CASA - Draft Proposal for Jabiru Aircraft

[ianboag]

Wasn't the 230 flown over there a number of years ago? Thought the starfish thing is a new Camit improvement so doubt it was the same.

I understood Jabiru have had a starfish setup for a while .... I could be wrong.

 

 

[Guest Ornis]

How would a very thick high tensile washer under the bolt heads be? Locktite would exclude the oil getting between the two surfaces . Wouldn't greasing the bolt threads get into the joint?

When we found out about the flywheel bolts breaking we replaced the aluminium vacuum drive with a steel spacer and fitted a thick steel spreader under the bolt heads. The AD requiring dowels applied only if the engine was removed. (The engine subsequently had a top end overhaul and went well until a valve broke.)

 

Grease is standard engineering practice, obviously just a little at the start of the thread. Loctite 620 means the bolts can't be retorqued.

 

The "secret" to a clamp joint is flat hard steel surfaces and stretching the bolts properly.

 

 

[dazza 38]

I understood Jabiru have had a starfish setup for a while .... I could be wrong.

Yup a chocolate star fish.

 

 

[jetjr]

Man this place is hard to get a correct point accross

 

Downunder no one said it was OK, but a reasonable upgrade. The real issue still needs work. Sticking valve guides has been a problem in engines since the first ones and still exists in the latest. It isnt an easy to fix. Until someone does the new pistons are a good thing.

 

I honestly doubt Jabiru could do anything to please many on here.

 

 

[facthunter]

The recessing is a good precaution. Valve guide gumming can be caused by some of the anti scuffing additives in some oils. When you remove the valve the guide has a fairly soft coating that reams out easily leaving the original guide bore untouched. This is different from high wear rates where the guide metal may be a bit soft and/or the valve to rocker geometry is incorrect . Nev

 

 

[Keenaviator]

Wasn't the 230 flown over there a number of years ago? Thought the starfish thing is a new Camit improvement so doubt it was the same.

I'm pretty sure that when Jab did this mod they called it a 'spider' fitting - made out of steel and used in conjuction with 3 dowels and 5/16" screws. And it was a few years ago.

 

 

[bexrbetter]

Your post only muddies the water. Bex Nev

 

Bex, just more repeated old info,

 

All of the issues you have mentioned .

... relax guys, was in context directly to the person's post I replied too.

 

You mention a large number of alleged "FAULTS" none too specifically,

"Alleged" my bum, this forum alone has every single one of those faults posted from owners over the years and this ain't the only source of them out there.

 

Even the largest Radials have allowable cracks and fin loss on heads and cylinders.

It's 2015, the cracks are not acceptable in the modern market.

 

Cracks, oil leaks, bitches to start or tune, i.e. "Character", is not something to be romanced about anymore, it is a completely different demographic today.

 

. But I find whenever modern composite aircraft are mentioned, there is always someone who would like to see them out of RAA. I really dont understand it.

No cracks, oil leaks, bitches to start or tune, i.e. "No Character", not a real plane.

 

 

[fly_tornado]

http://www.flightglobal.com/news/articles/casa-accused-of-quotmisconductquot-by-australia39s-light-aircraft-406591/

[ozbear]

... relax guys, was in context directly to the person's post I replied too.

 

 

"Alleged" my bum, this forum alone has every single one of those faults posted from owners over the years and this ain't the only source of them out there.

 

It's 2015, the cracks are not acceptable in the modern market.

 

Cracks, oil leaks, bitches to start or tune, i.e. "Character", is not something to be romanced about anymore, it is a completely different demographic today.

 

No cracks, oil leaks, bitches to start or tune, i.e. "No Character", not a real plane.

Are you guys talking about a old Harley or my ex wife ?

 

 

[ozbear]

Dowels schmowels trowels ... friction is what holds a joint together. Think of it as a zillion tiny dowels. Dowels can be handy for location but not much else.

Dowels take some of the torque off the bolts they are used in many high performance applications but properly harmonically balanced engines are probably the answer here it's not even driving a clutch /gearbox/tail shaft/diff or the vehicle.Making something bigger heavier and stronger isn't always the answer.

 

 

[Guest Ornis]

Dowels take some of the torque off the bolts they are used in many high performance applications but properly harmonically balanced engines are probably the answer here it's not even driving a clutch /gearbox/tail shaft/diff or the vehicle. Making something bigger heavier and stronger isn't always the answer.

What would happen if the joint had 9 dowels only instead of 3 of 14" dowels and 6 of 3/8" SHCSs?

