Thursday, August 02, 2012

Aircraft Hardware compared to Industrial Hardware

Hi John,I have your Mechanics Toolbox and found it very good and helpful. But I have been asked about what the SAE equivalents for Aircraft bolts and can't find the answer in your book or on the internet. I have a friend that wants to save money on prop bolts. I told him that aircraft bolts are manufactured to a stricter standard than SAE bolts. I also asked him, "Why take a chance?" However it would be nice to have a more specific answer. Do you have any tables showing what is equivalent? Also any reasons for using standard bolts or not would be appreciated.


There is no equivalency between standards organizations as the products are made with different objectives in mind. A quick example, much commercial hardware is installed in static structures and has a static load. Aircraft hardware is installed in dynamic structures and is subject to dynamic loads - An aircraft bolt,  "propeller" bolts" for example, will feel millions of stress cycles whereas a bolt used on a building will have constant stress and no load reversals. An important design consideration for aircraft hardware is fatigue strength - or strength when subject to millions of stress reversals. Pick fasteners for strength AND toughness.

Increased fatigue strength
Bolt Thread. One method of achieving fatigue strength is to round out the thread root so there is less of a notch. This is called the "J" thread and is used on almost all modern aircraft hardware (UNJF for example). Commercial grade bolt thread form (UNF or more technically UNRF).

  • The J thread increases fatigue strength by 40%.
  • The J thread increases shear strength by 10% (thread tensile stress area of 110.765 compared to 103.20 mm2 for 1/2-20 thread.

J thread
J thread has a more rounded root to increase fatigue strength.

If you just looked at "bolt strength" (usually ultimate or yield strength) it is unchanged. Using our propeller bolt for example, we are very interested in fatigue strength. The J thread is a rolled thread. Rolling the thread increases thread strength. Commercial hardware may use rolled or cut threads.More modern NAS hardware has other features to increase fatigue strength such as greater radius between the shank and head - which commercial or SAE hardware will not have.

More thread engagement for added strength
Most aircraft hardware has a class 3 thread engagement fit (UNF-3B) while most commercial hardware has a class 2 thread engagement fit (UNF-2B).

Better corrosion performance
Corrosion control and material compatibility is extremely important in aircraft hardware. Aircraft hardware most not be galvanic to aluminum. This is why lots of cadmium plated steel is used and almost no stainless steel bolts in aircraft. Along with corrosion protection, aircraft fasteners are often oven baked at longer temperatures to prevent hydrogen embrittlement. High strength bolts are particularly sensitive to corrosion and hydrogen embrittlement.

Different Torque values
Cadmium plating has excellent lubricity (low K factor) so that for a given amount of torque from your torque wrench, more tension is produced than would be if the plating were zinc. If you were to substitute a different bolt with different plating one would need to change the torque wrench setting. How would one know what torque value to use?

Increased thread shear strength
Bolt shear strength at the threads is greater. Another consideration is that much hardware store hardware is manufactured to a class 2 thread fit whereas aircraft is manufactured to a class 3 thread fit. Class 3 provides increased thread shear strength (approximately 10% increase in shear area).

Better alloy steel
Steel alloy's come in different grades - Generally, aircraft hardware uses "aircraft grade" alloy whereas commercial hardware uses "commercial grade" alloy. Not to say that "Aircraft" is superior to "SAE" as each is designed for a specific industries' requirements.

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