Cracking a bolt

[Michael GilliganParticipant @michaelgilligan61133]

Posted by vintage engineer on 04/12/2019 11:02:14:

Not all bolts are in tension. Sadly the RAF Red Arrows had one of their pilots killed because a bolt had been incorrectly fitted. The fitters had over tightened the bolt that held the parachute and ejection seat together rendering the parachute inoperable. The bolt was supposed to loose in the pivot but no one told the fitters.RAF Crash

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The important engineering design failure being nicely summarised within this brief paragraph from your link:

[quote]
Martin-Baker have since designed a small metal plate to go over the SFH which prevents accidental firing. This had been proposed a number of times but always rejected by MoD. Additionally they have created a new bolt that cannot be over-tightened and will hold the main parachute in place until the ejection seat is initiated and stabilised. This shoulder bolt had previously been removed from the Mk9 seat design, with the MoD's agreement.^[18] Training on ejection seats in the Royal Air Force was changed after this event.^[29]
[/quote]

Key term = ‘shoulder bolt’

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MichaelG.

[Howard LewisParticipant @howardlewis46836]

The bolt or nut comes loose with a "crack" because you are overcoming the static friction.

If you need to measure the torque applied when the fixing was tightened.

Scribe a line across the flat of the fixing and the area around it.

Slacken the fixing

Retighten until the marks align again. The torque needed to do this is the torque originally applied.

Yield tightening is used to maximise the efficiency of the fastener. (A 8 mm fastener in yield may replace a torque tightened 10 mm, and provide a more consistent elastic load in the fasteners.

Not all yield tightened fixings are single use.

Torque is one means of applying (not very accurately ), the load in a fastener.

A more accurate method is to tighten until a required extension is obtained. This was used for the Big End Bolts in Rolls Royce C Range engines, so was probably a carry over from aircraft practice. The bolts had reduced diameter shanks to provide uniform strength along the length.

When commissioning what was, at the time, the world's largest multispindle yield tightening machine, we found that we could retighten W range 1/2 UNF bolts up to nine times before failure. We thought that reusing six times would be as much as could be safe.

This is NOT advising this for all fixings. It will depend upon the material, and the dimensions of the particular fixings, as well how far into yield the fixing is taken.

Howard

Edited By Howard Lewis on 04/12/2019 15:49:48

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