Piston Length

[pockets]

Is there a mathematical relationship between the bore and/or stroke of a steam cylinder and the length, or height, of the piston? Does anyone have the formula?

Thanks, in advance, for your effort.

[andypullen]

Pockets,

Take the length of your cylinder. Subtract the length of the cylinder head locating bosses. Subtract the stroke and you will have the amount of space in the cylinder for a piston with a little clearance (about 1/16") on each end.

Hope this helps....

Andy Pullen

[pockets]

Andy, my friend, that's all well and good if you have a cylinder in hand. However, when it comes time to build a cylinder, the length of the piston becomes important. For example: 3.125 bore x 4.375 stroke + cylinder head bosses + piston length + appropriate clearance = cylinder length.

[GWRdriver]

Pocket,
I don't know that there is or ever was recommended proportion or formula for this. Maybe, somewhere, but it's usually a "whaetever works out best" situation.

As you are aware, there are several cylinder & motion dimensions/proportions which are interrelated, however when starting a cylinder design from scratch I initally key on two values for length . . . the stroke (obviously) and the thickness of piston required to accomodate the RING type and configuration I want to use and I minimize piston thickness to the extent possible without compromising ring strength and effectiveness. There is no benefit to having overly thick pistons, except as needed to hold and support the rings or packing.

Following this I "back-calculation" and add the piston thickness required for the ring setup I've chosen to the stroke (and the other length factors) to check to see if there is enough space for the finished cylinders. Sometimes, if it's early enough in the design process, the stroke may be shortened a bit to make it all work out.

So just about the fist thing I do after determining bore & stroke is to develop the ring design. This in turn is influenced by the cylinder size and sleeve material, the rings available, and just general good practice.

[pockets]

GWRdriver (sounds terribly British), thanks for the tip. It makes sense, since we're not talking significant cylinder pressures. At least not as compared to internal combustion.

[GWRdriver]

Yes . . . I have a lot connections there, but I'm in Tennessee.

[Curtis_F]

Pockets,

Let's see.... according to the book "Modern Locomotive Construction - 1892" the rule-of-thumb is that the outside to outside distance of your packing rings should be 1/9th to 1/6th the diamter of the bore.

Therefore using the 1/6th rule, with a bore of 3.75", the distance over the packing rings would be 5/8". Add 3/16" to the outside of the rings for shoulders to hold them in place and you get a piston 1" thick.


Other things the book notes about pistons:

<font face="Verdana, Arial" size="2">It should be made, in fact the whole pistion should be made, as light as possible, and consequently its metal must be judiciously distributed, so as to obtain the required strength with the least ammount of metal.</font>


On our little models, as GWRdriver pointed out, the thickness of the piston really isn't critical. On the big guys it was important because it's what helped keep the piston concentric to the bore and to reduce wear in the cylinder block.

Look at the big guys and you'll find that the bore on most engines is egg shaped. The piston is so heavy compaired to the diameter of the piston rod, and the strength of the packing material, that the piston basically rides on the bottom of the cylinder bore.

Here in the US where labor was realtivly cheap engines were regularly re-bored to correct this. In the UK, they didn't like to pay for the down time so you'll notice that their pistons have rod that go through the pistons and out through a packing in the front cylinder head.

That piston rod design also explaines why English engines have 4 perfectly square exhausts, and US engines have 2 soft exhausts and 2 loud exhausts. Without a piston rod on the front of the cylinders, US engines have more steam space on that side of the cylinder making for a louder exhaust.


Cheers,

Curtis F.

[GWRdriver]

&gt;&gt;&gt;In the UK, . . . . you'll notice that their pistons have rod that go through the pistons and out through a packing in the front cylinder head.&lt;&lt;&lt;

I really must disagree because this is simply not the case. If you study the British loco you will find that only a very tiny percentage of British locomotives ever had extended piston rods, no larger a percentage than on US locos (most of which were built between 1880-1900 and proved not to be of particular advantage, otherwise the practice would have continued.) On the Continent (Germany, France, etc) extended piston rods were used on a very much higher percentage of locomotives than anywhere else.

&gt;&gt;&gt;That piston rod design also explains why English engines have 4 perfectly square exhausts, and US engines have 2 soft exhausts and 2 loud exhausts. Without a piston rod on the front of the cylinders, US engines have more steam space on that side of the cylinder making for a louder exhaust.&lt;&lt;&lt;

There is a difference between "square" valve events and exhaust beat volume. Even (square) beats are the product of accurate wheel quartering, valve gear setup, and timing, assuming of course that both sides of the motion are identical. Uneven exhaust beat volume can be cause by several things, among them poor timing or unequal sides, blowby, malformed steam passages, or damaged components, but not by the volume of an extended piston rod. If that were the case it would be impossible for a non-extended piston rod cylinder to produce a square beat, but we know they certainly can.

