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Old Dec 11, 2015, 04:30 PM
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Careful!
Motor shaft circlips - replace if damaged

Hi folks, I would like to give a heads up for those who are starting to play with disassembling/ assembling of brushless motors (like me ). Last Friday I had a partial rotor/ stator separation in flight due to a circlip slipping from the shaft groove. It was during a knife edge spin which is a violent maneuver at full throttle.

The picture below shows how far the circlip moved from the groove. During the forward movement it damaged the winding somewhere (I couldn’t spot exactly where) creating a short. Luckily there was no damage to the plane from the emergency landing . Not even the cowling was damaged by the motor.

I have already rewound the motor (NTM42-38) and I put 18 flights this week without issues. This time I created a flat spot on the shaft with my dremel and installed a wheel collar tightening the grub screw with Loctite.

The root cause: I had previously disassembled this motor many times (re-gluing magnets, replacing bearings, rewinding, etc), deforming the circlip in the process. After repetitive bending/ straightening it obviously lost its strength due to material fatigue.

Lesson learned: if you’ve damaged the circlip, replace it or at least increase your safety by adding a shaft collar behind it. For most applications, if you were able to remove it with minor damage it may still be OK to use but in the end it is your decision. My planes have a 2:1 thrust to weight ratio (or more) and I push them really hard so I won't gamble again. A bent circlip will never have the same retaining strength as a new one.

As a mechanical engineer I always had the feeling that it could happen but was too lazy to do something to prevent it.

From now on I'll be using a double safety: new circlip + shaft collar installed 1-2mm behind it. If the circlip fails I'll be able to spot an axial play of the shaft.

I'll post a video of the failure soon.

Luckily I bought this pack of circlips one week before the crash.
http://www.ebay.com/itm/370609016762...%3AMEBIDX%3AIT
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Old Dec 12, 2015, 08:07 AM
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Here is the video, for just the failure skip to 2:15
The funny part was that we had weeks of non stopping rain for 2 weeks so my flying area was a real marsh. I had to take off my shoes and roll my jeans to go pick my plane. Luckly I had already left work so I could go straight home.
https://drive.google.com/file/d/0B77...ew?usp=sharing
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Old Dec 16, 2015, 08:35 AM
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I find even myself guilty of improper removal of these style clips, ie. forcing a screwdriver or other small tool inside the clip to pry it off. This improperly stresses one area and can easily and permanently deform the metal or weaken it.

It can also create burrs or other deformations in the groove. These must be carefully cleaned up before the clip can again be effective.

One other very important aspect is that circlips are stamped out, creating a round edge face and a sharp edged face. If you load towards the round edge it can force the circlip open.

That is the case equally between inner and outer types of clips.

ps. It seemed at certain throttle settings you had airframe resonance evident.
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Old Dec 16, 2015, 09:17 AM
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I share the same guilt, I don’t have a proper circlip plier (actually never found one small enough to fit model airplane motors).

I developed a technique using thin wire that is guaranteed to damage the circlip when extracting although it would never damage anything else. After replacing the circlip everything will be as new again.

I did notice the noise on that flight and later in the video but it never happened before neither after motor repair so I believe it was the first sign of the failure.

Thanks for the manufacturing info, I’ll try to check orientation before installation (kind of hard due to size)
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Old Dec 16, 2015, 11:05 AM
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Other than a sign of motor failure that resonance could be indicative of one or more structural conditions.

If the prop or motor has mass or aerodynamic imbalance that could be the source for vibrations fed into the mount and aircraft structure. It is only when that frequency matches that of another will they build up enough to be a problem, (this can quickly lead to destruction if the structure is not tolerant enough).

It may be that only parts of the structure vibrate in resonance. Your type of craft, built in what I call "Skeletonized" manner in that all excess material has been removed, is not as forgiving to some forms of vibration, (not enough mass) This is something I saw myself when a friend modded one like yours to nitro, (4-stroke with even more pronounced power pulses). All sorts of interesting effects started to display at certain RPM's.

If parts have been damaged or come unbonded the problem can get much worse. It may be prudent once you have the motor repaired to ground run to investigate where in the craft the resonance is exhibiting.

I fixed and flew full size planes and helis and especially in the latter a lot of engineering went into dampening vibrations. Helis are also weight critical to the point every excess gram is a factor. Add in that a higher majority of things are moving the sources are many and of varied frequency. To combat this parts of the structure are designed to vibrate at different frequencies, (hopefully away from what can be induced) so they dampen as many inputs as possible. Even the landing gear can be used to dampen longer cycle vibrations.

As an example of getting it wrong, one manufacturer had to alter the tires and pressures to eliminate ground resonance. Here is another where things went off the charts:

Ground Resonance - Rear View (0 min 38 sec)


ps. I'm in aerospace now and our company makes major driveline pieces for that one.
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Old Dec 16, 2015, 01:13 PM
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Thanks, as I mentioned that was the only time I heard this noise in over 200 flights on this airframe, Iíve put almost 30 flights since the event without any issues.
Iím a mechanical engineer and Iíve been working with gas turbines for over 20 years so Iím also familiar with vibration excitation and its effects and countermeasures. I currently work for a major manufacturer of helicopter engines so we probably had similar experiences along the career.
Considering how bad I treat my 3D planes with lots of high G maneuvers at full throttle such as blenders, KE spins, crankshafts, etc I believe Iíll face a major failure due to stress before anything else.
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Old Dec 16, 2015, 01:53 PM
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Did not realize you had some time on the airframe since.

Although I watched Sean Tucker at Oshkosh your type of maneuvers would break my neck Every so often I take the Super 'Bolt up to wring it out and have some fun, nowhere near as skilled or practiced at 3D as I saw in the vid. I would also imagine the forces imposed from the prop back to firewall from those moves.

I tend more to scale craft and flying now, with most efforts made to try and make them look like a pilots at the controls.

We should compare notes. I had experience wrenching on and flying behind, (or under) the PT-6 and -250 series, along with servicing the natural gas fueled GE LM6000 used for power generation, (Transalta).

Our company made parts for the original and upgraded Shuttle arms and also the original Mars Sojourner rover. In fact, the names of the entire family went with Spirit and Opportunity, as well as the Comet Impact and Japanese Selene missions.

Rather than clog up this thread, if you're interested send me your Email in a PM and we can continue this elsewhere.
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Old Dec 16, 2015, 08:04 PM
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Old Dec 19, 2015, 09:38 AM
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Took this picture to show how I create a flat spot now to secure the shaft collar.
I make a slightly deeper groove with the cut disk. This prevents the grub screw from sliding in the shaft. Make sure you apply blue Loctite before installation.
Tip, use masking tape to protect the motor and avoid metal particles being attracted by the magnets.
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