Clutch Repair

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 Other clutch related articles:
Clutch Bell Removal
Setting Clutch Clearance
Clutch broke? Try this:
Use a dial point indicator to center the clutch on the hub as shown <here>.

Increase the tension of the pinion bearing: http://www.raptortechnique.com/repair_pinionbbblock.htm

Next look to see if there is a washer between the fan and the front bearing of the engine, if so then remove it.

Elongate the engine mount holes of the plastic frame so that with the engine/fan/clutch assembly installed, the bottom edge of the clutch is at the bottom edge of the liner.

Also if you have more then 0.010" of clearance per side from the clutch to the liner then replace the liner with a thicker one or use business card (thick) paper under the liner to close the gap.

Also if your clutch bell wobbles when you spin it then get a new one. It needs to run true.
3-14-2 Strange thing happened, I was bragging on the fact I had a couple hundred flights on my current clutch (after tightening the pinion bearing block) and then it went out Thursday. The unusual part is the shoes are still great, the part that went out was the one-way bearing. The start shaft was locked up tight in the bearing inside the clutch. I almost was not able to get the shaft out! This is the first time I have had to change the clutch and the shoes were not cracked or broke off.
# tanks before failure Raptor #1 Raptor #2 Raptor #3
1st Clutch 103 tanks (TT clutch) 104 tanks (TT clutch) 129 tanks (TT clutch)
2nd Clutch 331 tanks (TT clutch) 33 tanks (TT clutch)  
3rd Clutch 345 tanks (TT clutch) 66 tanks (TT clutch)  
4th Clutch 11 tanks (Ron's clutch) 20 tanks (HHI/Quick)  
5th Clutch 14 tanks (TT clutch) 34 tanks (modified TT clutch)  
6th Clutch 204 tanks (HeliHut clutch) 25 tanks (TT clutch)  
7th Clutch (installed QuickUK clutch) 27 tanks (TT clutch)  
8th Clutch   20 tanks (modified TT clutch)  
9th Clutch   26 tanks (TT clutch)  
10th Clutch   29 tanks (TT clutch)  
<Here> is a step through for fixing a loose pinion bearing
12-22-1 It's odd how some people have problems with the clutchs and some don't. What's even more odd is that with all the Raptor pilots of all types of backgrounds (some mechanical engineers) that no one in all the time the Raptor has been out has figured out why! This is a complex problem since there's more then one thing that can cause clutch failures. One of my latest ideas that I think might explain it is that some have a loose fitting pinion bearing in the plastic frame bearing block. This can be because this area can be a little thick on some framesets and when the frame is bolted together it does not squeeze the bearing tight. This loose fitting bearing can allow the bell to tilt slightly at times during operation. Next if the clutch is very close to the inside top of the bell then the bell can touch the outside top edge of the clutch during the times it tilts. This pushing down on the clutch shoes fatigues it until it eventually develops a crack. I remembered that a while back someone told me his clutch problems were solved after changing the frameset. This was over a year ago that he told me, but at the time I did not see how the old frameset could break the clutch. Now I see that with a loose fit for the pinion bearing, then as the changing engine torque is transferred from the pinion gear to the main gear, the pinion gear is pushed away tilting the whole pinion/bell assembly. This in combination with a clutch that is too high might just be the cause of a lot of clutch failures.
======================
Possible causes of clutch failures
= Loose fitting pinion bearing
= Liner worn out, too much shoe to liner clearance
= Engine alignment crooked
= Too much liner dust
= Bell wobbles, not correctly threaded
= uneven liner thickness
= hub runout
= vibration
= up/down main gear
12-14-1 This page has a picture of the clutch I saw at the Sulphur, LA fun fly. http://utsunomiya.cool.ne.jp/rcheli/report/op9.htm to translate it, go <here> (Note: the translated version won't show the pictures). The added flexibility from removing most of the center is the secret to the long lifespan of this clutch. Here in the U.S.A. you can get it from Absolute Helis
