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Home✈️Aircraft🚁HelicopterCentury Radikal G20-30 N640 Hawk Predator › Drive Shaft Sub Standard Quality.
09-06-2006 03:54 AM  14 years ago
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BC Don

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Calgary, AB Canada

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Drive Shaft Sub Standard Quality.
Here's a before picture:

Then, on flight #26 on this bird I was inverted, about 50 feet high or so, putting in a lot of negative collective as I started some inverted flight which put some stress on the system and bogged the motor a bit when suddenly the motor started screaming and Heli started falling. I moved the collective stick around to no avail and hit throttle hold to stop the engine. Tried to move to maximum negative to do a negative auto (haven't even done a positive one) but the blades virtually stopped as the Heli fell out of the sky. Surprisingly the blades survived, likely cause they had pretty well stopped spinning and the Heli fell on the muffler, splitting it open and breaking the motor mount. Over $200 damage excluding the engine and muffler.

The cause of this was the drive shaft giving out. The top bearing had worn the diameter of the shaft to half of its original size.

Here's a picture of the break:

And here's a picture of the bearing. I've moved it down a bit so it is where it fits snugly. You can see how much is worn away.

I seem to recall someone else posting that they had this problem.

In any case, I've contacted Century to see what can be done. If I hadn't done something that was recommended to me by Century I can understand that there may be a problem but, this is my (lemme count now) - 6th Hawk (2 Sports and 4 Pros). When the cost of replacement parts is within 10% of a new unit I just get the new unit. This has never happened before so I wondering whether there was a bad shipment of Drive Shafts that are much softer than normal.

I've gone back and checked my other Drive shafts (and the one on my "down" Raven) and I can see some rubbing where the black on the shaft has rubbed away (much like you see on the main shaft) but nothing worse than that.
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09-06-2006 04:16 AM  14 years ago
trevorz

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Calgary, Alberta Canada

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Hi Don,

Looks like the bearing was not rotating, i.e. not fixed to the shaft. This will cause the shaft to spin inside of the bearing, instead of the bearing rotating and supporting the shaft.

This usually happens for one of two reasons:

1. The bearing is seized.
2. The bearing and shaft not exactly the same size (i.e. are ever so slightly loose), and the therefore, nothing to stop the shaft from just spinning inside the bearing instead of spinning the bearing.

From this, you can see why they make bearing retaining compound, or why it is often recommended to knurl the shaft to ensure good contact.

I have had this happen on my X-Cell 60, and my Raven, when I did not ensure the bearing were fixed to the shaft.

Unfortunatly, this reminds us all of how important pre-flight checks are too, as if the shaft was worn that much, it would clearly rock in the bearing, and the pinion would move.

Sorry for you crash Don, it really sucks. On those bearings (and most others) I always use a little bearing retainer, or at least mark the shaft so the bearing will not spin inside.

I don't thing the shaft being of poor quality here is to blame. Any shaft WILL wear if there is metal to metal spinning.

Regards,
Trevor.
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09-06-2006 02:45 PM  14 years ago
BC Don

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Calgary, AB Canada

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Thanks for the reply Trevor. Now that I know more I have a better idea of things to check for before (or after) flying. I did check the bearing when I had it apart and it would still spin but because of all the gunk in there it felt like it had heavy grease in it. I'm sure if I rinsed it out it would be fine so there must have been just a little too much space between the shaft and bearing.

It would be nice if the kit instructions told you which bearings were susceptible to this and what to do about it. However, when purchasing an ARF I would expect this to be done in advance for me. Doesn't make sense that I have to take the heli apart (split the fames) and do a bunch of work. That means that I have to do more work with an ARF than with a kit. Furthermore, with this being targetted (I think) to folks getting into the sport (why else would it be an ARF), they are the people are who are least likely to know about having to take it apart. Just doesn't make sense to me.
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09-06-2006 05:21 PM  14 years ago
jackheli

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Vancouver - Canada

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Interesting. I had the exact same issue happening to me.

The shaft broke and the head lost all speed while I was doing a stall turn 3 feet off the ground. My Raven when in hard, nose first...

I locktited the bearing to the new shaft while rebuilding...
It's easy to find an excuse to do wrong. Hard is not to find an excuse to do right.
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09-06-2006 07:13 PM  14 years ago
Gearhead

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sorry this had to happen, but if you look close you will see that the wear spot is wider than the bearing, the shaft was vibrating up and down, this is a close indicator that your motor was lean, when the motor is too lean it produces a/more harmonic vibrations, this vib is transferred to the counter gear and shaft..

Jim
Jim
Buzz Buzz Buzz
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09-06-2006 07:55 PM  14 years ago
BC Don

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Calgary, AB Canada

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Jim, I'll check the width of the wear when I get home. Hadn't thought of any up and down motion although it that had happened then I'd think I'd see in from both bearings - yes?

