Con-rods and length

A special section just for steam engines and boilers, as without these you may as well fit a sail.
Mike Rometer
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Re: Con-rods and length

Post by Mike Rometer » Sat Jan 19, 2019 10:33 pm

I did need the nudge to remember the valve gear, I hadn't exactly forgotten it but it had slipped down the pecking order. I'll work out two or three different lengths from 2.1:1 upwards, and I think I have a plan to keep the valve rods as long as pos with adequate guide length.
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Re: Con-rods and length

Post by steamboatjack » Sun Jan 20, 2019 8:27 am

If you are designing an engine from scratch as I do, then 2.5:1 is a minimum, my current engine build has 2.67:1
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Re: Con-rods and length

Post by Mike Rometer » Sun Jan 20, 2019 11:32 am

One (lonely?) thought that is running round my head is that the taller the engine is, the more flex there must be in the pillars; larger dia. will help but at some point diagonal bracing will need to be applied.
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Re: Con-rods and length

Post by barts » Sun Jan 20, 2019 7:55 pm

One (lonely?) thought that is running round my head is that the taller the engine is, the more flex there must be in the pillars; larger dia. will help but at some point diagonal bracing will need to be applied.
This is true. Flex in a beam varies with the cube of the length for a given cross section and side load. But the side load does down as the rod gets longer, and the stiffness of the pillars increases with the cube of the diameter, too. So if you make the connecting rod grow by 25%, do the same for the pillar diameters and the engine will be stiffer since the side forces have gone down.

Give yourself room for a proper size valve guide and gland - you'll be glad you did; unless there are real height limits or construction issues, making the engine taller has a lot of practical benefits.

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Re: Con-rods and length

Post by Lopez Mike » Mon Jan 21, 2019 11:23 am

I concur completely on the subject of valve guide design I have two ST 6A projects under my belt and both of them were entirely due to too short of an engine without an adequate valve guide. The H.P. valve side loads on the stock engine are too much for the rod. Excessive wear and valve rod breakage.

On both engines the fix was to go up a size on the valve rod. A balanced valve would have been another solution but that would have introduced it's own cascade of issues.

Your whole engine mass is unlikely to equal a couple of fat passengers. I wouldn't worry about a tall engine raising the center of gravity of the boat. My own craft has too low of a C.G. and has a sharp reaction to wakes. There is no way in hell that one of our launches is going to roll over. Water would be coming over the gunnels before they would capsize.

This is a subject much discussed and documented in the sailing community. I suppose because heeling is inherent in the use of sailboats. One of the things I hope to do when my equipment is in my new hull is to do some simple tests of the heeling moment with athwartship masses. One my old hull I have had as many as ten people aboard with them initially almost all on one side until I moved them about. We are talking about well over 1000 pounds of active protoplasm. The boat is only six feet wide and I doubt that it has healed 15 degrees.

I will most interested to hear how much warpage happens when you weld on your frame. I've always been terrified and ignorant on the subject. Keep us posted.
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Re: Con-rods and length

Post by DetroiTug » Mon Jan 21, 2019 1:09 pm

Quote: "I will most interested to hear how much warpage happens when you weld on your frame."

That can be controlled a great deal by fixturing and bracing components prior to welding, Weld on large open components like that unbraced and yes it's going to go all over the place, just a fact of fabricating. Whem unable to fixture, with practice you can learn how much to cheat the heat. Say you want a 90 degree joint and it's all welded on the outside of the corner, tack it prior to weld on about 87 degrees. It will pull back to 90 or very close to it. It will always pull on the welded side. In the case of the bed plate above, I would clamp it firmly to the weld table first, that will keep it relatively flat. Do the machining after the welding, leave some stock to remove and true up the weldment.

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Re: Con-rods and length

Post by Mike Rometer » Mon Jan 21, 2019 2:25 pm

DetroiTug wrote:
Mon Jan 21, 2019 1:09 pm
Quote: "I will most interested to hear how much warpage happens when you weld on your frame."