12-13-1 Another idea on the clutch is I wonder if a loose fit of the pinion bearing to the bearing holder(aka frame) contributes to the pre-mature failure? Just another thought. You might check if your clutch bell wobbles a lot next time you have the engine out.
12-7-1 I was thinking about another possible cause of the clutchs breaking. I wonder if liner dust contributes to failures. I think it's a good idea after the clutch has been ran half a gallon or less on a new clutch or liner to drop the engine/clutch and clean out the dust. I helped a guy a few weeks ago that could not get the helicopter to lift off the ground. The problem was too much dust from the new liner he installed. With this dust in there, the clutch was just slipping and building up heat. Plus if the liner thickness is inconsistent and the clutch is slipping inside it, that means the shoes are being worked back and forth 10,000 times or so per minute. That can't be good on it!
9-27-1 My latest idea is that if the engine/fan/clutch is installed too far up, then in flight when the frames flex from maneuvers and engine torque, that the clutch might have just enough angle so that one side of the top of the clutch might touch the inside top of the clutch bell. If this happens even for a moment with the engine running at 15,000 rpm, that might eventually break it. One thing I have noticed on ALL of my broke clutchs is that the crack starts from the top. This would correspond with the clutch touching the top of the bell. And it would make sense that with some installations some people could have a little more gap, and some not. This would explain why some helis have this problem and others don't. Again, I don't know if this is it, just a thought...
9-23-1 Raptor #1 broke a clutch, installed one from Heli-Hut
# tanks before failure Raptor #1 Raptor #2 Raptor #3
1st Clutch 103 tanks (TT clutch) 104 tanks (TT clutch) 129 tanks (TT clutch)
2nd Clutch 331 tanks (TT clutch) 33 tanks (TT clutch)  
3rd Clutch 345 tanks (TT clutch) 66 tanks (TT clutch)  
4th Clutch 11 tanks (Ron's clutch) 20 tanks (HHI/Quick)  
5th Clutch 14 tanks (TT clutch) 34 tanks (modified TT clutch)  
6th Clutch   25 tanks (TT clutch)  
7th Clutch   27 tanks (TT clutch)  
8th Clutch   20 tanks (modified TT clutch)  
9th Clutch   26 tanks (TT clutch)  
10th Clutch   29 tanks (TT clutch)  
9-16-1 The special oval holed, garenteed not to break super duper clutch broke on flight 11. Replaced with stock TT clutch.
# tanks before failure Raptor #1 Raptor #2 Raptor #3
1st Clutch 103 tanks (TT clutch) 104 tanks (TT clutch) 129 tanks (TT clutch)
2nd Clutch 331 tanks (TT clutch) 33 tanks (TT clutch)  
3rd Clutch 345 tanks (TT clutch) 66 tanks (TT clutch)  
4th Clutch 11 tanks (oval hole clutch) 20 tanks (HHI/Quick)  
5th Clutch   34 tanks (modified TT clutch)  
6th Clutch   25 tanks (TT clutch)  
7th Clutch   27 tanks (TT clutch)  
8th Clutch   20 tanks (modified TT clutch)  
9th Clutch   26 tanks (TT clutch)  
10th Clutch   29 tanks (TT clutch)  
Current number of
flights on last
installed clutch
9-17-1 just installed 9-17-1 129 tanks (HHI/Quick) 9-5-1 16
9-5-1 (flight 676) Replaced broke clutch and worn out liner on Raptor #1. The liner really needed replacing back when I converted to a 50 but I was too anxious to fly. I installed one of clutchs with the oval shaped flex holes.
# tanks before failure Raptor #1 Raptor #2 Raptor #3
1st Clutch 103 tanks (TT clutch) 104 tanks (TT clutch) 129 tanks (TT clutch)
2nd Clutch 331 tanks (TT clutch) 33 tanks (TT clutch)  
3rd Clutch 345 tanks (TT clutch) 66 tanks (TT clutch)  
4th Clutch   20 tanks (HHI/Quick)  
5th Clutch   34 tanks (modified TT clutch)  
6th Clutch   25 tanks (TT clutch)  
7th Clutch   27 tanks (TT clutch)  
8th Clutch   20 tanks (modified TT clutch)  
9th Clutch   26 tanks (TT clutch)  
10th Clutch   29 tanks (TT clutch)  
Current number of
flights on last
installed clutch
9-5-1 just installed 9-5-1 110 tanks (HHI/Quick) 9-5-1 16

One thing that might help the clutch is to take out any up/down slop in the main gear. My Raptor #1 does not have any slop, but Raptor #2 has about 0.012". Don't know if this is what causes them to break but certainly won't hurt to take out the slop.

