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karlelden
Regular Member


78 Posts
Posted -  21/01/2007  :  02:02

Valve gear on Corliss valves has  fascinated me for a long time, so I've been looking at the pictures here and wondering...

Are there two basic configurations of the Dobson block gear?  Looks to me like the gear on the Wiseman HP, Haslingden, Moss Mill and others is Dobson even though they have only one dashpot to serve both valves.  Is that the way it is, or are they a completely different valve motion than on the Bancroft engine?  Does that one dashpot contain two springs?  If so, does the "idle" spring cushion the one in motion, or is there other provision for cushioning?

Also, it looks to me like the Long Ing engine had slide valves all around.  Is this so?  If so, wasn't that rather uneconomical?  Seems like I've read somewhere that some mill engines had only slides, but I can't understand why.

Who can tell me what kind of valve motion was on the Hartford engine and Clover-Rochdale? They look a little strange and rather interesting.

Did you have any dashpots that worked only on vacuum?  That was common here in the US.

 




Karl Elden
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Stanley
Local Historian & Old Fart


36804 Posts
Posted - 21/01/2007 : 06:29
What a can of worms you've opened up here Karl......!!  I'll have a think and post you something.


Stanley Challenger Graham




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karlelden
Regular Member


78 Posts
Posted - 21/01/2007 : 20:31

Thanks, friend--No need to get all stirred up over it.  I just like those contraptions and appreciate the variety.  A little like listening to a Scarlatti sonata or some J S Bach.

I'll take any information I can get, but remember--no brain pain!

 




Karl EldenGo to Top of Page
Invernahaille
Regular Member


669 Posts
Posted - 21/01/2007 : 21:18

Karl. My understanding is that Corliss valve gearing is a rotary valve system, the rotary valve opens to allow steam into the cylinder, and then closes at peak pressure. I believe that most British Steam Engines worked on the slide valve system.

For the benefit of the non-engineering members I add an explanation of what Karl is asking.

The valve gear of a steam engine is the mechanism that operates the inlet and exhaust valves to admit steam into the cylinder and allow exhaust steam to escape, respectively, at the correct points in the cycle. It is sometimes referred to as the "motion".


In the simple case, this can be a relatively simple task as in the internal combustion engine in which the valves always open and close at the same points. This is not the ideal arrangement for a steam engine, though, because greatest power is achieved by keeping the inlet valve open throughout the power stroke (thus having full boiler pressure, minus transmission losses, against the piston throughout the stroke) while peak efficiency is achieved by only having the inlet valve open for a short time and then letting the steam expand in the cylinder (expansive working).

The point at which steam stops being admitted to the cylinder is known as the cutoff and the optimal position for this varies depending on the work being done and the tradeoff desired between power and efficiency. Steam engines are fitted with throttles (regulators in British parlance) to vary the restriction on steam flow, but controlling the power via the cutoff setting is generally preferable since it makes for more efficient use of boiler steam.

A further benefit may be obtained by admitting the steam to the cylinder slightly before front or back dead-center. This advanced admission (also known as lead steam) assists in cushioning the inertia of the motion at high speed.

In the internal combustion engine, this task is performed by cams on a camshaft driving poppet valves, but this arrangement is not commonly used with steam engines, partly because achieving variable engine timing using cams is complicated. Instead, a system of eccentrics, cranks and levers is generally used to derive the motion of a slide valve or piston valve from the motion of the main crank/big end. Generally, two simple harmonic motions with different fixed phase angles are added in varing proportions to provide an output motion that is variable in phase and amplitude. A variety of such mechanisms have been devised over the years, with varying success.

Both slide and piston valves have the limitation that intake and exhaust events are fixed in relation to each other and cannot be independently optimised. Lap is provided on steam edges of the valve, so that although the valve stroke reduces as cutoff is advanced, the valve is always fully opened to exhaust. However, as cutoff is shortened, the exhaust events also advance. The exhaust release point occurs earlier in the power stroke and compression earlier in the exhaust stroke. Early release wastes some energy in the steam, and early closure also wastes energy in compressing an otherwise unnecessarily large quantity of steam. Another effect of early cutoff is that the valve is moving quite slowly at the cutoff point, and this causes 'wire drawing' of the steam, another wasteful thermodynamic effect visible on an indicator diagram. An Indicator diagram is an engineering tool that allows the engineers to see if they are getting optimum output from the engines.


