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Questions Sent to Rotorway on December 22, 2003 by Matthew Dock, a registered
professional engineer in the state of Oklahoma.
1. In the first report, a stress concentration factor for fretting
corrosion was given a range between 0.24 and 0.90; however, RotorWay chose
0.8, which is the minimum number to allow the shaft any lifetime.
Therefore, why was 0.80 chosen when a conservative design would require
0.24?
2. The following reference, ASME Handbook, Metals Engineering-Design,
McGraw-Hill Book Company, 1953, also states that for severely corroded parts
that there really isn't an endurance limit, for severely damaged surfaces,
the part will fail at some point in its life. Therefore, why even risk
allowing the part to fret? Why not design the part in such a way that
fretting is avoided?
3. In the second engineering report, concerning "grooves" around the
shaft, I have significant confusion about the resistance to adding fillets.
4. If the shaft were fretted and had an alternating stress of 35Ksi, with
an endurance limit of 30Ksi, the part will fail within about 15 minutes of
crack initiation (the crack initiation would appear at some apparently
random time). However, if the part has a stress of 45Ksi, but an endurance
limit of 65Ksi, the part has infinite life. Which design is better for a
helicopter?
RotorWay was asked for the name of the engineering firm so that Matthew Dock
could communicate with that engineering firm directly. However, RotorWay
would not give any information about the engineering firm, other then to say
that they would pass these questions on to their engineers. Because
RotorWay will not divulge information about their engineers it is impossible for
Matthew Dock to communicate with knowledgeable personnel about this topic.
RotorWay's only response is that "the fretting problem is not apparent in
the current design." However, RotorWay has admitted that
one of their 35mm shafts has already shown fretting!
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