Shear Pins for Propellors?

[Rich Lemert]

Back when I was growing up on the farm, we used several powered
implements that received their power from the tractor through a power
take-off assembly. Somewhere on the device's power train, between the
PTO take-off and the "business end" of the apparatus, there was always
either a belt-drive or a coupling fitted with a "shear" pin. Both of
these systems were intended to protect that tractor (and the implement)
by failing if the implement bit off more than it could handle.

This morning, while driving past the airport on my way to work, the
thought occured to me that a shear pin could be used to protect airplane
engines (at least partially) from prop strikes. My understanding is that
the props on light singles (at least) are connected directly to the
engine's drive shaft.

Recognizing that this issue is driven as much by regulation as by
anything, I'm wondering if there would be any benefit to using shear
pins in these systems. Would it reduce the need for a complete (or
partial) tear-down after a prop strike? Would there be any benefit for
more complex propeller arrangements?

Any thoughts?

Rich Lemert

[Peter Duniho]

"Rich Lemert" > wrote in message
link.net...
> [...]
> This morning, while driving past the airport on my way to work, the
> thought occured to me that a shear pin could be used to protect airplane
> engines (at least partially) from prop strikes.

Frankly, having the prop keep moving is so much more important than
protecting the engine in the event of a prop strike, I can't imagine anyone
accepting the additional potential failure mode of a bad shear pin. We seem
to have a hard enough time making the crankshafts right.

Note that in a tractor, you are much more likely to suffer a sudden
stoppage, and the consequences for the premature breaking of a shear pin are
dramatically less. It seems to make much more sense for that application.

> My understanding is that the props on light singles (at least) are
> connected directly to the engine's drive shaft.

That's true of almost every propeller driven airplane. There are
exceptions, of course, mostly related to geared engine installations (found
on singles and twins).

Pete

[Morgans]

"Rich Lemert" > wrote in message
link.net...
>
> This morning, while driving past the airport on my way to work, the
> thought occured to me that a shear pin could be used to protect airplane
> engines (at least partially) from prop strikes.

I have also had shear pins fail for no good reason. Aviation thinks that
designing in another point of failure is a bad thing.
--
Jim in NC

[nooneimportant]

"Morgans" > wrote in message
...
>
> "Rich Lemert" > wrote in message
> link.net...
>>
>> This morning, while driving past the airport on my way to work, the
>> thought occured to me that a shear pin could be used to protect airplane
>> engines (at least partially) from prop strikes.
>
> I have also had shear pins fail for no good reason. Aviation thinks that
> designing in another point of failure is a bad thing.
> --
> Jim in NC
>

Don't some of the Rotax engines have a shear point to protect the engine? I
believe most vacum pumps have a Shear Wafer to protect the engine WHEN...
not IF the pump fails as well. But have ot agree that I'd rather have to
tear down an engine after a propstrike... than have the pin fail in
flight...

[Dale]

In article . net>,
Rich Lemert > wrote:

> Recognizing that this issue is driven as much by regulation as by
> anything, I'm wondering if there would be any benefit to using shear
> pins in these systems. Would it reduce the need for a complete (or
> partial) tear-down after a prop strike? Would there be any benefit for
> more complex propeller arrangements?
>
> Any thoughts?


I think the best protection for the engine would be to not land gearup,
taxi into super soft stuff, or do any of the other things that cause
prop strikes. <G>

--
Dale L. Falk

There is nothing - absolutely nothing - half so much worth doing
as simply messing around with airplanes.

http://home.gci.net/~sncdfalk/flying.html

[John T Lowry]

