Expansion link Stephenson valve gear

[ausdan]

Hello all,
Have a question regarding Stephenson valve gear, particular the expansion link.

I own a 5" Hunslet Don Young design, had it for close to 3 yrs and currently have the boiler off and remaking some of the valve gear as it very loose and worn. I know from the previous owner that they had modified the valve gear (as seen in the photo) at some time in the previous life...Im planning to replace it with the original valve gear from the Don Young drawings I own.

Question is what would of been the advantage in (current modified style)having the eccentric rod mounts closer to the mid section of the link on a different radius, opposed to the traditional mounting on the ends of the expansion link...the picture is the original link from the loco (top), and the partly finished link as per the original drawings (below).

THe loco performs fairly good, and is very lively(responsive) at the moment, I suspect I will get a slower valve event once modified ? Any other Hunslets owner out there or others care to comment.

cheers
Danny

[steam4ian]

G'day Danny.
Your old link shows it was set up a "launch" valve gear.
The two eccentric rod connections are close to magnify the eccentric motion, it allows a more compact arrangement.
If you change the connections to the link you will have to change the whole geometry, ie eccentric offset, eccentric angle, rod length, lap and so on.
If the loco works well with the old motion don't change it.

Regards,
Ian

[AndrewP]

Hi Danny
I've got a Don Hunslet but not reached the valve gear stage yet. No details in the build instructions in LLAS on the valve gear design which is loco link as Ian points out.
My manual on Stephensons link motion says that for launch link the eccentric throw will be half the valve travel and for loco link the throw and settings of the eccentrics will be different.

Cheers, Andy

[Shawki Shlemon]

There are two types of links Launch & loco type . ( top picture is launch and the bottom is loco ). With the loco type one needs longer rods and greater off set in eccentrics to produce enough travel for the valves . In a loco type you will require an off set in eccentrics greater than 1/2 of the valve travel while in a launch type you will require an eccentric off set less than 1/2 the valve travel .It is not a simple change over operation . You will need to calculate and work out the valve travel, eccentrics off set .and the length of rods , not mention the angular effect . The best thing to do is to stick to what it is and fix the parts that are worn.

[ausdan]

I see were you guys are coming from...if I choose to copy whats on my loco now (launch), I will have to estimate from the worn parts...(as it was originally loco type before that)

THe drawings I have are of the LOCO type, measuring/comparing whats on my loco now, both use the same eccentric sizes,eccentic positions, valve rod, reverse link..

so I will need to make another 2 expansion link, 2 eccentric rod and 2 lifting links...which are pretty worn anyways..

Would anyone comment on how you would think the loco will drive, using the LOCO link type

thanks for the input

[AndrewP]

Using a loco link expansion link with the same 7/16 throw eccentrics will reduce the valve travel to less than the 7/8 you would have been getting with a launch link, by how much I can't work out.
As you say that would be to drawing BUT are your valve and steam ports to drawing?
Isn't it great working on an engine that somebody else built with all the undocumented alterations from spec?

[steam4ian]

G'day ausdan.
Instead of making new gear can't you either make larger pins or bush the holes. I presume that to change the eccentrics you would have to press the wheels off the axles.
Open the steam chest and find the maximum valve travel, this will be shown by the wear on the valve face. Importantly find the minimum travel without the inlet ports being uncovered, ie 2 x lap. Your eccentric throw could be measured with a dial indicator when you rotate the shaft. Put the dimensions into a valve gear program and reverse engineer it.

That the loco worked well, even with worn linkages, suggests that the design is pretty sound.

I recall reading that for launch gear the eccentric rods are often crossed; others may be able to give more definitive advice in this regard.

Regards,
Ian

[ausdan]

Ok....
Im now convinced, will reproduce what I have (launch)...if it ran with all that slop, it should run better with some new parts ..

(I had re bushed and made a new guide blocks to get me out of trouble last time, but will go through and replace all, as pins worn, bushes worn, loose)

Want to her get back on the rail, so I can free up room in the shed for my 7.25 Phamton project