AIRCRAFT FASTENERS - WHAT IS ACCEPTABLE??

[ClearlyAfrica1]

Hi All,

Just replaced my propeller hub (last one got damaged somehow in the blade refurbishment process) -- I have a composite 3-blade prop -- and wanted to ask a few questions of those better qualified and experienced than I:

1. Does everyone agree that AN-type bolts and nuts should be used in this department, or not? Can a standard, non-AN, galvanised 8.8 bolt be matched with a simple (non-AN) nylock nut one can obtain from the hardware?

2. Are standard 8.8, galvanised, bolts acceptable (for fastening the hub to the gearbox output flange)?

3. Are 12.9 bolts acceptable, also galvanised, on any part of the propeller? In this case, these are the small ones holding the propeller blade root into the hub. Remember also that significant clamping force onto the blades is also provided by the M8 bolts holding the hub to the gearbox (as far as I understand).

The manufacturer informed me that the bolts supplied are perfectly acceptable, but I do receive objections from various other people in the field -- your input would be most welcome.

Thanks!

[Jean Crous]

Ok let us try and give a practical answer. Not all 8.8 bolts are equal. Get en engineering grade 8.8 bolt. with a rolled thread, not a cut thread. Reason being a cut thread removes material from the bolt`s structure, where a rolled thread displaces material to create the thread.
Remember that any "metallic coating" put on the bolt has the potential to weaken the bolt. Therefore use cadmium plated bolts, as these have been proven in use. Do NOT use 12.9 bolts, they are too hard and will succumb to fracture because of vibration and harmonics.
Also bear in mind the "grip length" of the bolt.....the shank. A bolt must NOT be in shear on it`s thread, this is where it will break, where the shank and the thread meet. Rather use an extra washer or two to take up the extra length. When a bolt is used on aluminium always have a washer under the head and the nut. Have at least 3 threads protrude past the nylon locking ring in the nut.
Last but not least, always tighten the nut, not the bolt...iaw... hold the head still and tighten the nut to the desired torque for the application the bolt is used in.
Dont take what I say as gospel, rather take it as safe practice
Jean.

[ClearlyAfrica1]

Hi Jean,
Thanks for your reply, much appreciated indeed.
On your last point, I cannot insert the bolts from the gearbox side (save taking the gearbox apart to allow for this) and so I am forced to insert them through the propeller hub and tighten the bolt with the torque wrench and hold the nut still with a spanner!
Steve