Thanks in part to wikipedia.




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karlelden
Regular Member


78 Posts
Posted - 22/01/2007 : 05:15

Yeah, thanks Robert for the added explanation.  I think the inlet valves close not at "peak pressure" but at a  point determined by the governor which senses the load on the engine.

If anyone is interested in a graphic display of some valve gears, here is a downloadable version:  http://www.tcsn.net/charlied/      It's easy to see the general differences between slide valves and rotary.

Since the man who made these displays is a model locomotive builder, he majors on the various loco valve types, but he has a couple rotary valve models too.  You can vary several parameters on the models to see what effect that has on performance.  Very well done and fun to play with.  Includes Stephenson, Bulleid and Caprotti for any of you Brit rail fans.

I just enjoy watching the poetry in motion.




Karl EldenGo to Top of Page
Stanley
Local Historian & Old Fart


36804 Posts
Posted - 22/01/2007 : 07:53
I'm still thinking about this one.  Sorry to say it but Robert has fallen into a common misconception about Corliss gear.  The term is used to describe both valves and gear but actually all Henry Corliss did was to devise a linkage which controlled cylindrical valves (which are in effect a type of slide valve) in such a way that the legth of time of the opening could be related to the power demanded by the governor and the closing was done by tripping the linkage and allowing a strong spring to slam the valve shut after which the steam continued to expand.  It was this intantaneous cut-off that gave the valve it's efficiency as the governor had accurate controll over the cut-off and the full power could be taken from the steam as it expanded, known as 'expansive working'.  The worst thing you can do, and a common mistake made by amateurs, is to cushion the closing of the valves to make the engine run quiet.  This converts instantaneous closure to gradual closing and destroys Henry's concept.  The cylindrical or circular slide valve was in use long before Henry came along.  More later.........


Stanley Challenger Graham




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Stanley
Local Historian & Old Fart


36804 Posts
Posted - 22/01/2007 : 15:38
STEAM ENGINE VALVE GEAR.

The earliest steam engine valves, on both Savery’s pump and the Atmospheric beam engine, were simple plug cocks and the ‘valve gear’ was a lad who opened and closed them in the right order. This was soon superseded by an arrangement of rods and catches taking their motion from the beam of the engine which operated the valves automatically. The same gear can be seen on any of the preserved Cornish Beam engines used as pumps, the rotative engines tended to use standard slide valves, either flat or piston, and a simple eccentric driven linkage from the flywheel.

In 1799 William Murdock patented the slide valve in both flat ‘D’ pattern and the cylindrical form. The sliding valve was not new, very ancient forms of the principle were used for regulating the flow of water out of a trough by a sliding board in the bottom obstructing a hole. I made such a valve in 2000 for a dolly scouring machine at Helmshore and I am sure I didn’t invent it! The valves of the original Trevithick engine were flat plates oscillating on a pivot and this, with a cavity, was used by Watt who called them ‘sliding valves’. Examples of drop, slide, piston and ‘D’ valves have been found in Watt drawings dated before 1783. This was a time of experimentation and various methods of controlling steam ingress and exhaust were being tried. In 1786 Murdock’s steam carriage used a piston valve which Watt said was similar to one used on a 12” engine at Soho Foundry. Towards the end of 1798 or 1799 Murdock proposed the slide valve but Watt and Southern rejected the idea on the grounds that the faces would ‘become fluted’ and be difficult to repair. They favoured drop valves. Be that as it may, Murdock patented the ‘D’ valve and within 2 years the slide valve was being used at the Soho Foundry. The first slide valves had to be faced by hand but a few years later the advent of a planing machine at Soho made the manufacture much easier.

