A Coarse Guide to the Steam Locomotive for ‘N’ Gauge Modellers

[Train Waiting]

A Coarse Guide to the Steam Locomotive for 'N' Gauge Modellers - Part 55


Hello Chums

Introducing VALVE - Valve Actuation Lazy Visualisation Exercise

I suppose it's time for me to attempt to show the principle behind piston valves.

Quick recap: A steam engine's double-acting cylinder requiring four valve events.  Piston valves - one valve with two pistons and three spaces.

Unfortunately, I've had to rule out the possibility of us all taking a trip to the NRM in York and watching the sectioned 'Merchant Navy', which means Poppy and I have had to come up with another approach.  We have used my coarse modelling inability and her keen supervision skills to create a lazy visualisation.  Proper modellers would, undoubtedly, have made something much more sophisticated.

With all the necessary 'contains unsuitable material for [Please fill in blank as you wish]' warnings deemed to be understood, let's have a look at VALVE:-





Picturingham 1: We can see the cylinder with its piston (coloured green) and the piston valve with its two pistons (coloured yellow).  At the top of the valve chest there are three openings. (These are analogous to the Three-Wire Control on Poppingham I mentioned in the previous part.) The opening in the centre is the inlet for live steam from the regulator (coloured pink).  The other two openings, one to the front and one to the rear of the valve chest, allow exhaust steam (coloured blue-ish) to escape - it will then make its way up the chimney. [I'll repeat the colours for this first example.] 

The piston (green) is at the front of of its stroke and the piston valve (yellow) is allowing live steam (pink) to flow into the front of the cylinder.  This will push the piston back down the cylinder.

Meanwhile, the piston valve is allowing the exhaust steam (blue-ish), at the rear of the piston to escape from the cylinder.





Picturingham b: The live steam has done its work and the piston is now at the other end of the cylinder. The piston valve is allowing live steam to flow into the rear of the cylinder. This will push the piston forward.

Meanwhile, the piston valve is allowing the exhaust steam at the front of the piston to escape from the cylinder.

Once this stroke is over, it will take us back to where we started - the piston has made two strokes, each under power, and the wheels have made a single revolution.  A bystander would have heard: 'wuff', [silence], 'wuff' [silence] or two exhaust beats per revolution of the driving wheels.

Except she didn't - as this is a two-cylindered engine, our cylinder has a friend at the other side - so what our bystander would actually have heard is: 'wuff', 'wuff', wuff', wuff' or four exhaust even beats per revolution of the driving wheels.  As you might recall, we discussed the cranks being set at 90^o in an earlier part. This gives the four regular beats of a two-cylindered locomotive.

Having inflicted VALVE and yet more wuffs on you, I'll beat a hasty retreat.  But I've not been cut off in my prime - I'll be back with some more about valves.  And (Warning!), VALVE might make a re-appearance.

ENDNOTE:

Although my analogy with the Three-Wire Control on Poppingham might seem a tad forced, you can see from VALVE that the live steam is admitted to the valve chest through one opening and the exhaust steam leaves it through two separate openings.

Using the middle opening for the live steam inlet is called 'inside-admission' and is the norm for piston valves.

'Outside-admission', with the live steam using the two outside valve chest openings and the exhaust sharing the inside opening is, perhaps, especially analogous to Three-Wire Control (common return, in other words).  Outside admission is normal with slide valves.

There is an interesting exception to this, the Bulleid 'Pacifics' had outside admission piston valves in their original form.  Mr Bulleid doing something different - what a surprise.  In modified form, Bulleid/Jarvis 'Pacifics' are even more interesting.  Outside-admission for the outside cylinders and inside-admission for the inside cylinder.


'N' Gauge is Such Fun!

Many thanks for looking and all best wishes.

Cheerio!

John

[chrism]

That reminds me of the very first job I was allocated when I started volunteering in the loco department at the Watercress Line - decoking the valve heads for BR Standard 4 no,76017. It was rather nice since it was the middle of the winter and the workbench was close to a nice warm stove 

Anyway, I was scraping away the carbon in the valve ring grooves and found a bit between two of them that seemed to be a bit out of place, so I asked if that was right - and the air turned blue as the foreman realised that the darned thing was cracked, not by me I hasten to add.

