My mCPX blade grip teardown. Incorrect factory installation of bearings.
I believe I see what is probably the matter with the blade grips and their bearings, and this problem with the mCPX.
After flying a couple times just fine, I was interested in the possible bearing issue affecting my new heli, so I dismantled my ends tonight.
First, I found that the bearings were installed as follows. (out is facing blade and order is assembled order)
Blade grip 1 - Gold bushing(with feathering shaft screw inserted, Philips head)
Blade grip 1 - Outer bearing(gold out)
Blade grip 1 - Inner bearing(gold in)
Main rotor hub side 1
Main rotor hub side 2
Blade grip 2 - Gold bushing
Blade grip 2 - Inner bearing(gold out)
Blade grip 2 - Outer bearing(gold in, with feathering shaft inserted, Flat head)
So, already a problem in my eyes. Pulled the bearings out and inspected them.
Horizon Hobby has a diagram of the blade grips here.
You notice on their diagram, it shows the gold bushing should be installed between the main rotor hub(rubber bushings) and the blade grips on both sides with the small flange from the gold bushing toward the bearing race. Mine had the gold bushings installed incorrectly. (look at my assembly order from factory)
Now, look at the bearings. (tapered race bearings)
You will see, the gold side is the cage holding side, and this is obvious because the race of these bearings are tapered. On the gold side, the thickness of the race is much less than the other side. Also, this means any thrust put on the bearing inner race(by feathering shaft screw head or slotted head) should be forced toward the thicker outer race which would keep the load so it's pushing on the thicker race side keeping the bearings together.
We also note the blade grips have slots where bearings seat, so the blade grip's thrust is pushed directly on the outer race of the bearings.
The way the diagram from HH states, means that all the thrust is directed on to the inner race of the bearings. This means, all thrust is put directly on the inner race and there is nothing to hold the blade grip if the bearings fail.
From my observation, the factory assemblers are installing the gold bushings to mate with the gold side of the bearings regardless of where or direction the bearings are installed. As my assembly had gold facing out on one grip, the bushing was also installed on the outer side of the blade grip mating with the gold side of the bearing. On the other grip, the busing was installed correctly between the rotor hub the blade grip. The installers are not looking at the diagram during assembly. My guess is a lot of them are this way.
I just purchased a new set of blade grips for the LHS. These new grips have the bearings all installed with gold side in, so if you look the grips on either side inside or out, you never see any gold. I believe this is also incorrect as this way the load is pushing on the bearing the wrong direction.
1) Horizon should issue a statement to fix the incorrect assembly of the blade grips asap as the current supplement to make sure the feathering shaft and screw are tight is not a fix for the problem with the bearings being installed incorrectly from the factory.
2) There needs to be a bushing between the feathering shaft screw head and the outer bearing, and on the other side the bushing would be between the feathering shaft slotted end and outer bearing. This would end up in four bushings total. This would apply pressure to the outer race of the bearing in the event the bearing fails and would be a fail safe to prevent the blade from flying off.
3) The feathering shaft and screw head should come with loctite from factory, mine had no loctite and was very loose when I disassembled. If the screw comes lose, then the blade is coming off no matter what you do.
UPDATE - 03/18/2011
Horizon Hobby has clarified(unofficially) that these bearings should all be installed gold(brass) side out, toward the blades.
Please feel free to comment.
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Last edited by redjoe; Mar 18, 2011 at 05:38 PM.
Great write-up, vey informative.
I got my hands on one of these little machines Saturday and was warned by the guys at the hobby shop about the blade grip failures and to look for the copper ring showing before I flew it.
I inspected mine and everything looked fine - only silver showing, so I gave it about a 3 or 4 minute first flight - nothing fancy - just hovering and getting a feel for it.
I decided to take a closer look at the bearings and noticed something odd looking about the outer bearing on the flathead-screw end of the feathering bar. It looked like the little ring that was between the races was working its way out of the bearing. I poked at it with my jewelers screwdriver and it rocked to one side, popping almost completely out. I slipped the screwdriver through the ring so that I could get a closer look at it. It looked like this:
“W” Type: One piece steel crown. Mainly used in very small bearings. This type of cage is inner ring guided. Material is type 410 stainless steel. The cage snaps in from one side over the balls. It has excellent performance in low torque, low speed applications. Generally, the one piece steel crown is not used in high speed applications.
It turns out that this ring is the ball bearing separator cage, the type used in very small bearings, that pops in from one side of the bearing. In the case of these blade grips, at least on mine, the centrifugal force exerted on the bearing has caused the separator ring to slip out on one blade grip, potentially leading to a complete bearing failure.
I was able to get the bearing out of the grip without pushing the inner race out of the outer one, and using a needle, reposition those tiny little bearings somewhat equally around the bearing and get that separator ring back in there - using the feathering shaft and one of those stepped brass washers (flat side towards the bearing) pushed and turned that ring back down so its fingers fell between the bearings.
When I put it back together, I flipped that one bearing over so that that separator cage is being thrown down into the bearing by the centrifugal force, but now it shows a brass ring on the visible side of the bearing. I don't believe that ball bearings are designed with any kind of taper to the races like roller bearings so I'm beginning to think that there's nothing to the brass side showing or not - yet. Keeping my eye on the other blade grip for signs of that separator cage working its way out. So far, so good.
It sounds like you have found the culprit ProtoDewd. If I understand you correctly the cage is being "thrown" out of the bearing, the balls bunch up on one side and the bearing effectively disassembles itself. If this is the case then it would seem to me that you want the "gold" retainer facing out away from the rotor head for both the inside and outside bearings in the grips. The odds of catching a bearing in "mid-failure" must be astronomical but its seems that you did. Prior to your discovery I hadn't heard a reasonable explaination as to why this was happening. All the photos of failed bearings I had seen so far showed no signs of destructive or catastrophic failure. Now I know why. Going to turn my bearings the proper way now!!!
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