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Prop Bolt Re-torqueing

G

glenn654

Well Known Member
I have a 3 blade Catto.....Happy Flyer...anyway, the Spring and Summer weather in GA is hot and very humid which affects the wood core. When having to adjust the prop bolt torque due to swelling, would it be a good idea to allow some time for the wood under compression under the crush plate time to absorb moisture and fully swell before re-torqueing the bolts?

Glenn Wilkinson
RV-4
N654RV @ MLJ
 
Glenn, that's an interesting question.

ANC-9 says propellers in service stabilize at a moisture content ranging from 12 to 15 percent (most regions) to as low as 4 or 5 percent (arid southwest).

ANC-19 lists shrinkage vs moisture content for various woods. For sugar maple in the radial axis (ie flat sawn boards in a prop hub), 7 points of moisture content is good for about 1.3%. For a 4" thick hub, that would be abut 0.050", certainly not insignificant.

However, I doubt you're going to get anything near than much variation here in Georgia and Alabama. As a practical matter, I'd estimate your whole possible range of moisture content to fall well inside the 12 to 15 bracket, and I would not expect it to vary on a rapid basis.

There are a great many variables to clamp force and frictional torque capacity. Personally, I would want to establish the specified clamp force with the prop hub at the low end of the local moisture range. Any subsequent swell due to increased moisture content would increase clamp force, desirable for the transmission of shaft torque. I don't think that swelling will damage the wood fiber due to exceeding the allowable crush value perpendicular to grain (1790 psi for sugar maple per ANC-18), but I've not run any numbers to prove it.

What does Catto recommend?
 
DanH said:
...What does Catto recommend?
Click to expand...
35 ft-lbs for my prop, others are different as I have smaller bolts due to the O-290-D2 I have installed. It is written on the prop hub from Catto.

As a practical matter, I try to re-torque my prop once a quarter and when I do it, I pull it off and inspect it. Thus all the bolts start backed all the way off.

In addition, I have marked my hub, prop, backing plates, and spinner left and right so they always go back on the same way.
 
Sensenich says...

glenn654 said:
I have a 3 blade Catto.....Happy Flyer...anyway, the Spring and Summer weather in GA is hot and very humid which affects the wood core. When having to adjust the prop bolt torque due to swelling, would it be a good idea to allow some time for the wood under compression under the crush plate time to absorb moisture and fully swell before re-torqueing the bolts?

Glenn Wilkinson
RV-4
N654RV @ MLJ
Click to expand...

...every 50 hours and when the seasons change.

The following practices will add to the service
life of your wood propeller.
1. Inspect and check propeller attaching bolt torque
at least every 50 hours according to the Bolt Torque
Check Procedure and Propeller Maintenance
sections. More frequent inspection may be necessary
when climatic changes are extreme, such as change
of seasons.

If it was mine, I would do it every 50 hours or quarterly, whichever came first...

http://www.sensenichprop.com/sen_html/aircraft_cet/install/cf-a.pdf

During the Flight Test period, AC 90-89a goes much further for torque checks during flight test --

A word about torque: A new, wooden
propeller should have the mounting bolts checked
for proper torque within the first hour of flight and
every hour for 10 operational hours thereafter.
(1) After 10 hours, check the bolt torque
every 5 hours thereafter. The mounting bolt torque
also should be checked prior to flight if the aircraft
has been in storage for a long period of time (3 to
6 months).
(2) If the bolts need to be torqued, it is
suggested all the bolts be loosened for an hour to
allow the wood to relax. ‘‘Finger tighten’’ the bolts
until snug and tighten the attaching bolts in small
increments, moving diagonally across the bolt circle.
It is good practice to check the propeller track (see
chapter 1, section 7) as the bolts are torqued down.
The torqued bolts should be safety wired in pairs.

http://www.faa.gov/library/manuals/aircraft/media/ac90-89a.pdf
 
Bolt life?

Just a thought here. With all this repetitive torquing going on at what point should you replace the prop bolts? Wouldn't they start to stretch after a while?:confused:
 
<<With all this repetitive torquing going on at what point should you replace the prop bolts? Wouldn't they start to stretch after a while?>>

Unlikely. Most wood prop installations would crush the wood well before the bolts reached the plastic region.

<<If it was mine, I would do it every 50 hours or quarterly, whichever came first>>

Yep, regular checks are good for a lot of reasons. Glenn's question went to allowing the wood to take on a new moisture content before retorque. I think he would wait a long time for any significant change.
 
Thanks for the info.....was really just curious to know opinions on this.
I recheck my prop bolt torque twice each year at oil changes in Spring and Fall. Only once have I found the bolts needing additional torque to get up to spec.

Glenn Wilkinson
 
Not so!

DanH said:
<<With all this repetitive torquing going on at what point should you replace the prop bolts? Wouldn't they start to stretch after a while?>>
Unlikely. Most wood prop installations would crush the wood well before the bolts reached the plastic region.
Click to expand...
I have seen prop bolts stretch, and yes, even break from the wood swelling. It's not real common, but it does happen. I recommend removing and inspecting prop bolts at least annually when using wood props. The bolts first begin to stretch right at the base of the threads and is visually apparent.
 