IIRC they had to have the whole head (and the other three) ultrasound checked, then have the broken bit refitted using pegs - twas cast iron so they couldn't just weld it back - then remachined.

[Nbodger]

Well Dear Chap, I am afraid you would not get a job as a Blue Peter presenter as they would have produced a working model that all of us kiddywinks could have copied.

Washing up liquid bottles spring to mind.

[Moonglum]

I dread to think what would happen if you put a bottle of Fairy Liquid (or similar) in a steam engine boiler! Smell nice I guess...

Anyway, John's graphics rather reminded me of "Vision On" with the late Tony Hart and Pat Keysell.

Tim

 

[martyn]

A brilliant explanation and working diagram.

Thanks again, John.

I'm pretty sure one LNER 4 cylinder class had inside admission on two cylinders and outside on the other two, with the piston valves driven by rocking levers.

But we're getting a long way from a beginner's guide!

Martyn

[Train Waiting]

A Coarse Guide to the Steam Locomotive for 'N' Gauge Modellers - Part 56


Hello Chums

Expanding on the Theme of Steam - Part I

Do you remember the Newcomen engine worked by the piston being pulled down by the vacuum caused by condensing steam?  The steam was about 5 psi or so.  Then came the use of 'strong steam' at higher pressures, resisted by James Watt.

You might recall from Section Two of this ever-so-brief mini-series that, at the time of the Rainhill Trials, 'Rocket' had a boiler pressure as high as 50 psi and that boiler pressures increased steadily throughout the Nineteenth Century.

The use of higher pressure steam transformed the steam locomotive as it gave two ways in which to make the steam work.  That's what we'll begin to discuss in this part.

The first way is simply the pressure of the steam pushing on the piston to move it along the cylinder.  A sort of brute force.

The second way is to use the high pressure steam's desire to expand to push the piston along the cylinder - this is known as 'expansive working'.

You probably have read or heard a fair amount about an engine working at 'such-and-such cut-off'.  Let's have a think about what 'cut-off' means.

The valve, whether slide or piston, moves to-and-fro in the valve chest, using power which derives from the piston moving in the cylinder, and is transmitted to the valve by some sort of mechanical contraption.  We'll probably encounter the Joy of these contraptions later.

However, the piston and valve are not fixed absolutely in relation to each other. The driver can adjust the mechanical contraption, which moves the valve, to alter the valve's position relative to the piston.  By moving a big lever or turning a wheel¹, the driver can make changes to the position of the valve, relative to the piston, which affects what the valve will do. The most significant change is which way the engine will run - forward or reverse.

The normal term for the device the driver moves is 'reversing lever'², 'screw reverser' or, simply, 'reverser'.  The reverser of LNWR 2-4-0 No. 790 Hardwicke can be seen at the bottom left of the picturingham.  Or, most of it can - my coarse photography strikes again.





In the simplest steam engines, the reverser will give forward, reverse and mid-gear - a sort of 'neutral' position.

But, in the sort of locomotives we are thinking about, the driver can adjust how much steam the valve allows into the cylinder. This can vary from during most of the length of the piston's stroke to a lot less.  The term for the distance the piston travels before the valve cuts off the live steam supply is 'cut-off' and it is stated as a percentage.

Jolly clever stuff and well worth reminding ourselves this technology is two centuries old.

In the next part, we'll resort to VALVE (kindly described by @Moonglum as reminding him of Vision On - I can remember the programme), to help us visualise this cut-off thing and then discuss why it's important.


¹ I admit to visiting 'Wald Disney World' in Florida. Thirty years ago. I was invited on to the footplate of one of the steam locomotives - three foot gauge I think.  I was discussing the controls with the engineer and mentioned the 'reversing lever'.  He looked at me for a second and told me they called it the 'Johnston Bar'. Fascinating.

² Sometimes a steam-powered reverser was fitted. James Stirling used a very simple and reliable one on his locomotives for the Glasgow & South Western and South Eastern railways.  Much appreciated by the enginemen.

