Spitfire wing bolt observation

As has been stated above, in the early days we would put the bolts in the freezer and knock them in. That changed when I found the bolt drawings in the RAFM archives.
The bolts were manufactured to a Fit “P” and were cadmium plated except for the “P” fit shank. Mod. No. Spitfire 1485 altered the bolts so that the end of the thread on the bolt was turned down to the root diameter to obviate stripping of the thread on assembly. The split pin hole was drilled during manufacture to align with the slot in the head of the bolt.
Mtl. S2 HTS.
Oversize bolts were manufactured in 0.004″ steps up to +.028″, the next being +.03125.

Although the seven bolts all reamed, transition fitted and bolted effectively make the spar booms ‘encastre’ with the frame five carry through spars, there is still a degree of flexure of the assembly and cracks regularly appear in the frame five inboard of the inner bolts. The cracks are permitted and can be drill stopped up to a certain point…then it gets expensive.

Mark

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I guess what I was trying to say is the load is taken by the reamed pins, the nut’s are purely there to stop the pins falling out?

I wouldn’t say that was correct exactly. Not unless the joint is very badly designed?

I’m willing to bet nobody’s ever seen a Spitfire crash with the wing off where any of these bolts are sheared through; they must be way stronger than the (cold-drawn?) alloy wing spar box-tube or the (forged / cold-drawn?) brackets either side (at frame five)?

All any of these bolts do is transfer the load from the spar-boom to the fuselage-bracket; it wouldn’t matter if there were four or four hundred bolts in a row the joint would still always fail above or below the end bolt hole in either the spar or the bracket!

Even with my theory that the spar-bracket friction (by torqued bolts) was the important factor doesn’t change that.

The more I look at this joint the less I understand it!

The three types of fit are:

Clearance – the hole is larger than the shaft, enabling the two parts to slide and / or rotate when assembled.

Location / transition – the hole is fractionally smaller than the shaft and mild force is required to assemble / disassemble

Interference – the hole is smaller than the shaft and high force and / or heat is required to assemble / disassemble

And to stop them rattling in their holes – most important that they don’t.
Keith

I guess what I was trying to say is the load is taken by the reamed pins, the nut’s are purely there to stop the pins falling out?

Correct.

Mark

Bad choice of words on my part, since there is no friction involved at the joint on the ground or in flight! I guess what I was trying to say is the load is taken by the reamed pins, the nut’s are purely there to stop the pins falling out? they would appear not to have a clamping load on the spar carry through? unless there was a method of shimming the shoulder to a set depth through the spar carry through? I’m not a designer I’ll grant you, just a mechanic trying to understand the designers method. Something we mechanics have been doing since designers started designing! It usually starts with “Why the F did they do that”!

Miniman
“It’s the friction of the pin/spar interface that transmits the load”

I’ve designed (and overseen) a fair few highly loaded joints on aircraft which are flying around today, including one which I watched tested to just under 700tons;- a very big aeroplane. I’ve never yet seen one that’s relied on friction to transfer load.

I urge anyone that wants to understand the reasons for the multi sized internal holes to read up on bolt group theory and stressing…….because you have a bolt group of 7 bolts.

I’m probably talking through my hat, but were there not counterbored washers fitted on the nut end of the bolts & that there were different depths of counter bore available – equivalent to fitting peelable shim washers?
Meteor spar bolt joints had different allowable resizes – there being a maximum before scrapping said spar, were Spits similar?
Keith

I was looking for some reamers on ebay and stumbled across these – there is no indication of their use, only the Spitfire part number that caught my eye. A lucky find. And for the record, although I am not an engineer I would say these are not bolts, but pins and agree with minimans.

I think we are all overthinking this? These are not bolts in the excepted term, they are shoulder bolts. Think of it as a retained pin rather than bolt. That’s why they are a reamed fit, It’s the friction of the pin/spar interface that transmit’s the load, not the clamping of the “bolt” I seem to recall that when the Spitfire arrived in India there were a number of failures of the spar joint due to incorrect reaming when the wings were reattached after shipment? The nuts were tight but the pins not?

As far as drilling split pin holes after installation and torque, it is pretty common in the British Aero industry, or at least it was.

You wouldn’t believe how many drill bits you would go through on a Westland Wasp when you put new (undrilled!) bolts in. Nearly everything had blank bolts. What a pain in the you know what that was!

Great find Elliott, Where did you find yours?

Bruce

Just as a matter if interest, does any one know if hollow bolts had to be fitted as complete sets or could they be just odd individuals? As to the design principle, “bolt-in-shear” would be normal and as Vega pointed out, multi-bolt joints require some very accurate fitting if they are to work properly, hence the reaming kits etc.

Wow!
What a beautiful piece of art!
Nice find sir

Andy

Following on from Bruce’s mention of the reamer set, I just found one too. Probably never even used.

[ATTACH=CONFIG]256158[/ATTACH]

[ATTACH=CONFIG]256161[/ATTACH]

[ATTACH=CONFIG]256159[/ATTACH]

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Thanks for posting that photograph; those holes don’t cut through as much of the spar-boom as I thought.

What I’d really like is some dimensions so I could do some basic calculations: just the diameter of the bolt and the outside size of the outer box-section of the spar-boom.

This is a slide on photo, but I think it is worth noting that this section of spar came from a BoB crash, I had the whole of both top spar booms with about 3feet of wing spar each side bent like a banana !
The point being the three bolt holes were still fully bolted and the holes are not fractured or pulled after hitting the ground vertically at high speed…..

Jules

[I”Guy Martin drilled these on his Spitfire TV show”.
][/I]

Must be true then! I’m sure I’d want bits of swarf floating around the engine bay!

Incidentally re torquing and split pinning, the Wessex employed shim washers on some fittings, you peeled layers off the washers until you got the desired position etc.

interesting read

http://www.pprune.org/tech-log/443530-limit-load-factors-ww2-fighters-eg-spitfire-mustang.html