For almost a hundred years the slide valve reigned supreme in British engine building, at first in the flat ‘D’ valve form but later as a semi-rotating cylindrical valve or a piston valve. Whatever configuration was used these were all at that time fixed event valves and any governing of the engine had to be done by throttling the steam supply.

It has always amused me to hear people talk about ‘the simple slide valve’. To the uninformed eye it looks dead simple, a metal plate that slides over a hole and opens or closes it. Believe me, it is far from simple. One of the great railway chief engineers (I think it was Churchward) said in his retirement speech that it was a pity he had to go as he thought he was just beginning to get to grips with the basic principles of the slide valve. I am not going to delve into this pit here but I know exactly what he meant. Apart from problems with angularity of rods and settings, the valve’s biggest problem is that both steam admission and exhaust use the same port and therefore the valve is cooled by the exhaust.

In the 19th century engineers were aware of the limitations of the simple valve and understood that if they could leave the throttle fully open but alter the length of stroke during which steam was admitted at full pressure they could gain the benefits of expansive working. This would be far more efficient and save a lot of fuel. The first serious attempt to address this problem to my knowledge was the Meyer system of varying the configuration of the ‘D’ valve whilst it was running so as to alter the length of admission. This system was used by some manufacturers and had considerable success.

Then along came George Henry Corliss. The Corliss Steam Engine Company was originally known as Fairbanks, Clark & Co. in the 1830s. In 1843 it was renamed Fairbanks, Bancroft & Co. when Edward Bancroft joined the company. In 1846 it was renamed Bancroft, Nightingale & Co. when George H. Corliss joined the company, and in 1847 it was renamed Corliss, Nightingale and Co. In 1848 the company moved to the Charles Street Railroad Crossing in Providence, Rhode Island. In 1857 the company was renamed for the last time to Corliss Steam Engine Company. By 1864 Corliss bought out his partners and was the sole owner of the company. In 1900 the Corliss Steam Engine Company was purchased by the International Power Company. In 1905 it was purchased by the American and British Manufacturing Company. In 1925 the company merged into Franklin Machine Company. By then Franklin Machine Company already owned the William A. Harris Steam Engine Company. During his time with the company Corliss developed a completely new system of valve operation and took the first of his patents for it out in 1849.

Corliss had one idea in mind, he wanted to improve the efficiency of the steam engine. The first thing he did was create separate admission and exhaust valves using cylindrical valves operated from a wrist plate driven by eccentric rods whose motion originated from the rotation of the flywheel. This meant he could separate admission and exhaust events and keep his steam port hot. He realised that if he could make a linkage to the steam admission valves capable of adjustment while the engine was running he could vary the cut off point of steam admission and achieve expansive working. He did this by incorporating a catch in the steam valve linkage which could be controlled by the governor to trip at any given point in the stroke allowing the spring-loaded cylindrical valves to snap shut. Once all these ideas had come together the Corliss gear was born. Notice that the valves are nothing to do with the operation of the gear, they are not Corliss valves, it is the control gear and separate ports that made the breakthrough.

The Corliss gear became the standard valve gear on just about every stationary steam engine in the world with enormous benefits in efficiency. The linkages varied, makers tended to fit their own version, but all adhered to the same principle of fixed exhaust events, variable admission and governing on the cut-off with the steam valve to the boiler wide open. The amazing thing about this simple and efficient principle was the way many ‘experts’ and engineers managed to reduce the possible efficiency. A very common fault was giving the cylindrical valves too much cover on the port opening. If you go looking you will find extensive tables for calculating how much cover a given size of valve needed for a certain duty. Without exception, all the ones I have seen are wrong. I can only assume they were written by theorists who had never run a Corliss engine in their lives and were adding on a spurious safety factor to make the engines ‘safer’. It’s quite common in these tables to find covers of as much as half an inch recommended.

