Author Topic: Kratmo Castings  (Read 3287 times)

Online Jo

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Re: Kratmo Castings
« Reply #15 on: December 28, 2019, 09:13:51 AM »
But not the same engine that Wolfgang has, his is bigger.

Indeed, but it is the only description of the Kratmo engines that I have and I do not speak German.


Surus has let me show you that the big engine has a lot more castings in its set than the little one in his collection  ::)  :pinkelephant:

I still need to put together drawing sets for both engines  :thinking: I need some time.

Jo
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Offline Admiral_dk

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Re: Kratmo Castings
« Reply #16 on: December 28, 2019, 10:06:22 AM »
Your article basicly says that the telephone didn't stand still for a moment after they published the article for the Satyr Old Timer airplane in 1983 and the majority where asking for a sutibile old engine for the airplane. Since there almost only where the Kratmo 10 konstructed by Walter Kratzsch - Krat(zsch)-Mo(toren) - back in the nineteen thirties in Germany (and for much off the World back then) - the FMT magazine had Roland Schwartz create a modern set of drawings from the old article and do a control build to ensure the quality before publishing them.

Offline Jasonb

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Re: Kratmo Castings
« Reply #17 on: December 28, 2019, 10:13:13 AM »
There is a bit about them in this old (and now politically incorrect) book towards the end

https://docplayer.org/56564840-Jahres-inhaltsverzeiehnis-band-8-lieft-seite.html

Those castings don't look too difficult to carve from solid, the most work would be in the cylinder casting but it may be possible to do that in steel and silver solder on some of the bosses which would make it quite easy. Also quite easy to make patterns if anyone wanted to cast them.

Online steamer

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Re: Kratmo Castings
« Reply #18 on: December 28, 2019, 12:41:52 PM »
There is a bit about them in this old (and now politically incorrect) book towards the end

https://docplayer.org/56564840-Jahres-inhaltsverzeiehnis-band-8-lieft-seite.html

Those castings don't look too difficult to carve from solid, the most work would be in the cylinder casting but it may be possible to do that in steel and silver solder on some of the bosses which would make it quite easy. Also quite easy to make patterns if anyone wanted to cast them.

The flapping wing model on page 128 is interesting as well.......
"Mister M'Andrew, don't you think steam spoils romance at sea?"
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Offline Wolfgang

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Re: Kratmo Castings
« Reply #19 on: December 29, 2019, 12:42:28 PM »
Many thanks to all who have responded, and special thanks to Graham for his generous offer to cast a replacement part! I am not sure yet whether I will have to go down that road, but in any case I will take a mold from the crankcase part before I start cutting metal.

My fist attempt to fix the over-bored part will be the JB weld method proposed by Tug, although I am not 100% sure about the outcome. Did some CAD modeling of the engine:



 and I think the highest stress on the crankcase walls will occur at the places of the little red circles:



While there is room for a reinforcement at the crankcase side walls, there is unfortunately very little room between the crank web and the back wall:



So the reinforcement can not go "around the corner", and I suspect the crankcase might fail at exactly this point. But we shall see...

That being said, my confidence in the Kratmo as a useful engine has dwindled over time. Biggest concern is the long and thin crankshaft, together with the large overhang from the front bearing. Any engine put on an RC model will have ground contact with the prop sooner or later, and even the slightest touch to that large prop will cause a bent crankshaft. To make the inevitable repair even worse the connecting rod is permanently attached to the crankshaft, a feature I have not seen on any other engine before.

So my current plan is to build the engine and see if it runs, and then it will probably end as a dust collector on the shelf.

My castings are almost certainly from the one of the replica batches, they just look to good for 1935. The drawing, however, seems to be a reprint of Walter Kratzschs original drawing. It is definitely hand-drawn, not produced by CAD. Print quality is rather poor, partly. It came with four pages of build instructions written by Walter Kratzsch himself. They are in German with a print quality far to bad for optical character recognition, so probably useless for most. But there is nothing special in there, and for an experienced engine builder the drawing alone should be sufficient. Got it from here:
https://manualzz.com/doc/4430645/antik-plan-liste---modellbau-and-kopierservice-gerold-kirchert
They also have plans for the smaller Kratmo engines, and a few others.

Wolfgang



Offline Ramon

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Re: Kratmo Castings
« Reply #20 on: December 29, 2019, 04:29:04 PM »
Hello again Wolfgang,

Well, I would agree with your synopsis the wall section does look extremely thin. That said the standard JBW is a very strong product and given that you don't intend to use the engine in an airframe on a continuous basis I still believe it would be worth a try - I certainly would have enough faith in it to try myself. If it's possible it might be better to bond in a solid blank and remachine the complete housing for a slightly smaller diameter, possibly thinner bearing to give a bit of extra strength?

Whatever - good luck with your JB repair  :ThumbsUp: - I strongly recommend the 'standard' over the 'fast' version - the former having much stronger properties and a higher tempreature resistance - the latter, though possibly not of great importance in this case, another factor in it's benefit.


Regards - Tug
"I ain't here for the long time but I am here for a good time"
(a very apt phrase - thanks to a well meaning MEM friend)

Offline Wolfgang

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Re: Kratmo Castings
« Reply #21 on: January 08, 2020, 04:31:12 PM »
Made a start on the crankcase repair. I turned a substantial ring from aluminum, making it a light sliding fit in the crankcase, degreased everything with acetone, roughed the glueing surfaces up with sand paper, and glued the ring in with JB Weld:



Let it dry for two days, centered it in the four jaw, and turned the overbored section to size:



The JBW withstood the cutting forces without problems, and while the repair is not invisible, everything is concentric and up to size now. The remaining clean-up of the opening towards the cylinder will have to wait until both crankcase halves are bolted together and bored out. I will probably support the ring with a close-fitting plug inside the crankcase during that operation.

Many thanks, again, to Tug for the tip!

Wolfgang
« Last Edit: January 08, 2020, 04:53:10 PM by Wolfgang »