"Rich Lemert" > wrote in message
link.net...
> Back when I was growing up on the farm, we used several powered
> implements that received their power from the tractor through a power
> take-off assembly. Somewhere on the device's power train, between the
> PTO take-off and the "business end" of the apparatus, there was always
> either a belt-drive or a coupling fitted with a "shear" pin. Both of
> these systems were intended to protect that tractor (and the
> implement)
> by failing if the implement bit off more than it could handle.
>
> This morning, while driving past the airport on my way to work, the
> thought occured to me that a shear pin could be used to protect
> airplane engines (at least partially) from prop strikes. My
> understanding is that the props on light singles (at least) are
> connected directly to the engine's drive shaft.
>
> Recognizing that this issue is driven as much by regulation as by
> anything, I'm wondering if there would be any benefit to using shear
> pins in these systems. Would it reduce the need for a complete (or
> partial) tear-down after a prop strike? Would there be any benefit for
> more complex propeller arrangements?
>
> Any thoughts?
>
> Rich Lemert
>
Sounds to me like an idea worth pursuing. Could have two shear pins,
with visually checkable integrity during preflight inspection, to
counter the problem of one failing.

John Lowry
Flight Physics

[Dave Stadt]

"John T Lowry" > wrote in message
k.net...
>
> "Rich Lemert" > wrote in message
> link.net...
> > Back when I was growing up on the farm, we used several powered
> > implements that received their power from the tractor through a power
> > take-off assembly. Somewhere on the device's power train, between the
> > PTO take-off and the "business end" of the apparatus, there was always
> > either a belt-drive or a coupling fitted with a "shear" pin. Both of
> > these systems were intended to protect that tractor (and the
> > implement)
> > by failing if the implement bit off more than it could handle.
> >
> > This morning, while driving past the airport on my way to work, the
> > thought occured to me that a shear pin could be used to protect
> > airplane engines (at least partially) from prop strikes. My
> > understanding is that the props on light singles (at least) are
> > connected directly to the engine's drive shaft.
> >
> > Recognizing that this issue is driven as much by regulation as by
> > anything, I'm wondering if there would be any benefit to using shear
> > pins in these systems. Would it reduce the need for a complete (or
> > partial) tear-down after a prop strike? Would there be any benefit for
> > more complex propeller arrangements?
> >
> > Any thoughts?
> >
> > Rich Lemert
> >
> Sounds to me like an idea worth pursuing. Could have two shear pins,
> with visually checkable integrity during preflight inspection, to
> counter the problem of one failing.
>
> John Lowry
> Flight Physics


Now you have added two additional points of failure.

[Jay Honeck]

> Now you have added two additional points of failure.

I'm sure Lycoming would be glad to pursue this, especially in light of their
recent failures in court.

Not.

Too bad, it's got merit.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[Rolf Blom]

On 2005-04-01 04:32, Rich Lemert wrote:
> Back when I was growing up on the farm, we used several powered
> implements that received their power from the tractor through a power
> take-off assembly. Somewhere on the device's power train, between the
> PTO take-off and the "business end" of the apparatus, there was always
> either a belt-drive or a coupling fitted with a "shear" pin. Both of
> these systems were intended to protect that tractor (and the implement)
> by failing if the implement bit off more than it could handle.
>
> This morning, while driving past the airport on my way to work, the
> thought occured to me that a shear pin could be used to protect airplane
> engines (at least partially) from prop strikes. My understanding is that
> the props on light singles (at least) are connected directly to the
> engine's drive shaft.
>
> Recognizing that this issue is driven as much by regulation as by
> anything, I'm wondering if there would be any benefit to using shear
> pins in these systems. Would it reduce the need for a complete (or
> partial) tear-down after a prop strike? Would there be any benefit for
> more complex propeller arrangements?
>
> Any thoughts?
>
> Rich Lemert
>

Hmm...I think this would mean a weakening of the shaft.
And how would it be inspected at preflight, as was suggested?

Maybe there could be a market for a protective skid, something like the
Avro 504K, if prop strikes are a common occurrence?

/Rolf

[[email protected]]

Rotax certified engines use a "slipper clutch" system to help protect the
crankshaft and gear box. It isn't a shear pin system, but rather a spring
loaded ramp-toothed afair that resets. Rotax has criteria for strikes that
still require teardown of the gearbox and/or the entire engine.

tom


> I'm wondering if there would be any benefit to using shear
>pins in these systems.
>Rich Lemert
>

[AES]

The propellors on outboard motors for boats have (or at least always
used to have) shear pins.

