Prop Bolts

[KrisW]

The total shear load on 6 bolts is unimportant. . IF they are tight.

You cannot go by the total tensile strength of the bolts to determine the shear strength. 6 bolts, which in this case can hold 9,000 lbs in a straight tensile strength test, will easily shear at a much lesser amount of force. But, it's not merely a constant twisting force between the prop hub and prop that shears the bolts .. it's work hardening and fatigue from constant flexing that eventually make the bolts fail. The less tension, and more slippage, the quicker the bolts fatigue, and the fewer back and forth cycles are necessary to make the bolts fail.

Figure a failure probablity in the realm of tens of thousands of cycles or revolutions, versus 100's of millions, if the bolts are not properly tightened, and the prop begins to move around, work-fatiguing the bolts in one particular location.

Which is why loose props come off. . and tight ones never go anywhere.

[dick hanson]

As long as there is an effective zero moment arm on side load to the bolts - --there should be no problem -looseness (side to side of any distance) will eventually lead to failure
So back to original question -- Did we resolve the practical methods for dermining the way to correctly tighten the props?

[KrisW]

	Quote: Originally Posted by dick hanson
	As long as there is an effective zero moment arm on side load to the bolts - --there should be no problem -looseness (side to side of any distance) will eventually lead to failure So back to original question -- Did we resolve the practical methods for dermining the way to correctly tighten the props?

Of course we did, Dick . .. Get some Unobtainium alloy bolts .a 400 ft.-lbs impact gun, some super-adhesive between the props, spinners, and hub, and hammer until the bolts quit turning. . . .

Oh. . wait .. that's for a Corsair hub. . .

for 5mm bolts. . 55-60 in-lbs of torque, crossing pattern

for 8mm center bolts. . get a wrench, pull as hard as you can. . it won't break (I've tried)

[Boulder]

I broke the center bolt on my ZDZ 80 a couple of months ago.

[dick hanson]

The Titanium bols they once offered would fracture - dune it
The std mild steel bolts - never saw one let go - but it apparantly happened
the torque for those locking nut setups need not be high 15-20 ft lbs then locked should do it
Please:always be certain to seat the stud in the crank - failure to do this can and will cause a problem -- I have seen one left unscrewed far enough to hold the spinner away from the hub - this setup a fail sure condition-- prop bolt s broke as the stud wound IN ,loosening all the bolts .
Stick around long enough and the possible failures increase with any setup.

[Boulder]

	Quote: Originally Posted by dick hanson
	The Titanium bols they once offered would fracture - dune it The std mild steel bolts - never saw one let go - but it apparantly happened the torque for those locking nut setups need not be high 15-20 ft lbs then locked should do it Please:always be certain to seat the stud in the crank - failure to do this can and will cause a problem -- I have seen one left unscrewed far enough to hold the spinner away from the hub - this setup a fail sure condition-- prop bolt s broke as the stud wound IN ,loosening all the bolts . Stick around long enough and the possible failures increase with any setup.

No doubt, but my stud was seated, I had a hell of a time getting the broken stud out because it was locktighted in. Got it out, though, and been flying it ever since.

Never have a problem with my bolts, just stripping the heads after time. I am constantly buying new allen wrenches so I don't have that problem. I'm sure I'm doing something wrong, I need to get a good set of T-wrenches.

[dick hanson]

There are some nice , inexpensive weighted handle ones on the market - which make it easy to spin the bolts into position- (nice flyheel effect)check with a local "Tool Shed" or similar shop.
I use these and just tighten till I feel the wrench seat then load up for a quarter of a turn. On glass props - that is tight .

[Boulder]

	Quote: Originally Posted by dick hanson
	There are some nice , inexpensive weighted handle ones on the market - which make it easy to spin the bolts into position- (nice flyheel effect)check with a local "Tool Shed" or similar shop. I use these and just tighten till I feel the wrench seat then load up for a quarter of a turn. On glass props - that is tight .

I'll check into those, Dick. My problem has been with my wood props, as you know you can make a few revs around the bolt pattern and still have them loosen when you tighten the center bolt. I guess "patience is a virtue".

