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Author Topic: How to Design a 1-10 hp Working-Class Steam Engine  (Read 24018 times)

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How to Design a 1-10 hp Working-Class Steam Engine
« on: February 13, 2014, 04:36:26 PM »

HOW DID I GET INTERESTED IN STEAM ENGINES?

A few years ago, I began a post about how to build a steam engine.
I have learned so much in the last few years, especially regarding foundry work, that I feel compelled to try writing an article again.

My initial interest in steam began as a child playing with my brother's Wilesco steam plant, and my interest increased when my Dad brought home a used copy of "Audel's Power Plant Engineer's Guide", Frank D. Graham, Theo Audel & Co., 1945.

I decided as a senior in high school to build my own steam engine and boiler as a science project, and while it did not impress the judges as far as scientific exploration, it did highly impress my science teacher, who had never seen such a thing. (photo of engine below, I don't have a photo of the vertical fire-tube boiler I build to run the engine).

My first steam engine and boiler were built with the help of my Dad, and it was his talent and experience with steam engines and boilers that made the project a success.

Some 30 years after my science project steam engine, my Dad died from cancer and took most of his steam engine and technical knowledge with him.  Dad was a very talented and very eccentric individual, and it is difficult to explain why so little of his work was witness or documented other than to say that he was an extremely private individual.

What was left after Dad died was his home machine shop, and about 18 of his model steam engines, as well as one of his Roper-replica steam bicycles.  Up to that point, I generally assumed that since Dad designed and built steam engines, then it must be somewhat rudimentary of a thing to design and build steam engines in general.  It was a rather naive thought.

I started studying steam engines and machining around 2007, and was able to aquire and relocate my Dad's machine shop to my garage, with great difficulty.  The more I studied Dad's engines, the more curious I became about how the old engines were actually designed.
The only clue Dad left regarding his engine designs were his hand-drawn on vellum geometric sketches for most of his engines.

So I began an internet search for information on how to build a steam engine, and basically came up empty handed.
The one valuable source of information that I found were the old scanned steam engine books, and luckily many of those had recently become available on the internet in PDF format.
Here is a list of 158 steam engine related books that I found, from various sources:
http://www.classicsteamengineering.com/index.php?topic=905.msg7676
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #1 on: February 13, 2014, 04:41:59 PM »

WHAT INFORMATION DID I DISCOVER FROM THE OLD STEAM ENGINE BOOKS?

As I began to read the multitude of old books about steam engines, I quickly realized how vast this topic is, and how quickly steam engine design advanced in a very short period of time.

To try and get an idea of the various types of engines which were built in the steam era, I began gathering screen captures from the old public-domain books, and before I knew it, I had a collection of some 1,000 engravings of steam engines.

These engravings including classifications of most of the types of old steam engines are categorized and included on this forum under the (RESEARCH) TECHNICAL LIBRARY SECTION.


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RESEARCH THE TOPIC

Finding good information about steam engines on the internet can difficult, and while there are some who know a great deal about steam engines and are always correct in the information that they offer, too many times one runs into "armchair steam experts" who make statements that sound very pausible and believable, but turn out to be totally false.

So how does one discern the informed steam expert from the uninformed steam expert.
A good way is to ask someone a question that you have verified in several old and notable book references.
I think you would be surprised at the range of answers, and the range of wrong answers.
If you find someone who gives the right answers every time, that is the individual to pay attention to.

And keep in mind at all times, I am not a steam expert, I simply get this information out of the old books and present my best understanding of it here.
I can read all the calculus where it frequently appears in the old steam books so that helps a great deal with understanding how steam engines operate.

Some of my favorite books are as listed below:

A good book for a general overview of steam engines, but light on the theory and math, with some incorrect technical data on valve gear.

"Audel's Power Plant Engineer's Guide", Frank D. Graham, Theo Audel & Co., 1945.


Some good overall general knowledge steam engine books:

Borne, John,
"A Treatise of the Steam-Engine",
Longman, Green & Co., 1868.


Clark, Daniel K.:
"An Elementary Treatise on Steam and the Steam-Engine, Stationary and Portable",
Crosby Lockwood and Co., 1885.

A footnote on Daniel Clark.  He was the person in charge of the London International Exhibit of 1862 where Charles Porter introduced the first high speed steam engine.  Clark asked Porter before the exhibition began how fast he intended to run his engine, and Porter responded 150 rpm.  Clark stated that no steam engine can run safely at 150 rpm, and Clark forbid Porter from operating his engine at more than 100 rpm.
Luckily Porter ignored Clark, and thus the first high speed steam engine was born, and it did operate at 150 rpm safely, and even at 300 rpm safely.  In 1862, there were only a couple of people in the world who believed that a steam engine could be operated at 150 rpm, but it was not long before the whole world followed Porter's design.  Such can be the impact of a single person who knows what they are talking about in spite of the entire world telling them they are wrong.


Croft, Terrell:
"Steam-Engine Principles and Practice",
McGraw Hill Book Company, Inc., 1922.


Header, Herman, Powles, H.H.P.,
"A Handbook on the Steam Engine with Special References to Small and Medium-Sized Engines",
D. Van Nostrand Company, 1902.


Hirshfeld, C.F.,  Ulbricht, T.C.:
"Steam Power",
John Wiley & Sons, Inc., 1916.


James, Walter H.,  Dole, Myron W.:
"Mechanisms of Steam Engines",
John Wiley & Sons, Inc, 1914.


Jamieson, Andrew:
"A Textbook on Steam and Steam-Engines",
Charles Griffin and Company, 1889.



A compelling personal story of a lawyer-turned-engineer who revolutionized the steam world with the introduction of the first high speed steam engine at the London International Exhibit in 1862.  This non-engineer turned the steam world on its head, and proved that most of the renowned steam engine designers of the day were wrong in their basic design approach for modern steam engines.