7-12-1 Here is a picture of the heavy duty clutch Ron sells:
7-10-1 Got a great tip from Geoffrey Woodward on how he keeps his clutch from breaking. His secret to success is to remove any runnout of the clutch or bell liner in any direction, axial or radial. Also he uses an industrial grade seamless liner in the clutch bell. To give the clutch a centered place to sit he puts the whole motor in the lathe and turns the crank with the chuck then the face and bore where the clutch sits is then machined dead true
The engine is mounted on the lathe bed with the crank on the centerline of the lathe. There is a delrin (plastic) part that goes into the crankcase that turns the crank around. The whole engine stays still and just the crank turns with the fan hub on the end. The clutch mounting face can then be machined true and the bore too.
<under construction 7-10-1>
7-8-1 Somehow that HHI/Quick clutch has lasted 98 flights! This is hard for me to believe since I got two at the same time and the first one only lasted 20 flights.
6-16-1 Ron has a heavy duty clutch that he will replace if you break it in the first 90 days or for $15 afterwards.
As a side note, I long time back I bought two HHI/Quick clutchs, the first one didn't even last as long as the stock ones do so I never used the other one until recently when I ran out of stock clutchs. Somehow, that 2nd HHI/Quick clutch has lasted 51 tanks so far!!!
Here is a picture of all my clutchs:
3-26-1 Another clutch broke after 30 flights.
# tanks before failure Raptor #1 Raptor #2 Raptor #3
1st Clutch 103 tanks (TT clutch) 104 tanks (TT clutch) 129 tanks (TT clutch)
2nd Clutch 331 tanks (TT clutch) 33 tanks (TT clutch)  
3rd Clutch   66 tanks (TT clutch)  
4th Clutch   20 tanks (HHI/Quick)  
5th Clutch   34 tanks (modified TT clutch)  
6th Clutch   25 tanks (TT clutch)  
7th Clutch   27 tanks (TT clutch)  
8th Clutch   20 tanks (modified TT clutch)  
9th Clutch   26 tanks (TT clutch)  
10th Clutch   29 tanks (TT clutch)  
2-25-1 Another clutch broke after 26 flights.
# tanks before failure Raptor #1 Raptor #2
1st Clutch 103 tanks (TT clutch) 104 tanks (TT clutch)
2nd Clutch 331 tanks (TT clutch) 33 tanks (TT clutch)
3rd Clutch   66 tanks (TT clutch)
4th Clutch   20 tanks (HHI/Quick)
5th Clutch   34 tanks (modified TT clutch)
6th Clutch   25 tanks (TT clutch)
7th Clutch   27 tanks (TT clutch)
8th Clutch   20 tanks (modified TT clutch)
9th Clutch   26 tanks (TT clutch)
12-9-0 The modified clutch broke after 20 flights. One thing I have noticed that is different about Raptor #2 is that the tail 'catches' during spool up. I think it is because of the belt. The 'catching' during spool up and possibly during flight maybe effecting the clutch. I will change the belt out and try that.
10-26-0 Another clutch broke. This was the one that was matched with a two liner bell that had been trued in a machine lathe to give a consistent thickness all the way around and allow for a smaller gap size (0.003" per side). I have a modified clutch that was given to me by Pete of Pete's Hobbies that I will try next. 10-27-0 I installed Pete's modified clutch. As per Pete's request, I put a new unmodified fan/hub, new start shaft, new pinion gear with bearing, new top start shaft bearing, new clutch bell with liner.
9-21-0 Another clutch idea. My friend Bruce thinks the clutch problem might be due to an uneven liner. At idle the shoes are a little closer to the liner. If the liner has a high spot, where the glue was a little thicker or the liner made a flat spot where the ends meet, then the shoes could bump the high spot and flex inward. This continual flexing might be what is breaking the clutchs. So the fix would be to do Sheldon's technique for installing a liner. It's at the bottom of my clutch page: He mentioned that his Xcell 60 will idle all day without turning the blades. My Raptor #1 will also do that and it doesn't eat clutches very often. Time will tell, I hope this fixes it.
9-17-0 Another clutch broke so I guess that blew the last theory about the auto hub bolt. Next I will try the lathe technique that Sheldon from YNT uDesign does. You can read about his clutch liner installation at the bottom of this page. I want to get a very very small gap size this time. I've been running 0.010". I don't have a lot of high hopes that this will fix it, but it won't hurt. My Raptor #1 has the clutch installed just like #2, but #2 eats twice as many clutchs. That's why I think it's something else, like maybe some vibration that is resonating at a frequency that kills the shoes.
# tanks before failure Raptor #1 Raptor #2
1st Clutch 103 tanks (TT clutch) 104 tanks (TT clutch)
2nd Clutch 331 tanks (TT clutch) 33 tanks (TT clutch)
3rd Clutch   66 tanks (TT clutch)
4th Clutch   20 tanks (HHI/Quick)
5th Clutch   34 tanks (modified TT clutch)
6th Clutch   25 tanks (TT clutch)
9-12-0 A kinda of top 10 list for clutch failures
