Aviation bolts

[biggles]

Some time ago I wondered about the purpose of the raised section beneath the heads of AN bolts . I asked my 94 year old uncle, who established Aeroswan Aviation in Swan Hill in the 50's and operated the busines for some 40 years , making quite a name for himself in the industry . As can be seen from the letter, Roy is a remarkable person for his age, and still has an interest in all things electrical and mechanical. It may also be of interest that, at 90 years of age, he published a book of his life with particular attention given to his early years working on properties in Central Australia . The book is titled " At A Cost " and makes good reading. One of his brothers,my father, was lost in a Catalina in the Solomon Islands in Feb 1943, two months before I was born. Appended is a letter written to various authorities, in an attempt to answer my question, but to date has received no reply. Hopefully someone on the site may be able to shed some light on it .

 

Bob

 

To whom it may concern.

 

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 licenced

 

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

 

centred 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.

 

I trust you will give me the time to answer this puzzling question as no-one so far seem to know. ( Retired LAME 4744 for some 45 years including WW2 RAAF )

 

Thank you, and I look forward to a favourable reply.

 

Yours faithfully,

 

Roy McFadyen.

 

 

[facthunter]

I would think that it's to prevent any bending load being applied by a slight misalignement of the surface the bolt head sits on. The calculation for bolt specifications would not be able to take that into account easily. Tension and shear loads are simpler to cover if it s not complicated by other factors. It could also be there to stop a plain washer spinning with the bolthead when tightening.. There would be a reason... That's my guess anyhow.. Nev

 

 

[Guest Maj Millard]

It's simply ment to be a washer under the head, which is why I get upset when the apprentice whacks another washer under the head !!! standard set-up is no washer under the head and one under the nut.............................................................Maj...

 

 

[old man emu]

This is the response from John Schwaner, the author of Mechainc's Toolbox:

 

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).

 

Crushed washer from tightening beyond material's yield strength.

 

Washer 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:

 

 

 

[old man emu]

Have a look at this link:

 

http://mechanicsupport.blogspot.com/2011/09/aircraft-washer-usage.html

 

Old Man Emu

 

 

[biggles]

Thanks Nev, Maj and OME .Have emailed your responses onto Roy

 

Regards ,

 

bob

 

 

[Tex]

It's simply ment to be a washer under the head, which is why I get upset when the apprentice whacks another washer under the head !!! standard set-up is no washer under the head and one under the nut.............................................................Maj...

I wouldn't know... but what about with OME's answer from John Schwaner who talks of washers?

 

 

[old man emu]

Tex,

 

If you read the article in http://mechanicsupport.blogspot.com/2011/09/aircraft-washer-usage.html you will see how the "bearing area" is used to calculate the load on the surface the bolt is going through.

 

By having a machined washer face it is possible to calculate the area of the face and from that, and the applied torque, the force that is being exerted on the surface around the bolt hole. If you just relied on the head of the bolt, the surface area could change with damage to the head and this could change the pressure on the surrounding surface.

 

Old Man Emu

 

 

[Tex]

Fair enough, so Maj has it wrong and the apprentice could have been right all along

 

 

[old man emu]

No. Maj is not wrong. He said: "standard set-up is no washer under the head and one under the nut..".

 

The standard set-up would occur if you were using a nut and bolt to clamp two things together, then you don't use a washer under the head. The maximum torque that you would apply to the nut/bolt combination would be determined by the bearing load of the material the bolt head was resting on, and this is affected by the area of the washer face. Therefore, you don't apply high torque.

 

If, however, you were bolting things to a wooden spar, which is easily crushed, then you want to spread the bearing load over a wider area. So you would use a penny washer (AN970) under the bolt head to spread the load. This is not what we would call a "standard set-up" under Maj's definition.

 

What about when you use a nut/bolt combination as an axle for a lever system, like a control column? In this case, you want the one of the parts to move relative to the other. In this case you would put a normal washer (NAS1149F****@) under the bolt head, and maybe washers between the two parts. The washers act as bearings for the parts to rub against. This means that the washers would wear out before the parts would. In this case you would use a bolt with a drilled shank and a castellated nut because the torque is not a factor in the assembly.

 

Perhaps Maj should have stated which set-up the apprentice was using that caused Maj to be upset.

 

Old Man Emu

 

 

[Tex]

Excellent, thanks OME, interesting stuff.

 

Sorry Maj... my mistake, you were wrong or maybe right, all along

 

 

[old man emu]

That's me. Nuts about bolts

 

OME

 

 

[facthunter]

I've decided that everytime I meet a nut, I should bolt. It's less complicated. Nev

 

 

[Guest Maj Millard]

By standard set-up I was referring to best, or ideal set-up, if one is using the correct length bolt. As OME states it is common to use additional washers under the head to distribute load (penny washer), or to adjust the grip length of the bolt if it is not the ideal length.9additional standard washer)

 

On seaplanes we will also often use aluminum washers to minimize galvanic corrosion as explained in the mechanicsupport blog that he posted, which I found to be excellent reading....you never stop learning !!..............In respect to the apprentice who is mature aged, but first year, you have to initially teach the standard and basic hardware set-ups and principles, and then the more involved later as they progress.................................Maj...

 

 

[old man emu]

Watch it Maj, you'll open up a Pandora's Box if you start talking about the different types of washer.

 

I'm just looking at my parts book now and see that there are 6 different material specifications and 5 different finish specifications for NAS1149 washers alone (new code for AN960 washers). Then we can move on to MS20002 countersunk high strength washers; MS27183 & MS15795 general purpose flat washers. Not to forget MS35333, MS35335 and MS35338 lock washers. And bringing up the rear are Countersunk Large Area washers, Flush/and Cup finishing washers, vulcanised fibre and nylon washers.

 

OME

 

 

[Guest Maj Millard]

I see your point OME...I reckon however, in 44 years of aircraft maintenance I've probabily seen most of them at least once !!....and then some .........................................................Maj...

 

 

[Ultralights]

What about 2600LW washers?