R-22 ROTOR SEPARATION? Florida Photo
I understand the concept behind measuring bolt stretch to determine preload (and why it's better than using torque in some applications), but I've also never seen it called for on the particular helicopter & airplane types I've maintained in the past.
So, no, I wouldn't consider it to be unusual for an A&P to never have done it - I fall into that category myself.
So, no, I wouldn't consider it to be unusual for an A&P to never have done it - I fall into that category myself.
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I've never heard of "stretching" a bolt
Maintenance Training The Robinson Way - AviationPros.com
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"Stretch- bolts" have been common in the UK motor-trade for~20 years!
When the first engines appeared with them, we assumed it was a racket to sell more parts
Up until their introduction, it had been normal to torque in stages and sometimes time-lapses between stages.....sometimes run to working -temp amd retorque hot, others cool and retorque .still others nail them down and send 'em out.
then came stretch bolts.... low initial torque , then mark all heads with a paint-stripe, then wind each one so many degrees in diffferent stages.....you'd feel them "twang!"..It needed a bit of research , (easy, nowadays with the internet.) A much more accurate way to tighten, as it virtually eliminates the problems caused by burred/damaged/dirty threads.
When the first engines appeared with them, we assumed it was a racket to sell more parts
Up until their introduction, it had been normal to torque in stages and sometimes time-lapses between stages.....sometimes run to working -temp amd retorque hot, others cool and retorque .still others nail them down and send 'em out.
then came stretch bolts.... low initial torque , then mark all heads with a paint-stripe, then wind each one so many degrees in diffferent stages.....you'd feel them "twang!"..It needed a bit of research , (easy, nowadays with the internet.) A much more accurate way to tighten, as it virtually eliminates the problems caused by burred/damaged/dirty threads.
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That is some intersting stuff. I'd doubt I'll see it on any of Sikorsky's stuff anytime soon since most all of our flight critical hardware is inaccessible and uses externally installed bolts threaded into inserts or barrel nuts, but I manage GSE across the H-60/ S-70i fleet so I always have my eyes open for new stuff.
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What you're describing Steve is very different. On the head of a Robinson you can get to both ends of the bolt and measure the bolt before it is tightened and afterwards. The correct tool just fits over the bolt and allows you to use a dial gauge DTi to measure the increase in thousands of an inch.
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bhawkh60 wrote:
You've got to be kidding me. You can read Sikorsky tech pubs or FAA hardware study guides all day long and will never find reference one to "stretching" a bolt. It's all TQ values.
As far as "Yikes, I sure hope you are not licenced to actually sign off on anything
":
What part of A&P confused you?
__________________________________________
Two points:
(1) But that's all TQ IS! You are simply stretching a bolt to a prescribed a value! And on really critical applications it is taken one step further to include special grease and a rotational limit in which you must get the prescribed TQ.
(2) Re your A&P - my point exactly. Where did you get it - out of a popcorn box?
You've got to be kidding me. You can read Sikorsky tech pubs or FAA hardware study guides all day long and will never find reference one to "stretching" a bolt. It's all TQ values.
As far as "Yikes, I sure hope you are not licenced to actually sign off on anything
":What part of A&P confused you?
__________________________________________
Two points:
(1) But that's all TQ IS! You are simply stretching a bolt to a prescribed a value! And on really critical applications it is taken one step further to include special grease and a rotational limit in which you must get the prescribed TQ.
(2) Re your A&P - my point exactly. Where did you get it - out of a popcorn box?
Last edited by oleary; 22nd Feb 2013 at 04:14.
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oleary,
You've hijacked the thread. bhawkh60 simply said that on the aircraft he has worked on, he has not applied the specific procedure of measuring bolt stretch as a function of measuring proper torque. Nor had I, after years as an AMT.
Had he been assigned to work on a Robinson, he would have seen the written procedure, and followed it in accordance with the manufacturer's specifications. That's what a good A&P does. Get off the tiny soapbox, please.
You've hijacked the thread. bhawkh60 simply said that on the aircraft he has worked on, he has not applied the specific procedure of measuring bolt stretch as a function of measuring proper torque. Nor had I, after years as an AMT.
Had he been assigned to work on a Robinson, he would have seen the written procedure, and followed it in accordance with the manufacturer's specifications. That's what a good A&P does. Get off the tiny soapbox, please.
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Just as an aside,,,,,There was no torque figure for any pre-war Rolls Royce engines. RR mechanics were trained to "know" how tight and "feel the bolts down".... proper engineers, how times have changed... 
Don't worry I'le get me coat......... !
E.
Don't worry I'le get me coat......... !