Porter, Charles T.:
"Engineering Reminiscences contributed to 'Power' and 'American Machinist' ",
John Wiley & Sons, 1908.



A good steam engine valve gear book:

Dalby, W.E.:
"Valves and Valve Gear Mechanisms",
Edward Arnold Pub., 1906.



A book about marine steam engines:

Yeo, John:
"Steam and the Marine Steam-Engine",
Macmillan and Co., 1894.



A book about balancing steam engines:

Dalby, W.E.:
"The Balancing of Engines",
Edward Arnold, 1906.



A steam engine governor book:

I.C.S. Staff,
"Steam Engines, Engine Governors",
International Textbook Company.



A steam engine indicator book:

Peabody, Cecil H.:
"The Steam-Engine Indicator",
John Wiley & Sons, 1900.



One of my favorite books, but difficult to find in PDF format.  I have hard copies of both volumes of the 18th Edition:

Sothern, J.W.M.,
"Verbal Notes and Sketches for Marine Engineer Officers, A Manual
of Steam Engineering Practice - Vol. 1 & 2",
revised by J.K. Bowden, 18th Ed., James Munro & Company, Ltd., Date ?.



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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #2 on: February 13, 2014, 06:07:58 PM »

GENERAL CONSIDERATIONS

The general considerations regarding steam engine design that are presented here are what I have gleaned from reading old books from the period.

Please do not mistake these ideas as absolute, all encompassing, or necessarily completely correct, but rather these ideas reflect my best understanding of how steam engines were designed in the late 1800's.

Some of these items have been topics of previous debates on forums, and there are often disagreements about any steam engine topic, but I generally try to adhere to information from some of the more recognized books of the time.

One must keep in mind though that there was not complete agreement between designers in the late 1800's about how to design a steam engine (it was a rapidly evolving technology), and furthermore, some of the books make incorrect statements, such as the correct suspension point for a Stephenson's link for minimizing link slip and equalizing valve events throughout the full range of the lnk travel, but generally what I state here is what I have found was in concurrance in more than one of the old books.

There are always differences of opinion on any topic, so keep in mind, this is just my opinion of how many of the steam engines from the late 1800's were designed, and often more importantly, I often select my favorite methods and configurations from a range of styles that were used for steam engines in the late 1800's.  The result is generally an engine that matches the look and feel of the era, without necessarily matching exactly any one engine.  I suppose it boils down to how I would design and build a steam engine if I lived in 1890.

Pick your own favorite methods and configurations as it suits your desires.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #3 on: February 13, 2014, 06:35:59 PM »

SIZE/SCALE, MATERIALS, FASTENERS

My dream has always been to build full-sized steam engines, and that idea generally collides with reality considering the size of most old steam engines.

However, there was a class of small steam engines built for miscellaneous purposes, and some of these engines can be reproduced at a 1:1 scale, so my focus primary lies with designing and building full-sized workshop-type engines at a 1:1 scale.

Some of the engines that interest me are simply too large to recreate at a 1:1 scale, so I do build scale models, however, I diverge from the modeler who builds an exact reduced scale model, and instead increase the thickness of various parts to create a scale model that is strong enough to be used for a workshop engine.

Workshop engines were used to power many things such as machine tools, sewing machines, and other equipment from the industrial era.
A good example of a small working class engine is the Cretors series of popcorn engines, which were designed in the 1/4, 1/3 and 1/2 horsepower range, and were designed to operate indefinitely without requiring frequent repairs.

The material of choice for most old steam engines was gray cast iron, since it was readily available, was easy to cast, had superb wear characteristics, and dampened the vibrations that were present in most machinines of the day.  Gray cast iron is also one of the easiest materials to machine and surface finish, which is a result of the graphite that is a part of gray cast iron.

Many modelers grumble about the black (graphite) dust that is created when machining gray cast iron, but there can be no doubt, gray cast iron reined the supreme steam engine building material for most of the steam era.

Cast iron parts wear well when operated against other cast iron parts (such as cast iron piston rings operating in a cast iron cylinder), and cast iron often wears well when operated against many other metals.

The bearing material of choice was Babbitt, invented by a man of the same name.
Babbitt bearings were often poured around the crankshaft while it was fixted in an accurately aligned position, as can be seen on the Cretors steam engines.  Careful examination of a Cretors Babbitt bearing indicates that there was no effort made to center the crankshaft in the bearing housings, but rather the Babbitt bearing was used both as a bearing and as a method to precision align the crankshaft to the other moving parts of the engine.

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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #4 on: February 14, 2014, 02:32:25 AM »

OVERALL CLASSIFICATION/CONFIGURATION OF ENGINE

The next item to consider is exactly which configuration you would like to build.

My all time favorite steam engine layout is the bottle engine, and specifically the Acme engine, which I am told was manufactured by Donegan & Swift.  Below is an engraving.  The Acme had an outboard bearing, and thus required more floor space, but there were similar styles that did not have outboard bearings, but rather just a bearing mounted on either side of the frame, with the flywheel and pulley cantelievered out on the ends of the crankshaft.

So I am building a Acme-like engine without the outboard bearing.

One consideration of omitting the outboard bearing is that a closed crankshaft will be required, ie: you cannot use a simple disk with crank pin mounted on the end of a shaft as shown in the Acme engraving, but rather must make up a crankshaft that has two webs and a central crank pin between the webs.

From a design standpoint, it would be easy enough to subsitute a simpler type of frame if that were desired, but this is the one that caught my eye.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #5 on: February 14, 2014, 02:37:29 AM »

CYLINDER/VALVE CONFIGURATION

My starting point for the engine I will use as an example is this engraving from an Audel's book.
It shows a cylinder configuration for the typical smaller steam engines of the period that I am interested in.

This section shows a typical unbalanced D-valve, and I generally like to use a balanced d-valve to attempt to attain most of the advantages of the piston valve while maintaining most of the simplicity of the D-valve/cylinder design.

I otherwise adhere closely to this format for the cylinder.