[Curtis_F]

<font face="Verdana, Arial" size="2">I really must disagree because this is simply not the case.</font>
I'll certainly take your word for it as UK engines are not my speciality. For one reason or another I was thinking it was a rather common practice through out Europe.

<font face="Verdana, Arial" size="2">There is a difference between "square" valve events and exhaust beat volume.</font>
Absolutly right. A valve gear being "square" is all about timing. I used the term to bring attention to the exhaust sounds, which is how most of us determine wether an engine is square or not and therefore a good common reference point.

<font face="Verdana, Arial" size="2"> Uneven exhaust beat volume can be cause by several things, among them poor timing or unequal sides, blowby, malformed steam passages, or damaged components, but not by the volume of an extended piston rod. If that were the case it would be impossible for a non-extended piston rod cylinder to produce a square beat, but we know they certainly can.</font>
Next time you're around a big steamer really listen to the exhaust, you can hear the difference between the front and back sides of the piston.

In reading trade journals of the early 1900s you can find references made by engineers about the posistion of the rods for maximum starting power. Namely having the engineer's side rods down and forward of center (on right-hand lead engines) by about 30 degrees. If piston rods didn't make an appreciable difference in the area of the piston and the volume of the cylinder there wouldn't be a "sweet spot" in rod posistion for starting an engine.


Cheers,

Curtis F.

[Bill Shields]

Aw heck, I just allow about 1/16" clearance at each end, and if for some reason things 'knock' a bit, I just pull the piston and put it in either the lathe or belt sander for a bit. There really isn't anything magic here for US design locos.
Now, OD clearance for expansion is another story....

[pockets]

I realize that some fuses may be getting a little short, however, for someone at my level of inexpertise, the sheer quantity of data coming over the dam is most welcome. Any discourse, even a warm one, is much better than none.

T H A N K S, guys.

[jon]

hello
i would disagree with the extended piston rod playing a major role in reducing cylinder wear. the pennsylvania railroad did extensive testing with locomotives equiped with front piston rod support and without it and found cylinder wear difference to be minimal
jon

[GWRdriver]

<font face="Verdana, Arial" size="2">having the engineer's side rods down and forward of center (on right-hand lead engines) by about 30 degrees. If piston rods didn't make an appreciable difference in the area of the piston and the volume of the cylinder there wouldn't be a "sweet spot" in rod posistion for starting an engine.[/B]</font>

And here all this time I thought it was because that was the approximate beginning point of the range of greatest mechanical advantage for a quartered engine, and that after movng through approximately 60 degrees on the RH side the LH side was brought to the same point of beginning of greatest mechanical advantage insuring a continuous power stroke for starting and that the rod volume had nothing to do with it.

[Curtis_F]

<font face="Verdana, Arial" size="2">I would disagree with the extended piston rod playing a major role in reducing cylinder wear. The pennsylvania railroad did extensive testing with locomotives equiped with front piston rod support and without it and found cylinder wear difference to be minimal.</font>
Jon,

It's been awhile since I read through the Pennsy's Altoona Shops test reports so I can't remember what their tests showed about extended piston rods vs. piston wear. Though what you say does sound familiar.

Just to be clear; I'm not posting my personal opinion here, I was mentioning a historical fact for the fun of it. References about the use of extended piston rods to reduce cylinder wear can be found in trade papers, Locomotive Dictionaries & Locomotive Cyclopedias of the late 1800s and early 1900s.


Cheers,

Curtis F.

[Curtis_F]

<font face="Verdana, Arial" size="2">Originally posted by GWRdriver:
And here all this time I thought it was because that was the approximate beginning point of the range of greatest mechanical advantage for a quartered engine, and that after movng through approximately 60 degrees on the RH side the LH side was brought to the same point of beginning of greatest mechanical advantage insuring a continuous power stroke for starting and that the rod volume had nothing to do with it.</font>
GWR,

By-the-numbers you're quite correct, there should be four (4) positions of maximum starting power on a two cylinder engine where the crank pins are 90 degrees apart.

However, according to reports and stories filed by hundreds of professional steam engineers over the past 150 years all say that of the four starting posistion there are two that are superior, and that there is one superior of those two.

On a left-hand lead engine; engineer's side, rods down, and forward of center. You get full bore power on the right-hand piston, followed by full bore power on the left hand piston, followed by the piston-rod side of the right hand piston, then the piston rod side of the left hand piston.

Guys who for decades ran steamers daily starting trains with massive tonnages gained experience that told them that they would get more peformance out of their eninges when starting a train where the first two power strokes were from the full bore, front sides, of the pistons.

This is historically documented information.

On a personal note, I have run some un-scientific tests on this topic using a 15" gauge consolidation starting a train from a dead stop on a 1-1/2 percent grade and found it to appear to be true. This October (once a few more cars have been built and the 1-1/2 percent grade is extended another hundred feet or so) I plan to take out the consolidation again for more scientific testing reguarding starting power and crank posistion.


