Catto Prop reTorque

[elippse]

prop torque, con't

looking at recommendations in engine manuals and propmakers, we see
torque values for the typical 3/8in prop bolt in wood props ,from about
150 to 250 inch pounds. Using 200, and our above stated scheme that
would be about 200/(0.22r*.35f)= @2600 lbs (not to far from my straw
man case above). In spite of the "drive lugs (bosses) " on the SAE
hubs, the primary drive forces, and especially the torque reversals
from four cylinder engines, are taken in friction. The clamped wooden
propeller acts very much like the friction clutch in your "stick
shift". Sensenich uses birch for their wood props, and produce a high
quality product with more experience in this field than almost anyone.
One of their recommendations for torque is the number of degrees turned
after initial "bottoming". They are seeking a compression of 0.006 inch
per inch of hub thickness. The modulus of elasticity for compression
perpendicular to the grain varies greatly with wood species, and even
at various points within the same log. Working values for the birch
used by Sensenich would appear to be between 200,000 and 300,000
psi/in/in. This would suggest a preload of roughly 1500 psi under the
"crush plate" which is pretty close to the crushing limit stated for
birch in the files of wood properties. With the beam deflection in the
crush plate, the effective area under each bolt would be less than 2
square inches, or a bit under 3000 lbf per bolt. Pressure on the
"clutch plate" driving the prop would be on the order of 36000 pounds.
With about a 0.4 friction and a 2.5in radius this would drive about
3400 ft lbs of torque without slipping - well over the steady state
value for most likely engines,

However, the result of this situation is a spring loaded system, the
primary spring being wood. A 0.006 in. per in shrinkage with moisture
change would completely relieve the spring force. Actually it is even
worse than that, since wood will notoriously "take a set" further
reducing this spring load value

SPRING WASHERS

As mentioned earlier, Paul was installing his prop with spring
(Belleville) washers. This is an approach that I had heard of being
applied to wood props on the McCulloch drone engines used on
gyrocopters, and one that I had often thought was a good idea for
retaining clamping forces in use. The spring washers can provide high
spring forces in a very compact package, and with a variety of stacking
techniques, provide tailored combinations of force and deflection. They
also provide visual monitoring of the preload on the bolt. The spring
washers he chose, were from McMaster Carr (a well known supplier of
industrial hardware). Two p/n 9712k32 . and one 9712K29 washers were
stacked in parallel under each bolt. In parallel, the spring forces are
additive. The specifications of these washers are:

Gardner# I.D. O.D. height thick deflection load flat load McMstr#

1187-105 0.406 1.188 0.125 0.105 0.016 1950 2812 9712K32

1000-105 0.406 1.00 0.118 0.105 0.010 1830 2657 9712K29



With 25 ft-lb torque (300 in-lb), the washers were not "flat", which
is as it should be. The specified total load for flat should be almost
8300 pounds, and the bolt load for 300 in-lbs torque would be expected
to be about 3600 pounds, or bit less than half the bottoming load. and
even a bit short of the 5700 rated displacement load. It would seem
that this stack is a bit on the "stiff" side. A tailored stack of
washers in combinations of parallel and series arrangement would
provide the desired preload with greater deflection possible without
losing too much preload. A selection of hardened flat washers should
also be included to protect the softer aluminum "crush" plate.

Checking a recent catalog from McMasters-Carr, I ordered an
assortment of the spring washers with ID values suitable for 3/8 inch
(typical propeller bolt size) bolts. I think That I will be excused
from any copyright usage I printing the selected portion of the
McMasters- Carr cataloge showing the variety of possible choices for
our 3/8 inch bolts. If you are using 7/16 inch or ? inche bolts there
are a similar number of choices 0n the subsequent page. Prices, are of
course, subject to change, but are quite reasonable for the potential
benefits derived.

Specifications of typical spring washers are:

ID OD Thick height deflect load flat load qty part# price qty part#ss
price

The units selected for testing - shown to scale on a 3.\/8 inch bolt

And the force/ deflection characteristics are"


The spring washers may be "stacked" in various combinations to match
the force and deflection characteristics that may be desired. Stacking
in parallel = like Dixie cups. Will increase the loads for a given
deflection. The forces are directly additive for this stacking
arrangement. To provide more working travel for a given force change, a
series stack can be used (point to point or "flare" to flare) The
series stack tends to be a bit unstable, with the points and flares
slipping out of alignment,O>C>Baker one of our KIS bulders suggested a
larger washer inserted at the flare to flar intersection to stabilize
the assembly (attached figures show this arrangement'. The force to
flatten the discs is the same, but the travel to get that load is now
doubled.

[Image removed]

Two washers in series with the central stabilizing washer.

[Image removed]

A four Belville stack combining series and parallel stacking.

With combinations of assemblies, a great number of force to
displacement 'curves" can be provided. For our purposes, we decided
that a fairly large displacement, with a rather "flat" force curve
would be desired. What this would mean is that the wood could change
dimensions (shrink or swell) over a fairly wide range without a large
change in clamping force. For a birch prop with 3/8 in bolts, the
characteristics of four number 428 washers in a series/parallel setup
appeared the most promising. The total assembly would probably consist
of a flat steel washer against the aluminum, the first two bellvilles
point down, the second two points up, and a flat washer under the bolt
head. This system appears to be nearly flat with roughly 200 inch
pounds of torque, and close to 3000 pounds of clamping force for each
bolt. Total deflection would be over .050 inch, and even a 010 change
in shrinkage would result in little loss in preload, Different bolt
diameters, and different wood hardness values would probably favor a
different selection.