Mr Bulleid fitted a steam reverser to his 'Pacifics', but it was not particularly reliable in operation.  Mr Jarvis replaced it with a conventional screw reverser on the locomotives which were modified.

Incidentally, I understand there was eventually a Federal requirement in the USA for all but the smallest steam locomotives to have power reverser.


'N' Gauge is Such Fun

Many thanks for looking and all best wishes.

Toodle-oo

John

[martyn]

Thanks again for an excellent explanation, John.

The ex NER also used a powered reverser, and Darlington fitted it to some early J39s. Not a problem on the ex NER region, but the RCTS history tells a tale where a driver unfamiliar with the gear couldn't get it out of 'full gear', ie the loco ran the trip with excessive use of steam, coal, and water. See Trainwaiting's next post-full gear means with maximum cut-off. A bit like trying to do 70mph in first gear.....

The later GER passenger locos also had a powered reverser, driven by the compressed air from the Westinghouse brake system: it also operated the tender water scoop and sanding gear. This brought an official complaint from Westinghouse as it meant the air supply to the brake could become compromised with too low a pressure. This was overcome by making the air receiver in two parts, with a non return valve between them: one part for the brake use, the other for the other gear.

It wasn't a requirement, but in the UK it was normal for a shunting engine to use the lever reverser, with screw version for 'main line' locos where frequent change of direction of the loco didn't happen so much. But, as ever, this was not an absolute rule, with many main line locos fitted with lever, as in your photo.

I've noted the pun in this post to go with those on your previous ones...

Martyn

[Train Waiting]

A Coarse Guide to the Steam Locomotive for 'N' Gauge Modellers - Part 57


Hello Chums

Expanding on the Theme of Steam - Part II

As mentioned in Part 56, it's time, I think, to inflict VALVE on you once again:-





Picturingham 1: The valve, which is about to close, is admitting steam for about a three-quarters (75%) of the piston stroke.





Picturingham b: The valve, which is about to close, is admitting steam for about a quarter (25%) of the piston stroke.


The first example is what is called 75% cut-off - the valve cuts off steam from entering the cylinder at 75% of the piston's stroke. Most of the movement of the piston is due to to the force of the high pressure steam.

The second example is what is called 25% cut-off - the valve cuts off steam from entering the cylinder at 25% of the piston's stroke.  This example will obviously use much less steam than the first. Most of the movement of the piston is due to the expansion of the steam admitted to the cylinder before the valve closed. Truly expansive working is generally considered to be 25% cut-off and shorter.

I suppose this begs the question, why would a driver use the first type of working when expansive working, as seen in the second example, is available?

*

Now for an anecdote.  About thirty years ago, we took my late Father-in-Law to the Strathspey Railway.  We stood and watched a train, behind an 'Austerity' 0-6-0ST, leaving Boat of Garten for Aviemore.  As it left the station, making a heck of a racket, Father-in-Law was shouting, "Wind her back" at the driver. This means adjust the reverser to shorten the cut-off.

It ought to be mentioned that Father-in-Law had been an engineman at Inverness in steam (and SuperSmelly diesel) days and knew well that stretch of railway. He was not impressed by the standard of engine working on the Strathspey.

*

Why was he so unchuffed at the poor engine chuffing loudly [Thank you!]?  I'll attempt a coarse explanation.  This is when many people mention the gearbox on a car to help explain the action of the reverser.  I prefer to avoid mention of gearboxes as steam locomotives don't generally have gears.  Yes, some specialist type do, but I've decided they are out of scope of this post.  But please feel free to mention them if you wish.

A steam engine and an electric motor share an important characteristic - they can move a load, within their design capacity, from a standing start - zero revolutions per minute.  Just try that with an internal combustion engine - immediate stall.

Cue gearbox and clutch to give a low gear and clutch slipping to gradually take up the load without stalling the engine.  If you ever have the pleasure of seeing and hearing a late BSA 'Gold Star' 'DBD34' 500cc single, fitted with a RRT2 close-ratio gearbox, start away, you'll notice the rider has to slip the clutch until well over 10 mph.  First gear will be used until over 30 mph.  There's not much fun to be had riding one of these motor-bicycles in a 20 mph speed limit.