Curtis F.

[mark rusche]

Get a copy of Steam Engine Design from Lindsay Publications . There are formulas in the book to help with piston and cylinder design .

[pockets]

This has been, and still is, great. Thanks to you all. I will look for the book.

[Jan-Eric Nystrom]

To get back to the very original question (which I assume is related to model sized, not full size equipment), I'd like to chime in with this:

If you design an engine from scratch, the stroke is, of course, 2 x the offset of the crank pins on the driving wheels.

The bore can vary, but you should take into account two things: The boiler steam pressure, and the adhesive weight. If the loco is light, there is no need to make large-bore pistons. You can never use the power they provide; the loco will slip.

There's a formula for the tractive force, you can find it here:

http://www.catskillarchive.com/rrextra/chapt26.Html

You can use about 2/3 boiler pressure for p in the formula.

The tractive force should not be much over 1/4 of the locomotive weight. If it is, the engine will easily slip on the track. OTOH, if you'd rather see the loco slip than stall when pulling too a heavy load up a grade, then you can use a somewhat larger cylinder bore.

My own 0-6-0 loco has a nice balance, 100 psi boiler pressure, 2.1" bore, 2.7" stroke, 6.25" wheels, weight about 260 lbs. According to the formula, it turns out to be a little more powerful than necessary (maximum tractive force is almost half the weight), and indeed, I can induce it to slip. See the article in the Sept./Oct. issue of Live Steam for more about this loco!

In addition, the 0-6-0 has extended piston rods, as did almost all Finnish steam locomotives.



[This message has been edited by Jan-Eric Nystrom (edited 08-20-2005).]

[pockets]

OMG, the dam has burst! Thanks to you all.As soon as I have time to sort it out, I will, undoubtedly find the answer to my question. LOL!

[pockets]

Okay, color me stupid. As regards the "sweet spot", I can understand the math and the cerebral satisfaction of even knowing that it exists and where it is. However, when you back down onto your train, you are going to stop where the pin drops and that is where you start from. Yes, I know about bunching a train to make use of the slack.

[andypullen]

Hi Jan,

I just finished re-reading your article. Very informative. The engine number gives me the willies a bit, but it's a very nice looking piece. Interesting processes, too. I have access to a shop with a laser that I may use in the future, too.

Where do you get that material that you used for your patterns? I'm doing the pre-engineering work on a future project and that stuff would save me alot of time...

Thank you,

Andy Pullen

[andypullen]

Hi again, Jan,

I'm also interested in how you "dummied" the heavier counterweights on your drivers. One pattern would be much easier than three to make.

I'm looking around on this side of the pond, so to speak for the urethane foam you used for your patterns. No luck as yet....

Andy Pullen

[pockets]

Andy, isn't that a terrific article? Jan, you truely humble a lot of us.

Andy, if you find a source for the foam be sure to spread the word. I have my ear to the ground, too.

Greg

[andypullen]

Yeah, Greg.

Jan's articles are clear and well written. Even the mistakes he's made have been interesting. Like the steam generator he made for the American....His writing is better than some American authors in my opinion.

I'm looking for that material. I couldn't find much doing a google search. I mostly found articles about the material and foreign suppliers. Mcmaster has urethane foam and I have an info request in with them. Hopefully, they'll get back to me today or tomorrow. Maybe a foundry supplier would have it. There's one in Baltimore, but the name escapes me at the moment.

Andy

[Jan-Eric Nystrom]

Oops - made an error, can't delete this. See below, I reposted the message.

J-E

[This message has been edited by Jan-Eric Nystrom (edited 08-27-2005).]

[Jan-Eric Nystrom]

I've not logged in for a few days, so sorry for the tardy response.

First, many thanks for the kind words about my article and writing - especially appreciated since English is my third language, after Swedish and Finnish.

To answer your question re. the urethane material; yes several brands do exist. From what I recently heard, Cibatool" has been renamed "REN":

http://www.mfcomposites.com/renshap2.htm

Another link:

http://www.freemansupply.com/PolyShapeIIPolyure.htm


The counterweights were "dummied" easily:

First, I drilled/tapped some holes in the existing cast iron counterweight, between the spokes, and put in small bolts to secure the additional piece yet to be made.

Then, I made a simple mold by cutting a strip of cardboard to fit between the spokes. A piece of adhesive plastic was attached to the back of the wheel. Some more tape kept it all together, like this:

http://www.saunalahti.fi/~animato/3003/IMG_6667.JPG

Then, I just poured in some polystyrene filler, taking care to avoid bubbles. When the filler had hardened, I milled it all smooth. Some hand filing between the spokes, and the wheels were ready for painting.

This took much less time than making two distinct patterns.

