Safety Alert!

[500e]

There is no thing as idiot proof, there is always a better idiot round the corner :noidea:

 

[barnstorm2]

Hello Kevin!!

 

[Jazzenjohn]

For a prerotator to be able to spin blades up to 300 RPM it needs to be in excess of 8 horsepower. 8 horsepower equates to about 4400 foot pounds force/second. That is quite alot of torque. Not quite enough to break the bolt by itself, but if the force is intermittant or cyclical as when a belt is intermittantly slipping or if a pulley is out of round it becomes a potent damaging vibration that could easily break the bolt over time. Whether or not the prerotator went to full RRPM isn't really necessary for damage to occur. The loads on the mechanical components are greatest at initial startup and full speed, but with an intermittant application of power, the prerotator would act like an impact wrench on the ring gear which is essentially twisting the bolt between the torque tube and the pitch block with a tremendous amount of mechanical advantage due to the extra large ring gear. The units that are most at risk are the ones where there is the most vibration/juddering, the ones with the longest, heaviest blades, and the ones abruptly spun up from a dead stop.

 

[cburg]

Bingo! You nailed it.

For a prerotator to be able to spin blades up to 300 RPM it needs to be in excess of 8 horsepower. 8 horsepower equates to about 4400 foot pounds force/second. That is quite alot of torque. Not quite enough to break the bolt by itself, but if the force is intermittant or cyclical as when a belt is intermittantly slipping or if a pulley is out of round it becomes a potent damaging vibration that could easily break the bolt over time. Whether or not the prerotator went to full RRPM isn't really necessary for damage to occur. The loads on the mechanical components are greatest at initial startup and full speed, but with an intermittant application of power, the prerotator would act like an impact wrench on the ring gear which is essentially twisting the bolt between the torque tube and the pitch block with a tremendous amount of mechanical advantage due to the extra large ring gear. The units that are most at risk are the ones where there is the most vibration/juddering, the ones with the longest, heaviest blades, and the ones abruptly spun up from a dead stop.

 

[Alan_Cheatham]

For a prerotator to be able to spin blades up to 300 RPM it needs to be in excess of 8 horsepower. 8 horsepower equates to about 4400 foot pounds force/second. That is quite alot of torque.

I beg to differ.

To spin 23 foot Dragonwings to around 300 rpm would require around 12 hp, that would equate to a torque of only 210 foot pounds.

Ultimately the amount of torque you can apply to the rotor is determined by the power source multiplied by the gear ratio.

.

 

[Vance]

What I feel is important.

What I feel is important.

I beg to differ.

To spin 23 foot Dragonwings to around 300 rpm would require around 12 hp, that would equate to a torque of only 210 foot pounds.

Ultimately the amount of torque you can apply to the rotor is determined by the power source multiplied by the gear ratio.

.

I checked John’s numbers and 8 horsepower equals 4,400 foot pounds per second.

I checked Alan’s numbers and 12 horsepower at 300 rpm equals 210 foot pounds of torque.

I feel that 12 horsepower is probably closer because of the excess power needed to accelerate the blades.

Either way it is a lot of torque and that makes having the pivot bolt properly torqued so the stack of stuff makes it stronger more important.

I feel any juddering increases the load on the bolt.

I suspect that any of this is well below the force required to sheer the bolt or cycle it enough to cause a fatigue failure even when it is not to the correct torque.

I am hoping people will not lose sight of the importance of maintaining the correct torque on any bolt, particularly those that have a stack of stuff to hold together.

I would urge everyone to check both their teeter bolt and their pivot bolt for the correct torque even if Chris’s bolt is discovered to be defective.

I feel a secondary form of retention is important for any bolt that has a twisting action on it.

In my opinion a castellated nut with a cotter pin is appropriate.

This does not change the value of the correct torque.

Thank you, Vance

 

[Jazzenjohn]

210 foot pounds of torque at what rpm Alan? I think the relevant number is the instantaneous torque at the start of the prerotation and the effect of it being intermittent or juddering, or whatever term you wish to use to describe it as not being smooth. I downgraded the HP number a little because I think most prerotation numbers are optimistic and because I didn't think it was likely to have full power on initial application like I would expect if it were a high power brushed motor. I know it is a very powerful system and it may well be 12HP or even greater.

 

[Jazzenjohn]

The 12 HP value is an approximate value of the power required to spin the blades at 300 RPM. The system is powered by the gyro's engine and the output available to the prerotator isn't a constant number. It is going to be the output of the Engine, minus the energy absorbed by the prop at the RPM it is running, minus losses. On a 4 stroke engine there is much more power available over prop load at lower rpms because of the flatter torque/power curve it has compared to a 2 stroke. On a 2 stroke the power curve and prop load are fairly close all the way up the rpm scale.

Sorry Vance, I still don't think the bolt torque is much of a factor in a double shear trunnion setup like it is.
Maybe Chuck Beaty will chime in and offer his insight on this.

 

[Vance]

No offence taken john.

No offence taken john.

Sorry Vance, I still don't think the bolt torque is much of a factor in a double shear trunnion setup like it is.
Maybe Chuck Beaty will chime in and offer his insight on this.

Hello John,

I am not saying that low torque is what broke the bolt.

I have noticed that often people don’t properly torque bolts that have a cotter pin.

I feel this increase both the stress and wear on the bolt in this sort of application.

I am seizing the opportunity to remind people of the importance of the correct torque.

In my opinion the picture of Chris’s bolt shows signs of insufficient torque.

I am waiting to read the report on the bolt.

I am having trouble imagining a -6bolt in shear going thru a piece of aluminum is the weak link in this system.