Note that this is a double-acting steam engine, ie: the piston is acted upon by the steam both during the downstroke and the upstroke.
Compared to a typical 4-stroke single-acting gasoline engine, a double-acting steam engine produces two power strokes per revolution of the crankshaft compared to only one power stroke for every two revolutions of the crankshaft for a 4-stroke gasoline engine.

A small steam engine can produce a surprising amount of power and torque for its relative size, and unlike a gasoline engine, a steam engine can produce maximum torque even when the rpm is low or even zero in some cases.

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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #6 on: February 14, 2014, 03:00:49 AM »

ENGINE OPERATING SPEED

Steam engine design went through some milestones in developement, from the old pumping engines, to beam engines, oscillators, locomotive engines, often with parallel development for marine, stationary and locomotive designs.

For stationary engines, which is what we are designing, there was a radical and landmark change in steam engine design methods when Charles Porter introduced the first high speed (150 rpm) steam engine to the world at the London International Exhibition in 1862 (photo below).
One of the special valves that was used in the Porter engine can be seen displayed in the engraving below, at the front of the engine.

Equally as important as Porter's high speed steam engine was his high speed isochrounous governor which incorporated both flyballs and a centrally located weight (examples shown mounted on Porter's engine in the engraving below).  This governor was to set the standard for years to come as far as precision and accurate steam engine speed control.

Although Porter was a lawyer by training, he had the mind of an engineer, and his keen observations allowed him to transfer the knowledge he gained from designing a rock surfacing machine into a steam engine that immediately and permanently obsoleted every steam engine built up until that time.

For the same amount of horsepower produced, Porter's engine required a fraction of the floor space of the most advanced steam engine designs of the day, such as the giant Corliss.  Porter had to carefully combine high strenth and rigidity into ever part of his engine, with careful attention to details that could more easily be overlooked on the larger and slower operating engines.  The Porter engine was also highly efficient, although not quite as efficient as the Corliss.

Porter also made significant contributions to the design and manufacturing of indicators, which he demontrated at the 1862 Exhibition.  Indicators were the high-tech devices of the mid 1800's, and allowed a great many aspects of the inner workings of a steam engine to be plotted on a piece of paper that was wrapped around a small drum, including the highly important and critical value of engine efficiency.

Few in the engineering world recognized the significance of the Porter engine at the 1862 Exhibition, and it would routinely dismissed and considered not a viable engine design due to its lack of a condenser.  All steam engine designs would soon follow the high speed design path after 1862.
The only steam engine design change more significant than Porter's high speed engine after 1862 was the introduction of the compound marine engine.  Although the steam turbine is considered a steam engine, and its impact on the world was and still is perhaps the most significant of all steam engines, turbines are a subject that deserve their own separate and complete discussion, and so are not included in the steam engine discussions here.

So to conclude this section, our steam engine design will be of the high speed variety, with "high speed" being defined as anywhere from 150 to 300 rpm.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #7 on: February 14, 2014, 03:31:54 AM »

CRITICAL CYLINDER FEATURES

There are several subtle but critical features that can be seen in the cylinder engraving above, and features that were typical of steam engines of the late 1800's, noted as follows:

1. Both ends of the cylinder are counterbored, ie: the end of the cylinder increases in size larger than the bore itself.  The counterbore allows the piston to over-ride the ends of the cylinder bore, thus preventing the buildup of a ridge at the ends of the cylinder as the cylinder wears.
More importantly, the piston ring itself actually over-rides the end of the bore, not just the piston itself, since the ridge will appear where the piston ring travel ends.

2. The steam passages enter the cylinder in the counterbore area, not in the bore area.  A passage entering the bore area would snag and break a piston ring.

3. For the same reason, the D-valve over-rides the valve seat (to prevent a ridge buildup as the vavle wears).  Not show in the engraving, but of the same nature is the fact that the crosshead over-runs the crosshead guides, to prevent a ridge buildup there.

4. The piston at top-dead-center and bottom-dead-center positions does not protrude into the steam passage area.  Porter noted in his book that high speed engines which have pistons that travel into the passage area will knock due to the pressure exherted by the steam onto the side of the piston at the beginning of admission of steam into the cylinder.

5. The flanges at either end of the cylinder extend significatly beyond the engine of the piston at its extreme travel locations, and this is to allow space for the steam passage to enter the wall of the cylinder.  In order to avoid having a large vacant space at either end of the cylinder, which would be wasteful to fill and empty with steam at every stroke of the piston, the cylinder heads are recessed down into the bore, and terminate very close to the piston when it is at its extreme positions.

6. The interior surface of the cylinder heads where the steam passages enter the cylinder are dished out to provide a smooth and consistent path for steam to enter the cylinder.

7. There are drain passages located at both ends of the cylinder, at the bottom of the cylinder in the counterbore area, and these were connected to drain valves, which were opened when the engine was first started and the cold cylinder produced objetionable amounts of liquid condensate (water).

8. The piston was generally very thin in section, and could be either a one-piece solid or two piece hollow design.  Typically two cast iron Ramsbottom-type piston rings were used per piston.

9. The piston rod was typically tapered where it mated with the piston, sometimes with a shoulder at the end of the taper to positively seat the piston onto the rod.

10. The piston rod nut protruded above the top of the piston into a recess into the upper cylinder head.  Sometime this nut was peened onto the rod to prevent the nut from loosening.

11. The exhaust passage beneath the valve seat (the valve rides upon the surface of the valve seat) wraps around a portion of the cylinder before it exits the side of the steam chest, and this allows the exhaust to exit the engine below the bottom of the steam chest.

12.  The sides of the cylinder were often insulated with strips of wood called lagging, and usually a flange protruded from the end of the upper and lower cylinder heads to retain this lagging.  Cylinder heads for small steam engines were typically not insulated, but heads for larger steam engines typically were insulated.