The argument against using them in airplanes on safety grounds seems
pretty compelling, however. Boats keep floating even if the shear pin
shears . . .

[EL]

In addition to the points made by others, let me throw this out:

Suppose the prop strike occurs when the engine is set to high power (maybe not the most likely scenario but quite possible).
The prop makes contact with something and the pin shears. At that very instant, the engine is set to high power but now has
no load on it. With no load, the engine will speed up a lot - way past redline. A likely outcome would be pistons,
connecting rods, and parts of cylinders/heads flying out in all directions, possibly followed by fire - considerably more
damage than from a normal prop strike!

Eric Law

"Rich Lemert" > wrote in message link.net...
> Back when I was growing up on the farm, we used several powered implements that received their power from the tractor
> through a power take-off assembly. Somewhere on the device's power train, between the
> PTO take-off and the "business end" of the apparatus, there was always
> either a belt-drive or a coupling fitted with a "shear" pin. Both of these systems were intended to protect that tractor
> (and the implement)
> by failing if the implement bit off more than it could handle.
>
> This morning, while driving past the airport on my way to work, the thought occured to me that a shear pin could be used
> to protect airplane engines (at least partially) from prop strikes. My understanding is that the props on light singles (at
> least) are connected directly to the engine's drive shaft.
>
> Recognizing that this issue is driven as much by regulation as by anything, I'm wondering if there would be any benefit
> to using shear pins in these systems. Would it reduce the need for a complete (or partial) tear-down after a prop strike?
> Would there be any benefit for more complex propeller arrangements?
>
> Any thoughts?
>
> Rich Lemert
>

[Eduardo K.]

In article >,
EL > wrote:
>In addition to the points made by others, let me throw this out:
>
>Suppose the prop strike occurs when the engine is set to high power
>(maybe not the most likely scenario but quite possible).
>The prop makes contact with something and the pin shears. At that very
>instant, the engine is set to high power but now has
>no load on it. With no load, the engine will speed up a lot - way past
>redline. A likely outcome would be pistons,
>connecting rods, and parts of cylinders/heads flying out in all
>directions, possibly followed by fire - considerably more
>damage than from a normal prop strike!
>

The engine's ECU should take care of that. (grin :)


--
Eduardo K. |
| "World domination, now"
http://e.nn.cl | Linus Torvalds

[EL]

Heh heh...

I once had a car where the rev-limit function was built-in to the fuel pump relay and it had a design quirk. Once the RPM
limit was exceeded, the fuel would be cut off but only after a short time delay. So the effective limit was 6100 plus
however many RPM you could get in about 1/2 second!

Eric Law


"Eduardo K." > wrote in message ...
> In article >,
> EL > wrote:
>>In addition to the points made by others, let me throw this out:
>>
>>Suppose the prop strike occurs when the engine is set to high power
>>(maybe not the most likely scenario but quite possible).
>>The prop makes contact with something and the pin shears. At that very
>>instant, the engine is set to high power but now has
>>no load on it. With no load, the engine will speed up a lot - way past
>>redline. A likely outcome would be pistons,
>>connecting rods, and parts of cylinders/heads flying out in all
>>directions, possibly followed by fire - considerably more
>>damage than from a normal prop strike!
>>
>
> The engine's ECU should take care of that. (grin :)
>
>
> --
> Eduardo K. |
> | "World domination, now"
> http://e.nn.cl | Linus Torvalds

[Rich Lemert]

Dale wrote:
> In article . net>,
> Rich Lemert > wrote:
>
>
>> Recognizing that this issue is driven as much by regulation as by
>>anything, I'm wondering if there would be any benefit to using shear
>>pins in these systems. Would it reduce the need for a complete (or
>>partial) tear-down after a prop strike? Would there be any benefit for
>>more complex propeller arrangements?
>>
>> Any thoughts?
>
>
>
> I think the best protection for the engine would be to not land gearup,
> taxi into super soft stuff, or do any of the other things that cause
> prop strikes. <G>
>

Agreed, but then again I plan on asking for some spin training when I
get back into lessons again.