auto hub too tight Anytime the rotor has more force then the engine, this should disengage the auto hub. But if the auto hub is not free, then some energy makes it from the rotor to the main gear, then pinion gear, then clutch bell. And during that moment the bell would be turned faster then the clutch. Since the shoes would still be engaged with the liner, the bell would be pushing the shoes back toward the flex region. Yes, I know if the rotor spins faster then the engine, then the engine will unload and also spin faster so the auto hub shouldn't disengage... but think about it this way, the engine does not output constant energy. It comes in pulses corresponding with each combustion. So about every 3.7 ms the main gear gets a push from the engine. This is like if you stood beside a merry-go-round and pushed it every one revolution. During the combustion and for a brief moment until the top of the piston reaches the exhaust port, there will be a sudden acceleration and the auto hub will be locked. I estimate this happens for less then 1ms. During the remaining 3ms or so the rotation slows down (not much, but the point is that it is slower), especially during the compression phase in the engine. While that happens, if the rotor is spinning faster then the engine can turn the main gear, then the needle bearings in the auto hub should rotate and unlock. If however something is wrong with the auto hub (like you have the bolt too tight), then the hub will remain locked for a longer duration. Before when the auto hub was locked, the engine/clutch/bell/main gear were spinning faster then then rotor so there was no problem. But before the next combustion the auto hub remains locked for a brief moment and now power from the rotor goes in the opposite direction. Normally the auto hub should act like a diode in an electronic circuit and only allow power to go one direction. But on this example, the main gear turns the pinion gear which turns the clutch bell which tries to rotate faster then the clutch. Since the shoes are still engaged, this forces the shoes toward the flex region. This only happens for a very short time, but the continual feedback pulse may cause the pre-mature failures.
frames not stiff Norimasa Ando of Japan had to replace his clutch 3 times in 6 months. He added some stiffeners, that his local hobby shop makes, to the frame and now has 150 flights on his current clutch.
clutch shoe to hub clearance The shoes clearly do drag the hub and you need clearance to allow the shoes to engage and disengage properly, but this alone will not cure the clutch breaking problem. I have broke many clutchs since doing this procedure.
clutch shoe to liner clearance This is important, but in my experience and from what I have heard from others, this alone will not cure the clutch breaking problem.
replace frames Chris Derboven from Belgium has reported to me that his clutch problem was solved after replacing his main frames. He is thinking that the frame was not holding the pinion gear solid.
end play in the engine crankshaft Dave Brown suspects that engine crankshaft end play could allow the clutch to move up/down inside the bell and break the clutch. That is a good idea, but when the fan is screwed on, that should eliminate the crankshaft endplay.
engine back firing Robert van't Riet suspects this will cause it.
carbon frame stiffeners, Robert van't Riet Robert also is wondering if the carbon frame stiffeners would stop the frames from twisting and remove undue loads. Since we don't know for sure what is the problem, we do need to take all things into consideration.
3d or general flying style This doesn't seem to matter. I have heard of both types of pilots have clutch failures. One note is that a lot of beginners ruin their clutch by using the built-in pitch gauge and end up causing the clutch to slip so bad for so long that it builds up a lot of heat and the shoes remain in the out position.
engine/start shaft alignment I had to drill out my engine mount holes, using an 1/8" bit, to allow engine/start shaft to align properly. If it's out of alignment, then the start shaft won't turn smoothly. This needs to be done for proper operation but it alone will not solve the clutch problem.
vibration Vibration is not a good thing. I did balance my fan/hub and blades, but there are probably other things like the start shaft and others that could cause vibration. I have not investagated this much.
turn in lathe for perfect roundness and smaller gap size My friend Raj Patel said he installed a liner with JB Weld, then turned it down, glued another liner in and turned it down so he had a 0.005" clearance on both sides. He has never had a clutch problem. Sheldon (of YNT uDesign) provided some excellent instruction for this at the bottom of this page.
check for up/down movement of clutch with dial point indicator I checked one of mine and it wasn't out any, but if you have a wobble that surely couldn't be good and would probably effect the clutch.
material problem Dave Brown and my flying buddy Bruce think it is a material problem. Either the metal is not heat treated properly of it's the wrong type. Aaron from California also thought it was a material problem until he ran a test that showed the clutch was in fact spring tempered. In addition, I have broke the HHI/Quick Hi-carbon clutch. So either both companies got it wrong using two different materials (which is possible) or it's not a material problem.
out of turn Aaron spun his clutch to measure if it was perfectly round. He found it was 0.003" out. This it a lot considering the speed the engine turns at.
elongated holes The Kalt Enforcer clutch looks just like the Raptor clutch, except for the elongated holes. It is not known for breaking so maybe if this was done to the Raptor clutch, the stresses would be spread over a large enough area to stop the breaking problem.
8-27-0 I have a new theory about the clutch failures. My clutch failed again on Raptor #2.
# tanks before failure Raptor #1 Raptor #2
1st Clutch 103 tanks (TT clutch) 104 tanks (TT clutch)
2nd Clutch 331 tanks (TT clutch) 33 tanks (TT clutch)
3rd Clutch   66 tanks (TT clutch)
4th Clutch   20 tanks (HHI/Quick)
5th Clutch   34 tanks (modified TT clutch)