That can be controlled a great deal by fixturing and bracing components prior to welding, Weld on large open components like that unbraced and yes it's going to go all over the place, just a fact of fabricating. Whem unable to fixture, with practice you can learn how much to cheat the heat. Say you want a 90 degree joint and it's all welded on the outside of the corner, tack it prior to weld on about 87 degrees. It will pull back to 90 or very close to it. It will always pull on the welded side. In the case of the bed plate above, I would clamp it firmly to the weld table first, that will keep it relatively flat. Do the machining after the welding, leave some stock to remove and true up the weldment.

-Ron
Pretty much the route I was taking. The other trick which can be very useful is a large gas torch and heat the metal opposite the weld, to remove the bow where it pulled. For instance welding a tee joint, the main will bow round the weld. Heat the outside and it will pull back on it's own, as it re-cools, it's just a case of how much heat to apply so start small and go bigger. Also good to remember that 1" of weld will hold a whole ton. so ask "do I need to weld it all up solid?". MiG or TiG is preferable to Stick or Gas, and much quicker, with less overall heat.

Off to try to get some weld preps done.

Oh yes, and don't continuous weld from one end to the other, that's asking for trouble!
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Re: Con-rods and length

Post by DetroiTug » Mon Jan 21, 2019 5:02 pm

Quote: "MiG or TiG is preferable to Stick or Gas, and much quicker, with less overall heat"

Not sure what is being referenced as preferable, but stick/ARC is the strongest type of weld, and MIG is the weakest. For strength between the two, TIG is slightly better than MIG.

Quote: "remember that 1" of weld will hold a whole ton"

That is a good rule of thumb, but it really depends on the operator, I've seen whole beads of MIG weld knocked off with a hammer. MIG is notorious for poor penetration and why I would never use or trust it for boiler construction. And that issue can easily arise from the subject being discussed, deformation from heat, a seemingly effective remedy would be to lower the weld heat to counter the effect, which will result in poor penetration.

Once understood, MIG is not all that easy.

-Ron
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Re: Con-rods and length

Post by Lopez Mike » Mon Jan 21, 2019 5:21 pm

I am somewhere near to being the world's worst welder. Partly due to faulty color vision but mostly due to lack of training and a poor attitude. Mig allows an ignorant machinist like me to stick non-critical stuff together.

I agree with Ron that for serious strength plain old stick welding is a good choice. My locomotive boiler was done to pipeline specs with a TiG root weld and the rest with 7018. Grinding out all starts and stops in both cases. NOT by me!

The problem with not welding all the way around is that it might leave a way for moisture to get in between the two parts. I think that you will be fine having all of your precision machine work done after the welding.

Is it worth cooking the whole assembly before the final machine work Ron? I have been told all sorts of stories of the need for stress relief. I was bandsawing a hunk of steel last year and the cut line opened up a half of an inch in six inches. Startling!
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Re: Con-rods and length

Post by fredrosse » Mon Jan 21, 2019 5:42 pm

Stress relieving of weldments that become a part of machinery is almost always required if precision is a requirement. When welding, a portion of the steel becomes liquid, and adjacent areas become butter weak. Then the material solidifies, still very hot. Then the material cools further, shrinking and distorting the steel, getting into a condition of severe tension stress near the weld, which will pull and distort nearby steel that wasn't even heated up much during welding. When any parts of the metal are further machined, the metal cross section is reduced, thus the amount of pull or push forces are changed due to the reduced metal cross section available for stress. The result is that the metal part keeps changing shape slightly as it is machined, a difficult problem.

Stress relieving by bringing the whole steel part slowly up to red heat, then very slowly cooling everything slowly and uniformly, results in a piece that is ready to machine without stresses locked in the material. I have often stress relieved weldments with a wood campfire, and must smother the entire part with a thick layer of hot ashes to make sure everything cools slowly and uniformly.
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