13. The steam pipe (the pipe bringing steam from the boiler to the engine) was typically connected to the side of the steam chest for horizontal engines, and often connected to the top of the steam chest for vertical engines.

14. The D-valve was not ridgidly connected to the valve stem, but instead was coupled with a sliding fit that alllowed the valve to move towards teh valve face as the valve wore.  The saying of the day was "D-valves wear in; piston valves wear out", which was derived from the fact that the D-valve leakage became smaller as the valve wore into the valve face.  Piston valves to not get tighter as they wear.  The advantages and disadvantages of a D-valve will be discussed in the "Valve" section.

15. From a structural standpoint, steam engine cylinders typically maintained a constant and minimum thickness, as did the cylinder heads.
There were no sharp corners or abrupt changes in metal thickness, since either of these conditions could cause cracking due to stress concentrations.  Large changes in metal thickness could cause cracking due to uneven shrinkage as the molten metal solidified.
The floor of the steam chest slopes to maintain a constant metal thickness.

16. Careful consideration has to be given to the size of the steam ports and passages, as well as the exhaust port and passage.
The steam and exhaust ports are the openings in the valve seat, and the valve seat is the plate that is beneath the valve.
The passages connect the steam ports to either end of the cylinder, and connect the exhaust passage to the bottom interior of the valve.

17. The passages were generally sized larger than the ports, and the ports located in the valve seat were machined to a final and exact size and location after the initial casting was complete.

18. Valve travel must be kept as short as possible to minimize valve wear, and the steam chest must be long enough to contain the valve when the valve is at its most extreme positions.

19. The steam chest typically had some spare capacity for use as a steam reservoir, to prevent a pressure drop at the beginning of the stroke.

19. The effective area on the bottom of the piston that the steam pressure could act upon was smaller than the area on the top of the piston, due to the entry of the piston rod.  For larger engines, a slight adjustment in the valve gear is made to compensate for this imbalance.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #8 on: February 14, 2014, 04:19:54 AM »

CYLINDERS - Continued

Below are other engravings of steam engine cylinders showing features similar to what are shown in the section above.

In each case, the cylinder is countebored at each end, and the piston ring over-rides the end of the bore.

The tapered end of the piston rod can be seen in the first engraving.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #9 on: February 14, 2014, 04:30:21 AM »

And my preliminary design for a typical small steam engine cylinder.

Bosses have been installed on three sides of the cylinder, at both ends to allow flexibility as to where the drain cocks could be installed.

Note the flange on the cylinder heads which retains the lagging (lagging not shown).
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #10 on: February 14, 2014, 04:41:52 AM »

Here is the cylinder with the lagging installed on the exterior of the cylinder.

The lagging was often strips of hardwood, but sometimes cast metal.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #11 on: February 14, 2014, 04:49:40 AM »

And here is an old hoist steam engine cylinder installed on a modern fabricated frame (the cylinder was the only item remaining of the engine, so the rest had to be fabricated).

You can see the location of the cylinder drain cocks, the entry point of the passages into the cylinder (rectangular in shape), and the cast lagging.

The piston in this case does not use a nut, and since the piston is furnished with holes in the top, it is assumed that the design intent was to not have a piston rod nut, but rather thread the piston directly onto the piston rod.

My preference is to use a nut on the end of the piston rod.

Cylinder head bolt patterns typically varied between five and six studs.
One reason that studs were used instead of bolts is that bolts tend to fail at the weakest point, which usually was at the threads.
Thus when a bolt broke off, it typically was difficult to remove the remaining part of the bolt from the cylinder flange.
A stud that sheared off at its upper threads could easily be removed if the cylinder head was removed.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #12 on: February 14, 2014, 04:23:22 PM »

THE PISTON

The piston for the smaller steam engine sizes, other than the uniflow variety, is typically very thin in section, and generally has two rings of the John Ramsbottom type.

Pistons for the larger engines were generally hollow, and consisted of two or more parts bolted together.
Pistons for smaller engines in the range of horsepower we are considering were typically one piece.

Pistons were typically fitted to the end of a tapered rod, and generally had a nut securing the rod to the piston.

High pressure pistons, which can be seen on the high pressure side of compound engines, were often conical in shape and one piece in construction.  This piston form was lightweight and strong, but required more complex cylinder head interior shapes, to match the contours of the piston on its top and bottom.

The engraving below shows one piece and hollow piston designs.

Hollow pistons could be designed as two piece, or cast hollow with a core and a plug to allow the core to be removed after the piston was cast.

For smaller engines, a flat one piece piston is often used, which allows the interior surface of the heads to be flat.



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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #13 on: February 14, 2014, 04:36:13 PM »

PISTON - Continued

This is my rendition of the piston style I would like to use.
It is a one-piece high-pressure type.   It will not be used at high pressure, but was selected for simplicity and light weight.

For this piston, the nut is semi-recessed into the top of the piston.
The diameter of this piston is 3 inches (76.2 mm).

The tapered joint where the piston rod meets the piston may need to be ground with a fine lapping compound to achieve a very tight fit.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #14 on: February 14, 2014, 04:43:17 PM »

PISTON RINGS

Piston rings often were notched so that the ends could overlap, and a better seal could be provided at the gap in the ring.
The notched ends could be rounded as seen in the piston engraving above, or square as seen on the Cretors Popcorn engine ring below.
The Cretors engines used a single wide piston ring with a staggered joint.

Steam engine piston rings are often wider then their combustion engine counterparts for the same bore size.

For model engines, the overlapping ends are often omitted, and a butt joint used.  This allows the rings to be turned as one piece at the same diameter as the bore, then the ring broken at the joint, the ring sprung opened, and heat treatment applied to allow the ring to maintain spring pressure in the sprung-open position.

For a piston ring with an overlapped gap, the blank ring must be turned slightly larger than the bore, then the overlapping gap cut or filed into the ends of the ring, then the ring compressed to the size of the bore and turned to final bore size on a special mandrel.

