Open-Column Marine Twin

[JorgensenSteam]

I have started a design for an open-column twin marine steam engine.

 

[zeeprogrammer]

Fascinating Pat.
I'll be following with great interest.

 

[kendo]

Wow, I can tell this is going to be one amazing build. Looking forward to seeing
the rest, All the Best of luck Pat.

Ken

 

[steamer]

I'll be watching this with fond nastalga!..... ;D

Best of luck to you!......holla if you get stuck.

Dave

 

[JohnLanark]

Will be a great engine! I'll follow with interest.

"The cylinder head is designed to keep the clearance space above the piston to a minimum" - sometimes this can be overdone and cause the cylinder pressure at the end of the stroke to rise above the steam pressure. This lifts the slide valve off its seat and makes an annoying rattle. The similar sized Stuart 6A can suffer from this in its HP cylinder and owners have been known to machine metal off the cylinder cover & piston. Of course, the volume of the steam ports comes into the equation too, and perhaps you will have large ports for good performance.
Just a thought, John.

 

[JohnLanark]

Pat - this is great stuff, you are certainly doing your homework!
What size of boat do you plan for the engine to push along? About 25 feet? John

 

[steamer]

One of the benefits of long valve travel is quicker valve event. The ports open and close quickly with less wire drawing of the steam.

Dave

 

[Dan Rowe]

Pat,
There is more than one type of taper attachment I have the telescopic one for my South Bend but some of them are plain where the cross slide feed screw has to be disengaged. The lathe manual needs to be checked.

Both types lock to the ways and guide the cross feed on a taper which is set on the end of the attachment. Mine has feet/inch on one end and degrees on the other.

Short tapers like a piston rod and piston can be done with the compound rest.

I have been reading this thread and you are planning a valve with different laps. I would use the Reuleaux diagram because it works out that type of problem very well.

If you are planning marine links then a lot of very good design data can be had by Radow's program which has the steam tables in the data and has the option to condense to a vacuum.
http://www.steamboating.de/valve/valve-maine.html

Dan

 

[JohnLanark]

Pat- set over your topslide and machine the rod taper with a tool set right on the centre height, feeding with the topslide. Then chuck the piston and drill a pilot hole; without disturbing the topslide, open up the bore with an upside down boring tool again set carefully on centre height, feeding with the topslide and cutting the "far away" side of the bore. You can check the bore size as you progress, using the mating rod. Hope this makes sense, John

 

[JohnLanark]

Pat - on the rod half inch thread, after starting the thread square by screwcutting, it would be usual to finish with a die held in a tailstock die holder. You will want a very good polished finish to these rods and it may be best to buy the material ready finished to your size.

If you are going with cast iron pistons, and still determined to have them hollow, you could make patterns and obtain one piece castings with cored-out holes. The patternmaking for this is fairly easy, I could post the method if you are interested. John

 

[Maryak]

Pat,

Just a thought. If you made the tapered hole in the piston and the tapered end of the rod to the same standard as a tapered pin you may have more control over the sizes due to the method of step drilling standard sizes in the piston before using the tapered reamer for the standard pin size. From memory all the details are in Machinerys Handbook.

Hope this helps.

Best Regards
Bob

 

[Angsiglar]

.....
Below are the calcs for the steam and exhaust ports and passages. The calculations for this engine are not so much for accuracy, but for the purpose of establishing a point of reference so that if something needs to change, you will have a defined point of reference to begin from.
.....

Given:

Bore: 3"x2
Stroke: 3.75"
RPM: 750 max.

Assume:
Steam port opening velocity: 8,000 feet per minute
"On engines when the steam port is used for both admission and exhaust, the port must be proportioned with respect to the exhaust, that is, the steam should be exhausted at less velocity than the velocity of admission, i.e. usually 6,000 feet per minute" Audel's Engineers and Mechanics Guide, Vol. 1, 1940, p. 185
Steam passage velocity: 6,000 feet per minute

Calculated:
The area of each piston is (pi r squared):
3.1416 * (3/2)^2 = 7 (sq. in.)
The volume of each cylinder that has to be filled with steam for each revolution of the crankshaft is:
7 * 3.75 = 26.5 cubic inches

Excessive piston velocity needs to be avoided. The piston velocity for this engine is:
(Max. RPM) * (engine stroke / 12) * 2
750 * (3.75/12) * 2 = 468.75 feet per minute
or divide by 60 to get 7.8 feet per second.

