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Jabiru Flywheel Bolts [Early models]

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Thruster87

Joined: 16 Apr 2008
Posts: 193
Location: Australia

Posted: Tue Nov 03, 2009 8:09 pm Post subject: Jabiru Flywheel Bolts [Early models]

Contributed by Anton Lawrence

Over the last few of years there has been an increasing awareness of a problem with the flywheel retaining bolts in the early model 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 wouldn’t 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 ½ 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 don’t 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, don’t 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 ½”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 won’t 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

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mhubel

Joined: 05 Sep 2009
Posts: 141

Posted: Wed Nov 04, 2009 6:44 am Post subject: Re: Jabiru Flywheel Bolts [Early models]

Is this going to be the "official" word from Jabiru? If so, will Jabiru be supplying the new bolt types and washers?



Given the use of loctite 620, replacing the bolts every 100 hours seems like a possible problem in itself. Getting these bolts out is a bit scary as a lot of heat must be applied to things. Is it being suggested that once the new bolt types are used, 100 hour replacements would not be needed? Also at this torque level is the use of Loctite 620 still required?



According to the manual, it sounds like removal of the timing gear for "inspection" is a minor engine tear down requiring the removal of the engine from the airframe. This does not sound like a practical 100 hour inspection, it effectively kills the use of this engine type. Is there a way to do it without removing the engine?



It would be good to post detailed pictures of "fretted" gears so those of us who are maintaining only one engine will know what we are looking for. 



Should the gear need to be replaced, it sounds like drilling for the 1/4 inch dowels is effectively a major engine tear down to remove the crank. Am I missing something?



100 hours is not a lot of time to put on an aircraft engine.

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_________________
Mark Hubelbank
N708HU
CH601XL
Jabiru 3300
Rotec TBI 40-3 carb
Sensenich ground adj prop.
240 hr TAF
Pictures at photo.hubbles.com

Stephen Smith

Joined: 02 Jul 2009
Posts: 13
Location: Santa Rosa, CA

Posted: Wed Nov 04, 2009 7:31 am Post subject: Re: Jabiru Flywheel Bolts [Early models]

What is meant by "early" engines?  It would be helpful to have serial numbers.



I don't want to ignore a potential problem but also don't want to go messing with critical bolts that seem to be working just fine.



Steve

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_________________
Steve Smith
N601WF
Zenair Zodiac XL
Jabiru 3300
600+ hours
Sensenich composite - ground adjustable

plus2s

Joined: 30 Nov 2006
Posts: 65
Location: NEW ZEALAND

Posted: Wed Nov 04, 2009 11:18 am Post subject: Jabiru Flywheel Bolts [Early models]

Hi All

  

 This does not come from Jabiru - It was sent out to RAANZ members in New Zealand by their technical officer over two years ago. Don't know why it has resurfaced. Early models I guess refers to engines with 5/16 bolts without the dowels. It is an opinion not a requirement. Check the Jabiru web site for their recommendations. www.jabiru.net.au service bulletin JSB 012-1 - The torque specs provided in the RAANZ bulletin are worth considering when changing the bolts. (Been there - Done that)

  

 Regards

 Keith



--- On Thu, 5/11/09, Stephen Smith <sRoydSmith(at)hotmail.com> wrote:



  	  Quote: 	
		  

From: Stephen Smith <sRoydSmith(at)hotmail.com>

Subject: Re: Jabiru Flywheel Bolts [Early models]

To: jabiruengine-list(at)matronics.com

Received: Thursday, 5 November, 2009, 4:31 AM



 --> JabiruEngine-List message posted by: "Stephen Smith" <sRoydSmith(at)Hotmail.com (sRoydSmith(at)Hotmail.com)>



What is meant by "early" engines?  It would be helpful to have serial numbers.



I don't want to ignore a potential problem but also don't want to go messing with critical bolts that seem to be working just fine.