Therefore, I've decided to avoid comparison with gearboxes and think of the humble upright push-bike.  With a big heavy frame and a fixed gear.  I can't afford to patronise Messrs Sturmey and Archer.

Getting away from standstill requires a lot of effort - maybe even standing up on the pedals for a few revolutions.  Then, once underway, we can sit down and the bicycle will begin to pick up speed.  As it does so, it becomes progressively easier to pedal until it arrives at a sort of bowling-along equilibrium. A nice late spring day, wildflowers out and birds singing.  Lovely.

Then, rotters, we come to a hill - a blooming steep one. Pedalling gets harder and harder until we find ourselves standing up on the pedals in a last gasp (almost literally) effort to breast the summit.  Which we manage to do, sitting back down on Mr Brookes' idea of a joke, and resuming gentle pedalling.

It's exactly the same for a steam locomotive.  A lot of effort is required to start from a standstill with '10 on', then, once underway, the going gets easier.  The driver starts off with a long cut-off to get the necessary force on the pistons and then progressively reduces the cut-off as the locomotive gets the train nicely on the move.  The same, in principle, as we did on the old boneshaker.

A poor driver might make a hash of this and not reduce the cut-off like a good engineman would.  The engine then uses a lot of steam and the poor fireman gets a wet shirt with all the shovelling he has to do to feed the hungry fire.

*

And, all these years ago, a not-quite-elderly gentleman, immaculate in collar and tie, sports jacket and flannels (he was in the RAF before joining the railway on de-mob, just after nationalisation - an Armourer on Lancasters and Lincolns), can be heard shouting at the driver.

*

If the train comes to a gradient, the driver will lengthen the cut-off to increase the power being developed in the cylinders.  Listen to a two-cylindered locomotive on Shap, Dainton, or a bank of your choice, and you will hear the sharp exhaust associated with working at a longer cut-off. Just like us making all that effort to keep the bicycle's pedals turning. 

The length of cut-off available to the driver depends on the design of the locomotive.  As an example, the BR 'Britannia' 4-6-2 class had a maximum cut-off, in full forward gear, of 77 1/2%.

As to minimum cut-off, a reversing lever has notches in a quadrant which fix it in place.*  To release the lever the driver pulls a handle - like a non-LNWR signal lever - and moves the lever to the required notch, then releases the handle to re-engage the catch in the notch. The shortest cut-off is the notch nearest to mid-gear.

On a screw reverser, there is a similar system to hold the reverser secure and that gives the minimum cut-off.

More practically, there is a limit as to how short a cut-off an individual locomotive can accept.  Some of this is due to the design of the valves and steam passages, some is due to the condition of the engine and, some is because individual locomotives, within a class, have their own characteristics.  Steam locomotives are like that.

Two-cylindered locomotives will not normally run as nicely on short cut-offs as engines with three or four cylinders and tend to become unhappy with cut-offs shorter than about 20%.  This was the minimum recommended by the designers for the BR 'Standard' locomotives with two cylinders.  Which was 998 out of the 999 built.

The enginemen were instructed as follows:

'The Driver should always endeavour to operate the locomotive in the most efficient and economical manner consistent with the work to be performed by the use of the regulator and reversing gear.'**

* Fixing the reversing lever stops the cut-off varying and protects the enginemen from a sudden, uncontrolled, movement of the reverser.  When 60532 Blue Peter had her catastrophic wheelslip at Durham Viaduct in 1994, the driver attempted to put her in mid-gear but the reverser whipped into full forward gear.  The force of the action severely injured the driver's arms.  I think both were broken. Unlike our tiny trains, steam locomotives are not toys and Thomas can kill you. Incidentally, neither driver or fireman had been on an 'A2' before,.

** Handbook for Railway Steam Locomotive Enginemen, British Transport Commission, London, 1957, p19.


'N' Gauge is Such Fun!

Many thanks for looking and all best wishes.