 

If the answer is "fail", what are the bolts doing? Providing clamp.

 

What happens to bolts to provide clamp? They stretch.

 

Why do bolts stretch? Because the nuts are torqued up.

 

Jabiru's 24 ft-lb for 3/8" Unbrako SHCSs is not enough. Needs to be 50 N-m (37 ft-lb) or more.

 

Is that making the joint "stronger"? Yes. Is it the answer? Yes, almost certainly.

 

 

[jetjr]

Not disagreeing, but Jabiru arent fools

 

Why would they not agree to such a simple modification?

 

 

[Camel]

Jabiru Logic. Valve rockers poor geometry, valve guides wear out, valves jam, valve hits piston, valve breaks hits piston and engine goes bang.

 

Jabiru fix, machine piston to clear stuck valves.

 

There are many more of these sonarios I am aware of.

 

 

[ozbear]

What would happen if the joint had 9 dowels only instead of 3 of 14" dowels and 6 of 3/8" SHCSs?If the answer is "fail", what are the bolts doing? Providing clamp.

 

What happens to bolts to provide clamp? They stretch.

 

Why do bolts stretch? Because the nuts are torqued up.

 

Jabiru's 24 ft-lb for 3/8" Unbrako SHCSs is not enough. Needs to be 50 N-m (37 ft-lb) or more.

 

Is that making the joint "stronger"? Yes. Is it the answer? Yes, almost certainly.

So what forces are acting on the flywheel to need all of that it's not driving anything only the engine when the starter is engaged is my point yes more torque on the bolts might be the answer I'm not arguing just asking

 

 

[ianboag]

So what forces are acting on the flywheel to need all of that it's not driving anything only the engine when the starter is engaged is my point yes more torque on the bolts might be the answer I'm not arguing just asking

It's getting bashed back and forth (in a torsional sense) by the pulsating load that a piston engine puts on anything attached to its crankshaft. If the joint is not sufficiently well clamped, then there will be (a tiny amount) of back and forth torsional movement which will eventually snap a bolt or two.

 

If there is enough clamp then the flywheel and crank act as a single unit. If you remove the flywheel and you can see thread marks in the bolt holes, this is not a good sign ......

 

 

[ozbear]

It's getting bashed back and forth (in a torsional sense) by the pulsating load that a piston engine puts on anything attached to its crankshaft. If the joint is not sufficiently well clamped, then there will be (a tiny amount) of back and forth torsional movement which will eventually snap a bolt or two.If there is enough clamp then the flywheel and crank act as a single unit. If you remove the flywheel and you can see thread marks in the bolt holes, this is not a good sign ......

You've hit the nail on the head Ian maybe a flywheel incorporating a harmonic balancer might be the answer

 

 

[ianboag]

You've hit the nail on the head Ian maybe a flywheel incorporating a harmonic balancer might be the answer

Who knows? OTOH if there's room to reef up the torque some more (and use proper lubrication on the threads) that might just sort it. This problem does not happen after 10 hours and not every engine does it. So it would appear that the present setup is close to being OK. "More clamp" would seem to be a sensible place to start ......

 

 

[facthunter]

Once the joint flywheel/crankshaft, starts to move the bolts are finished. It's only a matter of a short time It's relying on friction between the mating faces dependent on clamping force. The problem is there is another bigger flywheel on the other end of the crank (the propeller) and the operation of the crank is not a smooth even rotational speed, because of the power pulses but probably more from the fact that the reciprocating parts of the motor which provide intermittent forces due their inertia (which is considerable) that go from a max to a minimum twice per revolution The stress on the joint has nothing to do with the fact it isn't transmitting any real torque to a drive. Nev

 

 

[gandalph]

" Torque is a very indirect indication of tension and there are many factors such as friction, lubrication, surface texture, rust, material type, ...thread conditions, debris etc that affect the outcome.

 

The only way to determine the correct torque, especially for critical joints, is through experimentation under actual joint and assembly conditions using a calibrated torque wrench and a Skidmorer-Wilhelm type load indicating device to equate actual torque to desired tension.

 

Most of the torque/tension tables which haved been developed over the years have been calculated using the formula

 

T= K x d x P

 

where t = torque

 

K = Coefficient of friction factor

 

d = Nominal thread diameter (inches)

 

P = Tension (clamp load or pre load) induced in fastener (lbs)

 

The value of "K" can range from 0.30 for a rusted assembly to 0.10 or less for a clean well lubricated assembly using a proven proprietary lubricant. It should be noted that the accuracy of the 'K' factor is subject to many application variables eg. The industry accepted 'K' factor for plain fasteners is 0.20, however this figure reduces to 0.10 should the fastener be well lubricated and as a result an entirely different torque figure would result.

