Bolts-fatique, yield, torque, manufacturing

[lanichol]

I had the opportunity to visit with a Metallurgy Engineer and a Corrosion Engineer on the topic of a failed rotor retention bolt. It was amazing. Hopefully we can encourage comments from engineers and specialist in QC of these critical compenents.

[Attached should be Ortmayer's broken bolt pictures]

I did have some questions as to the rotor retention bolt.
1. Bolt size, spec, grade, torque specifications?
2. Is this a common size bolt used in the rotor head?

Given the bolt was in spec and the rotor head was designed for the load.
The bolt failure was from over torque or under torque. Most probably under torque.

Hear are some topic for discussion:
1. Manufacturing methods of bolts: Hex extrusion, rolled/machined.
2. Over torque – stretch pass yield strength.
3. Under torque – cycles of stretch before failure
4. Materials – foreign manufacture, low grade or counterfit
5. Load past yield –
6. Non destructive test - Mag particle inspection
7. Life cycle expectancy – eventual failure
8. Quality Control – reliable bolt manufacturers.

[scottessex]

That's ugly! does anyone know the recomended replacement times on rotorhead bolts? teeter bolts etc.?

[lanichol]

Per a discussion with the Corrosion Engineer, the bolt had a crack and the crack had existed for a while, long enough for corrosion to occur. A mag particle inspection might have caught the problem.

[lanichol]

So the story goes. The major failure problem with large pipeline compressors and large electrical turbines were the bolts.

A friend of mine was responsible for the maintanance program.
They reviewed torque policies and quality control of bolts from reliable vendors. They also did non destructive testing on the bolts.

They increased the life expectancy of the bolt associated with the wheel turbine. This is the bolts that connect the blade to the wheel.
Duty cycle was increased to 100,000 hours and thoughts are they could reach 300,000 hours. Failures were nearly eliminated. So should the rotor head bolts.

[Russell]

I have sent the pics of the bolts to an engineer at out plant, He does all the NDT and ISI testing also vibration analysts. He did e-mail me back asking questions . I forgot to send these home so I will next day I'M at work. Russ :eek:

[lanichol]

[Here is a response from a guy that studies failures]

Larry,

This has some classic fatigue crack indications. Would be interesting to know how long it was in service before failure. There are at least three crack fronts that grew in different directions, one after the other, with the middle of the bolt experiencing a linear crack path to almost 80% through wall before tearing occurred at the end of failure. Cracks went in numerical order as shown on photo. #1 indication is interesting from this photo – hard to call it a fatigue crack from this picture (although very well could be). Could also be some other kind of pre-existing defect, but very hard to tell (and this is less likely). If this is a pre-tensioned connection, I’d put my money on under-torque. Torqueing is interesting. Requires a very good operator, wrench, and ideal lubrication condition in order to get accurate reading. Also, inexperienced torque operators often under torque because starting a torque wrench actually requires a higher torque level than actual torque as measured during tensioning once the wrench is moving. Therefore, an operator may stop, slowly apply pressure, read min torque value and assume proper torque is attained when in fact the starting torque was never overcome and the reading was inaccurate. Hope this provides some interest.


[Thank you Chad]

[joeheli]

Scott :They don't have a time expecific to check of replace any screw or bolts. Here we never had a accident of that kind , that may be that they over torque the bolt, or was not a aircraft screw. Just checket every time that you can we have a gyroglider (bensen style) that is about 40 years old,(Every one has learn on that gyroglider) and never was nessesary to change any bolts on the rotor head. Just check it when ever you can , don't use screws and bolts that are not for aircraft, and DON'T OVER TORQUE IT! :D Remmember that on a industry you may have 1,000,000 screws
on good condition, but maybe one is defective! :(