Also, as far as being too lean I know I was quite rich on the low end and certainly had no tank foaming. On the high end, after a flight my head temps were at or just below 200F so I don't think I was lean on the high end either.
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09-06-2006 08:12 PM  14 years ago
BrunoB

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Quebec, Canada

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Hi BC Don,

I basically have to change this shaft every 15 gallons of fuel on my Hawk Sport and Raven 50. I alway use green loctite on the shaft and mount it with no up/down play but I still can't make it last more than 15 gallons. I also wish the shaft would be of better quality. I guess I was lucky and detected the play in the shaft the first time it happened, so I never had one break on me. Now I verify for "slack" all over at the end of the day and I can easily detect the wear before it's too late.

The last time I've replaced the shaft and bearing on my Hawk (one month ago) I modified the frame to allow space for a second bearing and removed the brass spacers as others did (SteveH comes to mind). If this modification makes the shaft and bearing last longer, I will also make the change to my Raven next time I split the frame.

Bruno
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09-06-2006 09:39 PM  14 years ago
BC Don

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Bruno, I may look at the extra bearing as well for my Raven. But, you say every 15 gallons or so. I had no more than 3 to 4 gallons through before it wore out and I'm wondering why so fast. As I indicated, flight #26 was when it happend. 15 gallons would be over 150 flights. If it lasted that long I wouldn't be so concerned.Got Money? Send it to me, I'm a Heli Addict.
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09-06-2006 10:28 PM  14 years ago
Gearhead

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Vt

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Don, maybe it's just how your holding the bearing on the shaft in the lower photo,, I do know when mine did this the wear patten on the shaft was wider than the bearing..

Jim
Jim
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09-07-2006 02:34 AM  14 years ago
adamsz06

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Geilenkirchen Germany

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I had this happen on my Hawk Pro also, but luckily the shaft let go before I got airborne. I replaced it with the constant drive upgrade from the Raven and made sure to locktite the bearings to the shaft; it worked great. I now have a Raven 50 also, and while building, fitted (2) more bearings to the shaft for extra support; it spins smoothly and the shaft is super rigid with no play. I have to agree about something being mentioned in the instructions in the ARF kit...they do tell you to use locktite in the Raven build. The Hawk was my first heli, and without the experience or the help of what to look for on my pre-flights, I had no idea anything was wrong; Century please address this!!!
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09-07-2006 04:33 AM  14 years ago
perfesser zero

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Dublin, VA., USA

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OK, now you've got my attention...
What is wearing here? I've looked at mine and I can't see anything out of the ordinary. I have about 140 flights on mine and it's been fine (up until now anyway). How do I go about checking these bearings and shaft. The counter gear has a little play up and down in it, about 1mm. To much? Just right? There is very little backlash between the counter gear and the engine/clutch gear. As far as I can tell there is very little play between the pinion gear and the main gear. Just what should I be looking for in all this? If I decide to replace all this with new parts, what all should I replace? If I do the added bearing upgrade what other bearings do I need? Thanks guys! Now you have me worried about flying the Hawk.

Later...

Scott!
Tiger 50 - OS 50SX-H
Hawk Sport - OS 37SZ-H
Raptor 30 - OS 37SZ-H
JR 10X/R950S
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09-07-2006 03:25 PM  14 years ago
Donald D.

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Central Wisconsin

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I have the same shaft set up in my Falcon 50, and here is my experience with it. I upgraded to the constant tail drive system two years ago, and after about twenty gallons of fuel I could hear this gear sound when I was descending in a power off descent. I noticed that when I grabbed the counter gear on each side and moved it up on one side, and down on the other side, I could feel play in the upper bearing on the shaft. I replaced the bearing but the shaft had a groove wore in it where bearing rides. I replaced the shaft then too, and at the advice of Steve H., I added the second bearing at the top of the shaft. I really noticed the added wear on the shaft after increasing the head speed, and I really think that the upper bearing is working hard, as when you increase power, it adds a lot of side pressure to that bearing. By beefing up that area with the second bearing, I would think that the problem would be solved. I only have about two gallons through it since I fixed it, so it is to early to have an answer. I will know more when I do my winter tear down later in the year. I would have thought by now Century would have addressed this issue, by adding a second bearing to their new kits.
Don
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09-07-2006 03:42 PM  14 years ago
BC Don

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Ahhhh, things are starting to come together I think. With my other Hawks (and my Raven), I've been running lower head speeds but recently, with the TOKI wanting to run high and with my upgrade to thrust bearings in the grips and full carbon blades, I've been running around 1900 as a head speed (before was 1800 or lower). Also, I've been doing a lot more - loops, rolls, flips, inverted flying.

In other words, I've been working the components a lot harder than just some simple hovering and flying around.