Here’s the SCG way of setting a Corliss valve. First of all, ignore any markings on the end of the valve when you take the back cover off. These are supposed to give you the opening point and the recommended setting for cover. With the engine stopped take up the adjustment in the linkage until you hear steam whispering through the valve, wind the adjustment back to give as little cover as you think you can get away with. On a single ported valve this will be about a sixteenth of an inch, on a double ported valve such as makers like Burnley Ironworks fitted, Newton tells me he has gone to below one thirty second of an inch. By doing this you ensure that the valve responds immediately when the linkage pulls it off its seat. It also takes a lot of strain off the linkage and bonnets because a large valve with full steam pressure on it is virtually locked on its seat and requires a great deal of force to shift it. The less effort required the less strain on linkage, bearings and castings. Newton told me that it was quite common for bonnets to fracture because of this repetitive strain. They always replaced these with heavier castings.

The beauty of the Corliss gear is the sharp cut-off which enables exact governing. This desirable attribute is commonly destroyed by engineers setting the dampers on the dash-pots to over-cushion the force of the spring returning the valve to the closed position to make the engine run ‘sweeter’. The damper is there to cushion the last half inch of return, not slow down the whole of it. In effect, an over-cushioned dashpot acts to alter the characteristics of the gear and make it little better than a slide valve. You can tell when an engine has minimum cover and sharp cut-off, it will run steadily on full boiler pressure under any condition of load. You find this out when you weave a shed out and end up running only twenty looms perhaps. Many engines were very dangerous running in this condition and I always like to hear the pistons slamming into the bottom of the dashpots. On an old engine, running can be improved by either fitting new springs or installing a spacer to increase the tension. Be careful if you do this, you need to be sure that the valve gear has enough travel to cope with the increased length in the pot. If you get it wrong it will pull the bonnet off the valve the first time you turn it over.

Continental engines were the first to use drop valves and some UK manufacturers followed them, particularly on that abomination, the Uniflow. They were an efficient valve but I am told needed more attention that a standard Corliss gear with cylindrical valves.

Notice that I have been talking exclusively about stationary engines. Locomotives were a different kettle of fish altogether because of the duty demanded from the locomotives under extremely varied conditions. Like stationary engines, the most efficient running was obtained by running with the regulator wide open and shortening the cut-off. Linking up in railway parlance. Conversely, when starting under a heavy load full cut-off was needed controlled by the regulator to minimise wheel slip. Add to this the need for a reversing gear in the linkage and I think you can begin to see that loco valve linkages offered tremendous scope for the designers. Two gears in particular probably accounted for most of the UK locos, Stephenson’s and Walchaert’s but there were dozens of others each with their own champions. The last improvement was the Caprotti gear which could possibly have eclipsed the others eventually but came in to late to get a foothold.

Funnily enough, one of the best places to look to find out about the various types of loco valve gear is in the back numbers of the Model Engineer. For many years a man writing under the pseudonym “LBSC” (Lillian “Curly” Lawrence) gave wonderfully clear descriptions of different gears and how to build them. If you want to dig further there are other good books; Youngson’s Slide Valves and Valve Gearing, 1948. Slide valves and Link Motions, Auchinloss, 1872. Valves and Valve Gears for Locomotives, Lake and Reidinger, 1940. I’ve never had any experience of loco gears so there is nothing more I can add. One thing is certain, the loco gears are far more complicated than stationary engine gears and it would take a lifetime to fully understand even the simplest of them.

SCG/22 January 2007



Stanley Challenger Graham




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Invernahaille
Regular Member


669 Posts
Posted - 22/01/2007 : 17:16
Stanley. I didnt fall into any misconception I was purely trying to expalin the workings in laymens terms. Valve gearing is no diferent than on an internal combustion engine. The only difference being that the explosion in an internal combustion engine is created by a spark therefore the cycle is static in that the valve gearing on a car is in ratio to the crankshaft cycle. Where as you rightly say there is steam expansion within the cylinder on a steam engine.The point I was attempting to make is that the corliss rotary valve was either open or closed whereas with a slide valve there was a certain amount of time where the steam inlet port remained partly open before the port was completely shut off. It is this that I believe Corliss was attempting to remedy. At the end of the day valve gearing is purely a timing concept. ie allow the correct amount of steam into the cylinder at the right time and pressure.