[Colonel Klank]

IMHO
Another, yet unexplored, reason for shear failure would be that the bolts were of inferior quality. Just because we might have the proper grade, torque, roughness+smoothness and we paid a lot of money for them, doesn't mean that the bolts are perfect. While bolts are being manufactured, the dies that cut the threads are continually wearing down causing an almost linear degradation in the quality of the threads. The bolt manufacturers know when to change the dies because their Go, No-Go guages tell them so and also before the linear degradation spikes to the "Slam Wore Out" phase. A perfect bolt will have a flat on the crest AND the root portion of the thread and the thread flanks will be smooth in appearance. The measurement of the "Flat Distance" is Pitch divided by eight. If the flat is buggered up, your bolts will not be as strong. ("buggered" is now widely recognized as a legitimate engineering term)
Moral of this story: All bolts are not the same even though the grade says so. Spend your money on the Big Brand Names and you can have confidence that your bolts will be of maximum strength. Next time you open the box of bolts, get out your magnifying glass and look at the threads closely. You will find differences.
(I think)

[KrisW]

	Quote: Originally Posted by Biff
	Another potential culprit is ignition issues. Saw three cases of this last year. It's UGLY!!

with ignition problems, you can lose the entire hub assembly, because the rattling of detonation will cause the hub to loosen on it's pressed-fit nose of the crankshaft, then start spinning, ripping out the woodruff alignment key (NOT anti-rotation .. forget that idea) and sometimes totally departing the aircraft. . .

NOT good. . . which is wy I insist on running higher octane gas to prevent detonation.

[Freddy Warbird]

No matter how one wants to slice it, I'll bet, that 90% of the time, the bottom line is, the fasteners are coming loose, or were never tightend properly in the first place. I see people whipping out the allen wrench everytime they come out to fly. Why? Are you afraid the prop is going to come loose? My theory with wooden props lyes with the prop hub. The hub is only made out of wood. -So what- if there is an aluminum plate on the face, and an aluminum face on the back . keep tightening, and the plates are going to sink into the prop hub face, and back. This will yeild unequal torgue, and weaken the prop hub. Once this happens, I believe the prop is junk, and unsafe. Likewise, by chrushing the prop hub, the bolt will rotate deeper into the bosses possibly bottoming out the threads giving a false torque reading, and depending on which prop one chooses to run, the bolt may be too long to begin with. If one chooses to use a torque wrench, I would suggest using a "dial" inch pound torque whench on wooden props, and a clicker inch pound torque wrench for carbon fiber props.Why? For wooden props, the hub is soft, so a clicker may never indicate torque. On the other hand, carbon fiber prop hubs are harder than wooden ones, so I would venture a clicker would be pretty accurate. One other thing, we are, at least most of the time, appling steel to aluminum. From what I have learned about fasteners, this requires some type of lubricant to the threads to prevent cold welding of the two pieces. Without some type of lube on the threads could also give a false torque value when applying aluminum and steel using torque wrench, or tightening by hand. Loctite is considered a lubricant when applied between aluminum and steel. I forget the percentage value change between wet vs dry, but I think wet torque percentage change vs dry is negligible with respect to what we are tightening. These are not torque to yield bolts.

This is what I do, and have had great sucess with regard to "wooden props."

1.) Drill prop with jig. (recommend use of a drill press) Feed fasteners through with each completed hole.
2.) balance prop.
3.) Clean all fasteners/ threads free of oil, grease, etc.... Wire wheel off any old loctite etc.....
4.)Apply Blue(242) loctite to the fasteners about 5/8" wide.
5.) Feed all the fasteners through the prop, and into the engine hub. Rotate them all down until flush.
6.) Now comes the part that take patience, and experience. With the allen wrench, start snugging them down evenly in a star pattern until snug.
7.) Go around in a circle several times choking up on the allen wrench, and giving the fasteners a little more torque. There is a point where the fasteners become tight, and no further tightening is reqired. I know where this is. It just takes practice. If you can't find this "feel spot", you starting to chrush the prop hub, or your not tightening the bolt enough. Either way, thats Bad!
Thats all I do!

Now, If I, for what ever reason, have to remove the prop to service, or inspect the engine, I repeat steps 3 thru 7. When I go to the field, I inspect the prop, and check that its not loose. I grab the thing while holding the spinner.If it doesn't rock, or have any play, its not loose. Why do I want to screw up a well torqued prop when there is no need to? Also, by going in there everytime one go's to the field, more than likely, your crushing the hub, and possibly unevenly causing runout. Its a spinning disc, so, runout applys which could cause vibrations. Thats also bad!
No problems yet!
Thats my two cents on this subject.