As David Ellis mentioned on the Raptor2 list, there is a difference between Raptor #1 and #2. Raptor #1 doesn't eat clutchs as much as #2. I had reviewed my flight logs but could not find anything that was different between the two machines that would cause premature clutch failures. Well today Raptor #2 broke another clutch. This was a TT clutch that my friend Bruce increased the chamber on. Also I was making sure the engine/start shaft was aligned all the time. At the field when we discovered this clutch failed, that blew our latest couple of theories, so we were out of ideas. Actually Bruce is thinking that it must be a material issue like Dr. Dave Brown mentioned on the Raptor2 list. I came home and all I could think about was how this could have happened and how the helicopter works. Of all the things I was thinking about, one thing stuck out in my mind. After it happened, I casually mentioned to Bruce that I knew it failed because I got the wah-wahs after I did a flip and then descended backwards. Bruce argued with me that the clutch doesn't cause that. I said it does, because I have noticed that on every failure. Well, I was thinking about that tonight and Bruce is right, but I am too. I know what I experienced, but from a mechnical viewpoint it should not have happened....UNLESS something is wrong with the autorotation hub. The wah-wahs are the sound you hear when the engine is overspeeding. During zero pitch range of the flip I did followed by the descent I did, the blades exceeded the rpm the engine was turning them at. With the low speed needle setup right that shouldn't have made the engine overspeed to the extent it did because under normal circumstances the autorotation hub should disconnect the main rotor from the engine anytime the engine is not turning as fast as the main rotor. At least that is how I interpet it. Bruce said that unless the engine is at idle, then the auto hub is engaged. He has been in r/c helicopters a lot longer then me and he maybe right. Bruce is saying that the only function of the autorotation hub is to let the blades turn free when the motor goes to an idle. But I'm also thinking that it also functions like a diode in electronic circuits. It only lets power go one way. From engine to rotor. But in my case something is not right with the autorotation hub and it is either intermittantly or constantly (while loaded) allowing power to travel from the main rotor to the clutch bell which spins the engine faster and makes the wah-wah sound. I knew my Raptor #1 wasn't eating clutchs as much as #2 so I put both of them up on the table and turned the main gear backwards to feel how free the auto rotation hub was. I couldn't tell a difference so I next felt of the belt tension. They felt about the same. I felt sure the hub was the problem so I re-examined it. Turning the main gear wasn't accurate enough so I held the main gear still and rotated the main blades backwards. At first it felt smooth, but then about 30 degrees later there was this very slight tick-tick feel. This was on Raptor #1. I did the same on Raptor #2 and the ticks were a lot more distinct. I loosened the bolt that goes through the auto hub and that nearly got rid of the ticks. I know of three things that can cause the auto hub not to turn freely. 1. the bolt is too tight. 2. the belt is too tight. 3. there is dirt in the hub. So why does this break the clutch... If the rotor is intermittantly (or constantly) transfering power back through the auto hub to main gear to pinion gear to clutch bell, then this causes the bell to turn backwards relative to the clutch shoes and is forcing the shoes toward the flex region. I'm thinking the reason I don't get any overspeeding while the clutch is still good, is that the liner slips across the shoes, but when one shoe fails, it changes the angle and makes the shoe dig in more to the liner so it can't easily slip. I will test this out on my next clutch and keep this page updated.

