Weak bolts

[old man emu]

I believe that Nylocs are a no-no forward of the firewall. If you want to use locking nuts, go full metal. 

 

An M8 bolt should have an 8mm diameter, a 5/16 (dash 5) imperial bolt has a diameter of 7.9375 mm. That's about a poofteenth's difference. If you use a good old AN5- bolt with MS21045-08 nuts. Or you could go the close tolerance route an use AN175-5 close tolerance bolts.

[BrendAn]

1 minute ago, old man emu said:

I believe that Nylocs are a no-no forward of the firewall. If you want to use locking nuts, go full metal. 

 

An M8 bolt should have an 8mm diameter, a 5/16 (dash 5) imperial bolt has a diameter of 7.9375 mm. That's about a poofteenth's difference. If you use a good old AN5- bolt with MS21045-08 nuts. Or you could go the close tolerance route an use AN175-5 close tolerance bolts.

i am sure they would not supply nylocs for hub kits if there was a problem.

[BrendAn]

[Thruster88]

For anyone not familiar with the Rotax prop flange it has metric M8 threads.

 

AN hardware including metal loc nuts CAN NOT BE USED.

[turboplanner]

9 hours ago, BrendAn said:

Can U explain please.

seat should have read "sea".

[turboplanner]

A Self Administering Organisation doesn't mean the people deriving income out of it can just flick all safety issues to their "customers", the pilots AND,  in RA, the maintenance training.

For a start doing that means the participants are not going to know what they don't know.

Apart from an in-flight prop failure, if someone is running an engine up airside and the prop flys off and into a spectator behind the fence and kills him or her, someone had the duty of care to prevent that. The lawyers would go after the person who bolted it on, but they would also be looking at why he wasn't trained and who had the duty of care to do that, and who owned the organization that had the duty of care to oversee that.

This is bigger than just a person shering a bolt.

 

[Bruce Tuncks]

Those bolts marked "8.8" are a metric bolt and pretty good too. I don't think Bolly would provide crook ones. In general, the higher the number, the better the metric bolt is.

I'm mystified by the references to nyloc nuts though. I've never heard of one coming loose, but I agree they are not used on Jabiru props. Personally, I would use some low-strength loctite and adjust the torque to suit the changed friction when tightening. If you use cotter pins and castellated nuts, you have to live with the thought that these items do nothing until the bolt has come loose a bit.

[old man emu]

7 hours ago, Thruster88 said:

For anyone not familiar with the Rotax prop flange it has metric M8 threads.

 

AN hardware including metal loc nuts CAN NOT BE USED.

I can see that prohibition being associated with the certification of the components for use in aircraft. Fair enough. But if someone wanted to go down a long an dank passage, they could get an Engineering Order to us Imperial-sized hardware. At least that way owners could get hold of hardware that had certification of quality and traceability.

 

As for differences in raw materials, a bit of metal rod of known tensile and shear strength doesn't know if it is going to be machined to make something to metric or imperial specifications.

 

Be careful when using the descriptor "AN" when ordering hardware. There are several other descriptors for aerospace hardware. These other ones post-date the AN descriptors in most cases simply for commercial reasons, nothing to do with the specifications of the item. https://www.flywithspa.com/docs/pbm/toc453317738.html

 

Certified General Aviation aircraft many times uses the old part number as that is what the aircraft was certified with, and experimental aircraft typically follows the old numbers because the certified ones do.  Various hardware supply companies will ship the hardware with different numbers from what you order on the website.  Aircraft Spruce will list AN365 in the catalogue, but ship it in bags labelled MS21044. You can't tell your AN from your MS.

[BrendAn]

5 hours ago, turboplanner said:

seat should have read "sea".

ok, now i get it.  i  did some more reading last night and realised my hub extension is not right anyway.  it fits over the rotax lugs but it does not have lugs on the prop side. so the the prop is side loading the bolts when the lugs are supposed to be taking that load. 

[old man emu]

22 minutes ago, Bruce Tuncks said:

If you use cotter pins and castellated nuts, you have to live with the thought that these items do nothing until the bolt has come loose a bit.

That is quite correct.

 

If you want to go the "belts and braces" approach, why not use an ANH7-xx  with one NAS509-8 nut with AN970-8 washers, then safety wire the heads and nuts?

[facthunter]

Yes it shouldn't drive on the bolts at all. There should be bosses that take the sheer loads. With a wooden hub there's a low limit to the clamping force. Lock nuts are another issue. The lock nut can equalise the torque of the bolt and take the load. Nev

[walrus]

What does the propeller manual say?

 

As far as regulations are concerned you require"approved data".

 

That is a hierarchy starting with manufacturers instructions followed by engine manual, airframe manufacturer, POH, etc. on downwards to AC43-13b.

 

Internet website comments don't necessarily have any credibility.

 

Having said that, my two cents worth: DO NOT over specify bolt strength because at the high strength end you may be sacrificing fatigue resistance for unnecessary tensile strength. That means your bolts may fail be cracking.