E.
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As said earlier, bolt stretch is a simple concept, easily understood and applied.
However, these are not tractors. These are aircraft. If the procedure is not called for in the OEM maintenance manual, then you do not perform the procedure. If it is, then the AMT performs the procedure. In many cases, an AMT can work on a variety of aircraft in his/her career where bolt stretch is not an approved procedure according to the OEM. That is the point you seem to be missing.
'Nuf said.
edited for Efirmovich: Don't forget the old German torque applied to the Bolkows..."Gutentite"
However, these are not tractors. These are aircraft. If the procedure is not called for in the OEM maintenance manual, then you do not perform the procedure. If it is, then the AMT performs the procedure. In many cases, an AMT can work on a variety of aircraft in his/her career where bolt stretch is not an approved procedure according to the OEM. That is the point you seem to be missing.
'Nuf said.
edited for Efirmovich: Don't forget the old German torque applied to the Bolkows..."Gutentite"
Last edited by Matari; 22nd Feb 2013 at 19:07.
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There was no torque figure for any pre-war Rolls Royce engines
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Overtorquing bolts also distorts and stretches the threads ..... whether in automotive , industrial , or aviation.
Do a little test on your next coffee break in the shop ..... and do it in front of all the musclemen who figure tight as hell is good enough.
Use common grade hardware store bolt , 3/8" by 2" long. ...... place a nice stack of washers on the bolt until there are only enough threads showing for the nut to be installed ...... use two wrenches and tighten normally ..... then as hard as you can .... tighten it a lot more.
Spin the nut off , remove the washers , and try to spin the nut all the way down the threaded area .... you will only get partway because the threads have been stretched and the nut will bind. You should even be able to visually see where the threaded area has stretched.
Next time you are having trouble changing the tire on your car because the wheel nuts are hard to turn ..... some muscleman at some gas station has stretched the wheel studs with a 250# impact wrench.
Do a little test on your next coffee break in the shop ..... and do it in front of all the musclemen who figure tight as hell is good enough.
Use common grade hardware store bolt , 3/8" by 2" long. ...... place a nice stack of washers on the bolt until there are only enough threads showing for the nut to be installed ...... use two wrenches and tighten normally ..... then as hard as you can .... tighten it a lot more.
Spin the nut off , remove the washers , and try to spin the nut all the way down the threaded area .... you will only get partway because the threads have been stretched and the nut will bind. You should even be able to visually see where the threaded area has stretched.
Next time you are having trouble changing the tire on your car because the wheel nuts are hard to turn ..... some muscleman at some gas station has stretched the wheel studs with a 250# impact wrench.
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If I remember correctly - and I often don't - the installation of the power turbine rotor in the LTS101 engine requires a combination of torque on the retainining nut (applied with a torque wrench) and stretch of the shaft (measured with a DTI). Both must be within prescribed limits to achieve an acceptable installation.
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If I remember correctly - and I often don't
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I also work hardens the bolt.
Robinson helicopter rotor head hardware is stretched, not torqued.
Not only that, the nut head is drilled and cotter pinned after the proper stretch is achieved. Since the bolt work hardens it can only be stretched 2 or 3 times before the drilled holes will no longer match up on the nut.
Then you have to use new hardware.
Bolt stretching is nothing new, the main crank assembly on radial engines required stretch back in the day.
Robinson helicopter rotor head hardware is stretched, not torqued.
Not only that, the nut head is drilled and cotter pinned after the proper stretch is achieved. Since the bolt work hardens it can only be stretched 2 or 3 times before the drilled holes will no longer match up on the nut.
Then you have to use new hardware.
Bolt stretching is nothing new, the main crank assembly on radial engines required stretch back in the day.
Last edited by 68fish; 18th Mar 2013 at 05:04.
Coning Bolts
If a coning bolt has failed to maintain tension due to fatigue/embrittlement/over-torquing, or for any other reason, then the immediate outcome is that the disk is not teetering about the teeter hinge, it becomes some point offset in the direction of the coning bolt that has lost tension. That is annoying in the hover.. in forward flight it would impose some pretty dramatic loads.
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in forward flight it would impose some pretty dramatic loads.
To explore; perhaps a major vibration with nodular points along the blades until it whips badly enough to cause the opposite blade to fail and begin tearing - the one with the good bolt - that causes a sudden rotational shock load - which causes its coning bolt and the first failing coning bolt to fail simultaneously?
The damaged blade stays in one piece because at the time of failure it still had not lost the torn piece and immediately had a different flight vector towards the blade tip thus allowing the torn bit which was attached at outer end - to stay still attached.
tet.