Rich Lemert

[Rich Lemert]

Dave Stadt wrote:

> "John T Lowry" > wrote in message
> k.net...
>
>>Sounds to me like an idea worth pursuing. Could have two shear pins,
>>with visually checkable integrity during preflight inspection, to
>>counter the problem of one failing.
>>
>>John Lowry
>>Flight Physics
>
>
>
> Now you have added two additional points of failure.
>

Just to play devil's advocate (1): I would think that the pins could
be designed to have a mean time between failures at least as long as
that of the engines themselves.

Just to play devil's advocate (2): Would failure of a shear pin
be qualitatively any different than any other engine failure?

Just to play devil's advocate (3): Logic would suggest that a
propellor shaft shear pin failure would most likely occur during
take-off, since that's when the engine is delivering the most power.
Just how much torque is the propellor applying to the shaft at this
time?

(Disclaimer: I really do understand everyone's concern about adding
potential failure points to the system.)

Rich Lemert

[Heywood]

A 300-hp direct-drive engine is producing 583 lb-ft of torque at 2700
rpm.

[Rich Lemert]

EL wrote:

> In addition to the points made by others, let me throw this out:
>
> Suppose the prop strike occurs when the engine is set to high power (maybe not the most likely scenario but quite possible).
> The prop makes contact with something and the pin shears. At that very instant, the engine is set to high power but now has
> no load on it. With no load, the engine will speed up a lot - way past redline. A likely outcome would be pistons,
> connecting rods, and parts of cylinders/heads flying out in all directions, possibly followed by fire - considerably more
> damage than from a normal prop strike!
>

This is something that I hadn't considered (and I suspect a lot of
others haven't, either). In fact, I think this answer is more compelling
than the primary answer I'm getting of "unnecessary potential failure
point", especially since you can probably design the MTBF for a shear
pin system to be at least as long as that of the engine itself, whereas
I don't see there being a simple way to implement an over-speed
prevention system.

Rich Lemert

[Matt Whiting]

Rich Lemert wrote:

> EL wrote:
>
>> In addition to the points made by others, let me throw this out:
>>
>> Suppose the prop strike occurs when the engine is set to high power
>> (maybe not the most likely scenario but quite possible). The prop
>> makes contact with something and the pin shears. At that very
>> instant, the engine is set to high power but now has no load on it.
>> With no load, the engine will speed up a lot - way past redline. A
>> likely outcome would be pistons, connecting rods, and parts of
>> cylinders/heads flying out in all directions, possibly followed by
>> fire - considerably more damage than from a normal prop strike!
>>
>
> This is something that I hadn't considered (and I suspect a lot of
> others haven't, either). In fact, I think this answer is more compelling
> than the primary answer I'm getting of "unnecessary potential failure
> point", especially since you can probably design the MTBF for a shear
> pin system to be at least as long as that of the engine itself, whereas
> I don't see there being a simple way to implement an over-speed
> prevention system.

Electronic rev limiters such as cars have had for years now... But that
is too modern a technology to be suitable in an airplane engine. :-)

Matt

[[email protected]]

The shear pin would fail at idle. The propeller on an airplane
is the engine's flywheel, and its inertia carries the crankshaft
between power strokes and smoothes its rotation. There's a
reciprocating force between the prop and crank flange, therefore, and
ANY looseness between the two will result in forces that will fail
attaching hardware. There have been cases of six 7/16" prop bolts
breaking when they weren't tight enough. There have been cases of
charring of wooden props when they weren't tight enough; the friction
created by small amounts of movement between the prop and flange heats
the wood. Props use six or eight substantial bolts for a reason: to
keep the thing on there tight. A shear pin in the system requires
looseness in the drive, and it just won't work. This isn't a boat ot
tractor. Imagine the damage when the flywheel of an outboard motor,
auto or tractor engine is a little loose.
Further, a sudden stop of the prop also pulls the crank radially
as the weight of the prop at its free end tries to keep moving. The
crank gets bent anyway, or the case fails, or engine mounts are
damaged. Any forward movement bends the prop backwards, too.