 

My understanding from the rotax owners website is that a grade 8.8 metric bolt is fine.

[onetrack]

Standard fasteners have a wide variability in shank dimensions, and should never be used to take the driving forces on their shanks, because of this feature.

That is why the prop drive forces MUST be taken by snug-fitting lugs - the bolts are only there to provide clamping force for the hub.

 

Bruces suggestion of using some low to moderate strength Loctite seems like a sensible suggestion to me. Nyloc nuts only hold over a small length of thread.

[facthunter]

Nyloc nuts are for Victa Lawnmower blades. The drive hub of a prop can get quite hot. Nev

[old man emu]

1 hour ago, walrus said:

What does the propeller manual say? As far as regulations are concerned you require "approved data". That is a hierarchy starting with manufacturers instructions followed by engine manual, airframe manufacturer, POH, etc. on downwards to AC43-13b.

 

No argument there. But it's good on internet forums to go wandering down the "Why is it so?" path. If BrenDan is doing work on his own aircraft, then wouldn't it be nice for him to have some background understanding of the hardware available to do the job it is doing?

[onetrack]

The regular Loctite thread retaining compounds will withstand stand a working temperature of 150°C, and you can purchase high temperature Loctite retaining compounds, if you need to.

[facthunter]

The only way to "release " them is to apply more heat  than I'd like to. Nev

[old man emu]

3 minutes ago, facthunter said:

The drive hub of a prop can get quite hot.

Stands to reason. The bolts are pulling the prop boss and hub together by creating forces acting in a longitudinal direction. At the same time there is that transverse force created by the rotation of the propellers matter. Over the length of time that the engine is running, per flight, minor variations in the speed of rotation can cause microscopic amounts of movement between the propeller boss and the hub. That little bit of "rubbing" creates the heat that facthunter has mentioned.

[facthunter]

To be fair the gearbox may not run as hot as the end of an engine crankshaft does but that would need confirming. Heat shrinks a lot of wooden prop hubs  (removes water) and they need retensioning. as a result. Nev

[Bruce Tuncks]

I've seen a Jabiru prop hub that has been made black by the relative movement between the prop and the driving flange....  and this was on a prop with driving bushes .   There was a letter from Jabiru where this movement could have been responsible for the trouble with fracturing flywheel bolts.

A hammering at one end of the crankshaft was transmitted to the other end.

On reading this, I used epoxy to take out any play in the driving bushes and also used a trace-glue ( nail polish ) to show if there was any movement going on...  yes, I did need to use a bit of heat to remove the bushes later, but this was not much.

You can't tell by turning on the ground if there is any play...  so I recommend the nail polish between the driving flange and the prop, and only use epoxy if you have to.

[BrendAn]

10 minutes ago, Bruce Tuncks said:

I've seen a Jabiru prop hub that has been made black by the relative movement between the prop and the driving flange....  and this was on a prop with driving bushes .   There was a letter from Jabiru where this movement could have been responsible for the trouble with fracturing flywheel bolts.

A hammering at one end of the crankshaft was transmitted to the other end.

On reading this, I used epoxy to take out any play in the driving bushes and also used a trace-glue ( nail polish ) to show if there was any movement going on...  yes, I did need to use a bit of heat to remove the bushes later, but this was not much.

You can't tell by turning on the ground if there is any play...  so I recommend the nail polish between the driving flange and the prop, and only use epoxy if you have to.

rotax lugs are pressed into the hub. 

[Old Koreelah]

3 hours ago, old man emu said:

Over the length of time that the engine is running, per flight, minor variations in the speed of rotation can cause microscopic amounts of movement between the propeller boss and the hub. That little bit of "rubbing" creates the heat that facthunter has mentioned.

Another factor I hadn’t considered is that direct-drive props, like Jabs, would cop much more conducted heat thru the crankshaft than via a gearbox.

[facthunter]

I don't know. Gears absorb POWER and the damper clutch handles a lot of energy. That aspect needs to be assessed.  The clutch doesn't slip in the sense of slowing the prop.

   Re the Jabiru. You shouldn't have a flywheel at one end and another (the prop) at the other end. They fight each other when that's done. BAD design in a way. One of the reasons for a wooden prop  and more than 4 cylinders.  The pistons act as flywheels. but badly in a four, (flat or in line . Vees are better. Nev

[Thruster88]

6 hours ago, old man emu said:

That is quite correct.

 

If you want to go the "belts and braces" approach, why not use an ANH7-xx  with one NAS509-8 nut with AN970-8 washers, then safety wire the heads and nuts?

Because the prop flange on a rotax has M8 METRIC, YES METRIC THREADS IN THE FLANGE. do keep up.

[BrendAn]

21 minutes ago, Thruster88 said:

Because the prop flange on a rotax has M8 METRIC, YES METRIC THREADS IN THE FLANGE. do keep up.

i would hate to see what the threads look like after forcing 5/16 bolts through them.