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karlelden
Regular Member


78 Posts
Posted - 22/01/2007 : 19:46

Stanley, when you say "drop valves" are you talking about what we call "poppet valves" or is it another animal?  Are these the valves they had on the Holmefield mill engine?

I agree with you on the use of dashpots.  Many engineers (I think) were afraid to let it drop with some noise.  I think the only potential danger was knocking the bottom out of the pot, and I never heard of that happening. 

I should probably not be so free with my comments on the dashpot matter.  I never was an operating engineer on a job, and my experience is limited to talking to engineers and reading old engineer journals and books and running and restoring engines for display purposes.



Edited by - karlelden on 22 January 2007 19:50:01

Edited by - karlelden on 22 January 2007 20:01:11


Karl EldenGo to Top of Page
Stanley
Local Historian & Old Fart


36804 Posts
Posted - 23/01/2007 : 06:41
Robert, my point was that it is a mistake to talk about the 'Corliss Rotary Valve' and you have done it again. The cylindrical valve is only a plug coak with parallel sides and a slot instead of a hole and was about long before George H worked his magic.  I think it makes the explanation clearer to get this out of the way.  It's the gear that does the job of influencing speed of opening and closing.  I've never heard of a dashpot being damaged by letting the spring do its job.  Besides, you can cushion the last half inch considerably by use of the damper valves.  The breakage point was the valve bonnets, particularly bad on engines running on superheat with too much cover.  Another thing that influenced it was the way cylinder oil was delivered.  Dripping it on the valves is useless, there is only one good way, atomise the oil into the steam pipe on the boiler side of the stop valve.  That way the steam does the lubricating and gets to every part of the engine including the LP.  The right grade of oil helps as well.........  Don't knock yourself back for being an amateur, alright, there is a difference between running engine loaded and light but you are still learning about them by running them.  There are very few men left who have rub commercially, we have all died!  As for the Dobson block, there are different configurations but they all work the same, a sliding block carrying hardened steel catches that engage with mating catches on the linkage, simple and effective if maintained properly.  Ellenroad had Craig's motion which was eccentric lifting cams driven by a rotating shaft and controlled by altering the attitude of the cams by linkage connected to the governor.  Both are food but on balance I liked Craig's better.  I suppose most engines used variations of Corliss's original idea of wrist plates and linkages.  Again, there were variations in the design, every manufacturer thought their way was best but the basic principle remained the same.  Drop valves aren't quite the same thing as poppet valves in design but are situated on top of the cylinder and act in the same manner.  Holmefield at Barrowford had drop valves.  Same principle as the poppet valve in an IC engine.  Many drop valves were balanced so as to cancel out the effects of boiler pressure on the speed of operation of the valve.  As a general rule, the best configuration of valves is steam in at the top and exhaust out at the bottom.  The cylindrical valve was great for this as it was at the lowest point in the cylinder and exhausted the free condensate as well as the steam.  Slide valves, whether 'D' valves or piston as at Ellenroad were a bummer in this respect as the ports are on the side of the cylinder.  Not a big problem on a loaded engine running hot but running light and cooler with vacuum back to the HP caused problems with retained condensate.  I used to have to run the LH engine at Eroad with the drains open after about an hour of running.  in this respect, preserved engines are more dangerous that loaded ones.  The only thing that saves most of them is the fact that there is enough wear in the bore to allow the water to get past under compression at the end of the stroke.  Long Ing was never altered and ran with slide valves all it's life.  No big disadvantage as it was running overloaded all the time and was running with the indicator figures crossing.  The governor never got the chance to wiredraw the steam.  The ability to govern on the cut-off has its greatest advantage on an engine running below full power for this reason.  One thing that is worth noting about Craig v. Dobson is that because Craig drove the gear by a rotary shaft driven by a gear he got rid of the complications cause by angularity in the eccentric rods.  Of course you still had the problem of the con-rod but that's common to every reciprocating engine.  I f that isn't clear, work out the piston speed by revs and stroke and then ask yourself the question 'is the speed constant?'  how can it be if it stops at each end........  Once you have grasped that, recognise that the same thing happens to any valve actuating gear run by eccentric rods.  Nothing simple about slide valves!