Cheerie-bye

John

[crewearpley40]

https://www.facebook.com/share/p/15j1nsTAGi/

Today, 21 January, is the anniversary of the birth of Egide Walschaerts in 1820. He was a Belgian mechanical engineer who invented the valve gear named after him in 1844. He died in 1901. The valve gear regulates the flow of steam to the cylinders in steam locomotives. In the 20th century Walschaerts valve gear became the most commonly used type, especially on larger locomotives. However, the Great Western Railway remained faithful to the Stephenson valve gear for most of its standard-design two-cylinder locomotives.

The exception was the GWR's four-cylinder locomotives – Star, Castle and King classes – which were built with Walschaerts valve gear, but unusually mounted inside, between the frames. One of the advantages of Walschaerts valve gear is that it can be mounted outside the frames where it is easily accessible for maintenance. Great Western locomotives with the valve gear hidden inside make a rather stately progress compared with the multitude of twirling rods of locomotives fitted with outside Walschaerts valve gear.

The British Railways Standard designs of steam locomotives built in the 1950s were virtually all fitted with Walschaerts valve gear. One of these classes, the 80 locomotives of the 4-6-0 class 4, was built at the Western Region's Swindon Works. This photograph is of one of the class under construction at Swindon in 1951, and shows off the valve gear very clearly.

Found this fascinating

Thanks John for the information

Chris

[Train Waiting]

A Coarse Guide to the Steam Locomotive for 'N' Gauge Modellers - Part 58


Hello Chums

Some More About Valves Part I

There's a few more matters regarding a steam locomotive's valves that I'd like to mention.  This is going beyond what is likely to be of much interest to most 'N' gauge modellers and is included for completeness.  The terms are often seen in books and magazine articles but, in my experience, few attempts are made to describe what they mean.

Please note my use of 'describe'.  I'll try to do this, but I won't make any attempt to explain the engineering principles.  These are beyond the scope of this exceptionally brief mini-series.

Before we discuss these matters, I'd like to spend some time considering something which one doesn't often see mentioned - the speed at which things happen.

*

Imagine you are driving a motor-car or riding a motor-bicycle, bowling along happily with 3,000 rpm showing on the tachometer ('tacho' or 'rev-counter').  That's 3,000 revolutions of the crankshaft each minute.  So? 

Please indulge me for a second and say 'One Mississippi' out loud.

Thank you.  The crankshaft would have made 50, yes, fifty revolutions in the time it took you to say, 'One Mississippi'. 3,000 rpm is 50 revolutions of the crankshaft per second!

*

Now back to steam locomotives where the parts don't move so quickly, but they are a lot bigger and much heavier. Let's continue to focus our discussion on locomotives with two cylinders and piston valves.

Imagine a 'Black Five' 4-6-0 out on the main line.  Perhaps you are in 'Premier Dining' and having a jolly fine day out with an agreeable companion of your choice.  You are both looking out of the window, with a nice drink in your hand.

A 'Black Five' 4-6-0 is restricted to 60 mph on the main line under present rules.  She has six foot diameter coupled wheels and 18 1/2 x 28 inch cylinders.

Time for sum somes:

Distance travelled each revolution of the coupled wheels: an apple pie x 6 feet = 18.85 feet.  Let's call it 19 feet.

When I was at school, Mr Ellis said there were 5,280 feet in a mile.  Divide that by 19, the diameter of the coupled wheels in feet, = 278.

This means that the locomotive's coupled wheels go round 278 times for each mile travelled.

60 mph is a mile-a-minute, so that equates to 4.6 revolutions per second.

Worth thinking about - these big coupled wheels revolving over four-and-a-half times every second as you and your companion enjoy the view and sip your tea/coffee/wine/'Nyetimber'. I rather like the concept of 'rpgN' - revolutions per glass of 'Nytimber'.  I think I'll experiment further.

You might recall there are two strokes of the piston for each revolution of the driving wheels.  Therefore, 9.2 piston strokes per second at 60 mph.

For a 28 inch stroke, the piston travels 56 inches each time the wheels go round.

At 60 mph, this is an average piston speed of 257.6 feet per second. (56 in x 4.6 rps = 257.6.)

