Hunslet Valve Gear

[baggo]

Rather than hijack Ron's cylinder bore thread, I'll start a new one.

Mike (Cotswold) mentioned shortcomings in Don Young's design of the valve gear for the 5" Hunslet. As I've been studying Don's (Ashton) valve gear design 'words and music' and playing with the design spreadsheets and simulation programs, I had a quick look at the Hunslet design.

I put the 'figures' for the Hunslet into Bill Hall's Stephenson valve gear simulator to see what it came up with. I had to 'guesstimate' the angle of advance for the eccentrics as this is not mentioned in the construction series. DY's method is to set the eccentrics 'by eye' once the gear is assembled rather than set them to the correct angle to begin with.

It is immediately obvious that there is something wrong with the original design as the cut offs at each end of the cylinder are vastly different e.g. a difference of 20% at approx 50% cut off. Having taken in some of DA's teachings, I think it's because the expansion link suspension pins are exactly on the centreline of the die slot whereas the suspension needs to be forward of the centreline. Playing with the simulator and moving the suspension forwards by 0.1" makes a huge difference and makes the cut offs nearly equal at both ends of the cylinder.

Putting the 'proper' figures into the design spreadsheet would obviously give a better result but I'm sure just moving the suspension pins on the expansion links would make a big improvement to the valve events without having to scrap the original and start again.

John

[cotswold]

Dec 25, 2009 17:30:45 GMT John Baguley said:

... DY's method is to set the eccentrics 'by eye' once the gear is assembled ...

Which brings us to problem number two. The Hunslet has balanced slide valves which obscure direct view of the valve face. Even a dentist's mirror is little help. I had to resort to hours of fiddling using low pressure compressed air to blow bubbles in a jam jar via plastic tubes attached to the drain cocks. It was a fortunate accident that I had read Don's book and my eccentric rods are 'Compensated' length so, at least, I was able to work toward equal leads with some confidence.

It's a long story but in the end, unaware of Don's site and spread sheet, I reverse engineered the calculations in his book and produced one of my own using the as built dimensions of the components, as built. I then set the eccentrics between centres on the lathe.

[donashton]

John, you've put your foot note to good use! I don't have DY's drawing but the original quarry engines have loco links and end suspension with the trunnion reaching down to the reverse pin. Bad news, as the reverse is fairly smoothed out by the lengthy lifting arm, but in fore gear the forward eccentric is dancing about. That's why the GWR made this lot 'upside down'.
I have contact with Mike, but others may like to know how to set without seeing the valve. A short distance before the piston reaches front dead centre the valve is exactly central over the ports, so from your known manufacturing make a mark or some means of being able to position the valve dead centre of travel without seeing the valve. Then look up your simulation figures for piston/valve relationships. This will tell you how far before front dead centre the piston must be. Clock it with front cover removed for the purpose and simply lock the valve centrally. All the simulation is then preserved.
The offsetting you describe, John, appears to be for links suspended on the horizontal centreline. You can offset end suspension but it doesn't cure its own errors. A point to watch is that you don't get equal events at 50% and rotten ones everywhere else, but that is up to care in proportions in design.
Incidentally, Mike did an excellent job of reverse engineering - an awful lot of work that should never have been necessary!
Don

[donashton]

John - a small additional note. Since Stephenson's gear normally has equal leads at only one setting, you can get this in theory by using the 'gets results' of Bill Hall at 50%, because his program sets the leads equal for you. That means the gear being in 50% when you come to the real valve setting.
D

[ron]

And here was me thinking the Hunslet would be a nice wee simple engine for a quickbuild after the Simplex

However, as the loco I'm building will probably only ever get used on the track in my back garden how important is all this??
Ron

[baggo]

Hi Ron,

If you haven't already made the expansion links, just make the pivot pins 0.1" further forward of the centreline of the links and leave everything else as is. That should sort out most of the inequalities of the cut offs.

I am sure there will be plenty of Hunslets running around with the original dimensions and the owners probably don't notice anything wrong anyway!

John

[cotswold]

I would also recommend making (or even re-making) the eccentric rods to the 'corrected' length necessary for equal leads (see Don's web site). With this done, the job of finding the correct adjustment of the valve in relation to the valve face becomes a simple matter of checking for equal valve openning at front and rear piston dead centre positions.

Cotswold Mike.

[ron]

Thanks John, I haven't made any of the valve gear yet apart from the eccentric straps and sheeves.
Mike, I've made some of the parts for the balance valve but I'm now wondering whether to go back to plain slide valves if it makes setting it up easier, basically I've plenty experience of making parts but very little knowledge of the theory, I just tend to follow the drawings so I'm a bit disapointed to find that Don Young's design leaves a little to be desired.
Ron

[teakfreak]

I've a 5" Hunslet to Don Young's design. I haven't had occasion to do anything to it since I bought it so I can't say whether it is was built faithfully to the original design, although I suspect it was. What I can say is that it is a super runner!