Stanley Challenger Graham




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Stanley
Local Historian & Old Fart


36804 Posts
Posted - 23/01/2007 : 07:52
Karl, I've been digging for you.  According the Arthur Robert's Black Book Clover was 1800hp by Buckley and Taylor, 1906.  Funny thing is that he says it was Corliss on all cylinders but the pic I have shows that the LPs were still slide valves.  There's a misapprehension somewhere but I don't know where.  Arthur is slightly erratic at times....  Anyway, the pic I have on the site identified as Clover has Buckley and Taylor's version of a wrist plate motion on the HP cylinders.  You say interesting, I say complicated!  As for dashpots run solely on Vacuum, never seen one although I can see how they would work, you would get away from springs weakening over the years but get problems with bore wear and loss of seal. 


Stanley Challenger Graham




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karlelden
Regular Member


78 Posts
Posted - 24/01/2007 : 13:11

Thanks for the thoughts, Stanley.  I'll be grinding this stuff in the mill for quite a while!

When you talk about angularity, are you referring to the speed of reciprocating parts varying with the position of a cam or eccentric on the shaft?  Valve gear necessarily slowing at the ends of the stroke?

I happened across another type of valve gear again on that Charlie Dockstader animation link I sent above.  Looks like what's on Clover and Hartford.  He calls it Spencer-Inglis.  Buckley and Taylor, you say.  Well, I suppose bottom line is like you mentioned.  Most of this esoteric knowledge is going into the graves with the guys who used to play with these toys.

I see some women running Ellenroad in some pictures.  Nice to see others coming up and getting involved.  In US we have a few women among the younger set of traction engineers too.

 




Karl EldenGo to Top of Page
tripps
Senior Member


1404 Posts
Posted - 24/01/2007 : 16:36

"nothing simple about side valves"

Your'e not kidding here are you? I have read it all, and don't understand any of it.  Thought for a moment Beachcomber had infiltrated the site. Full credit to those who do though. I stand back in awe.




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Stanley
Local Historian & Old Fart


36804 Posts
Posted - 24/01/2007 : 17:22
I was having a conversation with Newton Pickles once about the finer points of timing and engine and an inspector from English Heritage who heard us said to me afterwards "I assume you were speaking English?".  The combination of dialect and technical terms had lost him completely.  I'd be the same listening to two mathematicians talking.....


Stanley Challenger Graham




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karlelden
Regular Member


78 Posts
Posted - 24/01/2007 : 19:11
BTW, Stanley, that was ME up there in that post talking about "Corliss" valves, not Robert.  I understand what you're saying about them, though.  I just don't care enough to be  very precise in my terminology some times.  It's like when I was studying linguistics, and the instructor was trying to teach us some things about the functions of different words in a sentence and we smart-alecs were giving him a lot of grief, and he finally gave up and said "I don't care what you call them.  Call them peaches, if you want, just so long as you understand their function in the sentence!"


Karl EldenGo to Top of Page
Gugger
Regular Member


61 Posts
Posted - 05/05/2007 : 20:52

Stanley,
It is only now, that I discovered the topic about the Corliss Valve Gear.
In your writing dated 22 January 2007 STEAM ENGINE VALVE GEAR. your sentence “The Corliss gear became the standard valve gear on just about every stationary steam engine in the world with enormous benefits in efficiency.” caught my attention.

At the moment I am building a Corliss Cross Compound Engine and therefore I have spent a lot of time studying that type of engine.

In Europe the Corliss engine was dominantly used in Britain in the textile industry, but was very seldom found on the continent, where the trop valve engines were favoured. Of all the trop valve engines manufactured the Sulzer engine was the most important.

According to various literature the advantage of the Corliss system was the immediate reaction to quick changes in the load, a very important factor in textile mills. By comparison the trop valve engine had a better efficiency.

Walter Brunner




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