else.