Service Life of Bolts

Discussion in 'Maintenance Bay' started by MTWings, Oct 7, 2018.

  1. MTWings

    MTWings Pre-Flight

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    I was thinking of how engines, mags, vacuum pumps, ... all have recommended times before overhauls but was wondering what about attachment point bolts? Obviously they have been designed to for stress and shear requirements but you would think as an airplane gets older these bolts would get fatigued. Is there a serviceable lifetime for bolts?
     
  2. Unit74

    Unit74 En-Route

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    Ask Piper.... they probably can answer this question accurately at this point.
     
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  3. GRG55

    GRG55 Final Approach

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    I don't recall it was the bolt that actually failed. o_O
     
  4. MIFlyer

    MIFlyer Line Up and Wait

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    is the original poster really hankering for an AD for the fleet to disassemble every aircraft and replace all the bolts? i suppose that would drive sales of new $400k skyhawks as 80% of the fleet would get scrapped
     
  5. MTWings

    MTWings Pre-Flight

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    No just wondering since I have seen a few aircraft have U bolts snap. One was taxing back to the hangars and the bolt connecting the landing gear sheered off.
     
  6. Bell206

    Bell206 Line Up and Wait

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    If there is a service life for a bolt it will be listed in the aircraft's approved Airworthiness Limitation section. If it a standard bolt like an AN bolt, then it's either on condition or stated in the MM to replace the bolt at each installation.
     
  7. Dan Thomas

    Dan Thomas En-Route

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    What model was that? Some manufacturers put time limits on such hardware. American Champion, for instance, recooomends replacement of main landing gear U-bolts (or their replacement, the clamp bar NAS bolts) at 500 hours. If an owner ignores these recommendations, he might find out why they're there.

    Stationary bolts such as those that attach the wings and tail are fine if they're properly torqued and the airplane hasn't been overstressed in some way. Bolts that act as pivots wear out and should be replaced when worn. And I have often replaced bolts that I found way overtorqued, such as engine mount and brake caliper bolts. Too many mechanics think that if tight is good, tighter is better. They risk failed hardware and an accident.

    Corrosion is a much bigger failure factor than fatigue, I think.
     
  8. MTWings

    MTWings Pre-Flight

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    Okay thanks.

    The U bolt was either a Champ or a Citabria, it has been a few years, that snapped when taxiing.
     
  9. Dan Thomas

    Dan Thomas En-Route

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    Those airplanes are made by American Champion. See this: www.americanchampionaircraft.com/assets/sl135.pdf

    That's a 40-year-old Service Bulletin. American Champion long ago incorporated the bolt replacement requirement in its Service Manual as an airworthiness limitation, along with periodic NDI of the front wing strut attach fittings at the spar. It seems that so many shops won't buy and use manufacturer's manuals, and this stuff goes unheeded.
     
  10. Sundance

    Sundance Filing Flight Plan

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    I reciently purchased a project, 7AC Champ it came with several small boxes of hard ware, unfortunately none of it fit the Aeronca Champ, My guess was a 108 series Stinson. Buying hardware for an aircraft can turn into a very expensive part of the restoration but a 72 year old aircraft. Really should have all the hardware replaced during restoration. It can be sent to a cad plate shop, any cracks corrosion or any diformed hard ware is known. And identified, to replace. I did not do that this time, nothing to send in!
     
  11. NordicDave

    NordicDave Line Up and Wait

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    Good to remember bolts stretch as they are torqued to specs, and some occasionally over torqued. Many bolts actually have a service life. Examples include engine connecting rob bolts, which are replaced on overhaul. Corrosion and deformation like mentioned above wear out and have recommended service lives in some airframe service manuals. There are also non-specific calendar time-outs like "replace on condition" limitations.
     
  12. Dan Thomas

    Dan Thomas En-Route

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    Engine manuals often call for rod bolt replacement any time the rod is removed from the crank. That can occur on a top overhaul if the mechanic wants to look at the rod bearings to get an idea of what debris all the bearings might be getting through the oil.

