Paul A. Franz, P.E.
Joined: 02 Dec 2008
Posts: 280
Location: Bellevue WA
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 Posted: Thu Jan 01, 2009 11:19 pm Post subject: broken tailwheel suspension spring |
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On Thu, January 1, 2009 9:34 pm, gary.algate(at)sandvik.com wrote:
| Quote: | Noel
I think my spring was poorly designed - It has two leaves but the upper
(shorter one) was finished with a pointed end, as the lower leaf flexed at
this point it raised considerable stress leading to the failure.
I think I would check any springs out there and if it is finished like
mine I would grind the point off and leave it flat with rounded corners.
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Since that would effectively shorten the upper spring that would increase the bending
moment on the lower one and further shorten its life.
It would appear to me that the lower leaf needs to flex less. That would mean you need
to shorten it or replace it with a stronger leaf. If it is too stiff now, use a
thinner upper leaf to distribute the stress concentration at the fulcrum (tip of the
upper spring). Lengthening the taper rather than shortening it would be better - just
the opposite of what you're suggesting, provided I am understanding your description.
| Quote: | I did not see an signs of cracking prior to the failure as it propagated
from below the stress riser and at the bottom of the spring.
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That's the nature of fatique failure in hardened steel alloys.
Another geometry change you might consider is using a much thinner upper leaf and
lengthening it. This will reduce the maximum bending stress for the same effective
spring rate.
--
Paul A. Franz
Registration/Aircraft - N14UW/Merlin GT
Engine/Prop - Rotax 914/NSI CAP
Bellevue WA
425.241.1618 Cell
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_________________
Paul A. Franz, P.E.
Registration/Aircraft - N14UW/Merlin GT
Engine/Prop - Rotax 914/NSI CAP
Bellevue WA
425.241.1618 Cell
425.440.9505 Office |
|
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