That's interesting...

Mel said:
I have seen prop bolts stretch, and yes, even break from the wood swelling. It's not real common, but it does happen. I recommend removing and inspecting prop bolts at least annually when using wood props. The bolts first begin to stretch right at the base of the threads and is visually apparent.
Click to expand...

...if you just did a torque check, and the bolt didn't move, you might be tempted to just leave it and say "it's torqued and OK" when, in fact, the swollen wood is making it over-torqued....:(

I guess that is why the FAA AC had this bit in it...:) - post #4

...it is suggested all the bolts be loosened for an hour to allow the wood to relax....
 
<<I have seen prop bolts stretch, and yes, even break from the wood swelling.>>

Bruce's question was about repetitive re-torque, not wood swelling.

Still, interesting indeed. Recall post #2; the ANC info suggests a 4" thick hub can swell 0.050" with a move between US climate extremes, ie about 7% moisture content change.

Mel, the wood did in fact show evidence of crushing? And do you recall the combination; maple and SAE2 with AN6 bolts, or something else?
 
DanH said:
Mel, the wood did in fact show evidence of crushing? And do you recall the combination; maple and SAE2 with AN6 bolts, or something else?
Click to expand...
Dan, I don't remember the exact combinations. I believe the props were Maple, but I can't be sure. The bolts were standard AN. I've seen at least 2 prop that swelled enough to break 3 of the 6 bolts. And if I remember correctly, I think Van himself had this happen at one time.
 
Pierre,

My question as well. I would think the glass seals the wood so moisture can not result in "swell". I have a 3 blade Catto on my 9a and exceded Craig's recommendations for early checks on the torque (mine calls for 44 ft lbs). I checked it at 5 hrs, 10 hrs, 50 hrs, and then twice a year. Only once have I found even the smallest amount of additional torque required to meeet the spec. in 270 hrs of flight time. This result in an environment that varies greatly in atmospheric moisture content--San Antonio, Tx!!

Cheers,

db
 
Bill R.,

The bolt holes on my Catto ARE sealed with epoxy and finished! The finish in the opening is almost as good as the surface of the prop.

Cheers,

db
 
Dave how do you know that it was not overtorqued? Seems to me that unless you loosen the bolts, you can't be sure that they are torqued to the proper value. I always loosen mine prior to retorqueing. Note that I am not an AI/A&P.
 
db1yg said:
Bill R.,

The bolt holes on my Catto ARE sealed with epoxy and finished! The finish in the opening is almost as good as the surface of the prop.

Cheers,

db
Click to expand...

Not on mine. There is some paint overspray in the holes but not enough to keep the mosture out. BTW, I'm not talking about the opening, I'm talking about down in the holes. Yes, moisture can get past the crush plate and prop flange and get in there. My prop is off the plane at present and I can easily tell the holes are not sealed.
 
Hi Bill,

Here is the process I use--others may have a better technique to share:

I set the proper torque value and check the bolts for tightness (correct torque). If any of the bolts move I know they were loose and off spec. They are all then loosened and retorqued. I started this after reading about an episode where Van himself had several loose bolts break. I think he started feeling a vibration and the smell of burning wood!!!!!!

This being said, I have only found two bolts on one occasion that "moved" when checking torque--over 270 hrs.

Cheers,

db
 
Re coatings and coverings the old prop and wood books are clear. Coatings don't truly "seal" the wood. Eventually the moisture content will adjust to the environment, the only question being how long it will take. "How long" is one of the many wildcards of the wood prop design game. A prop coated in 30 year old shellac would obviously swing faster than a prop coated in epoxy/glass and finished with poly paint. Bolt holes, tip drains, and the big pilot hole are all further wildcards.

<<Dan, I don't remember the exact combinations. I believe the props were Maple, but I can't be sure. The bolts were standard AN.>>

Thanks Mel. Couldn't resist running some numbers this AM, and they are interesting.

Assume the common SAE2 hub size, which is what most everyone here is using. Further assume AN6 (3/8") bolts. Ok, the wood swells due to an increase in moisture content...so which fails first, the wood (by crushing) or the bolts (by plastic yield, not ultimate)?

The clamp area of an SAE2 is 22.83 sq in, bolt area included. Using crush values per ANC-18 for 15% and 20% moisture content, crush failure would be expected at a clamp force of:

maple 15% = 40866 lbs
maple 20% = 28994 lbs
birch 15% = 31505 lbs
birch 20% = 23287 lbs

Wood is far from homogeneous. The ANC table values are more or less adjusted averages, so the above could have a lot of scatter.

The yield strength of the AN6 (based on root thread) is about 6570 lbs, thus six bolts offers about 39,420 lbs of clamp at yield.

So, in the case of the maple prop at 15% and 3/8" bolts, it is possible to stretch the bolts without crush damage to the prop face. They will stretch at the root thread exactly as Mel noted.

Crush values decrease as moisture content rises. If the prop is really damp (a lot of swell) the probability of crush evidence is increased....a useful inspection tidbit. The opposite is also true. Even with a lot of data scatter, the above suggests all yellow birch props should show crush damage before reaching bolt yield.....but maybe not. Moisture content could be as low as 5% in the desert, which would push the crush values up, maybe even above the 39420 bolt capacity.