The port length will be 80% of the cylinder diameter (common range was 70-90% of cylinder diameter).
Port length will be:
0.8 * 3 = 2.4 inches

The steam port area required will be:
(piston area) * (piston velocity in fpm) / (steam port velocity)
(7 * 468.75) / 8,000 = 0.4101 (sq.in.) (minimum required steam port area)
The area of the steam port will is found as follows:
Area of steam port = (area of piston in sq.in. x piston speed in ft.) / 6000
or (7.07 x 4.68.75) / 6000 = 0.5523 sq.in.
The steam passage area required will be:
(7 * 468.75) / 6,000 = 0.5468 (sq.in.) (minimum required steam passage area)

For the port length given above of 2.4 inches (80% of the piston diameter), then the steam port width would be:
steam port area / steam port length
0.4101 / 2.4 = 0.1708 (inches) (minimum required steam port width)
0.5523 / 2.4 = 0.2301
The steam passage area would be:
0.5468 / 2.4 = 0.2278 (inches) (minimum required steam passage width)

When power is the main design consideration, and not necessarily efficiency, then many designers use a generous port/passage dimension in order to avoid an wire drawing, so we will use a steam port and passage dimension of 2.4 inches long and 0.23 inches wide.
Interesting that you have come up with the same amount

Standard multi-flute medium helix end mill sizes are 3/16" (0.1875) and 1/4" (0.25), and some would opt to use a 0.25" steam port, but we will use a 0.1875" milling bit, and use a 0.23" steam port width. Calculations are typically based on a port with square ends, but we will use a port with round ends, as was typically used. A square-ended port would be prone to cracking anyway.

The minimum steam pipe size needs to be at least as large as the steam passage we are using:
2.4 * 0.23 = 0.552 (sq.in.) (steam passage area)

A 3/4 inch schedule 40 steel pipe has an inside diameter of 0.824 inches, or an area of 0.533 (sq.in.), which would work, but since there will be a pressure drop between the boiler and the engine due to restrictions in the steam pipe, the next larger size pipe will be used for the steam line.

Using a 1 inch schedule 40 steel pipe for the steam line gives 0.8642 (sq.in.), which will work for our steam line to the engine from the boiler.

The exhaust port is typically twice the width of the steam port, or 0.46 inches.
The minimum area of the exhaust pipe should match the exhaust passage area. The exhaust passage area will be:

2.4 * 0.46 = 1.104 (sq. in.) (exhaust passage area)

A 1 inch schedule 40 steel pipe has an inside diameter of 1.049 inches, and an inside area of 0.8642 sq.in., which would be too small.

A 1.25 inch schedule 40 steel pipe has an inside diameter of 1.38 inches, and an inside area of 1.49 sq.in., which would work for our exhaust pipe.

The idea behind the steam port and passage calculations is to have ports/passages large enough to provide near boiler pressure for the entire stroke of the piston. If the ports/passages are too small, then the steam pressure will drop as the piston progresses down the cylinder, and this is indicated by a drop in the top indicator curve prior to cutoff. Ports/passages that are too small will also introduce "wire drawing" which is just a restriction in the flow of steam that prevents full boiler pressure from being applied to the piston.

The faster the engine operating RPM, and the faster the piston is moving, the larger the ports/passages need to be, since the volume to be filled with steam is changing rapidly.

The crankshaft throw will be 1/2 the stroke, or 1.85 inches.

I strongly recommend your acqusition of Frank Grahams book. The volume one uses the Bilgram diagram with an explanation that any mechanic/machinist can understand (that's what the name is about - it was meant to be carried in the hip pocket. There are several other errors/misconceptions that exist in your design so far. None will affect the ability of the engine to function, but based on the amount of detail you are going in to, I would think this might be useful. Sorry I didn't get to you sooner, but my computer was down for about 10 days due to computer "gremlins". If you want more info now, I will send you a same as reprints from the May/Jun and Jul/Aug 2010 issues of Steamboating Magazine (I can do this as I edit and publish same). But for anyone out there planning on building a steam engine from screatch, I STRONGLY recommend Audels Engineers and Mechanics Guide by Frank D. Graham, Vol. 1, any edition - I'm not sure if Google has copied it yet, but it is available online for a few bucks. He walks you through designing a small (by standards of 1900) steam engine, including ALL of the moving and stationary parts.

 

[zeeprogrammer]

I consider many of the guys on this forum as superb machinists, and you can tell just by looking at their work and reading their posts, but it would be a stretch to call me a machinist just yet.

No argument about the superb machinists on the forum. But if you're not a machinist...what does that make me? ;D Don't answer that.

I really enjoy your posts. Lots of information, tips...just downright interesting. Interesting projects and excellent work too. I wish I could contribute to them.

 

[steamer]

Hi Pat,

Are you going to use this engine in to power a boat?, and is it going to run at high speed? If so, I can recommend a ratchet feed pump to lubricate the connecting rods.

The pump is hard plumbed to the back of the standards, and then connected by flexible tube in the form of a large "U" to the cross head. I used plastic tubing and pushloc fittings here. From there it is hard plumbed down the side of the connecting rod to the big end bearing.

This works well in my boat, and some others I know, and avoids the need to drill the crank for oil passages.

I'll post something if it isn't clear.

Dave

 

[JohnLanark]

Pat -I think your engine and the boiler to steam it will suit a much larger boat than you have in mind. Take a look at the engine and boat sizes in the steamboat association register crossreference here:
http://www.steamboat.org.uk/register/html/x_e_mod.htm