[Fot] Gearbox woes

[Norlin Engineering]

Thanks to all that responded.  The driveshaft is at the specialists being
gone through and rebalanced.  I'm having a Spitfire housing bored out to
accept the larger GT6 bearing.  I'm also working on the design and
fabrication of an external support bracket that will hopefully reduce some
of the load on the casting.

I am a bit concerned that I may be approaching a natural frequency on the
driveshaft.  The driveshaft specialist measured the length and diameter and
said by his "rule of thumb" table, it should be good for 8500 rpm.  However
an engineer buddy of mine ran some numbers out and he thinks there may be a
problem starting at around 7450 rpm.  Even though I shift at 6800, I do hit
close to 7500 in 4th gear when we run Portland without the chicane.

What kind of maximum rpm are you Spitfire drivers taking the stock
driveshaft to in 4th gear?  Also, has anyone made up a custom shaft to help
resolve this problem?

Thanks

Jim Norlin

-----Original Message-----
From: fot-bounces at autox.team.net [mailto:fot-bounces at autox.team.net] On
Behalf Of norlinengineering at comcast.net
Sent: Sunday, April 19, 2009 9:30 AM
To: Fot at autox.team.net
Subject: [Fot] Gearbox woes


Well, my "woohoo" was premature.B  We broke a tailshaft housing on the GT6
gearbox in our Spitfire.B  This happened last year as well.B  In the past, I
remember tracing this to the nut on the output shaft working loose, but I'm
pretty sure that isn't the case here.B  I will get the driveshaft rebalanced
when I replace the housing.B  Has anyone else had this problem and can you
recommend a permanent fix?B  Has anyone tried to strengthen these housings
or add an intermediate mount or??



Thanks in advance.



Jim Norlin
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