Prying (Heel-Toe) Calculations 1

[jimmydee]

Hello all,

I was wondering if you could help me with a question on prying load calculations.

Flabel and all of the examples I've seen so for prying in a simple tension clip (bracket design) only consider one fastener. The classic prying calculation is shown here:
Link

I would like to perform the exact same calculations, but with 3 fasteners instead of one, equally spaced out so that there are 3 fastener reactions.

I have a simple design where the distance a = 1.0" between all 3 fasteners, and the distance b = 0.5".

I'm struggling to figure out how to do the sum of moments and sum of forces equations for multiple fasteners when prying is considered.

Could anyone please help me do this?

Many thanks,
James

 

[SuperStress]

James,

The answer to your question depends very much upon the stiffness of your tension clip. If it is very stiff,like a thick fitting, I would assume a linear deflection from the end of the clip all the way to the applied load. This would give you a linear relationship such that you could reduce the number of variables in your moment equation.

If it is more flexible, beware of assuming the clip effective over its full length (See Flabel Figure 5-51 regarding long base flanges). In this case, I would probably put all of the load on the first fastener, apply the reaction between the first and second fasteners, and assume two of the fasteners ineffective.

SuperStress

 

[jimmydee]

Thanks for your speedy reply SuperStress.

The bracket is 0.040" thick 301 1/4H stainless steel.
The load, P = 388 lb.

As it is a relatively small load and the bracket is made of 0.040" thick steel, I would assume it is more on the stiff side?

I have attached a simplee FBD pic.

Where would one place the prying reaction load in such a case? Would it be 2/3 the distance between the last fastener (P3) and the end of the Clip, or somewhere closer to the second fastener if assuming a linear deflection along the entire clip?

Much appreciate your help. Still very new to the prying concept.
James

 

[dik]

Not an aircraft guy, but the forces on the fasteners would depend very much on the stiffness of the bracket; other than an FEM model, I'm not sure how you would analyse this.

OP... thanks for the link; I didn't realise there was an AeroWiki

Dik

 

[rb1957]

I'd react the load like a beam, between P1 and P2; P2 = P/4, P1 = 1.25 P; one could say the compression reaction to prying is a triangular distribution from 1 to 2

and check the clip for bending due to the 388lbs applied load ... with a 0.25" offset and 0.04" thk, I think you're good for 160*(fty/40) fty in ksi (using the typical tension clip allowable curve from McCoombs Supplement to Bruhn (and copied many places)

another day in paradise, or is paradise one day closer ?

 

[SWComposites]

All of the vertical load should be assumed to be reacted by P1, and the prying part calculated using the length from P1 to P2 to add to the load on P1.

 

[dik]

SWC... that's the approach that I would have used.

Dik

 

[jimmydee]

Thanks for the great feedback.

After looking at it, I agree the most conservative way to analyze it is to assume all the load is reacted by P1.

I have attached a detailed picture of the actual structure that I am analysing.
As you can see there are two types of prying going on... the one we covered before (A), plus the prying in the vertical direction (B).

The 3 rivets in the vertical web restrain the structure, and the structure is eccentrically loaded by P = 388 lb.

I have analysed condition B in the second picture attached. Now, my question is: do I assume that the first rivet also carries all of the reaction load in this scenario given that it is so close to the load?

Or will it be divided amongst all 3 rivets?

Cheers,
James

 

[jimmydee]

Picture 1 is attached

Apologies, I could only seem to attach one at a time.

Thanks

 

[rb1957]

perhaps a logical way to combine the two offsets is to use the vector sum

another day in paradise, or is paradise one day closer ?

 

[MOHR1951]

rb1957,is it correct to simply vector sum offsets ? What rule or logic do you use or refer to? It sounds like a perfectly legitimate thing to do,but I don't recall being able to make this assumption,let alone being taught such practices.

 

[WKTaylor]

A few notes to compliment existing replies...

Even though P1 [F1] MUST absorb tension load of P [as noted], [2] or more fasteners on the attaching flange are always mandatory for 'stability' against any eccentricities tending to rotate and/or twist the fitting about F1... which could be dramatic. NOTE: in the real-world manufacturing and load ECCENTRICITIES will ALWAYS exist!

F1 MUST have a tension rated head and tail: bolt/washer/nut or HL pin/collar... especially since the load will concentrate on the edge of the fastener head or nut/collar [whichever]. Use of a solid driven aluminum rivet is unsuitable for this application since there are NO currently published allowables. Use of a CRES or Ti blind rivet or bolt in this instance [with tension allowables] is acceptable if well designed. F2 only hast to resist planar shear, or very minor prying forces, so a shear rating is fully acceptable.

I assume that this fitting sees cyclic loading so fastener loading in all cases MUST be held well-below yield of any part of the fastener installation for durability.

NOTE.
In this instance, I personally, would install a thick [rectangular] fillet-radius 'washer' ['radius filler' if You prefer that term] underneath the fastener head/tail into the fillet radius. The edge of the 'washer' must nest tightly into, and runs-the-full-length-of the fillet. It MUST have a matching radius to the fillet [or chamfer(s)], so it snuggles in very tightly and grossly improves tension bearing, at F1, while reducing the unsupported distance [0.25" now... much less depending on the fit into the radius] to virtually eliminate the 'prying' moment at the fastener [head or nut/collar]. Also this 'washer' allows for installation/fit of a nut or collar or fastener head VERY tightly into the flange fillet.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
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[GrandpaDave]

Will Taylor to the rescue... Excellent response. I don't know how we old guys survived in the olden days (1960s or earlier) with just a slide rule, Roark, Bruhn and those CRC tables. A classical hand stress analysis method, considering whether the tension clip is rigid or flexible, is correct approach. If I had modeled this on a computer I would have been fired.

My 2-cents I'll crawl back under my rock.

 

[jimmydee]

Appreciate all the great feedback and ingenious ideas to reduce the prying moment.

That first fastener is a CR7773S-04 blind cherry Ti maxibolt rivet which has a tension allowable of 600lb. The margin of safety on that fastener is very tight after doing the math in its raw configuration. I did a FEA to verify.

Interestingly, there is actually another fastener on the top flange which is not shown in the illustration. It is a nutplate attached with AD3 csk rivets accomodating a 1/8" diameter bolt hole on the top flange about halfway between the second and third rivet. Factoring this into the analysis seems to greatly reduce the reaction loads on that first rivet.

 

[MOHR1951]

We've lulled our way into thinking that if we don't have CAD,FEM.and all the other software that it can't be designed safely,or efficiently.Recall before we used standard engineering procedures,calculations,and healthy safety factors and for the most part it all worked out,we designed airplanes,they flew,most produced the results that theory predicted,and yeah,we probably weren't able to attain a closed solution to those PDQ's.but they flew for the most part.IMHO we probably still don't know what actually causes lift,so why are we all so possessed by trying to solve the PDQ's,what's the end game here ?

 

[rb1957]

yeah, I was going though a airplane museum with my brother, showing him all the little pieces added (VGs, fences, etc) developed mostly from flight test. I came away with the idea that our body of knowledge is mostly practical (ie passed by test).

and also agree that our tolls are showing us hotspots that have always existed by were below our visual (preceptible) threshold.

another day in paradise, or is paradise one day closer ?

 

[IRstuff]

"we designed airplanes,they flew"

We also crashed a lot of them. People tend to forget that. Test piloting is almost an office job today, but that wasn't the case 50 years ago.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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