The local bearing forces of the sharp edges of the Bellville washers
dictate the use of steel washers to protect the aluminum "crush plate",
and probably also the washer face on the associated bolt head or nut.
This total "stack" would add fairly measurably to the selected bolt
length for your installation.

[Image removed]

These curves were generated using wrench angle as an indication of
inches of compression. and reading wench torque value at each point.
This is admiditly a bit crude, but the direction and rough magnitude
can be seen. The four Belville stack of two 423 Belvilles in parallel
on each side of the stabilizing washer shows a great travel range with
low fall of in clamping force.



The result of such an installation would appear to be clearly a win,
win situation. Weight and cost is quite minimal, and there do not
appear to be any significant failure modes that have been increased by
this system.

[DanH]

Hi Paul,
Didn't know you posted on this forum. Good to hear from you. As we talked before, I'm gonna incorporate the Bellvilles into the M14 install. If you have a complete copy of Vance's article with charts and graphs, would you mind dropping a copy to me at danhorton(at)elmore.rr.com?

[roadrunner20]

Originally Posted by sprucemoose

FWIW, from day 1 I've noticed a vibration when throttling down from about 1800 to 1400 RPM or so. I re-torque several times per year, and the torque is never off by very much.

Jeff,
The re-torque may have made a very slight difference.
Although. it may just be my impression.
I. as you, am getting the same slight vibration when throttling down.
I can now move on. I believe it's probably due to the transition...
Thanks for the input from everyone.

[captainron]

Hi Paul, a very interesting article. I'm not enough of an "engineer" to dispute anything here, and it sounds logical enough. If this is a workable concept, then could the "re-torquing" simply consist of comparing the installed height of the stack against the original installation, using a depth micrometer? If you find during inspection that the stack height had "grown" due to fiber crushing or settlement, then you would simply run the bolt in until reaching the original stack height? Or, if no measurable change in height is noted, then there is no reason to put wrench to bolt.
One of the drawbacks that I see to this scenario is that the bolt is never "bottomed-out" solidly during this installation. In situations where I have seen Belleville washers used, usually some kind of "shoulder" bolt is employed and the
Belleville washer then simply provides a preload to keep a part in place, but have some movement available for it. The bolt, however, is solidly torqued in place.

[elippse]

Originally Posted by captainron

Hi Paul, a very interesting article. I'm not enough of an "engineer" to dispute anything here, and it sounds logical enough. If this is a workable concept, then could the "re-torquing" simply consist of comparing the installed height of the stack against the original installation, using a depth micrometer? If you find during inspection that the stack height had "grown" due to fiber crushing or settlement, then you would simply run the bolt in until reaching the original stack height? Or, if no measurable change in height is noted, then there is no reason to put wrench to bolt.
One of the drawbacks that I see to this scenario is that the bolt is never "bottomed-out" solidly during this installation. In situations where I have seen Belleville washers used, usually some kind of "shoulder" bolt is employed and the
Belleville washer then simply provides a preload to keep a part in place, but have some movement available for it. The bolt, however, is solidly torqued in place.

Hi, Ron! I just torque my bolts to 25 lb-ft. When removing the bolts, it's not like it is without these washers. Usually you just break the torque with a short turn and the bolt is free. With these washers, you have to turn the bolt several times with the torque gradually reducing the more it is turned. You may bottom out on a metal prop but not really on a wood prop because the fibers take up the force in their springiness until the fibers start to crush. It would be nice to see some of the more experimental types out there do some testing based on Vance's analysis and then report back to the forum on their results!

[bjohnson1234]

I know I'm chiming in pretty late on this, but getting the correct torque apparently requires good technique. If you don't reach the correct torque in one smooth motion then you end up getting static friction and the torque may appear higher than it really is. Even old bolts can increase the apparent torque on the wrench. Some (and probably all) manufacturers recommend lubricating threads, nuts and torquing surfaces as well. Perhaps when it was originally torqued it was not done properly?

Originally Posted by roadrunner20

I read a post last week about the Long EZ that threw it's catto prop after having slight vibration warnings prior to breakup.

I have 34 hours TT on my 7A.
I starting receiving a slight vibration on the floorboard on climbout. This was odd as I never experienced any in the past 32 hours. On downwind, throttling down to ~1300/1400, I noticed a vibration once again. The next flight, this ocurred again. Usually, throttling down creates a very smooth operation with this 3 blade.

Anyway, I pulled the prop today. All looked good although the torque values were off. They were all running about 30-35f lbs. They were torqued as Catto req to 45. This is my first wood/comp prop and I was surprised they were off that much. I thought by having the aluminum backplate, it would not be so sensitve to environment conditions.

Is this a common?

[humptybump]

Craig did an EAA Webinar recently. While much of the material was background on Craig and some of his designs, he delivered an eye opening segment on propeller safety wire and loss of bolt torque.

Here is a link: http://www.eaavideo.org/video.aspx?v=4387371098001

[Michael Burbidge]

Thanks for the pointer to that video. Craig said in the video, no "Krylon" on the backing plates. I assume he meant any kind of paint or primer; that the backing plates should be raw aluminum. Is that correct? Since I'm not at the airport I had to look up a picture of my prop installation to remember if I primed the backing plates. Here's a picture of my installation. I obviously have primer on the plates. I believe its the NAPA Primer on my backing plates.

Been flying 100 hours now, but I assume the safe thing to do would be to remove the prop and inspect the primer and probably strip it.

Dang, I just finished my condition inspection and did re-torque the prop bolts. But did not remove the prop.

Michael-

[humptybump]

I think the issue discussed was that any paint on the backing plate would turn to dust and act as a lubricant or abrasive. Bare aluminum is the correct installation.

[Vlad]

That was a great webinar thanks for mentioning it Glen. Interesting angle/lost torque percentage math.