I would expect the hole in the aluminum to become distorted long before shearing a -6 bolt.

I have never watched Chris spin her blades up so I am missing a big part of the picture.

Thank you, Vance

 

[Alan_Cheatham]

Sorry Vance, I still don't think the bolt torque is much of a factor in a double shear trunnion setup like it is.

At this point a bit of information on bolt theory may be of help.

http://www.boltscience.com/pages/basics1.htm

.

 

[Jazzenjohn]

The bolt in question isn't being used as a fastener, but as a pivot pin for pitch on the rotorhead. That is why none of the stuff on the boltscience page applies.

You did know that didn't you?

 

[Vance]

Pivot Pin?

Pivot Pin?

In my opinion the stack of stuff is used as a pivot pin, not the bolt so much of “the stuff” does apply.

Regards, Vance

 

[Jazzenjohn]

Perhaps you are right Vance. I don't think I'm helping this thread progress to a resolution so I'll leave it now.

 

[GyroDoug]

Alan,

That tutorial was very informative. Thank you for sharing that with us. Not sure I understood all of it but I certainly understand more than I did. It seems to me from what I read that over torquing a bolt would be preferable (and give a joint more strength) to under torquing it, (as long as you don't apply so much torque that you cause some deformation or damage to the joint in some way.) Am I thinking correctly or am I missing something?

 

[cburg]

Kent, the recent purchaser of my Monarch, inspected all the head bolts and contacted me to let me know that all the bolts were as good as new. There’s around 45 hours on them. He said he was swapping them out while he had it apart.

 

[daniel lecuyer]

I have a butterfly with a 503 rotax and the only mls with the 503, i have been running this since 2005. I taxi to the end of the runway about 2,000 plus feet with the mls engaged. I have been doing this for years,one thing i learned very quickly was when first engaging the engine rpm,s, when you hear the over running clutch kick in, wait for the blades to get momentem before applying more engine rpm.this truly gives you a smooth start. I pulled my bolt yesterday and found no issues with it,also i had the two aircraft mechanics inspect it as well. We also looked at their an hardware inventory, to my suprise, half of these certifed bolts are stamped 01do.new inventory bolts that just came in as well. All certified.

 

[birdy]

It seems to me from what I read that over torquing a bolt would be preferable
This talk about the torque value is shadowing the real aim of the tension.
As iv said before, you can tightn this bolt to wotever value you can, but if the sleaves aint sized proper, its still go'n to rotate on the bolt.
All thats needed is finger tight, + 1/2 max.
Move the hinge and check if the bolt is moven with the torque tube, If it is, its tight enough.
If it isnt, shimm it so it will.

 

[Doug Riley]

Based on my understanding of the makeup of this pivot joint, Birdy's got it.

The exact torque (hence internal tension in the bolt) is much more critical in a tension application than in a shear app. like this. What we're really talking about, though, is neither tension nor shear, but looseness that allows rotation between the steel sleeves and the bolt. This wears a groove in the bolt, which then breaks on the "dotted line" when exposed to the continual "chattering" of the prerotator.

But Chris's lab report will be the last word.

 

[Alan_Cheatham]

The exact torque (hence internal tension in the bolt) is much more critical in a tension application than in a shear app. like this.

Actually what you have is a steel bushing that is functioning as a short cantilever beam with the bolt acting in tension to keep that bushing pulled tightly against the flat bottom of the recessed hole it's pressed into. The bushing is what carries the bending and shear loads, the bolt holds the bushing in place, if it is properly torqued.

Torque spec for a AN6 bolt with standard nut is 160-190 inch lbs. per FAA.


To get an understanding as to why the bolt is in tension I submit the picture of the rotor bearing spacer, it's wider diameter makes the action easier to see. As long as the bolt is tight the bolt is not really carrying a bending load and if the spacer is partly recessed into a tight fitting bore the spacer would carry the shear load as well.

.

 

[GyrOZprey]

I spoke with Nick at Arrow lab doing the testing yesterday! He was sure it was a multistage stress fracture which progressed over a period of time ... confirming what Desmon & I saw in the dull old -metal break around the outer perimeter of the bolt & the fresh metal - shear in the center! He saw no issues like hydrogen embrittlement ( different kind of break) & the bolt was hardness 28 same as the comparison from a bolt from another manufacturer! - appropriate for grade 5 (AN hardware)! The chemical analysis was still being run & he'll mail me a final report!
I'm satisfied that it was EXTREME stress due to insufficient torque to hold the assembly tight to function as designed & discussed above - compounded by the aggressive MLS system & the probability that mine had performance issues not seen in Tim's ,William Clarks & the "Border Patrol Aurora" - who-ever is flying that by now! ... (Although none of those have reported back yet ... what they saw wear-wise on their pivot bolts & if they were correctly torqued!)?????
Today I spent some time preparing to remove the old lower pulley & replace with a nice CNC new design pulley Larry sent! & also figuring out how best to switch out the old slip-clutch connecting the gearbox to pto shaft - with the new one.
Desmon was able to compare the play/wobble in John Franklins with his memory of mine when visiting & looking over John's Aurora build last week - & noted Johns was much less 'sloppy"!
So looks like I'm fixing a vibration problem - hopefully ... then progress to rebuild rotor head with the new torque tube & replacement parts & all new bolts! Then replace the main rear keel - square tube ! ... finally replace the prop ... Larry offerred to have Powerfin send me 4 new blades - if I sent him my 4 ( 2 damaged ones) - his cost!!! - THAT was quite a surprise offer as I was about to go ahead & pay for 2 replacement baldes myself!

Back to building ... & it's our busy work season!