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HomeAircraftHelicopterHelicopter Main Discussion › Align 700N Clutch Mod to Make It Last
09-21-2013 02:14 PM  5 years agoPost 1
red_z06

rrProfessor

Dumont, NJ

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There is absolutely no reason why a clutch should not last the lifetime of the heli as it is really not a wear item like the liner.

With that in mind I first looked at the current clutch design limits to see the mechanism of the failure stated here.

https://rc.runryder.com/t746915p1/

So, we know that using a custom liner to decrease the gap to 0.007-0.010 will lengthen the clutch service life by effectively reducing the stress in the pivot area. But, as the liner wears, same situation of high stress will return.

This means the clutch wasn't designed with the wear of the liner in mind.

https://scontent-b-iad.xx.fbcdn.net...398874638_n.jpg

Since this isn't a redesign but a mod, you can only take away material. So, it won't be an ideal clutch but an improved one.

Challenge:

Reducing the area between the two arrows will reduce the stress at the same deflection by roughly two times the % reduction. On a bending beam one side undergoes compression while the other side goes under tension with zero stress line running through the middle.

So why not simply thin this?

There is a sectional moment of inertia to consider. This is what resists bending. I = 1/12 x b x h^3 where h refers to the thickness of the gap while b refers to the clutch height.

That means when you reduce the thickness by 1/2 (drastic stress reduction), you will have to thicken the clutch by 8 times so the I will be the same and the clutch will bend the same amount under centrufugal load. Not an easy task. Another way to put this is that, if you thin the diving board by 25%, you will have to double the width of the diving board to support the same weight.

If you were doing a new clutch, you would thin the stress zone and increase the height of the clutch some. But, in this case you will have to live with what you've got. Since the I which resists bending has been reduced (10% reduction in thickness is about 30% reduction in bending resistance), under same mass, centrifugal force will cause to wing to spread much quicker or at lower RPM.

So, you first find the thickness that will work with 0.015 gap at reduced stress and then you reduce the wing mass so the wing will spread exactly the same amount as before.

So, what is the side effect of this? The frictional force between the clutch and the liner depends on the force of the wing pressing against the clutch. This value will be reduced some. But, the positive thing is that the deformation of the clutch bell at high rpm will also be reduced.

End result is below.

https://scontent-b-iad.xx.fbcdn.net...041479016_n.jpg

If you need the clutch to grab earlier, the two lightening holes can be smaller.

Let's see what happens internally.

Origianl clutch at 0.015 gap

There aren't too many material that have 2x 75,000psi strength to make it last and even then with very very minimal liner wear allowed.

https://scontent-a-iad.xx.fbcdn.net...345567314_n.jpg

Below is a modded clutch at the same deflection of 0.015

There are quite a few material and treatment that will make this clutch work forever with tensile strength of 2x 55,000psi

https://scontent-b-iad.xx.fbcdn.net...988372846_n.jpg

Below shows comparison when 0.007 gap is used. It shows that the modded one can be carved out of a run of the mill cheap ass structural steel with no heat treatment and still work well.

https://scontent-a-iad.xx.fbcdn.net...527700605_n.jpg

Below shows tha the moded clutch will work well even with the liner wear down to the gap of 0.020

Even at 0.020 gap (tip spread about 0.027), the stress levels are lower than the original one at 0.015 gap.

https://scontent-b-iad.xx.fbcdn.net...304952839_n.jpg

That is if you start out with a 0.007 gap, you will be able to run the liner down 0.013 (about 1/3 liner thickness) without risking the clutch fracture.

www.JustinJee.com

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09-21-2013 02:35 PM  5 years agoPost 2
AgentOrange

rrVeteran

Jacksonville, Florida

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Man that is very detailed. I like it and I will have to try this out. Are these new clutches on the market.

Trying not to put them in the dirt.

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09-21-2013 02:39 PM  5 years agoPost 3
red_z06

rrProfessor

Dumont, NJ

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As I stated, it is a mod that you can do to the existing clutch. If redesigned, I will not make it this way.

www.JustinJee.com

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09-21-2013 02:39 PM  5 years agoPost 4
Ace Dude

rrProfessor

USA

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Great work red_z06! Thanks so much for not only refining a product to make it better, but for also sharing the how and why with the rest of us. Now this is engineering!

  

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09-21-2013 03:27 PM  5 years agoPost 5
TMoore

rrMaster

Cookeville, TN

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The question is; have you done one yet and tried it? The simulation is nice but the acid test is actually doing it and see how well it works.

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09-21-2013 06:05 PM  5 years agoPost 6
red_z06

rrProfessor

Dumont, NJ

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I do agree it needs to be built and tested. But, a 300 flight will run more than a year for most people to see the result and about 50 gal of fuel.

Simulation by itself isn't proof something (new stuff) works. But, in this case, first understanding why something breaks and reduce/remove the fault causing element purposes, it does the job well.

