A fascinating, and very thorough report

Useful reading for anyone using fasteners near their design limit !!

MichaelG.

If a fastener is over torqued, the material will go into yield and the extension vs torque relationship will no longer be linear. You feel the spanner go "soft" just before failure.

Tightening to yield is the most efficient use of the fastener, but tightening HAS to stop, as soon as yield is detected.

Having gone beyond the elastic limit, the fastener will have taken a permanent extension.

I spent six months testing and commisioning a 32 spindle yield tightening machine. We used LOTS of bolts which had been faced at both ends, identified and measured, before and after tightening.

A 1/2 UNF bolt in W range steel would take a permanent extension of a thou or so, indicating that it had JUST gone into yield.

In some instances, this means that a fastener can only be used once.

In industrial yield tightening systems the electronics monitor torque against angular rotation, As soon as the relationship becomes even the slightest non linear, tightening stops.

It also means that the clamp load applied by each fastener is consistent.

For this system to work, the parts being clamped together must not crush noticeably. If it does the control system will detectb the material going binto yield rather than the fastener.

In joint system involving a gasket, this alo has to mbe taken into account, although often the system is intended to mproduce a consistent clamp load on the gasket to improve sealing.

Howard

Interesting reading and worrying where threaded fasteners are being used in tension.

One would have thought that for this application the stud would have had the highest yield strength as replacing a stripped nut is far easier than replacing a stripped stud.

However the thinking here may have been that it is better to strip the thread on the stud than pull the stud out of the Ali crankcase if over tensioned. Unfortunately the designers thinking has probably been lost and the only info is the specification of the stud and nut.

CS

The Report's Conclusion states …

Multiple failures of cylinder base studs on the RR O-240 engine type have been recorded since 2014, but unless they resulted in engine failure in flight, they were not reported to the manufacturer.

Wow.

So were they at least reported to the Light Aircraft Association?

Ches.

Edited By Ches Green UK on 19/03/2023 19:19:28

The other question is: Whose roof did the missing cylinder and piston land on?

Nick

The thread doesn't necessarily take a working load if a fastener such as a bolt is taking its design load in shear ie orthogonal to the bolt axis in which case the nut is just tightened enough to stop the nut coming undone and if there are cyclic or torsional loads acting on the bolt a split pin is added

In the case of the cylinder holding down studs there is a load on the thread due to tightening plus cyclic tension loads during compression and ignition cycles plus complex asymmetric loads due to side thrust of the piston against the cylinder wall.

Most bolts in airframe applications including holding the wings on are working in shear mode. Having said that I had an aerobatic aircraft where the wing was held on by two 1/2 inch bolts in tension when the aircraft was pulling positive G which could be up to +6G !!! The weak point is the transfer of load between the bolt and nut and for this application there were two nuts per bolt.

CS

Howards contribution concerning tightening to yield convinces me that we really need better terms to describe how stretch bolts behave. When an ordinary bolt or stud is taken past its elastic limit it pretty much looses all ability to recover and stays stretched. A stretch bolt or stud is still capable of elastic recovery after yielding. It can't go back past the yield point but it still recovers elastically within its working range.

Which is why they are so popular and effective for things like head bolts on alloy engines, like the venerable pushrod V8 in my Range Rover. If the bolts didn't recover all head gasket pressure would be lost when cold after about 4 or 5 starts due to stretch from differential expansion when the engine is up to operating temperature.

The old Rover V8 is an excellent example of why correct installation ensuring the stretch bolt is always within its correct operating range is so important. Combine over torquing and with overheating and the bolt will go out of range so the head gasket fails. The margins are surprisingly slim too. Pushing the operating temperature up from 95°C to 100°C and restricting cold coolant flow to meet emissions requirements shifted the Rover installation from bullet proof to occasionally unreliable. The extra stretch between 95° and 100° is very small but just enough, when combined with stretch bolt production tolerances, to eat up the margins.

Clive

Edited By Clive Foster on 20/03/2023 10:28:40

I didn't read it that way. I thought:

• The stud was to specification (last updated in 1976)
• The nut met the specification, BUT,
• was made with a shouldered thread that resisted vibration loosening by bearing harder on one side of the stud's ordinary thread.

Although both met the 1976 specification, the combination put extra-stress on an already highly stressed fastener such that tightening the new nut cracked the stud. Lucky pilots got a new stud if maintenance broke the stud on the ground, whilst unlucky pilot had studs fail in flight. Vibration and heat from the running engine propagated the crack.

Interesting that something as standard as nuts and bolts are still being improved, but there's a risk that any change from the original could invalidate the original design. I guess all would have been well if a 1976 style nut had been fitted rather than a modern one. Anti-vibration nuts are good in an aircraft, but only if the studs are rated to take the extra strain. I don't suppose a maintenance engineer would know that the design stresses were exceeded.

Is it wise to be flying 46 year old engines? Materials age and components might change in a way a long dead engineer couldn't anticipate!

Dave

That's exactly what I thought at first, but the Spiralock diagram shows their female is cut full length with a wedge ramp:

The problem, I think, is what the wedge did to a stud that was already torqued close to maximum. As can be seen the wedge concentrates the tightening force on the tip of the male thread, giving it an extra hard time. Good idea, but a step too far on this engine! I wonder if the Spiralock instructions suggest backing off the torque when these are substituted for conventional nuts. If so does the maintenance engineer go with the new nut guidance or what's in the aircraft manual?

Dave