Gray cast iron makes an ideal material for piston rings, and matches the material that was traditionally used for rings in the old engines.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #15 on: February 14, 2014, 04:58:00 PM »

THE PISTON ROD

The piston rod is typically a straight shaft with a thread on the crosshead end, and a taper and thread on the piston end.
A high grade of steel is recommended for this part since it is under considerable stress, and is relatively thin with respect to the alternating compressive and tensile forces that it must withstand with every stroke.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #16 on: February 14, 2014, 05:37:43 PM »

THE CROSSHEAD

The crosshead supports the outer end of the piston rod, and travels along the crosshead guides, over-running the ends of the guides to prevent a ridge buildup.

Some crossheads are flat in the locomotive-style.  Others are hollow as shown in the following engraving.
Typically the crosshead has shoes that slide upon a tapered surface to allow adjustment of the crosshead in the guide, and to allow for taking up of wear.

The crosshead pin is often tapered on one or both ends.

The connection of the piston rod to the crosshead can be a threaded connection or a pinned connection.
For a high speed steam engine, a threaded connection for the piston rod, with a jam nut is my preference.

Below is the engraving I selected to be used as a guide for crosshead design.

It would appear that the crosshead below has sliding shoes on either side of the crosshead, and each shoe appears to have a replacable surface that contacts the crosshead guide.  The screws on the sides of the crosshead apparently lock the shoe in-place once it has been adjusted by the screws that face the piston.

A keyway is also noted in the bore for the crosshead pin, to prevent the crosshead pin from rotating.
For a tapered fit crosshead pin, this keyway would not be necessary.
A straight crosshead pin with built-in key and the associated hole in the crosshead would be much easier to machine than the tapered crosshead pins and associated tapered holes in the crosshead.

The shape of the outside was often rounded (the point where the crosshead shoes contact the crosshead guides), but sometimes "V" shaped.
The rounded and V-shape eliminated the need for the locomotive-style crosshead plates, since the plates are not needed to maintain the position of a rounded-exterior crosshead in the round bore of the crosshead guides.

Below is an end view and a section of a typical rounded-exterior crosshead.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #17 on: February 14, 2014, 05:40:45 PM »

THE CROSSHEAD - Continued

This is my version of a typical small steam engine crosshead, shown in various stages of developement.

The final design has tapered sliding shoes for adjustment, and replacable inserts at the contact point with the guide to allow renewal when the inserts wear too thin.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #18 on: February 14, 2014, 05:50:20 PM »

CONNECTING ROD

Charles Porter mentions that for a high speed steam engine, the one-piece connecting rod (a rod without removable straps at each end) is critical for strenth, and to prevent strap flex which can cause engine failure in a high speed engine.

The one-piece connecting rod requires an open crank so that the end of the rod can slide over the crank pin during assembly, and so for a closed crank arrangement, a rod with a strap or similar arrangement on the large end cannot be avoided.

As with many steam engine components, the connecting rod and its associated bearings are somewhat complex, and for the proper function of the rod over time, it is critical to pay attention to the details of the rod and bearings.

Rod bearings were typically a tapered wedge arrangement, adjusted with a screw which drove the wedge deeper into the taper.

Charles Porter points out that unless the adjustment wedges are both on the same side of the respective crank and crosshead pins, then the rod will change in length as the bearing wear, potentially causing a catastrophic failure of the engine when the piston strikes the cylinder head (for engines with a very close clearance, as was typical of many engines).
Porter mentions that although the wear on the crank pin bearing and crosshead bearing is not identical, it is close enough so that if the bearing adjustments are both on the same side of the respective pins, then the rod will not appreciably change length as the bearing wear.

Rod bearings are typically flanged on one side as a minimum, with the opposite side not flanged to allow insertion into the opening in the end of the rod (for a one-piece rod end without a removable strap).

A few engravings of typical rod configurations are shown below.

Typically the bearings were two-piece, with a gap between the halves to allow the bearing halves to move towards each other as adjustments are made to remove the slack due to bearing wear.
Bearings often had babbitt inserts or inner surfaces to give low-friction operation under high-load conditions.

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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #19 on: February 14, 2014, 06:27:51 PM »

Here is my rendition of a typical connecting rod for a small steam engine.

This design will use a split-cap type bearing with a steel cap on the large end of the rod to allow use with a closed crankshaft.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #20 on: February 14, 2014, 06:32:09 PM »

CONNECTING ROD - Continued

Here are details of the bearing that is located on the large end of the rod.

This bearing is similar to that typically used on the large end of a forked marine-style connecting rod, as shown in the engraving below.

For this type bearing arrangement, the bearing halves for the larger engines were typically keyed together with offsets, as shown in my design.  For a smaller type engine, these offsets were often omitted.
Several shims were inserted in the gap between the bearing halves, and take-up of wear was accomplished by removing shims as necessary.

If the bearing and/or associated rod did not wear evenly in a circular shape, as was often the case, then removing shims to take-up the slack could cause the rod to seize on the shaft and damage or destroy the engine.

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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #21 on: February 14, 2014, 06:46:33 PM »

CRANKSHAFT

A steam engine crankshaft can be the open-type with a simple disk and crank pin mounted on the end of a straight shaft, as shown in the engraving for the Acme bottle engine, or a closed type with webs on either side of the crank pin, and often counterweights mounted on the webs opposite the crank pin for balancing purposes, as shown in the engraving below.

Crankshafts can be machined from a solid piece of high strength steel, or more often formed in a built-up fashion where the shaft halves and crank pin are either pressed or shrunk-fit into holes in the webs, often with pins at these joints to prevent any slipping of the connections during high stress periods such as a hydraulic lock of the engine due to failing to open the condensate drains with starting a cold engine.

Some crankshafts have a thin shoulder where the webs join with either the crankshafts or the crank pin.