    Bolts are made of various alloys, and all of them have a bit of stretch. The metal has "yield" and "ultimate" points; yield is the point at which stretching the metal starts permanently deforming it, and ultimate is where it finally breaks. We want to avoid the ultimate at all costs, but we also want to avoid the yield point as well. We want that bolt to retain its elasticity; it will stay snug that way as the bolt itself and whatever it is holding will expand and contract slightly with stresses and thermal changes. Overtorquing it can take it beyond its yield point (to say nothing of crushing whatever it's holding together) and it can then work loose, or worse, loads might take it beyond the ultimate and it breaks.

    Overtorquing is a common problem with motor vehicle wheel hardware. If you see a large truck with rusty stains radiating outward from its wheel nuts, you're seeing the result of installation with a big impact wrench with no regulation. The studs have been stretched beyond yield and are now letting the wheel fret a little on the nuts and hub, grinding off iron powder and causing the stain. And once in a while a wheel comes off when the studs break and goes on a deadly trip across several lanes of traffic.

    Torque specifications are published for just about every fastener in the airplane. The standard torques listed in AC43.13 will apply unless the airframe manufacturer publishes something different for that same size. And all of those torques are much lower than what auto/truck/tractor mechanics are used to, because aluminum and other light metals crush a lot more easily than steel. Approved aircraft shops are required to have torque wrenches that are calibrated to national standards, usually on an annual basis. Harbor Freight and other cheap torque wrenches need not apply; their settings aren't reliable because their internal parts are rough and don't consistently indicate properly. They're false economy.
     
  13. Ray Eaker

    Ray Eaker Pre-takeoff checklist

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    If there is, it would be in the associated maintenance/operating manual for your airframe or power plant. SBs, SIs and ADs may also need review.
     
  14. bluerooster

    bluerooster Cleared for Takeoff

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    During the first annual since purchase, my A&P/IA told me that I should inspect the wing attach bolts. The heads were rusted. So I decided to replace them all with new. Cheap enough, and found no worries in the holes.
     
  15. vman

    vman Pre-takeoff checklist

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  16. Stewartb

    Stewartb Final Approach

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    Cub gear bolts? Replace every year. Cub tailwheel bolts replace every year. Cessna gear or tailwheel bolts? Inspect and fly. Different planes, different requirements.
     
  17. Craig

    Craig Pre-takeoff checklist

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    There are both nuts and bolts in some installations that are listed as one time use only. Generally they are located in either high stress installations or safety critical ones.
     
  18. PaulS

    PaulS Final Approach

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    Bolts have different ratings related to tensile strength, make sure you get the right replacements.
     
  19. Let'sgoflying!

    Let'sgoflying! Touchdown! Greaser!

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    I worry that we have forgotten how to make metal parts with the appropriate strength etc and wonder if old but undamaged and uncorroded hardware is actually better at times.
     
  20. Clip4

    Clip4 Ejection Handle Pulled

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    Some bolts have such a low torque application, they never wear out.
     
  21. Dan Thomas

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    For bolts loaded primarily in shear, yes, the torques are low. They're just enough to keep the parts from fretting against each other. For bolts loaded in tension the torques are higher to keep the parts from separating enough to fret when under load.

    Bolts or nuts that are frequently removed often have worn threads. If they get overtorqued, the threads can get deformed as well. Easy to see with a magnifier.
     
  22. Stewartb

    Stewartb Final Approach

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    AN bolts are stronger and more flexible than SAE bolts. AN bolts use rolled threads instead of cut threads. Use the correct bolts!
     
  23. Dan Thomas

    Dan Thomas En-Route

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    AN bolts have a strength of 125 ksi. Grade 5 industrial bolts are 120 ksi, almost as strong. However, the AN bolt is made of a top-quality nickel steel that has a higher ductility that the carbon steel used in industrial hardware and will bend or stretch more before breaking. That will let a good inspection find a problem before it gets serious. And most bolt threads, including industrial, are rolled rather than cut.

    AN bolt thread fits are also better than industrial. They're a "running" fit, while industrial stuff is usually "free" fit. Some cheap stuff is a "loose" fit.

    NAS bolts have tensile strength of around 160 ksi and are a closer fit. I have sometimes found AN bolts in places where the manufacturer specified NAS bolts.
     
    Last edited: Oct 13, 2018
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