Give me your educated guess as to what would or wouldn't happen.

www.JustinJee.com

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09-21-2013 08:32 PM  5 years agoPost 7
TMoore

rrMaster

Cookeville, TN

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I could do the machining here in the shop on a 700 clutch and set up a new bell with thicker lining to give the correct clearance. Have you modeled what the new engage speed will be?

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09-21-2013 11:29 PM  5 years agoPost 8
red_z06

rrProfessor

Dumont, NJ

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The lightening holes were calibrated to give exact same engagement as the stock. It just will be slightly lighter meaning touch less frictional force.

If your stock clutch was engaging a bit late, then you do not need as big of a hole.

After machining, the clutch needs to be balanced for the final check.

The key is matching the diameter of the pivot hole and location.

www.JustinJee.com

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09-22-2013 01:03 AM  5 years agoPost 9
TMoore

rrMaster

Cookeville, TN

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That's what being a Toolmaker and having your own CNC is for. Once I fixture the clutch and dial into center I can put the holes anywhere I want them.

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09-22-2013 02:04 AM  5 years agoPost 10
red_z06

rrProfessor

Dumont, NJ

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Nice.

www.JustinJee.com

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09-22-2013 03:06 AM  5 years agoPost 11
R.J.

rrVeteran

SF bay area, CA USA

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Did you already post the size of the holes and their positions that you used to do the calculations? Not sure if I missed it. Thanks.

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09-22-2013 03:38 AM  5 years agoPost 12
red_z06

rrProfessor

Dumont, NJ

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No. I have not done that yet.

The pivot hole center has shifted and enlarged. The lightening hole centers and sizes are not super critical.

www.JustinJee.com

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09-24-2013 09:47 PM  5 years agoPost 13
red_z06

rrProfessor

Dumont, NJ

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Little animation of the wing spreading and the stresses in the pivot area developing.

Watch at YouTube

www.JustinJee.com

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09-24-2013 11:27 PM  5 years agoPost 14
R.J.

rrVeteran

SF bay area, CA USA

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Cool video! I like it. Thanks for all the effort you're putting into this. Real engineering in action.

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09-24-2013 11:45 PM  5 years agoPost 15
RM3

rrElite Veteran

Killeen, Texas - USA

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something to consider...

instead of making the pivot area thinner (and thus making it weaker to the torque pulse of the engine)

why not just elongate the pivot area so that the flexing is spread over a longer "strip"...

then go with a reverse cut to the wings like Century did with thier G30 HP clutch so that it wedges itself in the bell under load?

showing a preference will only get you into trouble, 90% of everything is crap...

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09-25-2013 12:00 AM  5 years agoPost 16
red_z06

rrProfessor

Dumont, NJ

My Posts: All  Forum  Topic

As I stated, this is a solution for existing clutches. If a new design is done, it would be much different.

Why do you think the clutch would be weaker? With above setup, wings would open up exactly same amount as before. The load on above is a centrifugal force in FEA.

www.JustinJee.com

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09-25-2013 12:09 AM  5 years agoPost 17
red_z06

rrProfessor

Dumont, NJ

My Posts: All  Forum  Topic

If it is a new clutch, you can elongate the flexing area. But, this is not just constant thickness for the flexing zone. Since you want same stress for the whole flexing zone, it has to get thinner (very very small but calculated) as it goes toward the tip side.

If the elongated slot is constant thickness, the stress would be highest at the root.

What I'm trying to demonstrate here is that, when properly designed, even non heat treated cheap ass steel can perform well indefinitely as long as the max stress is below the fatigue stress limit.

www.JustinJee.com

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09-25-2013 01:30 AM  5 years agoPost 18
RM3

rrElite Veteran

Killeen, Texas - USA

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cause I think your only taking into account the stress from the centifugal force acting on the wing flex point outwards....did you include the torque pulse being transmitted though it from the engine to the bell as well as any forces that may be acting upon it as a result of the clutch and bell running in slightly different axis if and when there is a slight alignment issue between them?

showing a preference will only get you into trouble, 90% of everything is crap...

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09-25-2013 02:01 AM  5 years agoPost 19
red_z06

rrProfessor

Dumont, NJ

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Wing flex due to the centrifugal force is only a guide to determine the relative stresses in before and after the modification. Torque pulses (as mentioned in the previous post), and bell deformation due to wings forcing on them are the real reason for the fatigue failure IMO.

How much does it affect the deformation? That will require additional studies on the bell housing analysis and impulse forces from the engine.

Both of these are present before and after the modification so in relative terms of improving the design, they can be left out.

Ideally, you would do contact analysis of the whole assembly. But, doing above is sufficient in achieving lowered stress at same deformation.

www.JustinJee.com

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09-25-2013 02:09 AM  5 years agoPost 20
red_z06

rrProfessor

Dumont, NJ

My Posts: All  Forum  Topic

Suspecting that bell deformation adds to the cyclic loads, at given RPM, reducing the wing mass also reduces the centrifugal forces on them.

This will possibly reduce the bell deformation and improving the stress and not adversely affecting it, also was the reason to omit it.

www.JustinJee.com

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HomeAircraftHelicopterHelicopter Main Discussion › Align 700N Clutch Mod to Make It Last
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