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0. 12:33 AM - November List Fund Raiser (Matt Dralle)
1. 08:10 PM - Jabiru Flywheel Bolts Important Notice (Thruster87)
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Subject: | November List Fund Raiser |
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Subject: | Jabiru Flywheel Bolts Important Notice |
Contributed by Anton Lawrence
Over the last couple of years there has been an increasing awareness of a problem
with the flywheel retaining bolts in the Jabiru 2200 and 3300 engine.
The problem is simple enough to identify, the bolts are breaking and have caused
in-flight engine failure. The cause has been much harder to identify and has
been blamed on loose prop and prop extension bolts.
There can be no question that loose bolts in this area will transfer harmonic vibrations
down the crank and precipitate movement of the flywheel parts.
--------------------------------------------------------------------------------
Examination of the timing gear on engines with broken bolts has identified severe
fretting of the gear against the end of the crank, this cyclic movement is
the reason the bolts are breaking. The retaining bolts are 5/16th socket cap screws
property class 12.9, although some of these are threaded full length, which
wouldnt normally be considered best practice, all the bolts are fracturing
on the shear line.
12.9 bolts have an ultimate tensile strength of 1220 MPa and yield strength of
1100 MPa, it is normal to tighten these to within 90% of yield; this is to ensure
proper clamp pressure of the parts.
This would cause the Jabiru bolts to elongate by about 5 thou and is the mechanism
by which clamp pressure is maintained.
A company in Hamilton, Asseco has analysed this joint and come up with a torque
figure of 41 Nm for these bolts with lubricated threads and washer face. I have
used the formula from MIL-HDBK-60 and come up with a figure of 43 Nm using
the same lubrication and 56 Nm with no lubrication.
The Jabiru manual states a figure of 24 Nm for these bolts with out lubrication,
it is my belief that this where the problem partly lies. (Jabiru apparently
now recommend 32 Nm but this is still almost the maximum preload the bolts can
take.)
Contributed by Anton Lawrence
The CAA is working on an AD for these engines which will most likely require the
bolts to be replaced every 100hrs. It is very important that as part of the
replacement procedure the timing gear is removed and inspected, if there is any
sign of fretting the part should be replaced and equally important is that the
bolts are replaced irrespective of their appearance.
Jabiru are now fitting three 1/4 inch dowels into the end of the crank in an attempt
to prevent this fretting, any new gear will have holes for these dowels
pre drilled. The fitting of the dowels into the crank is a very precise job and
should not be attempted by anyone other than a qualified fitter, engineer or
toolmaker. Asseco is one company which has already completed ten of these dowel
fitting operations, I dont want this article to appear to be advert for one
company or an other, so if you want their contact details you can contact me
directly and I will pass them on to you.
Jabiru
In conclusion, if you have an unmodified Jabiru engine of either type in any aircraft
type you should immediately have the flywheel bolts and timing gear inspected,
if all looks OK you should replace the bolts (reminder, 12.9 bolts should
never be reused) and tighten them to between 41Nm to 43Nm using molybdenum
grease as a lubricant in the threads and under the head, dont get any grease
on the bearing surfaces. If you use Loctite 620 in the threads and grease under
the head you should tighten to 46Nm, Loctite 620 and no grease tighten to 53Nm.
If you are going to tighten these bolts to the above figures it is vital the bolt
has a diameter minimum hardened washer under the head, if not the bolt will
embed into the alloy parts and preload will be lost.
Check the prop bolts and prop extension bolts for correct tightening and also check
to ensure the extension is running true and the tracking is within 3mm, on
some aircraft these have been found to be well off centre. If you have 10mm
prop flange extension cap screws installed with Loctite 620, you can take them
to 89Nm to reach 90% of yield.
I have also checked the torque for the Crankshaft Prop Flange Cap Screws as these
have also been found slightly loose, Jabiru recommends 40Nm but these bolts
are capable of taking 83Nm.
If all these bolts are tightened correctly and all the parts are running true it
should be possible to eliminate the problem of broken bolts.
When tightening bolts you should tighten to halve the required amount following
the tightening sequence, leave for an hour (no longer if using Loctite 620) to
allow for local relaxation and then complete the tightening in one movement
so as not to get stuck with static friction at a lower level, I have calculated
the turn of the nut (5/16 cap screw) from snug tight to 43Nm be only 48deg so
take care.
Loctite
Some notes on Loctite :
Jabiru has apparently changed their recommendation of Loctite 262 to 620 for the above parts. 620 is not specified as a thread lock product but Loctite assure me it is good for the job as it has a longer time before cure and higher temperature capabilities, you can download all the data sheets from http://www.loctite.co.nz.
Unfortunately you wont find the friction coefficients or nut factors on these sheets,
I have had to dig deeper to get these directly from Loctite.
--------------------------------------------------------------------------------
Anton Lawrence, RAANZ Tech Officer
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=270894#270894
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