It's even worse than that - as the piston makes two strokes for each revolution of the driving wheels what is actually happening is something like this:

'Speed up, slow down, stop, change direction, speed up, slow down, stop, change direction' for each revolution of the driving wheels.

There's a lot going on in very little time. Let's, please, bear that in mind.

*

Over the years steam locomotive engineers learnt a great deal about how to make valves work better. And some put it into practice. In Great Britain, the first locomotive engineer to design really efficient valves was (you've guessed it!) GJ Churchward on the GWR.  He applied what he had learnt from US practice.

The first thing to consider is 'lead'.  Please remember the speed at which things are moving. In order to allow for this, the valve can be arranged to permit steam to begin to be admitted to the cylinder before the piston is static at front or rear dead centre. This is important for locomotives which will travel at high speeds where the valve events are happening  quickly.

This pre-admission of steam allows for maximum steam pressure at the start of each power stroke. The amount of lead provided on the valve is about 1/4 inch on a modern express locomotive.

The best analogy is probably ignition advance on a motor-car or motor-bicycle. Most people nowadays won't have experience of manual advance/retard levers, as this function is now performed automatically. Ignition advance means the points on the contact breaker or magneto operate before the piston is at top dead centre. Due to the speeds involved, as mentioned above, this permits the spark to be created at the sparking plug at the correct time.  The faster the engine is turning, the greater the ignition advance.  Just remember to 'tard it back before trying to use the starting handle or kickstarter.  This helps avoid a broken wrist or ankle.

*

'Lap' is also important.  It is the amount by which the valve overlaps the ports on the live steam side when the valve is in mid-position. Used in conjunction with long valve travel (which we'll come to next), the main advantage of steam lap is the movement of the valve is accelerated and the valve events become more sharply defined. The port opening to live steam is increased which permits the locomotive to work at shorter cut-offs, bringing advantages of economy. The amount of steam lap provided by the valve is about 1 3/4 inch on a modern express locomotive.

For locomotives which are expected to run fast, 'negative exhaust lap' or 'exhaust clearance' may be provided. This means both ports are open slightly to exhaust when the valve is in mid-position. This feature provides greater exhaust freedom at speed and reduces back pressure in the cylinder. The amount of exhaust clearance provided is small, generally around 1/16 inch. 

Here is my coarse attempt to show steam lap and exhaust clearance using VALVE.  Both are outrageously exaggerated to allow for the effects of my coarse photography and coarser workmanship:-





Hopefully, it can be (just about) seen that the valve is in mid-position.  Steam lap is how much the valve overlaps the port on the live steam side, coloured pink.

Negative exhaust lap, or exhaust clearance, is the amount the port is open to exhaust, coloured blue-ish, when the valve is in mid-position.

I think the important thing to remember is this 'tuning' of valve events is a recognition of how fast things are happening.  It's a far cry from a man or boy 'handing the valve' on a Newcomen engine to admit steam or release water from the cylinder.

Similarly, look at the camshaft for a four-stroke internal combustion engine and you will notice the inlet valve begins to open before the exhaust valve is fully closed. One might think this will lead to wasteful use of fuel, but it is a practical response to the fact that the crankshaft might be revolving 50 times, or many more, per second. 

The next part will discuss valve travel.  Then we'll move on to the mechanical contraptions that move the valves and, after that, more than two cylinders.

If you wish to find out more about valves, I recommend:

Reading Handbook for Railway Steam Locomotive Enginemen, British Transport Commission, London, 1957 and/or

Getting in touch with a preserved railway or steam centre.  In my experience, the wonderful people there will be absolutely delighted when someone takes a genuine interest in their work. But, please, ask permission first before going behind the scenes. And a couple of tenners in the collection tin is always hugely appreciated. 


'N' Gauge is Such Fun!

Many thanks for looking and all best wishes.

Toodle-pip

John

[martyn]

A wonderful description again, John, of a complex operation.

Many thanks.

Martyn

[Bealman]

Amazing. You have a talent for explaining this stuff, John.