The lower image below shows a preliminary (unfinished) design for a closed crankshaft.  Counterbalance weights will need to be appropriateely sized and added to the webs of this crankshaft.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #22 on: February 14, 2014, 07:03:51 PM »

CRANKSHAFT SIZE

Since the proper sizing and strength of the crankshaft is one of the more critical aspects of steam engine design, I generally pay close attention to this detail.

To avoid tedious force calculations, I generally try to size crankshafts, and often other engine components, no less that what was used for a similarly sized engine of the period.  It would be wise to use materials for steam engine construction that at least match the strength of the old materials, and preferably exceed those strengths, but without getting into a range of materials that may have problems with being brittle.

An idea of crankshaft sized can be gained from the following old bottle engine advertisement.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #23 on: February 14, 2014, 07:10:52 PM »

CRANKSHAFT BALANCE

The balancing of a steam engine is a topic of great confusion for many.

Some engine builders use a static form of balance which can be achieved by mounting the crankshaft shown above on level knife edges, and sizing the counterbalance weights such that they equal the weight of the piston, rings, nut, piston rod, crosshead, connecting rod, crank pin, and portion of the crank around the crank pin.

It should be noted that the piston, piston rod and crosshead move in a linear (straigh-line) motion only, while the connecting rod moved not only back and forth with the movement of the piston, but also side to side with the circular movement of the crank pin.
A static balance does not achieve sufficient operating balance in a steam engine, since it ignores the forces associated with the side-to-side motion of the connecting rod. 

To balance a steam engine, a dynamic balance must be achieved, and a dynamic balance is a compromise between balancing the horizontal and vertical forces that are created by the moving parts of a steam engine.

The equations used for balancing horizontal and vertical engines differ, and the appropriate equation must be used for the respective engine type.

The idea of designing a "perfectly balanced" reciprocating-type engine is a myth, although I frequently hear the statement in conversation.
There is no such thing as a perfectly balanced reciprocating engine.  The best that can be accomplished is a compromise between the vertical and horizontal forces present in reciprocating-type engine.

The best idea I have had suggested for verifying the dynamic balance of a steam engine is by using a 3D modeling program that has a dynamic readout of the center of mass forces of the engine.  A well-balanced engine will mininize the movement of the center of mass in both the vertical and horizontal direction while the engine is running.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #24 on: February 14, 2014, 07:18:42 PM »

PORTS, PASSAGES AND VALVE SEAT

Ports and passages can be calculated, or general rule-of-thumb layouts used.

The typical layout of the ports in the valve seat (the valve seat is at the bottom of the steam chest, and it contains the steam and exhaust opentings, and is the surface upon which the D-valve rides) is a bridge,1X-steam port,1X-bridge,2X-exhaust port,1X-bridge,1X-steam port, bridge, as shown below.

The ports are the openings in the valve seat, and the passages begin at the ports and terminate either in the cylinder (in the case of the steam ports) or terminate at the exhaust pipe fitting.

It should be noted that the steam passages that run from the valve seat to either end of the cylinder are used for both admission of steam into the cylinder and exhaust of steam about the same time from the opposite end of the cylinder.
In order to accomodate both the higher pressure steam (typically in the 100 lb. range at the boiler) and the low pressure exhaust steam (slightly elevated above atmospheric pressure for a non-condensing engine), the passages are generally designed in a larger section to suit the low pressure exhaust steam flow.

As can be seen in the engravings, the passages are generally larger than the ports.

The exhaust port is twice the width of the steam port, and this is due to the way the D-valve operates, where typically only half of the exhaust port is opened when the valve is at full travel.

The valve travel is often designed to give an 80% opening of the steam ports at full valve travel, although some designs were known to have a valve that over-traveled the steam port.

The exhaust port is typically opened at least 50% when the valve is at full travel.

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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #25 on: February 14, 2014, 07:54:44 PM »

PORT AND PASSAGE SIZING - A COMPROMISE BETWEEN ADEQUATE EXAUST FLOW AND ENGINE EFFICIENCY

The area of the ports and passages is dependent upon the bore and stroke (ie: the volume that must be filled with steam or exhausted with each piston stroke), the engine normal operating speed (how fast the cylinder volume must be filled and emptied), assumed nominal values for steam flow through the ports and passages (the maximum speed at which steam will flow through the ports or passages under the normal engine operating conditions).

So generally speaking, the port and passage size must increase as the volume of the cylinder increases, and also increase as the number of times the cylinder must be filled and emptied of steam per minute increases.

The intent of sizing the ports and passages is to be able to provide steam pressure to the piston that is as near the steam chest pressure as possible throughout the period during which steam is admitted to the cylinder.  If the ports and passages are sized too small, they will restrict the steam flow and cause a pressure drop, thus providing steam pressure to the cylinder at a lesser value that is in the steam chest, and limiting the power that the engine can produce.

A balance must be struck between sizing the ports and passages to allow the engine to be able to produce its rated power at its rated rpm, and limiting the size of the space between the piston and the head, and the space in the passages.
In the research I have done, steam engine designers often opted for a generous sizing of the steam passages and ports to insure an adequate flow of steam to the cylinder, and equally important to provide a sufficiently sized passage to accommodate the much lower exhaust steam.

Some designers were said to size the passages based on the exhaust steam flow only, since an oversized steam passage would not affect the power produced by the engine, but an undersized passage would restrict exhaust flow, and limit the engine power.

The length of the ports are typically 70-80% of the bore size, measured perpendicular to piston travel.

A rough rule of thumb for steam port width is 0.078 inches per inch of bore, where the dimension for port width is measured along the same axis as piston travel.  I generally round the steam port size up to the closest standard milling bit size if the ports are less than 0.125" wide.
The exhaust port would be twice the width of the steam ports, or roughly 0.156 inches per inch of bore.

