Prop Bolts

[Robert Brandi]

Hello to everyone: I have been putting on and off props on gas engines for some time. With that said has anyone out there ever had the prop bolts sheer off at the hub while engine is running ? If so what could cause the problem ? what is the solution? would appreciate comments on this one.
Thasnks Rob

[Anna Wood]

Prop bolts are a bit to long and though they look like they are clamping the prop they aren't and the prop quickly departs the airframe.

Regards,

Anna Wood

[sweetpea]

Had it happen.......its nasty if your motor vibrates......just ask my poor edge who is still under repair.


Things that can cause it--

Bolts are old
tighten too tight
tighten too deep
not tight enough

[Boulder]

	Quote: Originally Posted by Robert Brandi
	Hello to everyone: I have been putting on and off props on gas engines for some time. With that said has anyone out there ever had the prop bolts sheer off at the hub while engine is running ? If so what could cause the problem ? what is the solution? would appreciate comments on this one. Thasnks Rob

Hey Robert,

It's great to have you here!!

As for your question, all of the above is true with one additional cause. If your prop isn't drilled correctly, meaning your guide slipped when you were drilling, you might have to force the bolts into the hub. If the holes aren't aligned perfectly, it puts stress on the bolt and the vibration will snap it.

Welcome to the Giants!!

BTW, I'm going to move your thread to the gas engines forum, more exposure for your question.

[KrisW]

	Quote: Originally Posted by Robert Brandi
	Hello to everyone: I have been putting on and off props on gas engines for some time. With that said has anyone out there ever had the prop bolts sheer off at the hub while engine is running ? If so what could cause the problem ? what is the solution? would appreciate comments on this one. Thasnks Rob

The most usual problem is the bolts loosening, and then not holding the prop firmly in place. The prop then starts working itself back and forth on the hub, and eventually fatigues the bolts threads right next to the hub, to the point where they eventually fail. One fails, and it dominoes immediately to a total failure. Lock washers are NOT a solution for this problem, because the friction on the bolt threads should be more than sufficient to hold them tight. Loctite is likewise not necessary. The most common reason for "loosening" is that the aluminum of the thrust washer (where the bolt heads are) will crush and fatigue, allowing the tension to lessen. Hardened steel washers under the bolt heads is a good way to prevent this problem. Wooden props always loosen up a little bit, you have to go back and retorque them several times before they take a "Set" and stop crushing. Either be very conscientious of this fact, or switch to CF props with solid CF hubs (Biela props use a wooden "core" that can crush slightly over time).

Bolts will also fail due to repeated over-torquing, and the stresses will tend to build up at the point where the threads just exit the hub, working a certain area of 1-2 threads until it fails from fatigue as you are tightening the bolt.

Misalignment causes higher stress on the misaligned bolts (the ones that are binding), and again fatigue failure is the result.

Then there's the ever-present possibility of using bolts that just a touch too long, and which bottom the shank portion against the hub, giving just a little bit of tightness to the prop/spinner backplate, but not enough to do the job over a long period of time. It's always a good idea to double check to make sure that the shank portion is at least 2-3mm shorter than the depth of the prop/spinner combination.

I've seen people try to use 10-32 bolts in place of the 5mm bolts. NO NO NO NO NO!!!!!!! the threads are slightly different. . you may screw it together, but then you are also damaging the hub, and the bolts have a much higher chance of failing due to flexing.

That being said. . I've seen several of the "Big hame" flyers have repeated problems with sheared prop bolts. One guy I know of locally (National and Masters competition flyer) loses at LEAST one prop every year at Joe Nall. So it's not just your novice flyers that have problems with these bolts.

The threads of the bolts are less than 80% of the thickness of the shank, meaning a 5mm bolt has a 4mm thickness at the thinnest portion of the threads, so there is a lot of stress at the point where the bolt exits the hub. There is actually a rated "Torque" specification for tightening these bolts. A small inch-pound torque wrench is really the best way to go when tightening them, using a crossing pattern.

If someone has the Spec. it would really help. I've never used it. (have calibrated "Mechanics hands". . . . . . )

[dick hanson]

