HI
Went flying this morning, first flight of the year, the engine was hard to get wet enough to start. Then finally got started, went up in the air, sounded funny (lean), engine just stopped in the air.
Landed deadstick, 5 or 6 prop bolts sheared off, bent spinner, gouged prop and trashed spinner.
People at field thought engine backfired, has anyone had this happen?


Thanks in advance

Bill

The sheared bolts happen when the prop is compressed, so if the engine did backfire the loose prop did it...Sometimes when the ignition battery is low it causes funny timing......

I agree with RCIGN1, From personal experience!! (trashed all 6 bolts and disintigrated prop) I have purchased a reliable torque wrench and now religiously check ALL my engines before flight for proper fit and tightness on each prop.The power that these motors are putting out will easily break props and prop bolts if the proper tension on the bolts is ignored or GUESSED at !! Too loose and it becomes sloppy, Too tight and your stretching the bolts to a certain extent which eventually will cause failure.Overtightening is just as bad as undertightening especially in the larger horsepower engines.

Loose bolts and a wooden prop.
Loose bolts will fail quickly in fatigue - just like the wheel studs on your car if you don't tighten the lug nuts.

The socket head cap screws in the hub will not likely fail from tensile overload, they're just too strong for that.

Sorry Diablo but I have to disagree with that one,I have already done it myself to prove that to be true.Since these bolts are not heat treated like grade 5 or grade 8 bolts they will STRETCH if overtightened,and once they heat up they will eventually collapse and sheer from fatigue.

Hi Guys
I was using a Majzlic prop, what should the prop bolts be torqued too?


Bill

carbon fiber=40 inch pounds

wood=37 inch pounds

Fryfly:
Interesting theory.....but wrong.
I'm a metallurgist.

A tight bolt sees no cyclical stress, so it doesn't fatigue.

Overtightening the bolt may pull the aluminum threads out of the hub.

A 4 or 5 mm. bolt will EASILY STRIP the threads in aluminum.Been there & done it.Had to get longer bolts.Just try it & you will strip the aluminum threads if you keep turning....no way will you shear the bolt head off.
BTW,I see in the other thread(hey,good pun huh?) that it's been done on a 3W 70.

either way make sure the bolts are peoperly and evenly torqued to required specs.And check frequently to be safe.

Bolts are cheap..Why not get some grade 8 bolts from the hardware store ? Not plated like 3W ? So what, they're covered up by the spinner...

I have thought the same thing!! but what happens when you break one of those bad boys off inside,How do you get them out without completely tearing the whole front end apart or completely replacing the fron hub, magnets and all ?

Hi
Where can I get a torque wrench in inch pounds, a web address would be nice, not a whole lot of stores where I live.


Thanks

Bill

Here ya go Bill, Try below link:

http://www.harborfreight.com/cpi/cta...temnumber=2696


.......Mark

http://www.harborfreight.com/cpi/cta...temnumber=2696

this address takes you straight to the page you need,you can order online or simply call they take orders over the phone 7 days a week !

WOW Looks like I out typed you by a nanosecond FryFly

One thing that I always do, is track my props. If the prop is not tracked right it will cause some pretty good vibration.

Later, Scott

Let's say you use grade 8 bolts and they don't break.....what then is the weak point ie sacrificial point in all this. I'd rather break a hundred bolts than trash that aluminum hub.

your exactly right fancman ! grade 8 bolts are not that easy to just drill out,especially in aluminum. The front hub is $40.00 and the magnets are $6.00 each for a total of $12.00 and then shop time and shipping.Is it really worth the so called extra assurance you think you would get from grade 8 bolts? I think not,atleast for me it isn't.the grade 8 bolts might take a little more pressure but the side load that is put on these puppy's would still cause them to snap if they were loose, especially when were pushing ove 9 horse and up with these larger engines. I'd rather spend the money on the wrench and check them frequently,that's really all it takes. I'm not brave enough to experiment with the other bolts so I'm gonna stick with the supplied bolts that come from the manufacturer. If by chance I do lose 1 or 2 or so I have a better chance of drilling them out if need be versus dynamite on the grade 8's. Just a thought.

To the guys who say you can't break these botls by over-tightening them, I beg to differ. I've had several 3W bolts break off flush in the hub while being tightened. The fracture point was inthe threaded portion of the bolt, leading me to conclude that not only fatigue, but also the smaller diameter at that point (inner part of the threads diameter) tends to weaken the bolts in that area over a logn period of time.