The bridges between the steam and exhaust ports would be the same width as the steam ports.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #26 on: February 14, 2014, 08:51:07 PM »

LONG STROKE - LOW RPM versus SHORT STROKE - HIGH RPM ENGINE DESIGNS

Steam engines designed prior to Porter's "High Speed Steam Engine" (150 rpm) typically uses a stroke that considerably exceeded the dimension of the bore.  Since the older engines operated at lower rpms, often in the 50-70 rpm range, then the problems associated with excessive piston speed, which are often a result of too long of a stroke, could be avoided.

The long stroke provided time for the steam to be expanded after the steam valve had closed, thus extracting energy from the expansive power of the steam.

Porter had to reduce the bore versus stroke ratio of his high speed engine to keep the piston speed within a workable range, and high speed engine designs after Porter's engine tended towards lower bore vs stroke ratios.

The bore/stroke ratio for a small steam engine which avoid excessive piston speed could easily fall in the range of 1:1, up to perhaps 1:1.7.
For example for a 3 inch (76.2 mm) bore, acceptable ranges of stroke for a 150 rpm engine could be anywhere from a 3 inche stroke (76.2 mm) up to a 5 inch (127 mm) stroke.

A longer stroke is typically associated with a greater maximum torque that the engine can produce.  Since most standard-type steam engines produce an abundance of torque, even at low rpm, then a small loss of stroke caused by using a shorter stroke would probably have little consequence in most applications.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #27 on: February 14, 2014, 09:06:00 PM »

D-VALVE versus PISTON and OTHER VALVE TYPES

Generally, the stationary engine model builders of today use the standard D-valve, while the piston-type valve is often seen on marine type engines, especially for the high pressure cylinder.

Examples of steam locomotives can be found which used both D-valves and piston valves, and sometimes a balanced D-valve was used for locomotive engines.

The advantage of the standard D-valve is that it is a simple design with a simple steam chest layout, is well understood in its workings, and thus the standard D-valve was widely used in many of the low pressure steam engines of the period.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #28 on: February 15, 2014, 02:44:56 AM »

ADVANTAGES AND DISADVANTAGES OF THE SIMPLE D-VALVE

It took me a while to reach some understanding of how the D-valve operates in a steam engine, but basically a single valve is being used to control the steam admitted to both ends of the cylinder, and control the exhaust from both ends of the cylinder.

So an advantage of a D-valve is that you can manufacture a single valve to control all the steam admission and exhaust events required to operate a steam engine.

The disadvantage of using a D-valve is that the admission and exhaust functions are fixed in relation to each other, and one event cannot be varied without affecting all the other events.  The edges of the valves can be modified, and were often not symmetrical, especially in a larger vertical engine where gravity affects the upward and downward moving parts.

For better efficiency, a steam engine needs either four separate valves (two steam admission valves and two exhaust valves, such as a Corliss engine) each of which can be timed independent of the other (or a similar valve type/combination which achieves the same result).  The problem with the Corliss valve gear is that it is not designed to run at high speed.

The poppet and piston valves were reported to be of higher efficiency, as was the rocking type valve found on the Corliss, and the piston valve was often used with super heated steam on the high pressure cylinder of a compound steam engine.  D-valves do not work well with superheated stea.
Other variations of the slide valve used one valve sliding on top of the other so that the admission and cutoff could be independently controlled.

But it is the lowly and inefficient D-valve with its simplicity and reliability that is seen on most small steam engines.
The D-valve generally seals better as it wears into the seat, and this is a feature that is also seen on the Corliss-type rocking valves, but not seen on piston valves.

D-valves will also lift off of their seat to relieve any unwanted condensate buildup in the cylinders, whereas the piston type valve will not relieve excessive condensate from the cylinder.  Larger engines with piston valves are often fitted with condensate release valves at both ends of the cylinder to prevent destruction of the engine in the event that a large amount of condensate inadvertently enters the engine via the steam line while the engine is operating.

The variation of the D-valve that I prefer is the balanced D-valve, which adds an extention on the top of the valve to seal it against the steam chest cover.  A hole is drilled in the top of the valve at the exhaust dome section, and this keeps the pressure acting on the top of the valve at no higher than exhaust pressure, rather than having full steam chest pressure acting on the valve.  A balanced valve relieves much of the valve and valve seat wear, as well as wear along the entire valve gear train, including wear on the eccentric(s) which operate the valve.

Below is a standard unbalanced D-valve.

Often the D-valve for small engines has a slot through the center, and the valve rod has threaded nuts on either side of the valve.
The nuts are tightened to allow a sliding fit against the valve, and the valve can ride up and down in the slot.
The pressure of the steam in the steam chest holds the valve against the valve face, making a steam-tight fit.
 
 


 
 
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #29 on: February 15, 2014, 03:01:46 AM »

UNBALANCED D-VALVE

This is my rendition of a typical small engine unbalanced D-valve.

Note that the valve moves in the same direction as the valve rod which passes through the slot in the valve.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #30 on: February 15, 2014, 03:04:05 AM »

BALANCED D-VALVE

The balanced D-valve in a typical form requires a cylinder to be a cast on the top of a standard D-valve, and a piston is installed on top this cylinder, with the piston sealing against the inside of the steam chest cover.

Below is an example of a balanced D-valve.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #31 on: February 15, 2014, 03:11:03 AM »

BALANCED D-VALVE

This is my rendition of a typical small engine balanced D-valve.

Note that there must be a hole in the top of the D-shaped dome of the valve to maintain the top of the valve at no more than exhaust pressure.

Springs will be installed in the four round recesses to help keep the valve pressed against the valve seat.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #32 on: February 15, 2014, 03:52:09 AM »

ECCENTRICS

The eccentric(s) act the same as a small crankshaft, and provide the motion for the valve via the valve rod and often a valve rod guide.
The valve rod guide act similar to a crosshead, and prevents sideways forces from acting on the valve stem.

A single eccentric was typically used for non-reversing engines, or sometimes with reversing engines if the eccentric could be shifted.
Typical reversing-type steam engines were generally equipped with two eccentrics and some type of reversing valve gear such as a Stephenson's link.