[steammadman]

some people seem to "revell " in criticsising peoples work, if your so good,as you would lead us to believe, WHY DONT YOU design the perfect model locomotive?
or re-incarnate SIR Nigel Gresley, then you can pick the spots off his work too.

[baggo]

Don't get your knickers in a twist Steammadman, nobody is being critical of Don at all. Nowadays we have computer design programs and simulation for checking valve gear designs which such as LBSC, Don Young etc. did not have at their disposal. They did their best with what was available at the time. And NO they did not always get it right!! Neither did the designers of full size jobs.

Don is one of my favourite writers (along with LBSC) and is one of the people I would have liked to have met before his untimely death. Just because we find errors in their designs does not mean we are being critical. We just have the advantage of 'Modern Technology' and the ability to check.

John

[cotswold]

Dec 27, 2009 17:57:53 GMT ron said:

... Mike, I've made some of the parts for the balance valve but I'm now wondering whether to go back to plain slide valves if it makes setting it up easier, basically I've plenty experience of making parts but very little knowledge of the theory, I just tend to follow the drawings so I'm a bit disapointed to find that Don Young's design leaves a little to be desired.
Ron

I'm very happy with my DY Hunslet but I am equally happy to apply techniques and knowledge that were outside DY's field. So don't throw the baby out with the bath water!

In particular, balanced slide valves should be better than conventional slide valves for no other reason than a less convoluted path for the exhaust. But that is only my opinion, I have never built an engine with conventional slide valves. BTW, there are other ways of setting the valves than visually.

Stick with it. It is a good engine as it stands. How much one wishes to tweak it to for what ever reason is up to the individual.

Cotswold Mike

[Deleted]

Dec 28, 2009 0:27:53 GMT John Baguley said:

Nowadays we have computer design programs and simulation for checking valve gear designs which such as LBSC, Don Young etc. did not have at their disposal.
Neither did the designers of full size jobs.
John

Hi John. Are you sure you don't want to think about that one in the light of their modern technology!

[alanstepney]

It was standard in many/most engineering colleges, and no doubt at all loco works, to have various valve gear "simulators" for want of a better word.
(The only one I ever saw was around 2" scale.)

That is little different from LBSC's idea of making one from card and drawing pins, an idea I have used several times.

It will reveal gross errors, parts whacking against other parts at some point in the stroke, etc, but still leaves quite a lot unknown.

It was sorting out those "unknowns" that sorted out the top engineers from the rest.
That plus a lot of experience, some intuition, and in many cases, lots of experiments.

The modern computer programmes take most of the guess-work out of it and simplify "our" task, as well as being far quicker than the old ways.

In addition, recent analysis of valve gears, and other aspects of steam engines, some only practical with computers, have aided our understanding of what happens, or should happen.

Let's not throw away what we learned in the past, nor decry the old ideas and designs, but use modern technology to add to it.

[cotswold]

Dec 28, 2009 9:48:25 GMT alanstepney said:

... Let's not throw away what we learned in the past, nor decry the old ideas and designs, but use modern technology to add to it.

Here! Here!
On the newly completed narrow gauge line from Caernarvon to Porthmadog is K1 (usually, but incorrectly, regarded by most to be the first Garratt). During the rebuilding of K1 full full use was made of the original drawings. One focus of attention being the valve gear. Computer simulation indicated that little if anything could be done to improve upon the work of the original Beyer-Garratt drawing office team of 100 years ago.

One thing less for the rebuilding team to worry about, sometimes the old and the new can go hand in glove.

[havoc]

Nice photo Simplyloco. Look at all the places they can adjust on that thing.

balanced slide valves should be better than conventional slide valves for no other reason than a less convoluted path for the exhaust

I don't have the drawings you refer to. Could you post a scan/sketch of the valves? I once wanted to build the belgian type 53 which has balanced slide valves (see scan below). But the exhaust path is identical to normal slide valves. Just more complicated sealing. The biggest advantage is reduced friction from the valve at higher boiler pressures. I'd like to see the arrangement where the exhaust path is improved.

[baggo]

In the Hunslet type the exhaust passes through the middle of the valve and balance piston and exits through the top of the steam chest rather than have to turn back on itself to go through the normal exhaust port in the port face.



The exhaust port shown in the drawing is not used with the balanced valve. This was Don's first venture into balanced valves and he wasn't sure how successful they would be. Hence he included a conventional port just in case it was necessary to revert back to conventional valves.

John

[havoc]

Thanks John, makes sense that way. Neat way to balance as well.