 

.....

 

The most important point to remember when using a torque wrench is to make sure the instrument has been stored properly and has been calibrated by a recognized roque analyzer. In an experienced operators hand, tension control can be better than +/- 25% accuracy is possible using a good quality hand operated torque wrench. When used in conjunction with a Skidmorer-Wilhelm type device, an accuracy of +/- 5% is possible."

 

 

(abstracted from "The Fastener Black Book 1st edition p.78)

 

N.B the bold italics were added by me for emphasis.

 

My reading of the above would suggest that an experienced operator using a quality, well maintained & calibrated torque wrench could, when torquing a bolt to an indicated 26ft/lbs could achieve anywhere from 19.5ft/lbs to 34.6ft/lbs.

 

It follows that torquing to an indicated 36ft/lbs could result in an actual torque value between 27 and 48ft/lbs.

 

The book referenced above gives the following tightening torque figures for 3/8 unc grade 8 bolts:

 

dry: 44ft/lbs;

 

lightly lubricated: 35ft/lbs;

 

Well lubricated: 22ft/lbs

 

Without wanting to sound like the Jabiru service department, there could well be en element of operator " error" involved in the way we assemble components.

 

 

[facthunter]

You would get a more consistent result using an angle. Torque to say 10 and then follow up with a prescribed angle corresponding to the bolt stretch you would need. I do a lot of crankshafts with tapers. You don't oil them but they are not dry either as dry might pick up during assembly, depending on the metal(s) When new a torque will be specified but if distortion happens as in older used parts a fair bit of experience/judgement needs to be applied Nev..

 

 

[ianboag]

" In an experienced operators hand, tension control can be better than +/- 25% accuracy is possible using a good quality hand operated torque wrench. When used in conjunction with a Skidmorer-Wilhelm type device, an accuracy of +/- 5% is possible."

ARP make through bolts etc for a living. Zillions of them - all sizes and shapes. They go into high performance engines. ARP reckon that if you use their "ultralube" you get random variation that looks like under 5%. They compare results for other lubes as well. Their claims appear to be based on a simply ginormous heap of testing. They don't just say it.

 

It's all there - summaries, graphs, comparisons in http://arp-bolts.com/p/arpultratorque.php. These folk appear to have a seriously good knowledge of all this.

 

FWIW in the early days of the (ongoing) through-bolt hooha Doug and Sue were pointed at all this. We took their subsequent silence as disdain.

 

Go figure.

 

 

[turboplanner]

I agree FH, I do much the same, and because I work a lot in aluminium threaded casings, components, have developed a sixth sense to minimise thread stripping and at the same time optimising clamp.

 

Based on the details posted by gandalph, sure there's likely to be a considerable variation depending on the thread cutting, the person who prepares the bolt and thread for assembly, and the tightening technique used.

 

However, if someone has been tightening flywheel bolts for 40 years, by feel, and bolts are snapping on one make and model, that's fairly compelling (not say that's the case with the Jab bolts because the reports have come from different people who haven't talked about their previous history/success with other flywheel bolts/other flywheel assemblies.

 

These variables are one of the reasons a manufacturer tends to overdesign bolts in critical areas.

 

 

[Downunder]

I struggle to believe a modern torque wrench would be more that 5% out of calibration.

 

Far more important are the bolt, thread and fitting conditions (relative to tension)......People fitting a flywheel should have some mechanical knowledge of this I would assume.

 

I did see a chart along time ago with about 6 lubricants and the effects of torque relative to tension viewed as a percentage.

 

As in percentage of increased tension over "dry" for the same torque.

 

Quite variable results with different lubricants. I've never been able to find it since...

 

Off the top of my head! Would studs be an improvement over bolts on the flywheel?

 

Less likely to get lube in the joint?

 

 

[facthunter]

I can't see studs as offering any improvement. There is a limit to how many holes you can put in a small dia shaft and torque them up without distortion, or weakening of the shaft. (Like a telephone dial)

 

Torque wrenches should be recalibrated regularly. I do it any time some thing is critical.

 

There would be a big variation with lubricants. You should specify a new torque for using one.

 

If you torque a lot of bolts you can FEEL when they have gone past the elastic limit indicating already stretched too much previously?? use NEW ones each time in critical applications. Nev.

 

 

[sandman]

Rotax never have this problem, even the little 582 is unreal.