8-26-0 I was wondering if the headspeed makes a difference. I used to run near 2000 rpm on Raptor #1 for a long time. Maybe that accounted for 330 flights I got on one of my clutchs. If the engine speed is higher then the outward force on the clutch shoes would be greater and the clutch would experience less slip on the liner. The more slip there is, the more heat there would be. I was thinking that if the clutch got too hot, then it might weaken the flex area.
Another thing to think about came from a Raptor pilot from Belgium. Chris emailed me that it might be the frames. He is thinking that the frames are not holding the pinion bearing solid and therefore let the assembly move too much. Each time the pinion gear turns againist the main gear, it is forced toward the front of the heli. Each power on power off would move the assembly. I suppose if the clutch bell (attached to the pinion) gets moved back and forth a lot, that might lead to the failure.
One person on the Raptor2 list said he still thinks it's a material problem. He thinks the Quick Hi-Carbon clutch failed because too much carbon makes steel brittle. And the reason the TT clutch fails is because the steel used has too much sulpher or other impurities and is just too low grade for this application.

as a side note, I'm still making sure my engine/start shaft in alignment and I currently have 50 flights on Raptor #1 and 29 on Raptor #2

8-19-0 Was discussing the clutch problem with a fellow Raptor pilot (Rick Smith) and he has been doing the engine/start shaft alignment all the time and still has had clutchs fail. He learned to do this alignment from his Shuttle helicopter days. I'm still monitoring mine and hope it does not fail, but from the sound of Rick's experiences, another broke clutch maybe inevitable. I'll keep this page updated with any of my results.
8-6-0 Thought maybe the distance from the engine case mounts were further from the centerline of the crankshaft, so I ground the engine mount in hopes the crank shaft might line up with the start shaft. It did not, so then found it was an angle problem. Drilled out engine mount holes, using an 1/8" bit, to allow engine/start shaft to align properly and hopefully take the stress off the clutch. The start shaft turned much easier after that.
# tanks before failure Raptor #1 Raptor #2
1st Clutch 103 tanks (TT clutch) 104 tanks (TT clutch)
2nd Clutch 331 tanks (TT clutch) 33 tanks (TT clutch)
3rd Clutch   66 tanks (TT clutch)
4th Clutch   20 tanks (HHI/Quick)
8-5-0 HHI/Quick Hi Carbon clutch failed after 20 flights.
Why? I don't know. It cracked in the same place my TT clutchs did.

I'm considering the following other possible causes. I have no idea if they are the key to the solution or not. I'm just trying to use 'brainstorming' technique to find the answer.
1. Alignment of start shaft into one-way bearing of clutch. This is something I never paid attention to until Howard Moftich mentioned it, on the Raptor 2 email list, in reply to someone wanting to know if the engine had an alignment that needed to be done when installing. If the start shaft goes into the clutch one-way bearing crooked, does this cause the shoes and clutch not to track together. In other words, if say the clutch bell is off by say.... maybe half a degree. Now as the shoe rotates to the back, then for the shoe to remain flat with the liner it has to twist up to a half degree, then as it rotates to the side, it is flat again, then rotate to the front and now has to twist up to a half degree in the opposite direction. Even if it is less then a half degree, could this twisting cause the crack... could the shoe twist enough to matter... is it even a twist problem... or is this non-tracking causing a vibration that is breaking the clutchs???
2. Elongated holes for the flex area. Maybe it needs the elongated holes so as each shoe flexes outward, the stress is spread over a larger area. The TT and HHI/Quick clutch are made of two different materials, so maybe no material is strong enough for this flex area, so it would require the design to spread the force by way of the elongated holes to provide more area to flex.
3. Holes too close to edge. The holes are closer on the HHI/Quick clutch then on the TT clutch. Maybe the clutch would have lasted more then 20 tanks if the flex area was thicker???

