High Quality Metric Fasteners

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Flow

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Here is an over view. I think there is a bit of confusion around the difference between class and grade. Basically you spec your bolt to elastically stretch the required amount for the torque enabled by the work and then ensure your nut has a proof load greater or equal to the bolt. ie don't use a stainless class 70 with an 8.8 bolt.

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User27

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All good, but there are basically 2 types of metric hardware that are usually specified for aeroplanes DIN & LN. For nuts LN 9025 is much superior (and substantially more expensive) than the DIN equivalent. Don't worry about the Airbus part numbers, they are corporate specific numbers for standard hardware.
 

Flow

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LN 9025 is a washer, no? Does anyone have a table that shows us what all of these codes actually translate to?
 

Matt G.

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Edit:
Nevermind...other posts that showed up when I refreshed the page answered it better.
 

User27

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LN 9025 is a washer, no? Does anyone have a table that shows us what all of these codes actually translate to?
Yes, my apologies, nyloc nuts are LN9338. Typical price in Europe for an M6 nyloc is >$1 each
DIN 985 are cheaper
 

Flow

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OK now I am looking for some M8 x 40 Class 8.8 cap screws for the prop spacer. Does anyone know of an Airbus or Diamond part number for something like this?
Also do we think it is OK to use 10.9 fasteners on a propeller or are they not tough enough and I should I stick to 8.8?


Fasteners and bolts with hexagonal socket head
vis-cylindrique-aero
plan-chc-EN3303
– Standard, without locking hole: NFL 22 225
Standard, with locking hole: NFL 22 224
– With shank, without locking hole: NFL 22 221, L22 223, EN3303
With shank, with locking hole: NFL 22 220, L22 222
 
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Flow

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Looks like 22220BE080040L is an Airbus part number for a shanked and drilled M8x40 cap screw . Does anyone know where to find the specs on this, in particular the class?
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Flow

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Nothing in the Tecnam or Aerospool IPCs and nothing in the online interactive Diamond IPC either.
http://ipc.diamond-air.at:8080/ipp/app? This is quite cool as they often have the LN ISO or DIN number in their part numbers.

This looks to be spot on but the difference between an M8 being rubbish and stripping and being good seems to be 7.80 - 7.82mm at the threads = rubbish while 7.88 - 7.90 = good.

I found these locally from Wurth but again no way to know if they have quality threads or not.

I bought some locally sourced M8x110 hex bolts that even came with certificates of conformity that turned out to be the junker 7.80mm threaded sort that just ended up in the bin.

Essentially it look like I am looking for:
DIN 912 / ISO 4762 / NFL 2222[0|1] / L 2222[2|3] / EN 3303
I think these are pretty much the same M8x40 cap screw.
 
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malte

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Würth has usually a good quality, not sure about close tolerances though. Home improvement stores in Germany also have DIN 912 M8x40.

For the strength classes, the first number is the tensile strengh divided by 100 in N/mm². The first and the second number multiplied with each other and multiplied by 10 delivers the yield strength (up to class 6.8) or the 0.2% offset yield strength/proof stress (from 8.8 on).

Class 8.8 has a tensile strength of 800 N/mm² and a 0.2% offset yield strength of 640 N/mm².
Class 10.9 has a tensile strength of 1000 N/mm² and a 0.2% offset yield strength of 900 N/mm².

Unless you want to utilise the elasticity or tensile strength of the bolts, I would say you could substitute an 8.8 bolt with a 10.9 bolt, as it can carry more.

Keep in mind, that torque values differ if you change strength classes. A DIN 912 M8+1.25 thread is torqued with ca. 25 Nm for class 8.8 (delivering 16.5 kN preload force) and with 35 Nm in class 10.9 (delivering 23.3kN preload force). Make sure, your structure can bear the increased preload force.


Here is a table of the torque moments and preload forces of different DIN 912 thread sizes and strength classes: Vorspannkräfte und Anziehmomente für Metrische Schrauben
 

Flow

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Thanks Malte. I am constantly amazed at the depth of knowledge here. I went with the Würth 8.8. I had been told by a number of people not to use higher tensile hardware for propellors. The structure is some high quality hard annodized forged alloy so I think it could take the prepload. Maybe folks are worried about the higher tensile fastners being too brittle? There is also the issue of making sure the nuts can handle the extra preload required.
 

JimCrawford

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I'm not sure what the preload / torque tables represent in the reference above. If they are typical working values then I think the higher preload for 10.9 rather than 8.8 is irrelevant if a different preload is specified for the propeller, then that would be the requirement.

Jim
 

Flow

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Indeed, I have asked the propeller manufacture, DUC, if 23.3Kn is within the design parameters for the hub and if the threaded insert in the prop spacer have sufficient proof load (I doubt it)

Of note the Würth 8.8 cap screws were of reasonable quality all >7.88mm some not so round but got 6 good ones out of a pack of 25.

The new DUC propeller bolts I ordered were actually hopeless being both bent and threads on the 7.80 end of the spectrum. So trying to see if I can use the 10.9 Würth ones there. (We don't have stock of the 8.8s in that length here in NZ)
 
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