Fuse pin 1

[feajob]

Hello, We need to design a fuse pin for a main landing gear. The goal is to achieve controlled shear failure to protect the aircraft. Our focus is on achieving a predictable failure in shear mode rather than tension or bending, as shear failure provides more controlled separation. To control the failure location, internal or external grooves (or notches) can be incorporated. I am interested in your insights on this matter: would you recommend using an internal or external groove design for the fuse pin, and why? Thanks, AAY

 

[SWComposites]

External is way easier to fabricate and inspect in service.

Note, design of fuse pins is not easy. And you cannot rely solely on FEA. You must test them.

 

[rb1957]

I've done this before. It requires a lot of testing (to match the specific strength of the lot to the required failure load ... depending on your tolerance for failure load).

An internal grove ?? sounds way (sorry, Way) harder to produce than an external grove. Failure should be evident (and not need inspection). You may need inspection to check for corrosion or other damage.

Many landing gear already use this design ... go have a look at one, find what engineering data you can. I suspect there are papers written on this (like for trade conferences).

 

[WKTaylor]

Suggest Google 'FAA Fuse Pin'... lots of info embedded in the MANY AD's, service letters, etc... on them... used in flight control systems, landing gears, wing-to-pylons-to-engines, etc. Sigh.

Some other info...
SAE AIR4566 Crashworthy Landing Gear Design

FYI...

Fuse pins can be as simple as a rivet that shears at a prescribed torque over-load... or an expensive/complex large hollow pin... calibrated to cross-shear at some god-awful over-load 'liberating' a component to move-away or tear-away-completely. Calibrating tests are needed for every production lot/batch of pins.

Alloy steel has very predictable HT response and is easily 'calibrated for shear... BUT internal [bore] inspections mandate specialized/removable corrosion protection... to see and NDI for fatigue cracks and corrosion in the Pin bore... a moisture-trap. A specially smoothly radiused cut-groove helps shear at exact values, quickly and efficiently... but are hard to inspect and are corrosion prone. Also, high strength hollows pins... even with Fty close to Ftu... can deform 'out-of-round'... so one level of inspection is to simply remove them for major inspections... which will reveal any deformation very quickly... and can be NDI [EC, FPI, X-ray, etc.

SStls & Heat resistant alloys are virtually immune to fatigue and corrosion... but response to heat treat [aging] is some times hard 'nail for all the raw-pins each HT-lot... even for parts made from the same materials and HT in the same oven at the same time. IE: the pin location in the oven and quench/air-cool variations occur, frequently affecting the Ftu and Fty... 'just-enough' to be a nuisance for 'calibration'.

OH yeah, all shear pins MUST be inserted 'wet' with a grease [or other lubricant] that has anti-wear and anti-corrosion additives... then they should never be too difficult to remove from the joint for hands-on inspection. CAUTION/WARNING... all load MUST be relieved from/at the shear-joint to ensure the joint is 0/zero stress when pulling the pin. Obviously if the lubricated pin is 'stuck'... then suspect deformation. Only way to remove a stuck/deformed pin might be to: (a) pull-it and... all bushings in the structure holes... out... a mess; or (b) step-machine thru the bore to carefully obliterate it from the inside. Gag.

Simple 'rivet shear pins would benefit from being installed 'wet with epoxy primer' and the bucked-tail being 'set' by calibrated mechanical 'rivet-squeezer' [not hand bucking. PS: Use of blind rivets or blind bolts when shear strength can vary-a-bit... and accessibility to the solid rivet protruding tail for forming is 'hard'... are sometimes optional work-arounds.

Just saying...