Long travel valves

[Peppercorn]

Good afternoon,

 

Like many others, I've read a fair number of books on steam locomotives. There is often a reference to 'short travel' and 'long travel' valves, the latter being preferred, it would seem, by Churchward amongst others. What exactly does it mean? On the face of it, it refers to the length of travel of the valves, but what makes it 'long' as opposed to 'short'? Further, why would it be advantageous - is it because the port would be open for longer with the long rather than the short? 

My apologies for asking something that may be blindingly obvious.

 

Cliff H

[Wickham Green]

hmmmm ........ one of those things we take for granted !  Obviously if the valve - generally a piston valve in this context - has to move further, it has to move faster so it covers and uncovers each steam port more quickly and hence gives more precise control to the admission & exhaust. If there's anything more to it than that ( and no doubt a lot more COULD be said ) I'm sure someone else will pipe up.

[LMS2968]

It can give 'snappier' opening and closing, but the longer travel also allows larger ports, and for these to be open for a longer period. Steam, like all fluids, has inertia and takes a finite amount of time to pass through the ports. As speed increases, that time diminishes, so a longer opening allows more time, and so more steam to enter the cylinder to produce more power.

 

This doesn't work well with slide valves as they have a large contact area and corresponding high friction, so moving them at speeds absorbs ever more power. Piston valves are different, and this was a situation not always understood by various drawing offices with the switch piston valves; short travel was retained as this was assumed to limit wear of valves, rings and steam chest liners.

[62613]

Of as much advantage with piston valves, surely, was that the valve heads could be widely spaced on the valve rods, allowing the steam passages between the the valve chest and the cylinder to be straightened. 

[Right Away]

There's heaps of info on Don Ashtons website; URL below

 

 http://www.donashton.co.uk/index.html

[Gibbo675]

Hi Cliff,

 

Long and short travel valves are precisely that, the amount by which the valves move. Later designs of piston valves move a lot more that the earlier designs of slide valves.

 

Looking at the details for the valve gear of the Stanier Black Fives, which are typical of long travel valves, the dimensions are as follow:

• Valve diameter 10"
• Lap of valve 1.5"
• Lead .25"
• Exhaust clearance .0625"

 

A longer travel valve allows for larger port openings at any given cut off, this allows for a lessening of resistance to the flow of steam in and out of the cylinder as noted by LMS2968 above. This is especially important for the passage of exhaust steam out of the cylinder to the base of the blast pipe as it does not have as much energy which reduces its velocity leaving steam in the cylinder and causing back pressure in the cylinders. The compression figures to the right of the diagram will indicate why the exhausting of the steam is important to avoid back pressure that retards the engine.

 

The values for short travel valves are usually approximately half that of the above figures.

 

There is a link to the best book upon the subject:

 

https://www.amazon.com/Locomotive-Valves-Valve-Gears-Jacob/dp/0951936719

 

Gibbo.

[Bucoops]

I don't know a great deal on the subject apart from the implication of the name. But the N7 I am building is a long travel variant - can you spot the difference?

 

Original:

 

 

LTV:

 

[Peppercorn]

Thank you all so much for answering a question that I feared would be considered risible. 

It's interesting - and perhaps inevitable - that the further we get from a technology, the links to understanding the technology in question can weaken. And older texts do take for granted one's understanding, another example being "cut-off" a term pretty liberally used in their books by the likes of O S Nock ( a writer I have always found pretty tedious to read) but others, too. 

With the insights - and links as well as the table above - so kindly provided by you, I'll go back and re-read.

Cheers,

 

Cliff

 

[PenrithBeacon]

https://www.amazon.co.uk/Locomotive-Valves-Special-Treatise-Setting/dp/0353419982/ref=sr_1_11?keywords=locomotive+valve+gears&qid=1565429041&s=gateway&sr=8-11

 

The best book on loco valve gears, but it was written before poppet gears were used on steam engines.

 

https://www.amazon.co.uk/Valves-Valve-Gears-Locomotives-G-S/dp/0905100360/ref=sr_1_25?keywords=locomotive+valve+gears&qid=1565429176&s=gateway&sr=8-25

 

[Peppercorn]

1 hour ago, Bucoops said:

I don't know a great deal on the subject apart from the implication of the name. But the N7 I am building is a long travel variant - can you spot the difference?

 

Original:

 

 

LTV:

 

Nice looking model - is this a kit, or scratch-built?

[Bucoops]

42 minutes ago, Peppercorn said:

Nice looking model - is this a kit, or scratch-built?

 

Thanks  I'm going for the record on the longest time to build a kit. It's a Stelfox kit. I've done a few tweaks to it, but it's mostly as they intended. Not sure how old it is, but it seems to be maybe 1980s.

 

[Peppercorn]

Thanks. I have a Connoisseur kit for the N7 (in 4mm) awaiting my dubious attention. Be nice to see your's when you've finished it.

Cliff

[Bucoops]

I'd like to see it finished too! The Connoisseur is the same part 3 as this but a lot more modern (and hopefully easier to build!).

[LMS2968]

8 hours ago, Peppercorn said:

Thank you all so much for answering a question that I feared would be considered risible. 

 

Never shy away from asking a question. Some on here know a little, some on here know a lot, but no-one knows everything, and we all learn that way!