A cheap , reliable torque wrench in inch plounds: Harbor Freight- fo Nineteen bucks .
That said, to specify torque these prop bolt setups is just about impossible -you can look up a given bolt and torque within it's designed operating limits but again a problem--
Back to basics:
1- measure stack height of washer and prop and spinner backplate.
run a threaded bolt into the hub -
2--find bottom -or 3/4" whichever comes first.
3-Select prop bolts which engage 3/8" to 1/2" when all assembled.
4-Clean all bolts and holes but don't lube anything.
On multiple patterns -tighten to a firm grip on a Tee handle . On a 4mm Tee handle for 5mm bolt about 8" long ( mixed measurements ,I know) the wrench will flex the shank about a 1/8-1/4 a turn when tight
On glass prop -this comes up abruptly
On wooden prop -this is gradual and keeps on going each time you tighten it
What is that torque ?pretty in low foot lbs not over 10 ft ft pounds- more like 5+
If you now have the inch /lb wrench - try some settings and get a feel for it .
Here are some comparisons : a spark plug in a ZDZ-tapered seat NO gasket - is 15 ft pounds - that is a LOT
On a typical spark plug with a crush gasket,under 10 ft pounds-
Now, a typical single bolt 8mm model prop retainer setup-10 lbs again no more tha15 even on one of those POS four strokers that try to slip/ backfire and cut your hand off.
Let's say 15 lbs is really enough on a good single 10mm bolt
that is total compression to engage clutch force needed to secure the prop ( MVVS 26- BME 40 etc)
So----15/6=less than 3 -meaning that about 5 lbs one each bolt is likley all you really need.
On wood because it compresses constantly- ya gotta check em!!
between assimilating the above threads and also learning to drill the prop for a drop in fit on all six bolts - you should be ready to rip
Any other ideas?

[KrisW]

Cmon Dick. . I'm sure there are "Engineered" torque specs for these bolts. . .I'll see if I can find some.

[dick hanson]

sure there are -for the bolts - charts abound !
I was addressing the application - the needed torque is way below bolt specs (the big problem is guys ratting out the drive hexes on some of the softer ones)
However I have never gotten a "spec " from the engine guys
Hope you can pin some down
Like pinning down tire milage --------------"it depends"

[KrisW]

	Quote: Originally Posted by dick hanson
	sure there are -for the bolts - charts abound ! I was addressing the application - the needed torque is way below bolt specs (the big problem is guys ratting out the drive hexes on some of the softer ones) However I have never gotten a "spec " from the engine guys Hope you can pin some down Like pinning down tire milage --------------"it depends"

Surreeee . . ..

For 5mm bolts (prop bolts). . with either 8.8 grade or 10.9 grade metal, use 4.5-5 Ft/Lbs of torque. This is 54-60 In/lbs or 860-960 In/ounces. The difference is very small between the grades of metal at this low diameter.

For 8mm bolts/studs, use 19 ft.lbs for 8.8 grade and 32 ft.lbs for 10.9 grade metal. This equates to 228 In/lbs for 8.8 grade, or 384 In/lbs for 10.9 grade.
This equates to 3650 in/ounces for 8.8 metal or 6144 in/ounces for 10.9 grade.

These specs are 75% "yield" tension on the bolts, about where they take a 'set" on tension when you tighten them. Going higher does not improve clamping force, going below 60-65% will allow things to possibly come loose.

So, for a nice round number. . . let's call it 55-60 In-lbs for 5mm prop bolts, and 19-20 FOOT/lbs for 8mm single-bolts for the prop. Studs can go 25% higher. . 24-25 ft/lbs for studs.

[KrisW]

OTHER things to remember. . . . . .

When screwing a steel screw into aluminum. . you want at LEAST 1.5:1 ratio length of engagement to diameter of the bolt . .. . .

So. . for a 5mm bolt . .8mm of engagement, and 12 is better. Call it 3/8-1/2 inch of engagement. . MINIMUM.

If you do not have enough engagement depth, you can actually pull the threads right out of the aluminum before the bolt fails in a catastrophic situation, such as a crash. You should prefer to break the bolts, than yank threads out of the base metal.

A couple of Engine builders had problems with bolts loosening, or pulling threads out of crankcases, while the engines were running in flight until they started using longer bolts during assembly. With all that metal sitting there, it's hard to imagine WHY they would use short bolts in such a critical area.

For Steel-Steel engagement, you can get away with a 1:1 ratio, but it is "preferred" to have 1.25-1.5:1 ratio, or 7-8mm of engagement on a 5mm bolt, or about 5/16-3/8".

[dick hanson]

Sounds like we are reading from the same page -
One more thing --some guys think these bolts are actually pins, which turn the prop .
Not so- They are simply tensioners for clamps-so the bolts passing thru the prop are best if they drop thru the entire stack of frontwashers prop spinner backplate and into the hub
the wrench should spin freely till tensioning occurs- this assures that all force applied is in tension.
Agreed?
All in favor ?
All opposed?
OK --moved and seconded ------

[Boulder]

	Quote: Originally Posted by dick hanson
	Sounds like we are reading from the same page - One more thing --some guys think these bolts are actually pins, which turn the prop . Not so- They are simply tensioners for clamps-so the bolts passing thru the prop are best if they drop thru the entire stack of frontwashers prop spinner backplate and into the hub the wrench should spin freely till tensioning occurs- this assures that all force applied is in tension. Agreed? All in favor ? All opposed? OK --moved and seconded ------

Agree 100%