TEETER BOLT

JETLAG03

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Hello. I have a 8.8 grade M10 teeter bolt and whilst there seems nothing wrong with it, simply because I don't know its history, along with all the other critical bolts I am checking sizes to replace them. I would like to use AN grade but cannot find metric sizes.

So, if staying with non- an bolts I find the sheer strength increases rather dramatically with the 10.9 and more with the 12.9 grade bolts.

Looking for advise, would you move up to 10.9 or 12.9, if not why?

I'm not an engineer and more than happy to be told my logic is flawed and why rather than simply change them and create a huge problem of myself.

regards

Phil
 

XXavier

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If I remember correctly, the 'Jesus bolt' of my ELA is 10.9
But if it were 8.8, I wouldn't be much concerned. A simple calculation shows that such a bolt may hold five tons in tension, without deformation. That's ten times the weight of my gyro. Even accounting for some extra load in certain moments, and for the fact that the bolt is loaded in shear, I believe that there's a wide margin...
 

Brian Jackson

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I have no expertise with this application yet, other than to mention that harder materials tend to be more brittle. AN hardware, though softer, is designed to bend before it breaks. I believe AN is similar to grade 5 but as I understand use a "UNJ" thread form that provides greater fatigue strength. AN have rolled threads instead of cut which would degrade their strength. Though I can't offer any valuable advice, these are a few things to consider before substituting hardware.

My first move might be to contact the manufacturing company of your gyro and pose this question to them.
 

thomasant

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Which one is the "Jesus Bolt"? Is it the main spindle bolt that holds the head to the torque bar, or the teeter bolt that holds the hub bar to the head towers? I have heard these two bolts used with the same name.
 

AirCommandPilot

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Which one is the "Jesus Bolt"? Is it the main spindle bolt that holds the head to the torque bar, or the teeter bolt that holds the hub bar to the head towers? I have heard these two bolts used with the same name.
The Jesus bolt is the one going vertically through the main bearing.
 

fara

AR-1 gyro manufacturer
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Hello. I have a 8.8 grade M10 teeter bolt and whilst there seems nothing wrong with it, simply because I don't know its history, along with all the other critical bolts I am checking sizes to replace them. I would like to use AN grade but cannot find metric sizes.

So, if staying with non- an bolts I find the sheer strength increases rather dramatically with the 10.9 and more with the 12.9 grade bolts.

Looking for advise, would you move up to 10.9 or 12.9, if not why?

I'm not an engineer and more than happy to be told my logic is flawed and why rather than simply change them and create a huge problem of myself.

regards

Phil
AN bolts do not come in metric sizes. M10 Class 8.8 (not grade 8.8) metric bolt is closest to 3/8 or AN6 bolt but its slightly larger in diameter than AN6. In theory its stronger than AN6.
AN would be roughly equal to Class 8.8. Class 10.9 is a stronger bolt but harder and a bit more brittle. Its best to stay with what engineer had selected for your application. Harder bolts bend less before breaking. In aircraft you want things to bend. Having said that we use Class 10.9 M12 teeter bolt for Averso rotor teeter block also. The rest of our hardware on AR-1 is mostly SAE or AN hardware. Just make sure you are not getting a low quality metric bolt.
 

JETLAG03

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Virollet 17260 FRANCE
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300+ flexwing (pendulaire) training for gyrocopter (autogire)
@Brian Jackson thanks for your reply I can ask the manufacturer ... but it's me and I don't know. It's a kit machine from the annuals. No technical data available. The bolts will be taking the strain in the sheer so I suspect their thread is less important other than to keep the bolt in place as there should be little tension force along the length of the bolts.

@fara Thank you for your explanation of the bolts. The original specs for the bolt is unknown to me, class 8.8 is in situ at the moment, but looking at the tables for the difference in sheer strengths between the 8.8 (39kn) and the 10.9 (51kn) caused me to pose the question.
 

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fara

AR-1 gyro manufacturer
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@Brian Jackson thanks for your reply I can ask the manufacturer ... but it's me and I don't know. It's a kit machine from the annuals. No technical data available. The bolts will be taking the strain in the sheer so I suspect their thread is less important other than to keep the bolt in place as there should be little tension force along the length of the bolts.

@fara Thank you for your explanation of the bolts. The original specs for the bolt is unknown to me, class 8.8 is in situ at the moment, but looking at the tables for the difference in sheer strengths between the 8.8 (39kn) and the 10.9 (51kn) caused me to pose the question.
Ok. Got it.
Just to give you some simplified numbers. A M10 class 8.8 bolt in double shear carrying 1000 pounds of hanging weight in shear will break at approximately 20 Gs.
You will be ok. Just change it every few years. Also make sure that the shank or grip length is enough and carrying the load in shear and no load is put on the threads at all. Use washers under the head and under the threads and use a castle nut only hand tight with a locking safety pin or cotter pin. The reason for changing it every couple of years is fatigue as depending on your rotor head the bolt shank surface may be being used as a bearing surface.
 
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JETLAG03

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Virollet 17260 FRANCE
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300+ flexwing (pendulaire) training for gyrocopter (autogire)
WOW !! 20Gs ... That does go a long way in giving confidence in these seemingly small diameter bolts. Thank you for the data @fara. Also for the advice for tightening the bolts only hand tight
 

AirCommandPilot

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"only hand tight"
Shouldn't the bolt be tight enough to lock the bushings/bearings in the tower plates to the tower block so the rotor teeters using the bushings or bearings instead of the bolt?
 

Alan_Cheatham

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Shouldn't the bolt be tight enough to lock the bushings/bearings in the tower plates to the tower block so the rotor teeters using the bushings or bearings instead of the bolt?
It depends on the head design and if the teeter bearings are in the towers or teeter block.
European typically put bearings in the teeter block and are hand tight, others use bearings in the towers and the bolt is torqued to specs to keep the "Hats" tight against the teeter block.
 

kolibri282

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In designs with high cyclic loads that I have been working on (mostly railway stock, some aerospace) the critical design parameter usually was the fatigue strength of the material. You should check if the bolts with higher elastic limits do not have a lower fatigue strength. Ultimately the designers of the aircraft would be the ones who could answer questions like this.
 
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XXavier

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FW Savannah XL + ELA R-100 and Magni M24 autogyros
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1394 FW + 526 gyro (Oct. 2019)
It depends on the head design and if the teeter bearings are in the towers or teeter block.
European typically put bearings in the teeter block and are hand tight, others use bearings in the towers and the bolt is torqued to specs to keep the "Hats" tight against the teeter block.
It's exactly so in the ELA. The bearings are in the teeter block, and the bolt's nut is tightened by hand, fixed in its position with an Allen screw, and then secured with a pin.
 
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