Valve Gear Questions-For the technically adept

Superheater · **Posted:** Wed Jan 29, 2003 7:38 pm

I was wondering if some of the readers with some mecanical acumen could answer a few questions.

A.) In valve gear, (specifically walshaerts) it seems that the effect of the eccentric crank is to reduce the diameter of the circular motion imparted by the main crank, thus reducing the horizontal motion of the eccentric rod and associated components.

1.) Am i correct with regard to above?

2.) what determines the length of the eccentric crank

3.) What determines its angle, why not just have a shorter eccentric crank that points directly at the axle-producing the same circular diameter?

4.) does the angle have any effect on lead? lap?

B. what purpose does the combination lever and union link serve? How do they accomplish that purpose?

If anybody can answer in novice terms, I'd be appreciative.

Superheater@rrmail.com

dave · **Posted:** Wed Jan 29, 2003 7:59 pm

Oh Boy.......

No doubt many can do better than I but here goes.

The crank imparts motion to the link. It has to be hung outside of the rods otherwise the rotation of the drivers would shear it off. Its location is not only based on the designed valve travel - how far it moves back and forth - but also based on at what point within the rotation of the wheel the valve is made to move.

Walschaerts gear is direct in one direction and indirect in the other - that is to say, in the forward (or reverse) corner the valve is driven in the same direction as the crank is pushing the link, and in the opposite corner it goes the other direction. Since the same crank activates the valve in both forward and reverse gear, using it to create lead isn't going to work real well - advance in one would be retardation in the other.

Lap is designed into the valve and passage openings and not valve gear controlled. The union link, etc provides lead in both directions equally, by using the motion of the crosshead to alter the link-directed movement of the valve.

There is a great book Gresley wrote for British live steamers out there that explains it very nicely.

Dave

Roger Mitchell · **Posted:** Wed Jan 29, 2003 9:53 pm

There is yet another outside valve gear called the Young valve gear. The Young gear was mainly intended as an after market replacement for the Stevenson gear but many locomotives were also built new with it. The Young gear takes it's motion entirely from the crossheads and elimenates the eccentric cranks of the Walshaerts and Baker gear and the axle mounted eccentrics of the Stephenson gear. UP #5511 is one of the last surviving examples of an application of the Young gear.
Roger Mitchell
Master Mechanic
Fort Collins Municipal Railway

Fort Collins Municipal Railway home page
n0mcr@netzero.net

John Stewart · **Posted:** Thu Jan 30, 2003 10:01 am

Superheater:

"Use the source, Luke"... Try the following software - it gives a neat overview of valve gears. Live steam modellers use it quite a bit.

It runs under Windows, or (if you are like me) under the Wine emulator on Linux.

If a picture is worth 1,000 words, this software is worth at least 100,000.

BTW - I don't think your assumption about the eccentric crank is correct - view changes with the software. ;-)

John Stewart

> I was wondering if some of the readers with
> some mecanical acumen could answer a few
> questions.

Valve gear software.
freewrl-1@rogers.com

Superheater · **Posted:** Fri Jan 31, 2003 12:19 am

> Superheater:

> "Use the source, Luke"...

Thanks John. Apparently, Charlie is Ben Obi-Wan, perhaps we should ask him to do an article !!!!

P.S., would LOVE to tell M$ to take their register-ware and shove it-but can't get my Suse 8.0 to run on my year old 1.6 P4 the way it ran on my old K-6-2 350. Alas, I am stuck with the Windrool-especially because NTFS file structure isn't partitioned as easily as FAT 32-so I can't dual boot until I find a Linux modem driver!!!!

Superheater@rrmail.com

James Hefner · **Posted:** Fri Jan 31, 2003 11:11 am

> P.S., would LOVE to tell M$ to take their
> register-ware and shove it-but can't get my
> Suse 8.0 to run on my year old 1.6 P4 the
> way it ran on my old K-6-2 350. Alas, I am
> stuck with the Windrool-especially because
> NTFS file structure isn't partitioned as
> easily as FAT 32-so I can't dual boot until
> I find a Linux modem driver!!!!

Not to get too far off-topic here, but Red Hat Linux lets you set up a Unix file system as a file on a NTFS volume.

I did so with a temporary Linux installation on a Windows NT 4.0 workstation (wanted to see if my "Surviving World Steam" programs would work under Wine). It seemed to work OK.

-James Hefner
Hebrews 10:20a

Surviving World Steam Project
james1@pernet.net