Con-rods and length

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Mike Rometer
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Con-rods and length

Post by Mike Rometer » Thu Jan 17, 2019 5:42 pm

"And now for the twin"

I like to keep my thinking in front of what I'm doing, daft maybe, but it helps me concentrate. Hence my thoughts are now turning to the con-rods, which will be required before I can assemble the crankshaft, which I haven't totally sorted out yet.

My background is 'Infernal Combustion' and I don't need telling that what goes there, doesn't always fit with steam. Looking at the few drawings I have for steam engines, the con-rods seem VERY long, with stroke/rod length ratios over 2:1. The original designer of this engine used 2.5:1 with a stroke of 2.5". Most I.C. engine run about 1.75:1 + or - a touch.

My thoughts are try to keep the engine as short as practical, without too much compromise. Would a ratio of 2:1 be out of the question? I realise that it will increase cross-head loads some, but surely just beef them up a bit? (Increase in weight!)

Stroke is 3.25.

2:1 would give a rod length of 6.5"

2.1:1 = 6.825", 2.5 = 8.125"

1.75:1 = 5.6875" So we're talking a height difference of way more than an inch. Is that kind of difference significant?

As an aside, I originally though I might use ali for the rods. My thinking is drawing back to steel, partly due to availability.
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Re: Con-rods and length

Post by Steam Captain » Thu Jan 17, 2019 6:44 pm

The question is why are con rods so long. The purpose of long connecting rods is to have little sideways pressure on the slide, because the slide is one of the points, where a steam engine can loose a lot of power if it is poorly designed. The shorter, the more sideways pressure, and thus friction develops.

This is one of the few things I would take my time to calculate at least roughly in the head for a steam engine just to see what I can expect. I think I would simply calculate the force/power loss created by the connecting rod's sideways force onto the slide in relation to the connecting rod's length and see what it spits out. If I can live with the calculated friction/sliding loss, I would continue with that lenght.
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Re: Con-rods and length

Post by Lopez Mike » Fri Jan 18, 2019 2:06 am

There is something to be said for more symmetrical valve events. Longer rods, less angularity.
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Re: Con-rods and length

Post by RGSP » Fri Jan 18, 2019 9:16 am

Lopez Mike wrote:
Fri Jan 18, 2019 2:06 am
There is something to be said for more symmetrical valve events. Longer rods, less angularity.
Seconded.

With such a widespread and effective engine, it is inevitable that there will be a cottage industry criticising Stuart 6As, and those criticisms are at least partly valid.
The con rods on 6As are often thought to be too short, and while the engines do run quite well with the short rods, there are a lot around which have been modified to make them an inch or so longer. I can't remember what the figures are, but experience suggests that standard 6A rods are just about OK, but a bit longer might be better.

The other way of looking at it is to ask whether an extra inch on the height of the engine would do any harm, and only in rather rare circumstances will it do so.
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Re: Con-rods and length

Post by Mike Rometer » Fri Jan 18, 2019 9:54 am

RGSP wrote:
Fri Jan 18, 2019 9:16 am
Lopez Mike wrote:
Fri Jan 18, 2019 2:06 am
There is something to be said for more symmetrical valve events. Longer rods, less angularity.
Seconded.

With such a widespread and effective engine, it is inevitable that there will be a cottage industry criticising Stuart 6As, and those criticisms are at least partly valid.
The con rods on 6As are often thought to be too short, and while the engines do run quite well with the short rods, there are a lot around which have been modified to make them an inch or so longer. I can't remember what the figures are, but experience suggests that standard 6A rods are just about OK, but a bit longer might be better.

The other way of looking at it is to ask whether an extra inch on the height of the engine would do any harm, and only in rather rare circumstances will it do so.
So, from that comes the thought, if the ST6a rod is just about adequate, what actually is the rod ratio? The stroke is 3".

I don't have a problem with a taller engine as long as it's justifiable.


Late Edit!

From this forum (it's all there if you look hard enough! :lol: ) and my copy of "Steamboats and Modern Steam Launches", the rod length for an ST 6a is 6" which is of course a ratio of 2:1. From that I will draw the conclusion that, that is a short as one should go; and so above that starts to have an advantage.
Last edited by Mike Rometer on Fri Jan 18, 2019 10:33 am, edited 1 time in total.
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Re: Con-rods and length

Post by Steam Captain » Fri Jan 18, 2019 10:14 am

But this is not, what Lopez Mike meant.

He meant to say, that a change of the connecting rod's length will also cause changes in the valve timing in relation to the piston travel, because a crank doesn't produce a perfect sinus curve of the piston movement. In valve timing, a few milimeters can already mess a lot with the correct steam timing. In theory.
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Re: Con-rods and length

Post by barts » Sat Jan 19, 2019 4:24 am

Longer rods are generally better; 2.5:1 is not too long by any means. Herrreshoff engines were well known for long connecting rods. If you have the space, go right ahead. Long rods may need slightly larger sections to prevent Euler buckling.

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Re: Con-rods and length

Post by Mike Rometer » Sat Jan 19, 2019 9:12 am

barts wrote:
Sat Jan 19, 2019 4:24 am
Longer rods are generally better; 2.5:1 is not too long by any means. Herrreshoff engines were well known for long connecting rods. If you have the space, go right ahead. Long rods may need slightly larger sections to prevent Euler buckling.

- Bart
And add some to the reciprocating weight. All things to be considered. I have no time currently to get back to this side of things, Domesticity is getting a hold. I was thinking 2.1:1 would be a starting point and work out from there.
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Re: Con-rods and length

Post by RGSP » Sat Jan 19, 2019 11:20 am

Mike Rometer wrote:
Sat Jan 19, 2019 9:12 am
barts wrote:
Sat Jan 19, 2019 4:24 am
Longer rods are generally better; 2.5:1 is not too long by any means. Herrreshoff engines were well known for long connecting rods. If you have the space, go right ahead. Long rods may need slightly larger sections to prevent Euler buckling.

- Bart
And add some to the reciprocating weight. All things to be considered. I have no time currently to get back to this side of things, Domesticity is getting a hold. I was thinking 2.1:1 would be a starting point and work out from there.
For what it's worth, my memory (which may be wrong) says that the Anthony Beevor modified 6As had 8" rods. It isn't the actual rods so much that matter, but the length of the bits of the (Stevenson's) valve gear, where fitting everything in means the angles to start to get uncomfortably large. Getting the valve events right for all cut-offs in forward and reverse is not really possible, but the engines still give good service. A rare but inevitable problem is that if the valves become stiff for any reason, there is a likelihood of bent valve gear.
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Re: Con-rods and length

Post by barts » Sat Jan 19, 2019 9:10 pm

Audel's suggests that the normal range is from 2:1 to 2.5:1 - you're in good company no matter where you go.

Reciprocating weight is seldom an issue on our engines other than for vibration. I do wish the Burleigh compound in Rainbow were taller; there's little room for proper valve guides & glands on top of the reversing gear.

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