Can a connecting rod be any length?

[sedge]

Hi
If I make a steam engine and follow the plan for these dimension :-
1The cross head.
2 The crank throw.
3 The eccentric.
Can I make the connecting rod and valve rod a multiple of the original (i.e twice as long, three times etc) and keep the timing the same?
Ken

 

[cncjunior]

Hi Sedge,
As far as the length of the connecting rod and valve rod on a steam engine can be any length as you want. There are two things to note. The most important is that the cylinder support be spaced appropriately. The second is just a weight issue. The longer your rods are the more mass you have moving around. This could cause balance problems. This isn't a problem as you need look no further than steam loco's and their connecting rods and the wheel counterbalances. I believe that on a Stanley Steamer Car the engine has long con rods as well. The timing will be the same regardless of the lengths if there is a simple eccentric strap with no reverse. If there is a Stephenson valve gear with reversing or other valve gear system then care would be needed that the right rods are lengthened.

Daniel

 

[OrangeAlpine]

The workable minimum for a con rod is 1 1/2 times the stroke. Shorter than that and side pressure becomes a problem. I don't think there is a max.

Bill

 

[sedge]

Hi
Thanks for the replies and information.
I am a beginner at steam engines, and want to make the con rod longer on the engine I am making.
Ken

 

[Tin Falcon]

According to my reprint of ICS steam engine design text book the connecting rod should be 5.5 to 6 times the length of the crank arm . but may be shorter in a marine engine like 3.5 or 4 .

this would correspond to 2 -3 times the stroke.

the problem with longer than this is added stress .

Think of a con rod as as column that is holding up a building . the taller the column the more it will tend to bend. now imagine the building is jumping up and down. This is the type of stress we are looking at.


Ok reality check we are talking a small model . so the question is what will the model be used for . and what is the operating pressure on the piston and piston diameter. also is this working engine or demonstration engine that will never see a load.

Hypothetical calculation .

area =Pi *r^2 so a 1" piston @30 psi 3.1416 * .25 *30 = 23 lbs of force applied to the con rod double or triple as a safety factor.
so you need a con rod short enough not to bend under 60 pounds of compression force.

if you look at the corliss engines some of them had fairly long con rods heavy in the middle and tapered at the ends this allows for more strength but clearance at the connecting points.


Things are usually deigned the way they are for a reason. making a con rod two to three times the length in the design is likely not a good idea. If nothing else the model will be way out of proportion.

Tin

 

[OrangeAlpine]

Tin, your not considering the impact of the resulting angle. As the con rod gets shorter, the side pressure on the crosshead increases and the forces on the con rod change, becoming less compressive and more bending.

Bill

 

[Tin Falcon]

Tin, your not considering the impact of the resulting angle. As the con rod gets shorter, the side pressure on the cross head increases and the forces on the con rod change, becoming less compressive and more bending.

Bill

You established that a con rod should not be shorter that 1 1/2 times the stroke. I am not arguing that. but mentioned 2 times as a more conservative figure.

The workable minimum for a con rod is 1 1/2 times the stroke. Shorter than that and side pressure becomes a problem. I don't think there is a max.

I am telling you based on published data a con rod should not be longer than 3 times the stroke. to long or too short will introduce undue forces. Not sure what your point is here.

You seem to be arguing your own statement.

You ok there ??
Tin

 

[OrangeAlpine]

Tin, I'm fine and not arguing anything.

I construed this statement "so you need a con rod short enough not to bend under 60 pounds of compression force" to be an argument for short con rods, when in fact, shortening the rod increases other stresses that you were ignoring.

It's a very complex subject of which I know little, but I'm perfectly at ease with the concept of rod lengths being 1 1/2 to 3 times the stroke.

Peace?

Bill

 

[Tin Falcon]

It's a very complex subject of which I know little, but I'm perfectly at ease with the concept of rod lengths being 1 1/2 to 3 times the stroke.

Peace?

Bill

That is the real point here !

I was not trying to ignore the problems cased by short con rods just did not feel it it was the topic of discussion. "the question was how long can I go " and you had already established the limit for how short.
I am certainly no expert either that is why I rely on my collection of technical books..
Tin

 

[Gedeon Spilett]

too short a rod also increases the rod angularity, making the symetry of the steam distribution and thrust on both side of the piston difficult to maintain for a double acting engine.
this effect vanishes practically for a rod/stroke ratio above 5

 

[sedge]

Hi All

Before we get too technical and complicated let me explain my question.

I am attempting to build a model of a Steam Horizontal Capstan Donkey engine using the Dolbear 1862 patent application and a copy of a drawing of a later Winch Donkey Engine with full size measurements.

This is my first attempt at making a steam model and I am working to a scale of "about" 1" = 1' and making it up as I go along.

I was basically trying to find out if their was a recommended multiple of any thing to work too for the conrod length, perhaps the twice or three times was not good analogy.

The engine I am thinking of using has a bore of 1/2" and stroke of 3/4" and I need to lengthen the conrod to fit it all onto the sledge.

Working to the information give above the conrod would be 3/4" x 3 = 9/4" = 2 and a 1/4".

From this a full size engine would be 6" bore x 9" stroke with a 2'-6" conrod.

Thanks for all the information
Ken

 

[Tin Falcon]

hope we helped and did not confuse you.

you may at some point want to pic up the ICS steam engine design book . I purchased my copy from PM research.

or you should be able to find a digital copy.

I was basically trying to find out if their was a recommended multiple of any thing to work too for the conrod length, perhaps the twice or three times was not good analogy.

well 2* 1.5 = 3 doesnt it. so not really off base.

Tin

 

[sedge]

Hi Tin

Is this the book you suggested?

Steam Engine Design and Mechanism
International Correspondence Schools
ISBN-13: 978-1603861106

Ken

 

[Mechanicboy]

Do not forget!
The length of connecting rod has affect on piston speed:

Short Rod is slower at BDC range and faster at TDC range.

Long Rod is faster at BDC range and slower at TDC range.

The generally rule of length of connecting rod:

Short Rod -- Min Rod/Stroke Ratio -- 1.60 Max Rod/Stroke Ratio -- 1.80

Long Rod -- Min Rod/Stroke Ratio -- 1.81 Max Rod/Stroke Ratio -- 2.00