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rotax 912 crankshaft

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ndibiase

Joined: 19 Oct 2008
Posts: 8

Posted: Fri Oct 29, 2010 2:12 pm Post subject: rotax 912 crankshaft

Until recently i believe that rotax 912 crankshaft was bomb proof.



But, after speaking with other owners of this engine, i was warned that there had been some failures on crankshaft for overspeed, early wear etc. 



Because rotax 912 crank is composite (almost incredible for a 4 stroke engine of so high price) i'd like to know from rotax list experience if someone else has experienced crankshaft problems on his 912 engine.



I am specially interested in knowing witch are the symptoms, if failure is sudden or if it is people that has flied for a long time with partial compromised crankshaft etc.



thanks 



Nicola di Biase

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jetboy

Joined: 22 Jul 2006
Posts: 233

Posted: Fri Oct 29, 2010 5:15 pm Post subject: Re: rotax 912 crankshaft

http://www.atsb.gov.au/publications/investigation_reports/2007/AAIR/pdf/aair200700054_001.pdf

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lucien

Joined: 03 Jun 2007
Posts: 721
Location: santa fe, NM

Posted: Sat Oct 30, 2010 7:30 am Post subject: Re: rotax 912 crankshaft

 	  ndibiase wrote: 	
		  

Because rotax 912 crank is composite (almost incredible for a 4 stroke engine of so high price) 	




Not really. The rationale is probably the captive rods, which probably leaves you with the only economical alternative for assembling the crank to be the press-fit multiple pieces. 



I'd bet the advantages of the captive rods outweighed the drawback of the composite crankshaft. Captive rods don't have the extra weight and bulk at the big end that capped rods do, so that's less rotating mass. Also the crankcase can be made smaller because you don't have to enlargen the passages to accomodate the caps, shoulders and studs. All that reduces size and weight, both of the engine itself and the rotating assembly.



That's probably how Rotax has managed to make a 5500rpm all the time motor do so for 2000+ hours.. 



There may be a way to manufacture a one-piece crank with captive rods, but I bet it's not economically feasible.....



I mean, I don't really know what's in the mind of Rotax on this, but this seems to me to be why they do this on their aircraft motors?



LS

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Titan II SS

trigo(at)mail.telepac.pt
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Posted: Sat Oct 30, 2010 7:56 am Post subject: rotax 912 crankshaft

To put it clearly, the word “composite” here doesn’t mean the crankshaft is made of fibreglass, carbon fibre or any other composite material. It is metallic !!  

The word “composite” here means press-fit multiple pieces, not one single piece crankshaft.  

   

(English is indeed a very nice language but also has some wording issues….)  

   

And Lucien explained the reasons why Rotax must have chosen this construction method  

   

Carlos  

   

[quote] --

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rickofudall

Joined: 19 Sep 2009
Posts: 1392
Location: Udall, KS, USA

Posted: Sat Oct 30, 2010 8:06 am Post subject: rotax 912 crankshaft

Lucien, all, No, actually there is no other way to make the crank of the 912. Look at the relationship of the rod journals to the crank throws. The throws are not straight as they are in the VW say, they are offset. This results in a shorter crank, for the same reasons as you noted about the big ends of the rods, less weight. The problem is, how would you go about grinding the rod journal with the crank throw covering about a third of the journal for about half of its area? 

Rick Girard

On Sat, Oct 30, 2010 at 10:30 AM, lucien <lstavenhagen(at)hotmail.com (lstavenhagen(at)hotmail.com)> wrote:

 	  Quote: 	
		   --> RotaxEngines-List message posted by: "lucien" <lstavenhagen(at)hotmail.com (lstavenhagen(at)hotmail.com)>

 ndibiase wrote:

 >

 > Because rotax 912 crank is composite (almost incredible for a 4 stroke engine of so high price)

 Not really. The rational is probably the captive rods, which probably leaves you with the only economical alternative for assembling the crank to be the press-fit multiple pieces.

 I'd bet the advantages of the captive rods outweighed the drawback of the composite crankshaft. Captive rods don't have the extra weight and bulk at the big end that capped rods do, so that's less rotating mass. Also the crankcase can be made smaller because you don't have to enlargen the passages to accomodate the caps, shoulders and studs. All that reduces size and weight, both of the engine itself and the rotating assembly.

 That's probably how Rotax has managed to make a 5500rpm all the time motor do so for 2000+ hours.. 

 There may be a way to manufacture a one-piece crank with captive rods, but I bet it's not economically feasible.....

 LS

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