How Do You Calculate Connecting Rod Length?

[warmstrong1955]

Not sure what you're doin' Bill, but you got me curious, and I was searchin' a bit....

http://www.john-tom.com/MichelNiggel/Mi ... elTwin.pdf

Metric.....easy enough to convert to inch if you need, and easy enough to change the scale.

More here.... http://www.john-tom.com/html/SteamPlans.html

I just googled 'Steam Engine Plans'.
Let your fingers do the walkin'....and maybe do some good ol' reverse engineering....

Other Bill

[seal killer]

Bill--

Thank you for the links. I like the plans at the link for the engine!

My immediate goal is to experiment with a valve idea I have floating around in my mind.

--Bill

[Al_Messer]

K. N.Harris reccomends that the length of the Connecting Rod should be 2.25 times the stroke of an engine.

Al

[warmstrong1955]

K.N. Harris book.... http://www.fastonline.org/CD3WD_40/JF/424/19-423.pdf

Bill

[johnpenn74]

Rod Length

A longer rod is better, right? maybe not.

Many mikados / consolidations have the main rod connection on the third driving axle. This far back, should reduce the angle of the main rod from the cylinder centerline, thus increasing the verticle the force applied to the crosshead guides. Remember that force, is going to turn into friction and power lost. If you look at earlier engines 1890 c19s & c16s consolidations I believe the rods were typically placed on the 2nd driving axle. Later this was changed 3rd axle as designed evolved.

It should be noted that this solution of making the rod longer doesn't always work. N&W k3 4-8-2 were designed with the main rod located on the third driving axle. Unfortunately, the material required to keep them from folding up added so much weight that the hammering of the rods beat and bent the rails to death. The engines were limited to 35 miles an hour. There's some good discussion about this design flaw in Kings book, "Mercedes of steam"

JP

[seal killer]

Al and Bill and JP--

Thanks for the help . . . I just ordered "Model Stationary and Marine Steam Engines."

--Bill

[Al_Messer]

Another helpful book, if you can find a copy, is "Model Mechanical Engineering" by Steele. It is a British publication and has lots of technical data.

[Loco112]

When trying to detemine rod length you also need to start with your "cleareance volume" in mind.

http://www.google.com/imgres?hl=en&sa=X ... zAQ&zoom=1

[seal killer]

Loco112--

Thank you for that tip!

--Bill

[Dan Rowe]

Bill,
My assumption is that you are talking about a double acting steam engine so my answer is only considering that type of engine.

You make no mention of engine configuration, but as a general rule vertical engines have shorter rods then horizontal engines. This is to keep the engine from being to tall as they need room for maintenance considerations.

The shorter the connecting rod is made with respect to the crank throw the greater the angularity of the connecting rod. This is one of the greatest errors in valve gear events as the cylinder will not be at mid stroke when the crank is at 90 degrees.

Marine engines and Shay locomotives have short rods. Longer connecting rods are desirable but there is no room. The Shays I have checked are as low as 2.5 to 1.

K. N.Harris reccomends that the length of the Connecting Rod should be 2.25 times the stroke of an engine.

Al, I thumbed through Model Stationary and Marine Steam Engines by K.N. Harris and could not find that quote. As he usually gives an engineering reason for any statement, I would love to read the whole context. Can you supply a page reference?

Dan

[seal killer]

Dan--

Thank you for the guidance! I am still studying. The engine could be either horizontal or vertical, I prefer horizontal merely for cosmetic reasons. In fact, the engine is not my goal. I am playing with a valve design. Of course, I need an engine to find out if it works!

--Bill

[Al_Messer]

K. N.Harris reccomends that the length of the Connecting Rod should be 2.25 times the stroke of an engine.

Al, I thumbed through Model Stationary and Marine Steam Engines by K.N. Harris and could not find that quote. As he usually gives an engineering reason for any statement, I would love to read the whole context. Can you supply a page reference?
Dan

Dan, I am at a loss: I know that I read those figures some place not five minutes before I replied to your query, but I have read Harris' book tonight until I am half blind and cannot find the statement. At this point, I haven't the foggiest idea where I read those figures. I gotta go get some memory pills I guess.

Al