Very interesting.

Ok, different subject. Needed some bolt calcs for the above, a sideline of which address Bruce's question. We don't torque prop bolts to yield. Assuming a nut factor of 0.19 for cad plate, 30 ft lbs preloads an AN6 to 77% of yield. It would take 39 ft-lbs to reach yield. You can loosen and re-tighten an AN6 as many times as you want without stretching it, as long as you stay well under 39 ft-lbs.
 
To Pierre,
I assume my prop is typical of the 3-blade Cattos, it has a hole in the center of the hub about 1 1/2" so it can easily absorb moisture.

Glenn Wilkinson
 
<<typical of the 3-blade Cattos, it has a hole in the center of the hub about 1 1/2" so it can easily absorb moisture.>>

If it is on a standard SAE2 hub, the hub center pilot hole is 2.63". Trivia, more or less.

<<... If it's too tight, you won't know it.>>

Precisely what made the thread's original question so interesting. We quite naturally tend to focus on the question "Is it tight enough?".

Ya'll remember that big prop I carved for an M14? It wasn't just a prop carving project. The engine and prop hub is a new design, a modified SAE4, worked up from scratch after a lot of research and detail design.....bolt preload vs torque, wood compression stress, clamp force, you name it. Even ran new coefficient of friction tests for hub finish materials, since nobody really had a hard number for modern coatings like epoxy varnish or polyurethane on steel. Sensenich was kind enough to supply their current equations for frictional torque capacity as well as the original Henry Rose paper on the subject. The last serious government publications on wood prop design predate the FAA, thus the ANC publications. I'm not a pro engineer, so I had my buddy the Phd looking over my shoulder as well as the chief engineer at the M14 plant. A serious effort, with a great many "what if" questions considered. The natural focus was on torque transmission ("is it tight enough?") and structural reliability.....but I don't recall any of us asking "What if the wood swells a lot?"

Interesting.

At this point I'm not yet convinced wood fiber in a prop hub sees any significant change in moisture content merely by loosening the clamp force and waiting an hour. For starters, such a change would depend on the environmental conditions during that particular hour.

I am sure moisture content can and does change over longer periods, the actual rate of change being highly variable. As Mel noted, the result can be catastrophic, and preliminary numbers back him up. The ANC stuff suggests a 7 point change in moisture content can swell a 4" hub 0.050", while a 4" AN6 bolt would stretch to yield in about 0.010".

So, listen to Gil. Don't just pull prop bolts with a torque wrench to see if they are tight enough, because they may indeed already be too tight due to wood swell. Slack them off first, and like Mel said, take them out and inspect the root thread. If the hub shows some crush, be suspicious; either the bolts were overtorqued or the wood swelled at some point.

Education and recreation, eh?
 
Hi Gil,

Sorry, I just noticed your question/comment:

"If its too tight you won't know it"

I use the process I outlined in post #18. After checking for loose I use the same wrench to back the bolts out for re-torque--it is still set to 44 ft lbs.
As mentioned, the Catto does not seem to get out of spec. Maybe due to the .08-.25(thicker on the leading edge) of glass/epoxy over the prop, or maybe the wood has stabilized ref moisture absorption----I personally think it is the epoxy!! As Bill mentioned, the only place moisture might be absorbed is through the center or bolt holes, if this area is not sealed.

Nonetheless, it will continue to be checked at least twice per year or every 100 hrs.

Cheers,

db
 
overtorqued prop bolt

Here is something to look at. This prop bolt cameoff of a Cherokee 140... All of the bolts were like this. What sort of torque do you suppose stretched these bolts ?? Also shows how much they can be stretched without failing.....How soon was this accident about to happen ? This airplane was imported and was going through the import inspection.... I'd say this was a "good catch"...:eek:

http://picasaweb.google.com/MarcDeGirolamo/Safety?feat=directlink
 
Great Example!

Hey Marc,
Thanks for that picture. I often tell folks about stretched prop bolts, but don't have a good example picture. If it's OK with you, I would like to use your picture as an example.
 
prop bolt

Mel said:
Hey Marc,
Thanks for that picture. I often tell folks about stretched prop bolts, but don't have a good example picture. If it's OK with you, I would like to use your picture as an example.
Click to expand...

Mel...sure by all means go ahead......
 
Here's what I do on my two bladed Catto. Oil changes @ every 25- 30 hours. Loosen prop bolts while the oil is draining, tighten after change and final inspection before cowl goes back on. I always let the oil drain overnight so that gives the prop a while to stabilize. I change out prop nuts @ annual.
Tom
 
Hold on a minute !!!

Hey Tom. That's my technique. I posted that here at least a year ago and claim a patent on the process. I suppose I could make an exception for a fellow RV'er though :D.

Seriously, It's usually done on a Friday night and has the plane ready to go Saturday morning. I will be adding a careful inspection of the prop bolts though. I'm still not convinced that a wood hub could stretch steel bolts. But it sure won't hurt to check while I'm in there anyway.....

John
 
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