The Unofficial Mamod and Other Steam Forum

Steve_S · Posted: Mon Apr 02, 2012 8:32 pm Post subject:

I have always belived that the explanation is as Roly has stated. If the big end bearings are not parallel with the crankshaft (i.e they are skewed) then the con rod and piston must twist as they rise and fall. They can't not!

mogogear · Posted: Mon Apr 02, 2012 9:12 pm Post subject:

And I shall be the lone contrarian.

I believe the twist is caused inside the hollow pistons by a wrist pin following in a twist groove.

I see the lower end of the piston rod is articulated but with with a pin 90 degrees from the crank journal. If it is pinned - I do not see how it could rotate.
And I see no seam in that coupled sleeve separating it into two halves ( u and L) .

This allows some nutating action of the rod ( also allows the piston to be assembled while it moves up and down and the piston twists.
I think the magic lies inside the piston cavity..

I could be a fool on a fools quest...

One day we shall see one disassembled.

Sorry to be a nay-Sayer- I still think the story is untold
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scorpion2nz · Posted: Tue Apr 03, 2012 5:42 am Post subject:

kno3 · Posted: Tue Apr 03, 2012 9:52 am Post subject:

I have to agree with Mogogear and Scorpion2nz. I still think the engine works the way they said (and I also said that on page 1). I'll get the engine soon enough and will use all non-destructive methods to disassemble it.
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Roly Williams · Posted: Tue Apr 03, 2012 10:04 am Post subject:

Bugsy · Posted: Tue Apr 03, 2012 10:05 am Post subject:

As Roly said, looking forward to a definitive explanation.

Phil

mogogear · Posted: Tue Apr 03, 2012 5:12 pm Post subject:

No matter what -If there was an engine in the world I would love to see if I could reproduce- it would be this one. The crank is a simple construction method- only the cast end-plates would need known Australian casting abilities.

And if I stand with Dennis and Calin- I am in good stead!!
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http://gerddi.blogspot.com/

Cowboy · Posted: Tue Apr 03, 2012 5:33 pm Post subject:

I've studied the pics a bit more, and read the explanation here: http://www.btinternet.com/~sylvestris/steam/BM.html My first attempt at explaining it was basically right, but I was wrong about the rod big ends being slotted. That would also work, but would wear out quickly. The big end is bored to fit the crank, and then the rod is pinned to the big end 90 degrees from the crank pin. This allows the the big end to "rock" when at TDC or BDC, and to "twist" when at 3 o'clock or 9 o'clock.

Elvis Presley had one of these when he was a kid.

Edit: Hey I just noticed I've been promoted, from Forum Newbie to Junior Member.

STEAMPROPULSION · Posted: Wed Apr 04, 2012 6:48 am Post subject:

mogogear · Posted: Wed Apr 04, 2012 4:38 pm Post subject:

Roly Williams · Posted: Wed Apr 04, 2012 5:42 pm Post subject:

Roly Williams · Posted: Wed Apr 04, 2012 6:01 pm Post subject:

Maybe this quick sketch will make things a bit clearer. I've drawn it looking from above with the tilt much exagerated. The twisting action can be seen clearly.

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Daryl Canada · Posted: Wed Apr 04, 2012 8:28 pm Post subject:

Fascinating thread to follow. Roly's diagram looks OK but is something more needed? Assuming 180 degrees is BDC and the end of the power stroke, the exhaust stroke would then start with both valves closed and therefore compression would exist and I would not expect that to be helpfull. Perhaps this is OK on steam engines. Is it not usuall for the exhaust to open before BDC and the intake to open before TDC on most two stroke. If this is also true for steam then a timing adjustment would be required, but how. Angling the slots in the piston could work but these appear to be straight? Likewise if the crankshaft pins were at right angles at approximately 160 and 340 degrees that might work but I can't model that in my head. Balancing the crank would be challenging I think.

I'm sure glad no one is depending on me for the answer. Please, someone take an engine apart.
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mogogear · Posted: Wed Apr 04, 2012 8:54 pm Post subject:

Excellent drawing and it would help me buy in ...but:

In the picture shown my Giovanni on the first page( this one)

The piston on the right is shown to be turning to the right with respect to the grooves for the port. If it is turning and the rod is turning the upper most pin at the lower end of the rod would also be turned at the same degree as the groove.

In this picture the piston on the left side seems to be in close to full upward position also-or close to the same approximate position except reversed from photo 1- the top pin also appears to be aligned straight and not turned.

I pinched this fabulous close up from Odilon's site ( Moose - let me know And I will take it down) But I wanted the best close ups all together

Does this agree or not with your drawings Roly? ( which I truly appreciate)

To my eyes it still appears to be parallel to the crank disks- not skewed. Of course I realize that this maybe be too subtle a turn to notice- --so I am still in the other camp for now .

It feels to me as if there should be "rifling" grooves in the bore of the cylinders that turns the piston back and forth- or something like that at work ....?

Shutting up and muddling on. I hope I have explained. If I am off course -I am happy to be set straight!! I promise my reasoning clearly

Cheers to all my friends enjoying this mental puzzle- I do really love puzzles

BTW- here is where the quote comes from Basil Harley on our site :

http://modelsteam.myfreeforum.org...s-steam-engine__o_t__t_31125.html

I have cropped the specific paragraph here:

I can visualize a inclined female "ball-joint hole" in the piston.
The small end of the rod has a ball on it and a pin that projects through the ball at"some angle" 45 degrees is stated. A the pressure from the admitted steam forces the piston downwards- the piston is forced to travel "down and incline " and thus twist in a certain direction.

Now I can see this--!!

There is some "slop" in this design but not much travel is required to rotate the small piston a few degrees in rotation as the pin slides up or down the inclined internal groove. I wonder if this is a separate piece machined and then soldered or clipped inside??? the piston looks thin walled and hollow mostly

These being soldered in seems odd- I would think a small internal cirlcip would be easier- I wonder if the piston discussed earlier was a repaired version.

Have I lost everybody????

I wish I could CAD what I see in my head!!

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Mo (greg)

His Most Noble Lord Admiral Mo, the Apocalyptic of Old Tonbridge Wafers

http://gerddi.blogspot.com/

hellequin · Posted: Wed Apr 04, 2012 9:23 pm Post subject:

Is that slot in the piston straight?and why are the marks on the piston vertical and not radial?

The slot for exhaust is longer than the inlet as B Harley states that it has cut off on admission
Well theres only one way to prove a theory make it

but not this year
Still cannot understand how the crank rotates the piston if thats the case why does it have a ball fitting and not a simple pin Im still missing some thing simple

It has to be a simple solution as it was a toy sold at a price to reflect this fact .Back to K.I.S.S and a rethink for me.
best wishes

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