I've taken to buying 30 or so 5mm bolts of the proper length from Microfasteners, and keeping them in sets of 6. I always "feel" the tension while tightening the bolts, and I've stoppe dshort several times as a bolt got "soft". When that happens now, I jsut toss the entire set of bolts and install all new ones.

The bolts from Microfasteners are Grade 5 bolts, if memory serves me correctly. Also, as stated above, about the only way you will break an entire set at once, while the engine is running, is by them being loose, or by not having a full set installed. I've actually considered drilling my 3W hubs for the smaller 100cc pattern, and installing hardened anti-rotation pins in the inner diameter hole pattern. Probably tremendous overkill, though.

originally posted by Kris
I have to agree with you on that one Kris, but I wasn't going to argue with Diablo on that one especially when he's a metallurgist !! I figured if I said anything I was merely beating a dead horse !!!!!! I have broken them too,and I'm not the only one,and regardless of popular metallurgist belief bolts can weaken/stretch/twist/break off if they are tightened too much or past their capability. I would think that if they were impervious to breaking under these conditions there would be no reasoning behind the torque specifications that comes from the manufacturer.I don't believe that point is just for the aluminum threads in the hub, but also for the specific bolts supplied. And again that's just my theory on the subject.

If a bolt weakens over time, it happens by growing a fatigue crack. The tensile prestress (tightening the bolt) reduces the fatigue stress and prevents the growth of a fatigue crack. Bolts are designed to be prestressed by tightening so that the stress on the bolt is below the yield strength in the elastic range of the stress-strain curve. If the bolt is initially tightened to a lower stress level (lower foot-lbs of torque on the wrench), the fatigue cracks will grow faster.

If you want to understand this a little better, take a look at the Metal's Handbook - Edition on Failure analysis. There's a nice chapter explaining bolt and fastener design theory.

Hey Diablo,I don't really want to get into it that far,you pretty much went over my head with that last comment !!! All I know is that if I do it wrong it will end up breaking, it's that simple.
I guess if we follow the criteria set before us from the manufacturers as far as torque specs go,we should be just fine.
I have broken all 6 bolts before on one engine and 3 on another,that was the whole reasoning behind me purchasing a torque wrench just for prop tightening.The reasons behind the failures are pretty much my own fault for not properly tightening them with a good wrench,my advice to all is to invest in one and use it !! RELIGIOUSLY !!!!!!!!!!!!!!!!

In my line of business (automotive repair) we have "throw away" bolts for certain applications, especially for head bolts on certain engines. They are a one-time use item because they are tightened to a "Torque to yield" specification where you first tighten to a lower than nominal tension, then another 90 or 180 degrees of rotation to preload the bolts against further stretch and compress the head gaskets. You CAN re-use these bolts, but it is highly recommended by the manufacturers that they be replaced.

This is just one example of bolt "failure" or a bolt developing questionable second-time use characteristics because of initial over-tightening.

The same is true for ANY bolt. I guarantee that I can snap off a brand new 10-32 or 5mmx.8 thread bolt by merely tightening it with a T-handle, and the bolt will not break in the thicker shank area, but in the threaded area directly adjacent to the material it is being screwed into. That being said, it is silly to tighten any bolt that tight in our applications. The desired procedure is to pre-tension the bolt so that a maximum of bolt "stretch" is achieved without permanently changing the bolts length. This is possible because steel is mildly elastic, and will allow a small amount of "stretch" or pre-load without incurring permanent physioligical change. However. . "stretch" these bolts, with the focal point for that stretching, and eventually they will work-harden that small area and it will become brittle or start to fatigue. When THAT happens is when you will start seeing failures during tightening of the bolts, but it is highly unlikely that a bolt will fail (unless grossly over tightened) while the engine is running. That little game still belongs to non-preloaded bolts that have not been tightened properly, and are basically moving around a bit and developing rapid fatigue stress in the localized area next to the screwed into material (prop hub). It doesn't take very many cycles to develop a failure mode, in this instance.

Best bet. . tighten the prop bolts TIGHT, and worry about breaking them from overtightening them the next time around if you change props or have to take the current propellor off. Even better is the use of a torque wrench, but it's still not totally possible to make sure you have enough bolt "stretch", although, in theory, torquing to a known "proper" value is much better than not getting there at all.