Steve



--------

Steve Smith

N601WF

Zenair Zodiac XL

Jabiru 3300 

600+ hours

Sensenich composite - ground adjustable





Read this topic online here:



http://forums.matronics.com/viewtopic.php?p=270946#270946m/contribution" target=_blank>http://www.matronics.com/contribution[b]h 



	
  [quote][b]

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Tonyplane(at)bellsouth.ne
Guest

Posted: Wed Nov 04, 2009 1:00 pm Post subject: Jabiru Flywheel Bolts [Early models]

If I recall correctly, the problem with the bolt failures on the NZ aircraft jabJabiru engines was eventually attributed to "bad vibrations" caused by a prop not approved by Jabiru Aus.  Ibelieve I have a report somewhere in my archives and the report should be available, maybe in the Jab archives. I incorporated the Jab recommended retorqueing and loctiting about two years ago and have rechecked the bolts on my last two annuals and will continue to do so with my s/n engine (I think-do not have my log book available) #644. 

Tony Graziano

Zodiac 601XL, Jab 3300; 541 hrs

Sent from my iPhone



On Nov 4, 2009, at 1:17 PM, Keith Pickford <kpickford(at)xtra.co.nz (kpickford(at)xtra.co.nz)> wrote:



[quote]Hi  All

  

 This does not come from Jabiru - It was sent out to RAANZ members in New Zealand by their technical officer over two years ago. Don't know why it has resurfaced. Early models I guess refers to engines with 5/16 bolts without the dowels. It is an opinion not a requirement. Check the Jabiru web site for their recommendations. www.jabiru.net.au service bulletin JSB 012-1 - The torque specs provided in the RAANZ bulletin are worth considering when changing the bolts. (Been there - Done that)

  

 Regards

 Keith



--- On Thu, 5/11/09, Stephen Smith <sRoydSmith(at)hotmail.com (sRoydSmith(at)hotmail.com)> wrote:



  	  Quote: 	
		  

From: Stephen Smith <sRoydSmith(at)hotmail.com (sRoydSmith(at)hotmail.com)>

Subject: Re: Jabiru Flywheel Bolts [Early models]

To: [url=mailto:jabiruengine-list(at)matronics.com]jabiruengine-list(at)matronics.com (jabiruengine-list(at)matronics.com)[/url]

Received: Thursday, 5 November, 2009, 4:31 AM



 --> JabiruEngine-List message posted by: "Stephen Smith" <[url=mailto:sRoydSmith(at)Hotmail.com]sRoydSmith(at)Hotmail.com (sRoydSmith(at)Hotmail.com)[/url]>



What is meant by "early" engines?  It would be helpful to have serial numbers.



I don't want to ignore a potential problem but also don't want to go messing with critical bolts that seem to be working just fine.



Steve



--------

Steve Smith

N601WF

Zenair Zodiac XL

Jabiru 3300 

600+ hours

Sensenich composite - ground adjustable





Read this topic online here:



[url=http://forums.matronics.com/viewtopic.php?p 70946#270946]http://forums.matronics.com/viewtopic.php?p 70946#270946[/url]m/contribution" target=_blank>http://www.matronics.com/contributionh 



[/url] 	
[url=http://forums.matronics.com/]   	  Quote: 	
		  

	


[b]

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selwyn

Joined: 12 Mar 2008
Posts: 102
Location: Australia

Posted: Sat Nov 07, 2009 3:24 am Post subject: Re: Jabiru Flywheel Bolts [Early models]

Likewise, I can't really see where this has come from as a new issue.  The information on what is an "old" engine and the rectification process is all in JSB 012-1 dated 10/06.  If this is followed then I believe the problem has basically gone away.



There a couple of points I would make.:

JSB 012 does not call for any sort of grease on the assembly and given that the joint relies on friction between the parts, why would you risk grease contamination?  The bulletin calls for 24 ft lb (32.5 Nm) so using grease to achieve ultimate screw tension does not seem to be an issue.

and

There is a comment earlier in this thread relating to partial tightening to allow "local relaxation" then tightening to torque up to an hour later. Although this time period is consistent with the Loctite literature my experience with 620 has been that after an hour you will be tightening against the Loctite and not the bolt tension.  This is implied in the revised prop flange attachment procedure in JSB 022-1 which says;

"Fit the flange to the crankshaft & fit the screws by hand, then tension the screws using a calibrated torque wrench immediately"

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_________________
Cheers, Selwyn
Kit 66 VH-ELZ

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