Dan

[George Patterson]

wrote:
>
> The shear pin would fail at idle.

How about this one. The prop would have a clutch plate on the front with a hole
in the center of both prop and plate large enough to fit over the end of the
crank. The prop/clutch assembly would slip over the end of the shaft and another
clutch plate assembly would bolt onto the end of the shaft. The latter would
have pivoting weights to increase pressure on the clutch as rpm rises (much like
some older CS props have to adjust blade angles). When the engine is producing
little power (as it would during landing), the prop would only be loosely
connected to the engine. Add power, and the clutch engages more and more
completely, both from the pressure added by the weight mechanism and by the
thrust pushing the prop forward.

This system would also allow the prop to windmill without attempting to turn the
engine in cases of engine failure.

You'd have to thread the inside of the crankshaft end and perhaps have to
rethink the current method of driving CS props with engine oil, but I doubt that
the problems are unsolvable.

George Patterson
Whosoever bloweth not his own horn, the same shall remain unblown.

[David CL Francis]

On Fri, 1 Apr 2005 at 02:32:55 in message
. net>, Rich Lemert
> wrote:
> This morning, while driving past the airport on my way to work, the
>thought occured to me that a shear pin could be used to protect
>airplane engines (at least partially) from prop strikes. My
>understanding is that the props on light singles (at least) are
>connected directly to the engine's drive shaft

It occurs to me that a shear pin could only work effectively against a
rotational stoppage. Since a normal prop either fits over a crankshaft
end or is bolted to a plate, a shear pin could do little or nothing.
What loading cases would you propose for the shear pin failure?

In a minor accident which way are the prop tips usual bent?

I somehow feel that the possibility of a prop detaching in flight should
not be encouraged. :-)

--
David CL Francis

[David CL Francis]

On Fri, 1 Apr 2005 at 13:06:26 in message
>, John T Lowry
> wrote:

>Sounds to me like an idea worth pursuing. Could have two shear pins,
>with visually checkable integrity during preflight inspection, to
>counter the problem of one failing.

Good idea but when both were in place the shear failure load would be
twice as strong?
--
David CL Francis

[[email protected]]

>This system would also allow the prop to windmill without attempting
to turn the
>engine in cases of engine failure.

So what do we do for a flywheel?

Dan

[George Patterson]

wrote:
>>This system would also allow the prop to windmill without attempting
>
> to turn the
>
>>engine in cases of engine failure.
>
>
> So what do we do for a flywheel?

What about it? On my old Maule, it's attached to the crank just in front of the
engine case. The prop is attached to a flange that's located several inches in
front of the flywheel. I propose to replace the flange and rigid prop mounting
with a clutch arrangement. The flywheel location and method of attachment would
not change.

George Patterson
Whosoever bloweth not his own horn, the same shall remain unblown.

[[email protected]]

That isn't a flywheel. It a ring gear mount for the starter
that also drives belt for the alternator (if it's a Lyc). It's aluminum
and weighs about four pounds. An engine such as the O-320, with its
four huge cylinders (compared to a Chev 350's eight much smaller ones,
for instance) would need a wheel of about 50 pounds in that small
diameter.
In all direct-drive aircraft engines the prop is the
flywheel. In most geared certified engines, the prop is the flywheel.
Geared ultralight engines like the Rotax have a separate flywheel, as
do many of the converted and geared or belted auto engines. They have
other problems with resonance between the prop and flywheel that can
lead to belt or gear failures if not properly designed and operated.

Dan