[Temptation]

	Quote: Originally Posted by Boulder
	I broke the center bolt on my ZDZ 80 a couple of months ago.

On July 9th 1987 at 2:36.26 seconds I was assembling my Sig Kadet. When I was installing the 1/4-20 nylon wing bolts, I over tightened the left rear one causing the bolt to snap. At that point I was wishing that I had built the plane with a rubber band retention system like the instructions called for. Now, if I would have known the correct torque specs, and had a torque wrench that was calibrated by Rockwell, along with knowing what the shear limits were on the bolt this event would have never happened. To this day I still hold the wings on all my planes with rubber bands, including my 40% planes. Kris buddy, where were you when I needed you back in 1987?

[KrisW]

	Quote: Originally Posted by Temptation
	On July 9th 1987 at 2:36.26 seconds I was assembling my Sig Kadet. When I was installing the 1/4-20 nylon wing bolts, I over tightened the left rear one causing the bolt to snap. At that point I was wishing that I had built the plane with a rubber band retention system like the instructions called for. Now, if I would have known the correct torque specs, and had a torque wrench that was calibrated by Rockwell, along with knowing what the shear limits were on the bolt this event would have never happened. To this day I still hold the wings on all my planes with rubber bands, including my 40% planes. Kris buddy, where were you when I needed you back in 1987?

HAHAHAHAH Almost too funny, Dude!!!!

Lets see. .1987. . . I was either in N. Korea putting the final details onto the death of Kim. . . languishing in a prison in Cambodia while finding leads on the Heroin smuggling rings. . . or perhaps it was tht brief romantic interlude with a soon to be famous woman (then teenager) named Elle Macpherson, on the shores of Northern Aussieland. . . .

Ahh. .so many possibilities that year. . How could I have POSSIBLY forgotten to send out my blanket snail mail statement to all AMA members concerning Bolt Torque specifications???

Well. . now we all know. . so you better NEVER forget again!!!!!

You guys want to know how we can stop wooden props from crushing? Simple. . drill the 5mm holes out to 1/4", install stainless steel sleeves that have an 1/4"OD/5.1mm ID, then face the front and back of the hub with a 1/8" aluminum disc, JB welded in place .. .No crushing. . no slippage. . no more problems. .

[Temptation]

[quote=KrisW]
Well. . now we all know. . so you better NEVER forget again!!!!!/quote]

Kris, I didn't forget, and I won't forget. I never knew the nylon 1/4-20 specs to begin with, and I still don't because you didn't tell me what they are yet. That's o.k. though because I am still using rubber bands...remember?

Honestly dude keep the info coming, I find it very interesting. I just felt like being a little bit of a smart ass this morning.

Mike Darr

[KrisW]

[quote=Temptation]

	Quote: Originally Posted by KrisW
	Well. . now we all know. . so you better NEVER forget again!!!!!/quote] Kris, I didn't forget, and I won't forget. I never knew the nylon 1/4-20 specs to begin with, and I still don't because you didn't tell me what they are yet. That's o.k. though because I am still using rubber bands...remember? Honestly dude keep the info coming, I find it very interesting. I just felt like being a little bit of a smart ass this morning. Mike Darr

OH. . you DEFINITELY Succeeded!!!

There is no spec for nylon bolts, as they are not considered to be a "structural" bolt, and have very limited ability to withstand stresses. I've NEVER used nylon bolts for wings . .always steel bolts .. even on my Sr. Telemaster I used 8-32 steel bolts in 4 places to hold the wings on.

The only thing I use nylon bolts for is canopy retainers through the side of the fuselage. I use 8-32 "thumb screw" nylon bolts for this purpose. Anything else is steel.

The best spec I can give for nylon bolts. . . when it touches the surface, give an additional turn to seat things, and that's it (assuming things all mate perfectly flat). If you have to "wind it down" into place using the bolts, as in some wing mounting designs, I recommend using STEEL bolts, and large diameter fender washers to spread the loads. I have yet to see any plane, with a bolt on wing, such as a trainer, or 60 sized 4-star or comparable plane, that NEEDS more than an 8-32 steel bolt to hold the wing on. Using 1/4-20 nylon is, IMO, begging for disater because the nylons bolts CAN break . .planes don't fly very well in cases such as that.