RotorWay's Sport Helicopter Article

Red lettering is actual RotorWay comments from their article


Most RotorWay owners have probably received Issue 46 of Sport Helicopter, in which, RotorWay discusses "The RotorWay Secondary Shaft System."    In that article RotorWay says "the stresses that are acting on the secondary shaft are below the allowable endurance strength level of the shaft, therefore no fatigue problem is anticipated."  Again, the assumption is that fretting corrosion only reduces the endurance limit of steel to 80% of its original value, which is inconsistent with engineering texts that state that fretting corrosion can reduce endurance strength to between 24% and 90% of its original value

What evidence does RotorWay have that the fatigue strength will only be reduced to 80%?

RotorWay also says that "On the 30mm shafts that did fail fretting corrosion was seen underneath the bearing or bearings on the shafts."  Ok, RotorWay accepts fretting corrosion as the cause of the 30mm shaft failures.  However, changing the shaft size from 30mm to 35mm will only reduce the relative sliding motion between the bearing and the shaft, only one factor in fretting.  There will still be high bending stress at the point of bearing-shaft contact, therefore, fretting will still occur. 

It may take a little longer for the endurance limit to drop to the point of failure, but if other factors such as shrink fit, environment, or contamination come in to play, the shaft will fail.  RotorWay stands behind their statement that they have 3878 hours on nine shafts at the factory.  However, Tom Smith has told Matthew Dock that one of the shafts did show fretting corrosion under the shrunk fit sleeve, a point of failure for a number of shafts.  Remember that this is in Arizona, perhaps the least corrosive environment available.

RotorWay's own ship has had fretting corrosion on a 35 mm shaft, something not talked about in their recent article.






John Spurling, Pres.
Route 3, Box 12 C
Cleveland, OK 74020, USA
Tel. (918) 243-7635
Fax: (918) 243-7882