4. Bad material. Well, it doesn't seem likely that two different companies using two different materials would both get it wrong, but...
5. Fan not balanced. Don't think this is it, but listed it, just cause I'm trying to come up with ideas.
6. Bad luck. Maybe I just got the famous one-in-a-million bad one in the batch.
7. Something else I haven't thought of... yet :)
7-23-0 Clutch failed after 66 tanks :-( Argh!!!!! Saturday I had 3 great flights with no sign of problems, then when I cranked up for the 4th flight, the clutch was dragging a lot. I inspected it but could not see anything wrong so I cleaned out the clutch dust and reinstalled. But it was still dragging so installed Quick Hi Carbon clutch. It was dragging also so later that night I checked the clearance and found it to be 0.012'' on both sides. Looked for anything that might cause both clutchs to drag. Relooked at old clutch and it's hard to tell but I think the famous crack has started in the usual spot. If it's not a problem with clutch material then it might be a vibration issue. At this point in time I'm leaning toward bad metal. If the Quick Hi Carbon clutch breaks too, then I will know it's a vibration problem. As for why the Quick clutch was also gripping, it appears to spring out at a lower rpm. The holes drilled for the flex area are closer to the edge then on the stock clutch. The next day it worked fine but grips more than I like but I figure it will wear in.
The clutch mod does still need to be done to allow any clutch to work properly. Shoes that drag can cause things like not letting the engine return to idle quickly in an auto and cause it to require a high rpm to engage. I also had one person say it cured an RF glitch problem. Unless you want to rebalance the fan hub, then a shim is the best bet.
Heliproz sells one pre-shaped and holes drilled for less than $5. I have also heard some who made their own out of a coke can and one who used paper.