Hope this helps!

Regards, Jan-Eric




[This message has been edited by Jan-Eric Nystrom (edited 08-29-2005).]

[pockets]

Jan-Eric, thanks for the links and the how to. Boy, I wish my camera took pictures that nice! LOL

[Jan-Eric Nystrom]

<font face="Verdana, Arial" size="2">
I wish my camera took pictures that nice!</font>

FYI, it was shot with a digital 2 megapixel camera with automatic flash - already hopelessly old-fashioned...

regards, Jan-Eric

[pockets]

Thanks, Jan-Eric.

Don't worry about obsolete technology..., Look at what you're building!

I don't always view a new technology as progress. Some engineering ( of all disciplines ) feel it necessary to justify their existance, so make changes just because they can. Sort of like politicians!

[GWRdriver]

&gt;&gt;Some engineering . . . . feel it necessary to justify their existance, so make changes just because they can.

That's just not so. Rubbish actually. I've worked with engineers for over 30 years. My father was one. Engineers tend to be very conservative, safety, and comfort oriented people who given the option would do the same proven thing over and over again and wouldn't change anything, ever. Most of them do not like "going where no man has gone before," at least on their own, or because they have nothing better to do. These days most design engineers make changes because some owner, developer, customer, or bean-counter has charged them with a task, typically "We have to make/build this complex thing cheaper, but without losing performance," or "We must improve the product without increasing the cost." Otherwise, once a design worked well, the typical engineer would never want to change anything.

[pockets]

GWRDRIVER, No offense intended. At least nothing personal, but, as the owner of a prototyping company for a number of years.....

[GWRdriver]

None taken, . . . I just thought that was a little bit hard on the engineers, and being and architect myself sometimes there's no love lost between me and an occasional engineer. :-)
I know what the field & shops folks like to think, but the reality is that us designer types practically never change things because it's been a slow day. There is always a reason, and it usually has reducing cost or improving performance behind it. If we're going to spend someone else's money to make a change, the change better be worth the trouble and expense.
But isn't that what "prototyping" is for, to work out bugs and changes before it costs somebody REAL money?

[pockets]

GWRdriver, That's exactly what prototyping is for. "Proof of concept" were key words in my business. The most asked question was, "What, E X A C T L Y, do you want?" Try getting that one answered!

Without trying to sound too cynical, I've had my best experiences with Plant Engineers and Production Engineers. They've been slapped around by reality enough to be pretty level headed. It's the 25 year old, freshly graduated engineer that made me crazy. There may be some out there,but, I haven't met one, yet, that didn't dream of inventing the replacement for the wheel.

Perhaps I was a little harsh about existance justification....They would come to me with a concept or, rarely, a complete plan to produce something faster or cheaper, or both, but never better. That can only come from, ultimately, the bean counters. That's a whole 'nother rant.

You might enjoy http://www.thegeekgroup.org/

Respectfully,
Greg B.

[GWRdriver]

Greg,
What kind of prototyping, were you involved with aluminum extrusions by any chance?

PS: Kind a like looking for the right woman . . . "Well what exactly are you looking for? . . . I don't know, . . . but I'll know it when I find it."


[This message has been edited by GWRdriver (edited 08-31-2005).]

[pockets]

My company ( North Ottawa Imagineering ) primarily did prototype production machines. Occasionally we would work on the actual product prototypes. My customer list ran the gamut from chemical manufacturers to the Big-Three auto companies. NEVER again.

I shut down and moved to Florida. Just batting the breeze, over Margaritas, at a sidewalk cafe resulted in a job offer. My last position, before retiring, was QC Mgr. for a defense contractor through the 9/11 mess. That will send you home crying!

I don't know if this forum will facilitate back channel communications. If not, you can find me over at Chaski.

Regards,
Greg.

[Jan-Eric Nystrom]

<font face="Verdana, Arial" size="2"> My company ( North Ottawa Imagineering )</font>

"Imagineering"? Isn't that a trademark of Disney's?

Just curious - regards, Jan-Eric

[pockets]

As near as I remember, the term was coined by Bill Kee in the NMRA Bulletin. I know that in susequent years Disney used it. Whether, or not, they copy-righted the term, I can't say. Not being a fan of Disney Inc., I didn't give it much thought.

[Jim]

<font face="Verdana, Arial" size="2">Originally posted by andypullen:
Yeah, Greg.

Jan's articles are clear and well written. Even the mistakes he's made have been interesting. Like the steam generator he made for the American....His writing is better than some American authors in my opinion.

I'm looking for that material. I couldn't find much doing a google search. I mostly found articles about the material and foreign suppliers. Mcmaster has urethane foam and I have an info request in with them. Hopefully, they'll get back to me today or tomorrow. Maybe a foundry supplier would have it. There's one in Baltimore, but the name escapes me at the moment.

Andy</font>