[Train Waiting]

A Coarse Guide to the Steam Locomotive for 'N' Gauge Modellers - Part 59


Hello Chums

Some More About Valves - Part II - the Slow Adoption of Long Lap - Long Travel Valves

As mentioned in Part 58, we will now discuss valve travel.

In slide valve days, valve travel, which is the maximum amount the valve can move within the valve chest, was modest - somewhere between three and four inches was typical.

As piston valves began to be widely used, locomotive engineers started to grasp the importance of longer valve travel.  The valve will move a greater distance for any given angular movement of the crank from which it derives its motion.  This, in turn, accelerates the initial movement of the valve in either direction and, in conjunction with increased steam lap, enables the valve events to be better defined and the port opening to live steam increased.  Which allows the locomotive to work at shorter cut-offs, resulting in increased economy.

Trust Mr Churchward to Lead the Way

Much influenced by developments in the USA, Mr Churchward understood the importance of long-travel valves and, as early as 1902, had designed 10 inch diameter piston valves with valve travel of 6 inches.  Steam lap was 1 5/8 inches.  Typical contemporary practice was four inch travel and one inch (or less) steam lap.

These valves were used in locomotives from 1903, including 4-6-0 No. 98 and 2-8-0 No. 97.  These were the prototypes of the 'Saint' and '28xx' classes.  The long travel - long steam lap valve had been introduced successfully to Great Britain.

The valve travel was increased in the 'Saint' class to 6 1/4 inches.  Slowly but surely, the GWR refined the Churchwardian principles.  Mr Hawksworth's 'Modified Hall' class 4-6-0 had almost 7 inch maximum valve travel and a steam lap of 1 3/4 inches. GWR practice eventually reached 7 1/2 inch maximum valve travel with the 'County' class 4-6-0.

Unfortunately, Mr Churchward's views went largely unheeded for getting on for twenty years.

Mr Hughes' Near Miss

George Hughes on the L&Y came close to defining the modern locomotive with four 4-4-0s, built in 1908, with both high-degree superheating and piston valves with six inch travel and 1 1/2 steam lap. The locomotives had a remarkable performance and were diagrammed for duties normally performed by an 'Atlantic'.  Unfortunately, the old worry about valve lubrication caused them to be converted to short-travel valves.  A near miss.

Ashford Laps Up Churchwardian Ideas

Down at Ashford on the SECR, REL Maunsell built up a small technical team with Derby and Swindon influences, and longer steam lap and longer-travel valves were used on the 'N1' 2-6-0 and the 'E1' 4-4-0 rebuilds. Mr Maunsell carried on with these good practices after being appointed Chief Mechanical Engineer of the Southern Railway. As an example, his 'King Arthur' class 4-4-0 had 6 9/16 inch valve travel.

The LMS Dithers Over Its Direction of Travel

Matters on the LMS were not so positive - with even modern-looking locomotives like the 2-8-0s for the Somerset & Dorset Joint Railway having short travel valves - 3 3/4 inch in that particular instance.

The appointment of Mr Hughes as its first Chief Mechanical Engineer led to an important development in the sturdy form of his 'Horwich Mogul'.  Unfortunately, Mr Hughes was glad to retire to his garden before the class entered service and his successor, the thoroughly Midlandised Sir Henry Fowler, tinkered with some parts of the design.  But, fortunately, not the important ones.  The class had been designed at Horwich, where they had been reading a a couple of recently-published books, from the USA, on valves and valve gear.

The cylinder and valve design followed recent American practice with 11 inch diameter piston valves having 6 3/8 inch travel and 1 1/2 inch steam lap.  These are strikingly modern figures for a locomotive designed in 1924.




[A thoroughly competent design - the LMS 'Horwich Mogul'.  This example, featuring original livery and number, is a ProperlyPoole Graham Farish model.]
 

I might write a future post regarding the strange goings-on in Derby Drawing office - design work had moved from Horwich to Derby after Mr Hughes retired - but some new locomotives, principally the '2300' 2-6-4T passenger engines introduced in 1927, had excellent cylinder and valve arrangements, influenced by the 'Horwich' 2-6-0s. And other new designs didn't.

