AN Bolt Head Design

Dear Sir/Madam,

Some 12 months ago I was asked, 'what is the reason for the machined section under the head of some bolts' ?. This I should have know as a licensed aircraft maintenance engineer for more than 40 years, yet the question took me by surprise because I never had cause to question the problem.

Today I'm in my 94th year and all of my working life has been in the Automobile and Aircraft fields, of course the Department of Civil Aviation (now CASA) has tested all engineers involved that they know much about bolts, and in particular AN bolts (Air Force/Navy). We must know of metalurgy, tensile strengths (UTS), yield points, cadmium plating, and the dangers of chrome-plating bolts the requred knowledge seems endless to the stage where BOLTS as such is a complex science, and this leads me to writing this message.

In all my years working with bolts I would question most of them, especially in aircraft-but in all those years I never asked why many bolts today have a slightly raised circular section under the head of the bolt within that part of the imagined circle within the under side of the hexagon flats. The raised machine section is only a few thousandths of an inch proud, in the order varying about : 004" to :008" in the ones I've measured.

To answer the question I said I didn't know! adding that I had never been asked, nor had queried the reason for the raised section, further adding that I'd find out!

With the lack of better knowledge, I suggested to the question that the raised/proud section might be to ensure the first 'bite' in tensioning a bolt fastener is centered immediately close to the bolt shank and that increased tensioning would gradually spread outward from the bolt shank thus ensuring that the initial axial loading would essentially take place radially over the raised portion of the under-head hex, with or without a washer.

To me it makes a lot of sense to have this section machined thus preventing sharp edges from the hex edges gouging into the clamped pieces, yet of this
I'm unsure.

AN bolt showing washer face

This area is called a "washer face" and defines the bearing area for the bolt head. The bearing area is useful for calculating bearing loads on the washer and/or faying surface so you do not exceed the material's yield strength (crush the joint or washer).  The washer face provides a flat machined bearing surface.  Chamfering the edges at the points prevents the points from digging in and affecting torque.

Crushed washer from tightening beyond material's yield strength

Washers resting on washer face

I would guess that it is easier to achieve a specified bearing area by machining a circle than by beveling the edges of the hex nut. Another alternative is this:

Here is a link to an article I wrote a while back that touches on the subject of washer unimpressive strength.


Companion article link: AN Nut Head Design