Con-rods and length

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

Post by DetroiTug » Mon Jan 21, 2019 6:24 pm

Quote: "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."

Yep, poor attitude can negate a whole lot of good training :lol:

Quote: "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."

Stress relieving of weldments is industry practice. but I can attest it is rarely done. Generally it's not necessary for simple parts, but for components with long moments and precision is required between perhaps dowell hole location in relation to another surface etc, then yes it is imperative. Just as Fred notes, internal stresses can fight one through the whole job right through to the finish machining, each layer of material that is removed alleviates some stress and invites further deformation. The machinist just keeps jumping back and forth between operations and arriving at a compromised result that will suffice, otherwise referred to as a "bad day" :?

As Fred noted, brittleness along the welds poses a definite need for stress relieving, however in most of our hobby designs we typically overbuild with safety margins as high as 10 times the service requirement, and why we get around dealing with internal stress relief to any extent where component strength is the concern.

-Ron
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Re: Con-rods and length

Post by Mike Rometer » Mon Jan 21, 2019 6:32 pm

Once properly understood (and trained) MiG is easy, and efficient. I used to train welders in MiG and we always reckoned anyone could be trained to do a joint in a very short time. That doesn't mean you've trained a welder, only an operative that can do THAT joint. Prep, as always, is most important and a gap between parts to allow for penetration is part of that, and so is machine maintenance. I will show the process I use in the other thread for those with an interest and perhaps less experience. I too would be careful about MiG for boilers, though some codes allow for it.
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Re: Con-rods and length

Post by DetroiTug » Mon Jan 21, 2019 7:00 pm

Quote: "Once properly understood (and trained) MiG is easy, and efficient."

Yes it's definitely easy to lay down a bead, but it's a questionable weld due to the penetration issues. Sheer mass of the part can affect the penetration as localized temperatures of the material can often not be easily changed and heavy mass can serve to stabilize temperature and prevent the necessary preheat temperature of the metal for proper penetration and fusing.

Quote: " anyone could be trained to do a joint in a very short time. That doesn't mean you've trained a welder, only an operative that can do THAT joint. "

Yes repetitive welding of the same type of joint, and that is what makes them well suited for production welding, but not always the best for jobshop welding where the conditions are continually changing. I'm not claiming that the MIG welder is unsuitable somehow, I use one very often, I was just addressing the assertion that it is somehow preferable overall to other types of welding. It's good for some things, but not for everything.

There are many areas of welding where MIG is not allowed. Steel erection - bridges, Heavy construction equipment, high pressure gas piping etc, it is Stick only with rods of specified alloys.

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Re: Con-rods and length

Post by Mike Rometer » Mon Jan 21, 2019 8:07 pm

With modern machines poor penetration is generally down to operator incompetence. Poor technique, lack of proper prep, or set-up, and on occasion, trying to go too fast.

My main problem these days is lack of practice. welding is definitely one of those skill that needs to be honed (poor eyesight doesn't help much either!).
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Re: Con-rods and length

Post by TriangleTom » Mon Jan 21, 2019 8:53 pm

With all the concern about deformation and weld stress, why not build up an engine using threaded connections? It seems like the easiest way to avoid weld issues in the engine would be to avoid welding the engine at all.
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Re: Con-rods and length

Post by Mike Rometer » Mon Jan 21, 2019 10:33 pm

My contention would be that the only 'issues' would be from something one forgot to allow for. I'm not expecting any problems, but isn't that anticipation one of the big thrills of playing these games, and proving oneself to oneself (and others if they're interested)?
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Re: Con-rods and length

Post by Mike Rometer » Thu Jan 24, 2019 10:24 am

The most suitable/logical rod length (currently: I'm still thinking) seems to be 2.46:1, giving a rod of 8", but having just looked a Ramon's new crankshafts I notice that the rods do not split (probably fitted with rollers, common on 'strokers'). I was going to use sealed ball-races anyway so perhaps I might go that route myself. I can see a disadvantage from a strip-down point of view, but if there was need for that it most likely would be for a new bearing, which would mean splitting the crank anyway. The advantage is a smaller dia. big-end with less reciprocating weight, negating the added weight from the longer (than anticipated) rod.

Any comments?
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Re: Con-rods and length

Post by RGSP » Thu Jan 24, 2019 12:42 pm

This one goes on for ever, and I doubt there will ever be full agreement. It's really about fabricated (and splitable) crankshafts), once that is accepted, having a one-piece con-rod is only sensible, and needle roller bearings (or similar) become easy.

One side of the argument says that forged or fully machined (from solid billet) crankshafts are always going to be most robust and strongest. This is almost unarguable.

On the other hand, built-up crankshafts were (perhaps still are) common practice in high performance motorcycles, and given the right jigs and presses they have certainly proved to be more than adequate. I cringe at the thought of "truing up" a crankshaft using a soft heavy mallet, but I know plenty of people who've done it, after pressing it together, and ended up with a good and reliable engine.

I think it's a case of "You pays your money and takes your choice". People are always going to argue one way or the other, with good reasons on both sides.
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Re: Con-rods and length

Post by Mike Rometer » Thu Jan 24, 2019 2:46 pm

RGSP wrote:
Thu Jan 24, 2019 12:42 pm
Edit : -
I cringe at the thought of "truing up" a crankshaft using a soft heavy mallet, but I know plenty of people who've done it, after pressing it together, and ended up with a good and reliable engine.
This is nothing new to me, I used to tune Villiers 8 and 9E engines for Go-karts, that pulled 11000- 12000 RPM, and that is how their cranks were done. All you need really is a reliable D.T.I. and a rotatable chuck (lathe). There was a slight danger of moving something you didn't mean to as you pressed in the locking pins, but as with all things there's a bit of a knack to it, though I must say . . . . . it's been a while!
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Re: Con-rods and length

Post by Mike Rometer » Wed Oct 16, 2019 3:26 pm

Yes it's taken a while, but the decision is made and no going back (I've cut metal). After long deliberation and head scratching, I've settled on : -

8" con-rods, giving a stroke/rod ratio of 2.42424 etc.

I don't think it will look ungainly or over-tall.

Now I'm wondering should I make them a fancy section . . . . . :roll:
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