HeliProz shims - Yes, the shim is technically thicker then it has to be, but I have heard from several who use it with no problem. As long as your clutch is not touching the inside top of the clutch bell then it will be fine. If it is dragging, then first make sure the fan is screwed all the way down on the engine crankshaft. If it was all the way down, then unscrew the fan from the engine and remove the washer that's installed between the fan and front engine bearing. This will give you the extra clearance you need. The washer is not needed, because the fan has a built-in ridge so that it rides on the inside race of the bearing.
There are 3 groups of thought as to the clutch failures.
1)clutch shoe to clutch bell clearance
2)clutch shoe to Fan hub clearance
3)quality of clutch material
1 & 2 are important and BOTH need to be done correctly to ensure any clutch (stock or upgrade) will have a chance to work properly. I describe below how to shave the hub to fix #2, but an excellant alternative has been made by Heliproz. They made a kewel shim you put on under the clutch. It's less then $5 and simple to install. They also have a very good page comparing the Kalt clutch and fan hub to the Raptor's. These are very similar designs, but the Kalt does not have the failure rate the Raptor does. Read about it at
http://www.heliproz.com/Rapclutch.html
Click
<here> for instructions on setting the clutch shoe to clutch bell clearance and <here> for fan hub clearance
3 is still an issue. Most clutch failures maybe a result of not doing 1 & 2. It will be interesting to find out the long term effect of this. If it turns out that clutch failure is still high, then there is another step. Heliproz will be selling a new direct replacement clutch part #HHI5410 (note: I don't know if this is the part number you order by, do a search on their webpage for 'raptor clutch', It will be listed as 'QWW Heavy Duty Raptor Clutch') The current price as of 7/17/0 is $29.99
My friend Bruce looked at his clutch with a high power microscope that he has at work. He found cracks starting to form all around the edges. He has not done the fan hub mod because he wanted to do his own test. He found three cracks were on top and one on the bottom. One could be seen but the others required the microscope. One of them was around the bolt hole area so this would suggest that this is a material problem. If you continue to have problems with the clutch after setting the clearances, then Heliproz now have a new direct replacement clutch part #HHI5410. It is a high carbon clutch manufactured by HHI/Quik.
Why does the clutch fail?
At first I thought this was just a problem with everyone else since I had over 300 flights and counting on the clutch in Raptor #1. I kept hearing more and more reports of the clutch failing. The only problem I had was that the clutch failed on the 103rd flight on Raptor #1, but I replaced the bell/liner/clutch and had over 300. I just figured it was a clearance problem. The gap size should be between 0.008" and 0.012". Everything was going fine until I was having engine problems with Raptor #2, I started swaping the engine in #1 back and forth with Raptor #2 to figure out the overheating problem, but in doing so I would always keep the #1 clutch with #1 heli and #2 clutch with #2 heli. I did not want to take a chance on a different gap size ruining either clutch. Unfortunately this did not help. My 300+ clutch broke. Not long after I got Raptor #2 running right it's clutch failed. This was on flight 104 and I thought maybe it failed because of a clearance problem. I replaced the clutch bell/liner/clutch and set the gap to 0.010" on each side. Well guess what, it failed. This time on flight 137, only 33 flights. I knew there was more to this clutch thing then clearances.
I still had all of my broke clutchs, including one that just began to fail, plus a brand new, never used one. So I evaluated many situations to determine if they could produce the effects seen in the clutchs I had.
1. Gap size too large
---Well a large gap size is not good, but this was not the case in the last 2 clutchs.
2. Liner too soft and the gap gets too large
--- Possibly, but not in my experience. I had a crash in which the engine ran full throttle for at least 15 seconds. The clutch bell was very hot. I was sure this ruined the liner, but I checked the clearance and it was 0.010" on both sides. I was amazed this had not damaged it.
3. The material is not flexible enough.
--- Don't think this is it either, if so I think the other side should at least show signs of cracking and none of my broke clutchs have both sides bad.
4. Tool marks - on corners
--- This is a theory by my friend Bruce. Mechanical engineering teaches that if you use two 45 degree angles, they will see half the force that a single 90 degree joint will. TT used the 45 degree technique correctly on the top and bottom edges of the clutch, but the machine that cut the angle left small nicks. If a nick occurs in the flexing area of the shoes, then the forces will concentrate at the nick and eventually form a crack. If you score a material, it will break at that point.
This made sense, but every one of my clutchs broke in the exact place. I found a better theory.
5. Tool marks - on sides
--- There are tool marks on the side, one that goes from top to bottom and another that only goes half way. This seemed like the answer, but I found the cracked clutchs didn't break on this line, but was a 1/16" away.
So what's the answer... the fan hub! I know what you're saying, WHAT!!! Read on, I'll try to explain.
These three are all broke, the two on the left have shoes completely seperated and the third one has the beginning of a crack. On all three the crack began at the top. You can tell that by looking at the broken surface. The rough area is where the crack began and the smooth area is the final part that snapped. Notice that I have the one on the right turned upside down so you can see the bottom. See the shiny area on the top shoe. This is on the shoe that has the crack. The other two clutchs also had these shiny areas.
This is a better view of the broken surface. The rough area on the top is the side the crack began on and the bottom area is the last part to snap.
The shiny area on the bottom of the clutch shoes is where they were rubbing on the top of the fan hub. It left black marks on the hub where the bottom of the shoes were up against it. When the shoe is slung outward, the bottom half is being restricted and initially causes the shoe to twist until it is fully engaged with the bell. This repeated twisting causes the top to stretch until it breaks at its most weak point.
Here you can see that A and B point to the shiny marks where the shoe was dragging. C shows where the shoe detached. D shows where a crack has begun.
I colored the tool marks red.
The tool marks are not the problem because the cracks have all been a 1/16" away from the mark. The red arrow points to where a crack has begun on the top of the clutch.
The solution is to shave off enough of the fan hub so the shoes don't drag. Only file down the part where the clutch shoes were dragging. It would do no good to shave the entire top surface. And be sure to remember to place the clutch back on so that the shoes are over the shaved area.
Clutch shoe to clutch bell clearance
I have some instructions at Setting Clutch Clearance
Also Sheldon (of
YNT uDesign, makers of the excellent BC6 flight monitor) provided the instructions listed below. I was told the idea came from Juan Rodriguez, so thanks Juan :)
Assuming that you either have a lathe or a friend with a lathe this is really easily done...
  1. Remove clutch bell from machine
  2. Mount clutch bell in lathe and remove .020"-.030" of the existing clutch liner. The amount isn't that critical at this point.
  3. Wipe down newly cut liner with acetone and let dry
  4. Wipe down new clutch liner (to be installed) with acetone and let dry
  5. Trial fit new liner in bell and cut to length as necessary
  6. Mix up a small amount of JB Weld, you only need a very thin layer 
  7. Install new liner right over the existing liner and secure in place using your favorite method
    • I removed enough material from my existing liner so that I could use  the clutch (wrapped with 1 layer of scotch tape to protect from JB Weld) to tightly hold the new liner in place while the JB Weld cured
  8. Allow to cure overnight
  9. Mount clutch bell in lathe. The stock Raptor clutch measures 1.410" O.D. You want to remove material (SLOWLY) until you have a finished ID of 1.422" - 1.430". This will give you a final clearance of .006" (a tad snug but will wear in) - .010" (seems to be just about right according to most people).
  10. Re-install clutch bell in machine using a thin layer of grease or Never-Seize on the threads. This will allow you to (fairly easily) unscrew the bell from the pinion in the future.