From the comments I've heard this seems to be a common factor. I like the idea of putting in a second bearing at the top. Don, do you have pictures of how you did that? Did you have to Dremmel anything or?????

I've emailed Century as well but haven't heard back yet. I don't want to repair anything till I know that what I do will work.
Got Money? Send it to me, I'm a Heli Addict.
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09-07-2006 03:50 PM  14 years ago
catzazz

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Vancouver Island, BC, Canada

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Funny thing this happened, I had the same thing happen on a hawk pro frame with 300 flihts on it. Kinda expected a bearing to go but the bearing was fine and the shaft was worn out....

Cheers, Mark M,
http://www.pdqflyers.com
Fly like you borrowed it!! Land like you stole it!! visit www.virc.ca
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09-07-2006 04:55 PM  14 years ago
SteveH

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Texas

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Yep, that top bearing will beat the shaft out bad over time. It's worse with a lot of head speed, a lean carb setting, or a high horsepower engine. Keep in mind that this design, as well as a lot of other 30/50 size helis today, was originally designed with the power of an OS .28 in mind. We now run Hyper 50's in the same drive system. Even the .32 with a high head speed and/or a lean carb will do it.

I recently tore down my number one Peregrine Falcon because of a bad crash (it was caused by an electrical problem....a short circuit between my eyes and my right thumb), and the bearing and shaft were in perfect shaft after more than two years of flying. I have the two bearing mod done to all my helis....don't even build one without it any more.

It's easy to do. Just use an exacto knife and cut out the top lip that holds the top bearing in place, delete the two brass spacers, and add an additional bearing.
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09-07-2006 05:45 PM  14 years ago
jackheli

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Vancouver - Canada

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Hi Steve,

Would it be possible for you to post some pictures of the mod? I've never heard about it beofore... Weird!

Sounds like a great idea.

Thanks.
It's easy to find an excuse to do wrong. Hard is not to find an excuse to do right.
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09-07-2006 06:04 PM  14 years ago
Donald D.

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Central Wisconsin

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Hi Guys
The upper bearing modification is actually very easy. When SteveH first mentioned it to me, I thought it would be hard to do, but it is quite simple. Just remove the upper lip that holds the bearing in place in the plastic frame with an exacto knife, just be careful to not go too deep. Then add a third bearing to the shaft, and remove the two upper brass shims that were there. The top bearing will rest a little above the slot in the frame, this is normal. The upper bearing now is just below the gear. I don't have any pictures, but it is a simple procedure that makes good sense.
Don
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09-07-2006 08:40 PM  14 years ago
automan

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Topeka, Ks - USA

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SteveH,

In previous threads regarding this issue, it appeared that a possible solution was to install 2 additional bearings between the 2 existing bearings. Is your method of adding a single bearing above the existing top bearing, a better/simpler alternative or am I missing the picture? I was a little confused when you mentioned - "I have the two bearing mod".

Regarding your method, if the top lip is cut out that holds the top bearing in place, doesn't that place additional vertical stress on the top lip of the bottom bearing? (maybe it's not a factor)

Since I'm in the process of completing my build on the Pro, I've debated on whether this mod is really necessary. It sounds like the answer is "yes" - especially if something bigger that a OS .32 is going to be used.

Thanks for your help!
-Dan
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09-07-2006 08:57 PM  14 years ago
SteveH

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Texas

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Dan,

I've only heard of adding one additional bearing. Maybe the reference to two bearings was about the two bearings at the top. The bottom bearing/shaft area does not have a problem. If you study the way the transmission works, the top bearing takes all of the load/torque of the engine, and absorbs any ratcheting back and forth from an out of tune engine.

Once you take the frames apart you will see there is no vertical load what so ever on the system. As a matter of fact, I like to install the intermediate shaft system with no vertical play, and I distinctly remember the instructions of one of my first Century helis saying to do so. Additionally, if you don't have the driven tail option, it is usually necessary to take the intermediate shaft out and grind the flat farther down the shaft to be able to get all the vertical play out. The thing is, with vertical play, the pinion gear is only allowed to go up and hit the bottom side of the main gear, which in itself, puts additional and unnecessary side loads on the shaft.

Again, I would not assemble any small Century heli without the extra bearing...it works that good.
The government cannot give you anything without first taking it from someone else.
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09-07-2006 09:44 PM  14 years ago
automan

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Topeka, Ks - USA

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SteveH,

Thanks for the explanation - makes sense.

As you suggested, I had already taken the frames apart and removed the vertical play from the shaft. I also, used Loctite 609 on the bearing/shaft interface.

As info, here's the link to one of the threads I was refering to regarding the additional of 2 new bearings between the 2 existing bearings.

http://www.runryder.com/t265851p1/?

-Dan
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