gradual crankshaft

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gilteva
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gradual crankshaft

Post by gilteva » Mon Nov 28, 2016 12:08 pm

The crankshaft gathers the momentum power from 4 pistons.

The momentum power increases as we go closer to the end of the crankshaft.

1. Is it true?

2. The diameter of the crankpin is large, and the big end of the con rod is large.

Can the crank pins and the connecting rods
gradual.jpg
diameter be gradual?

For example, the first is 20 mm, the second 25 mm, the third 30 mm and the fourth 35 mm?

Only from the mechanical aspect. not oil etc.

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ctwo
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Re: gradual crankshaft

Post by ctwo » Mon Nov 28, 2016 1:52 pm

I think it could be made that way.
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spro
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Re: gradual crankshaft

Post by spro » Wed Nov 30, 2016 8:51 am

Gilteva. You may be on to something revolutionary but I cannot grasp it. Why would the left main bearing be smaller? Convention is that each piston cycles a full stroke and in balance with each other. There is something else going on here that I don't understand. The weight saved by reducing the rod bearings would have to be made up by heavier counterweights unless this is actually a two stroke, two stage type of design.
Obviously you've given this some thought so I claim befuddlement on my side.

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NP317
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Re: gradual crankshaft

Post by NP317 » Wed Nov 30, 2016 10:29 am

Ditto on spro's comments.
??
Might a non-symmetrical shaft assembly have disturbing harmonic vibration issues?
I've already experienced crankshaft failures on a simpler 2-cylinder crankshaft assembly.
~RN

sicivicdude
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Re: gradual crankshaft

Post by sicivicdude » Wed Nov 30, 2016 12:52 pm

While you COULD thin down the non-PTO end crank journals, they generally aren't for two reasons.

One, interchangability (although this reason is going out of favor with cracked rods and "painted" bearings with .0001" clearances, it still sort of stands true)

If you made an inline 4 cylinder crankshaft with full main bearing support you will have 4 rod bearings and 5 main bearings. If you sized down each journal to only what was absolutely necessary to take the load needed, you'd end up with 9 different sized bearings. Most manufacturers end up with 2 different sizes; one for the rods and one for the mains.

Two, balance. When you are talking about harmonics it's not as simple as just more or less counterweights. You are talking crank flex, walking, chatter. It's difficult enough for some major manufacturers to make "simple" crankshafts without major defects as it is.

Now, that's not to say what you are talking about isn't possible (and indeed maybe even been done before). It most certainly is and for SOME applications it might be worth it. If you were talking about an ultralight racing airplane engine, you might want to save that extra few pounds of reciprocating mass. This may be one of the many many secrets in a formula 1 engine that makes them so easy to rev? I can just say it's not common in mass production engines.

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ctwo
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Re: gradual crankshaft

Post by ctwo » Wed Nov 30, 2016 1:02 pm

At first you might think that the crank at the output side experiences just the force of the last piston, then the force of the last and second to last, until the the output side where the force of all pistons are transferred. However, because of the way the pistons are timed, there is only force of one piston at any time.

You may also consider the force of the first piston is transferred direct to the output, where each piston back has to transfer through more shaft and another connecting pin, resulting in more bending/twisting of the shaft, so more rigidity would be needed. An engine with a million pistons - when the last piston fires, the entire shaft would not rotate as it would just twist in place.

I had a thought that the diameters could be reversed with the small end at the output and leaving material where it would be used.
Standards are so important that everyone must have their own...
To measure is to know - Lord Kelvin
Disclaimer: I'm just a guy with a few machines...

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BigDumbDinosaur
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Re: gradual crankshaft

Post by BigDumbDinosaur » Thu Dec 01, 2016 1:25 am

ctwo wrote:At first you might think that the crank at the output side experiences just the force of the last piston, then the force of the last and second to last, until the the output side where the force of all pistons are transferred. However, because of the way the pistons are timed, there is only force of one piston at any time.
That's only true for engines with less than six cylinders. With six or more cylinders, power strokes overlap and torsional stress on the crankshaft, although still pulsating, is continuous, not intermittent.
You may also consider the force of the first piston is transferred direct to the output, where each piston back has to transfer through more shaft and another connecting pin, resulting in more bending/twisting of the shaft, so more rigidity would be needed.
This is a main reason why automotive V-8 engines forced in-line eights into extinction. While straight eights ran very smoothly, with no primary imbalance and plenty of power stroke overlap, crankshaft torsional vibration and whip were significant problems, further exacerbated by those engines' long-stroke, under-square design. The combination of torsional vibration and whip would caused fatigue cracking in the mains, especially the rear main. Running a straight-eight too fast almost always resulted in the rod big ends smacking the crankcase and causing engine fatality.
An engine with a million pistons - when the last piston fires, the entire shaft would not rotate as it would just twist in place.
Not true. Assuming no mechanical losses, the torque generated by that piston will eventually find its way to the flywheel, as the crankshaft would act like a torsion bar and transmit torque without loss to the other end. It would just take a while. :D
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NP317
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Re: gradual crankshaft

Post by NP317 » Thu Dec 01, 2016 10:52 am

This discussion sure makes radial engines sound like a good idea!
:lol:
~RN

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ctwo
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Re: gradual crankshaft

Post by ctwo » Thu Dec 01, 2016 11:45 am

Someday I want to build a running engine. I just don't want to build the same old thing that's been done already. Need something new and improved! It takes some people an entire lifetime to get something done.
Standards are so important that everyone must have their own...
To measure is to know - Lord Kelvin
Disclaimer: I'm just a guy with a few machines...

JackF
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Re: gradual crankshaft

Post by JackF » Thu Dec 01, 2016 7:08 pm

I haven't seen anyone build a rotary engine yet. That doesn't mean it hasn't been done before but it would be unusual. :wink: :)


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spro
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Re: gradual crankshaft

Post by spro » Thu Dec 01, 2016 9:08 pm

Since this topic has evolved into different designs and radial engines, I had to step back. Perhaps some don't know how America scrambled to find a suitable engine for tanks 35-40 tons. There were 9 cylinder, air cooled radial engines by Wright Continental and the Chrysler A-57 water cooled. Go to see these things. The A-57 was five 6 cylinder engines arranged radially to a 30 cylinder engine. Each had their own distributer and linked to common main shaft. Info says two banks could be destroyed and the Sherman would still run.

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BigDumbDinosaur
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Re: gradual crankshaft

Post by BigDumbDinosaur » Thu Dec 01, 2016 10:31 pm

NP317 wrote:This discussion sure makes radial engines sound like a good idea!
:lol:
~RN
Well, they were, as long as you were willing to put up with their idiosyncrasies and large, bluff frontal area. The latter wasn't too much a problem in aircraft after the NACA cowling was developed, but it sure created some interesting design challenges for the few folks who used a radial to power an automobile.
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