Mr Gresley Learns from the GWR

It is well-known that Mr (later, Sir Nigel) Gresley's 'A1' 4-6-2 design, for the Great Northern Railway and perpetuated enthusiastically by the LNER, was greatly influenced by the Pennsylvania Railroad's 'K4' 4-6-2, detailed drawings of which were published in the journal Engineering in 1916.

With the benefit of hindsight, it seems strange that the 'A1' had 8 in diameter piston valves with 4 9/16 inch travel and a steam lap of 1 1/4 inch.  This valve design was too restrictive to take advantage of the excellent capability of the boiler to produce steam.  But, Bert Spencer, Technical Assistant for Locomotive Design, realised this in 1924 and produced a design for longer lap and travel valves.  Mr Gresley decided not to implement it, although the 'A1' class was proving to have a higher coal consumption than expected.

After the well-known interchange trial between 'A1' and GWR 'Castle' locomotives in 1925, in which the 'Castle' had proved superior, especially in coal consumption, Mr Spencer's improvements to the 'A1' design were put in hand. Valve travel was increased from 4 9/16 inches to 5 3/4 inches with steam lap increased from 1 1/4 inch to 1 5/8 inch.  Lead was 1/8 inch.  Incidentally, the inside cylinder's valve had an additional 1/16 inch lap. This, and the increase in boiler pressure¹mentioned in an earlier part, gave rise to the 'A3' 'Super Pacific'.

The cost of the valve modifications was modest - £150 - £190 per locomotive - and performance was greatly improved.  Coal consumption reduced from about 50 lb of coal per mile to 40 lb. Evidence of one benefit of this is the introduction of non-stop working from King's Cross to Newcastle from the summer of 1927 and to Edinburgh from 1 May 1928.

From 1927, three of the 'Big Four' companies had adopted long travel valves with long steam lap.  The LMS hadn't made up its mind.  Fortunately, the 'Royal Scot' 4-6-0 class of 1927, designed in collaboration with the North British Locomotive Company, paid heed to the valve design of the 2-6-4T with 9 inch piston valves having 6 5/16 inch travel and 1 1/2 inch lap.  Lead was 3/16.




[Evidence of the long travel valves fitted to this LMS Fairburn 2-6-4T can be seen in the extended front cover for the piston valve chests, located directly above the cylinder.  This allows for the movement of the front piston valve head at maximum travel. The rear cover is similarly extended.  Thanks are due, once again, to the lovely people at the Lakeside & Haverthwaite Railway where both of the preserved examples of this class can be found².]


The next part will conclude our discussion of valves and summarise the importance of Mr Churchward's contribution to British locomotive practice. Then we will move on to other things.

¹ From 180 psi to 220 psi.

² The final LMS development of the 2-6-4T locomotive which began with the '2300' class mentioned ante.  Charles Fairburn was appointed Chief Mechanical & Electrical Engineer (CM&EE) of the LMS when Sir William Stanier formally resigned in 1944. He had been seconded full time to the Ministry of Production as Scientific Adviser since 1942. Mr Fairburn was a distinguished electrical engineer and Mr Ivatt the Younger looked after the steam locomotive side of things.

Mr Fairburn died in late 1945 and Mr Ivatt was appointed as CM&EE with effect from 1 February 1946, having undertaken the role in an acting capacity since Mr Fairburn died.

Whilst clearly Ivatt engines, the convention is that the head of the department takes the credit (or blame) for locomotives introduced, even if they had little to do with the design.


'N' Gauge is Such Fun

Many thanks for looking and all best wishes.

Pip-pip

John

[martyn]

Thanks again, John.

Just one thing-in paragraph 3, shouldn't the locos be able to work at shorter cut off for economy? Typically, the LNER Pacifics seemed to have run at 15-20%. Or is this something to do with higher speeds generally obtainable from these, and other passenger express, locos?

Martyn

[Train Waiting]

Just one thing-in paragraph 3, shouldn't the locos be able to work at shorter cut off for economy?

Yes indeed.  An especially silly typo - now corrected.  Thank you very much.

With all good wishes.

John