Allowables for saddle clamps

[sdiguana]

I'm writing up stress notes for an electrical conduit in the fuel tank, and one item of contention is that the saddle clamps have no allowables. The prime does not have test data, and as best I can tell hand waves them as having worked in the past. (the joys of systems stress!)

Analytically, I am having trouble on how to treat them... If I look at a load parallel to the fasteners (vertical), I can treat it as a pair of tension clips, and get an allowable of ~75 lb (using FBU). Hoop capability is 7800 lbf. I don't really believe either one of these that much. Probably closer to the tension clip, but it seems low to me as well.

for ref, the saddles are t=0.02", w=0.5", tube d=1.25", material 2024-T6 sheet




any thoughts or suggestions?

 

[sdiguana]

typo: t=0.04

as an aside, I don't see an edit button for the post..?

 

[Sparweb]

I have seen an analysis of these clamps. I don't know why it was done, it seemed unnecessary at the time, but the engineer had already done it... Used the tension clip model like you suggested and the small load was plenty for a conservative analysis with positive margin. Are you saying 75 pounds is not strong enough?

The simplest analysis is the one you don't have to do.

When using a mil-spec clamp such as this, for its intended purpose, arranged in the correct manner, having made sure that the materials are correctly selected, then there's usually not much stopping you or the approving authority from accepting the installation. Unless your case is different (you mention a fuel tank...) for some reason such as extreme vibration, chemical attack, weight or flexure of the conduit, trouble sealing... If there is a specific problem that is driving this, then the analysis can be targeted, to collect the information needed to answer the question.

Otherwise I'd just say, use the tension clip analysis, or do your own test, or find someone else's test results (good luck with that).

STF

 

[sdiguana]

I am finding the tension clip is inadequate for an allowable. Looking into element averaging my enforced displacements to try and bring the loads down as well as using the tangent modulus to calculate stiffness of the clamp instead of elastic range.

I am starting to believe the tension clip analysis. Kind of depressing. The side load is the one that hurts. I'm pretty sure one side of the clamp resists alone while the other side only reacts hoop loads.

 

[sdiguana]

Ok I have solved this. What enlightened me to it, was that the stiffnesses are incredibly low, so obviously that attracts very little load. In this instance I had just assumed 1000 lb/in in both directions for the clamps when running enforced displacements. When I calculated it; it was 1400 lb/in parallel to the fasteners and a whopping 53 lb/in when normal. So corresponding that horrible allowable in the normal direction is a low stiffness to not attract much load. ends up okay.

as you'd mentioned, as long as the design guide is followed, the tube will fail long before the clamps, so they can essentially be hand-waved.

now if only I had seen the error of my ways sooner... But on the bright side I have proven it to myself, so I can feel comfortable with hand waves instead of just relying on 'that's how its always been done'.

 

[tbuelna]

I recall a similar situation where there was a small diameter (.50") fluid tube attached every 24" or so to a 12" duct using P-clamps. After building a fairly detailed FEM of the arrangement, what we found was that the small tube was too flexible to create any significant moments/strains at the P-clamp attachments.