BR Standard class 6 No. 72010 'Hengist' and Clan Discussion Thread

As I undertstand it the reason the outside valve gear is better than the inside valve gear on the Bulleid pacifics (and illustrated in your appended picture) is the fact that the top of the combination lever is suspended from a swinging link, unlike the arrangement for the inside cylinder which is simply connected to the end of the valve rod which, in turn is carried in guides, an arrangement used on the majority of Walschaerts fitted locos.

 

There are two basic valve gear arrangements on the BR standards as I'm sure you'll know. The Pacifics (71000 excepted!) and the Cl.5 & Cl.4 4-6-0's have the radius rod suspended from the lifting link. The remaining ones have the radius rod path constrained by a die block arrangement situated behind the expansion link. The former arrangement gives good valve events in fore gear at the expense of poor events in back gear due to excessive die block slip in the expansion link, as I mentioned in my previous post referring to the S15's.

 

You have missed the 9Fs off the list, Steve. I wonder if there is any mention of the valve gear design in Cox's book?

 

Any chance of some pics plse Steve?


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I spent a bit of time looking for clear pics from @Steve description (not my photos):

"The Pacifics (71000 excepted!) and the Cl.5 & Cl.4 4-6-0's have the radius rod suspended from the lifting link." (Photo is 75078)

"The remaining ones have the radius rod path constrained by a die block arrangement situated behind the expansion link." (Photo is 80151)

Tom

 

Perfect, and I should have known having worked on 80002 and helped restore 75078. In fact I’m a member of the Std4 Loco Soc!

 

Here's the relevant section from the LH motion arrangement drawing for class 7 and class 6. The motion is identical to both classes.



As Jamessquared says, the radius rod is suspended from the lifting arm.

Our valve gear expert comments that the valve travels at different cut-offs have variations from end to end, although they make no difference to performance at less than 50% cut off - at 78% the difference in port opening front to back is 19/64", at 50% 6/64" and at 20% 1/64". The geometry may not be perfect, but that is common with many other versions of Walschaerts on a multitude of engines worldwide. 70000 and 70013 have the same motion and are running satisfactorily.

 

Thanks Tom, saved me the trouble. It's interesting to put the relevant parameters into Dockstader and see just how much die block slip there is in reverse with the lifting link arrangement.
If you've driven the S15 on the B.B. and it is anything like 825 on the NYMR the 'kick' at the reverser in back gear is very noticeable. In forward gear there is nothing but put it into back gear and, if you run without the reverser shaft lock actuated, it is very easy for it to jump out of the catch and move into forward gear. This is what happened a few years ago resulting in the unfortunate death of a guard. The Standards are nothing like as bad but the reverser screw arrangement is totally different. The N & the U have the radius rods similarly suspended but the valve gear arrangement on these is back to front in any case so I don't know what effect it has.

 

As a non-expert in valvegear matters, I’m still trying to rationalise all this into a form my ageing brain can understand.

Am I right in thinking that the gear which utilises a sliding block to move the rod up and down in the expansion link gives totally accurate positioning, whereas the pivot link will result in some shuffling up and down within the expansion link as the rod moves to and fro?

If I’m right, doesn’t this accelerate the wear between the expansion link and the dieblock over time?

 

It's more complicated than that.

Assuming a hanging link from the lifting arm, this will swing like a pendulum as the radius rod is moved backwards and falling, so the rear of the radius rod is also rising and forward with this pendulum movement. However, the point of contact within the expansion link changes, partly due to the vertical movement of the rear of the radius rod, but also because the expansion link is also swinging like the aforesaid pendulum, but not necessarily in phase with the radius rod, and certainly not when in back gear. With the radius rod working in a sliding die block in the lifting arm, the rear of the radius rod does not move in the vertical plane with its longitudinal movement, but the expansion link still has its pendulum movement, so the point of contact between radius rod and expansion link is constantly changing as the link face moves up and down but the rod doesn't.

 

As Steve says the N and U classes are built so the die block is in the upper part of the link in forward gear. The valve events are more even in fore gear than in back gear and very close to equality at about 40% cut off.

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Now that confuses my simple mind. With a suspended radius rod the suspension link is going to prescribe an arc as it moves back and forth and this will have the effect of lifting and lowering the die block in the expansion link, especially in the top half. I understood that for consistent valve events between fore and back gear, the die block should not move in the expansion link and any die block slip will affect this. Looks like I need to gen up a bit more.

 

A couple of points, the outside admission gives a very short and much straighter path for the exhaust. I remember looking down into the exhaust passages of the Q1, with the blast pipe not yet fitted, you could see the inside of the cylinder covers.
The gear on U's and N's has been discussed here before, I think Don Ashton explained it on his website, thee arrangement does give good events as stated. I think people at Ashford really knew their stuff on valve gears as other engines like the P's show.
As for driving 847 backwards, well it's mostly down hill and the reverser does kick a bit. It's long been a mystery why the SR and Western (based on my knowledge of the Dukedog, don't know about other types) could design a nice light and quick screw reverser but the Standards had such an unpleasant type to use.
I haven't done any valve setting for a while, it would have been interesting to try equalising cut offs at a good running point say 40%
As this thread is about the Clan build, can't the gear be fine tuned and then considered as part of improvements to the first of the next batch?

 

The other unusual point is that the link tail pin backset is negative (the pin is in front of the zero backset position). You are correct about the lifting and lowering of the die block. The arrangement looks wrong and is counter intuitive but it provides good valve events in the normal running position of the loco. The valve openings and cut offs are tabulated on the valve gear drawing. I have checked them using the Allan Wallace programme from Don Ashton's web site and the figures correspond. Whoever designed this valve gear was very skilful. I believe it was Harold Holcroft but he does not mention it in his autobiography.



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Are we talking about the N & U Classes RLinkins ?

 

Before I mentioned the U & N classes I went to Locomotive Adventure to see what Holcroft said about the subject and was quite surprised to find no mention of the subject. As you suggest, I'm quite certain that he was the brains behind the design

 

This is the same Holcroft as in the Gresley gear?

 

Yes - also the same Holcroft who to all intents and purposes designed the GWR 43xx mogul - one of the Churchward “bright young men” who left Swindon for Ashford to work on the SECR and then SR under Maunsell.

His two-part autobiography, Locomotive Adventure, is well worth seeking out.

Tom

 

Yes we are. I have a copy of the valve gear drawing dated about 1928, the original is in the NRM

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He also was the originator of the so called Gresley conjugated valve gear.

Later on he designed a conjugated arrangement that operated the inside valves on a 4 cylinder loco with the cranks set at 135 degrees from the 2 outside sets of valve gear. This gave 8 exhaust beats per revolution

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