The advantage of the eccentric (in lieu of a small crankshaft to actuate the valve) is the eccentric(s) can be installed on the crankshaft anywhere outboard of the main bearings, and generally somewhat inline with the center of the steam chest when possible.

Eccentrics for smaller engines are often solid disks, and for larger engines, a spokes are used to lighten the eccentrics.

Below is an engraving for a typical eccentric and strap (the strap is the part that fits around the eccentric).

Charles Porter noted that if the slot is recessed into the strap, then oil will be retained in the slot (see first engraving below), and eccentric wear will be reduced.  If the slot is located in the eccentric, then it will not retain oil and will wear more quickly (see second engraving).

Eccentrics were generally keyed onto the crankshaft once the exact position for them had been determined.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #33 on: February 15, 2014, 04:02:29 AM »

ECCENTRICS - Continued

My rendition of an eccentric and strap.
The eccentricity (throw) of the eccentric must be carefully calculated prior to making these parts.

The strap will be a two-piece arrangement, and will bolt onto the eccentric.

The eccentric rod will bolt onto the strap.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #34 on: February 15, 2014, 04:17:13 AM »

MAIN BEARINGS AND BEARING CAPS

The main bearings for small steam engines often were configured as shown in the engraving below.

The bearing cap had shims installed to allow adjustment as the bearing surface wore.

Bearing material was generally Babbitt, and grooves were generally cut into the Babbitt to promote oil flow evenly across the face of the bearing.

If the bearing pillar is not adjustable, then consideration should be given to pouring the Babbitt bearings after the crankshaft has been precision aligned, as was done with the Cretors popcorn engines.

There was usually an oil cup built into the top of the bearing cap, as shown in the second engraving.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #35 on: February 15, 2014, 04:22:12 AM »

SPACE FOR ADDITIONAL FUTURE MATERIAL
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #36 on: February 15, 2014, 04:32:06 AM »

VALVE GEAR

The topic of valve gear generally follows the "Pandora's Box scenario", as I call it, since the subject of valve gear is so vast, and opinions about it vary so widely.  My experience has been that once a discussion about valve gear has begun, agreement and concensus may never be reached by the discussing parties.  Luckily those other folks are not here, so we can party on with impunity.

Everyone must certainly have their favorite form of valve gear, and I certainly have mine, for reasons similar to why I like the balanced D-valve.
My choice for valve gear for a small steam engine is the Stephenson's link, and as Dalby mentions in his book about valve gear, there are few valve gear types that are as simple, reliable and consistently accurate as the Stephenson's link.

The arrangement I use is somewhat standard among small steam engines, and uses open-links, and a link which is radiused towards the crankshaft.  Several examples of the Stephenson link can be seen in the engravings below.

The main feature of the Stephenson's link is that it allows the engine to be easily reversed using the reverse lever.
Additional benefits of the Stephenson's link is that it allows the steam cutoff to be completed earlier in the stroke, so that the steam can be allowed to expand and give up energy that would otherwise be wasted if the steam was cut off late in the stroke.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #37 on: February 15, 2014, 04:50:43 AM »

A few more diagrams for typical Stephenson's link configurations for steam engines.

Many small engines such as marine engines were designed to operate in full forward gear most of the time, with only occasional operation in reverse.  The suspension point of the link for some engines used in this application can often be seen at the end of the link, or other convenient locations.

Locomotive and automotive engines had to operate well in both forward and reverse operation, and so greater attention was given to the suspension point of the Stephenson's link for non-marine engines, and you can see in most of the engravings below that the suspension point for the link is slightly off-center of the link centerline, and offset towards the eccentrics, and located at the center of the link, not at the end of the link.

Any link suspension point that differs from the center of the link and offset slightly towards the eccentrics, is not a design that wil equalize valve travel throughout a range of link positions with the engine running in both forward and reverse directions.  A further benefit of carefully selecting the link suspension point was that you can minimize the amount of slip that the link block experiences as it slides inside the slot in the link, as the link is rocked back and forth by the eccentrics.

The second engraving illustrates a link suspension point that was chosen for convenience, not for equality of valve events throughout the full range of link travel.

Adjustment to the link suspension point can also compensate for other inequalities of a steam engine such as the non-linear motion of the piston as related to the revolving crankshaft (due to the angularity of the connecting rod), and the presence of the piston rod on one side of the cylinder only.

Note the clever offset in the forked end of the valve rods in the fourth engraving, which allows the rods from both eccentrics to be connected to the link without resorting to bending the rods.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #38 on: February 20, 2014, 01:11:39 PM »

The general approach I use to design steam engines after I establish the bore, stroke, and the general scale of the engine, is to I begin at the piston and propogate outward, generally maintaining the proportions of the various parts to those normally used in the era, with considerations for maintaining sufficient strength and minumum thicknesses of materials if the engine is scaled.

I am sure everyone has their own favorite method.  This is the approach I use.

Here are a few screen captures of the approach I use for roughing out the cylinder design.

If you don't begin the design from inside the engine and work out, chances are that your engine will not achieve all the advantages that adhering to the old designs can give.

This is important if you are building a working engine such as a launch or locomotive engine, where power output and longevity are critical, but from a broader perspective, why not build steam engines using the same proven designs that were mastered years ago by the great designers of old steam engines?

Note that the dimensions shown in these sketches are generic, and you will have to determine the dimensions needed for your exact engine.
Also note that generally the cylinder heads are dished out where the steam passage meets the head, and I need to revise these sketches to reflect that detail.
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Re: How to Design a 1-10 hp Working-Class Steam Engine
« Reply #39 on: November 22, 2014, 02:51:04 AM »

I have condensed this series of posts into a single topic and tried to clean it up to keep it more on-topic.

There is much more work to be done here, and hopefully I can get back to this topic and better describe my understanding of the process of designing a working steam engine.

« Last Edit: August 01, 2017, 08:37:55 AM by admin »
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Smarter not Harder