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Saito 82a Ceramic Bearing Failure

Old 05-17-2009, 07:19 PM
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SeamusG
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Default Saito 82a Ceramic Bearing Failure

I need your input - looking for the cause of my bearing failure ...

I recently replaced the rusted bearing set in my Saito 82a with ceramic units from rc-bearings.com. Ifollowed Paul Macintosh's suggested procedure to install the bearings - freeze the bearings and bake the case at 350 degrees for 30 minutes, Both bearings literally fell into place with no pressure. During engine assembly I used air tool lube freely to pre-lube all of the components.

As the engine also got a new ring I ran a couple of tanks thru the engine (on our club's engine run-in table) to help burnish the ring. The engine then ran (apparently) well for 6 flights. On the 7th (this afternoon) very loud grinding noises could be heard from the ground - I landed straight away.

Observation:
. the fuel tank pressure line had come free from the muffler (unclear why it came loose)
. I think that inadvertently left a plug in the end of fuel line connected to the case breather but it was apparently blown out during flight (or Imay have forgotten to plug it in the first place)

Later, during disassembly on the bench, when I removed the crank the bearings and plastic retaining device fell out. The inner race was on the crank and the outer race remained in the case. The crank throw had started to chew into the inside surfaces of the case - a bunch of aluminum chips and slivers were found throughout the case. The cam "bay" and the inside of the case were both very well lubed (nothing was dry). The stuff under the rocker covers was very well lubed and wet as well.

I'm wondering if the prop "drive flange and tapered collet" installation might have caused the bearing to fail. When installing the prop the first time after reassembly the prop nut applies pressure to the prop which in turn applies pressure on the drive flange. The drive flange puts pressure on the tapered collet causing it to bite into the crank. (what stops the collet from being forced against the front bearing?) While the prop nut is being tightened it seems to me that it is also pulling the crank forward against the back side of the main bearing. The tighter the prop the more pressure being applied to the back side of the bearing.

Since I'm going to put new bearings in the engine and fly it again I'd hate to make the same mistake with potentially worse results.

Again, thanks for your input.

Edited ...

I just added the picture of the crank, the front bearing, the tapered collet and the drive flange.

Notice that the crank (from the right)has threads, smooth section, raised bearing seat and then the cam drive teeth. I slipped the tapered collet over the front of the crank. It seats against the shoulder created by the bearing seat. So, it makes sense that when the prop is tightened it forces the drive flange against the tapered collet which in turn wants to "seat" against the shoulder at the front of the bearing seat. However, the bearing's small race is wider than the machined section of the crank for the bearing seat so the tapered collet seats against the bearing's small race. The crank is then pulled into the backside of the small bearing's race.Seems that by tightening the prop (blah blah blah)will pull the crank snug to the backside of the small bearing's race and force the tapered collet to the frontside of the small bearing's race.

IMHO, the only way that the tightening of the prop (seating all of this junk) would affect the main rear bearing would be if the main bearing seats were not set flush in the case or the main bearing seats were too wide. Either condition would result in the crank being pulled against the small race (perhaps)forcing it out of alignment with the outer race. Subsequent side forces and vibrations encounted during "running" would cause a shearing affect between the races.

Maybe ...

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Old 05-17-2009, 07:31 PM
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w8ye
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Default RE: Saito 82a Ceramic Bearing Failure

Get the plain bearings from Paul and see how they hold up
Old 05-17-2009, 07:41 PM
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gkamysz
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Default RE: Saito 82a Ceramic Bearing Failure

ORIGINAL: SeamusG
I'm wondering if the prop "drive flange and tapered collet" installation might have caused the bearing to fail. When installing the prop the first time after reassembly the prop nut applies pressure to the prop which in turn applies pressure on the drive flange. The drive flange puts pressure on the tapered collet causing it to bite into the crank. (what stops the collet from being forced against the front bearing?) While the prop nut is being tightened it seems to me that it is also pulling the crank forward against the back side of the main bearing. The tighter the prop the more pressure being applied to the back side of the bearing.
This system of retaining the front bearing is properly designed. If it was not assembled correctly or manufacturing tolerances are not within spec there may be a large preload on the bearings. That preload would be very noticeable as a tight engine. The step on the shaft where the inner race of the front bearing seats is what prevents the collet (or in keyed engines like OS, the drive washer) from binding up the bearings as the prop is tightened.

It sounds like you had a damaged or defective bearing.
Old 05-17-2009, 07:50 PM
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Default RE: Saito 82a Ceramic Bearing Failure

w8ye & greg - thanks for your input. I was editing the original post while you guys were posting

I have ordered a new set of standard bearings from Paul at rc-bearings.com. I may call him in the morning.

Cheers,
Old 05-17-2009, 08:40 PM
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Campgems
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Default RE: Saito 82a Ceramic Bearing Failure

I've got to second W8YE's advise. Get a set of standard bearings.

The ceramic bearings are high performance bearings, IEhigh speed. How many high performances things are as durable as the stock items? None.

If a guy was flying Pylon, I could see the need for the bearings. I mean these guys burn up a piston every other flight, they are pusing the engine so hard, so a set of bearings every few flights is just the price they pay.

In my view, Ceramic bearings have there place and nothing will replace them there, but that place is high RPM and lighter loads. That doesn't describe our day to day flying, given a prop strike now and then. .

You have to ask yourself this. Paul sells a set of bearings for about 30% of what you would pay for the replacement from the manufacture. They are as good as the orginal bearings, I've used them in several engines.. Those bearings probably lasted many years, unless you had some hard nose front crashes. Why pay the extra bucks, Unless you are racing, for bearings that are no where near as tough and cost more??? I've gone through this argument with myself the last couple time Ibought bearings. I ended up buying the standards.

BTW, the photo looks like the front bearing spun on the crank, which would be expected if a ball let loose and locked the races.The outer race also looks as if it may have spun. Check the fit on the new bearing befoe you assemble them. Feel the ground bearing surface on the crank, and if there is any roughness, use a fine stone lightly to knock them down.

A last though just came to mind, if you didn't get the bearings completely seated against the shoulders in the crank case, and the crank fully seated against the rear bearning, you were putting a high stress on the bearings when you seated the tapered hub. Everyting is machined to put the right load on the bearings, and more load means early failure. This may be what happened to your bearings.

I am a journeyman Machine Repairman from way back when, and have been replacing bearings since 1960. I've screwed up more than my share by trying to seat a bearing on the shaft by pressing on the outer race and vise versa. Even when things look right, they may be off by a few thousandts. After seeting the bearings and every thing is assembled and tight, a couplesharp whacks on the back of the crank and then one on the threaded end, protect the trheads by having a nut screwed on at flush withe the end of the shaft, This will make sure things are where they are susposed to be. After finall assembly, if the bearing shows any roughness, it isn't going to last long. I should be smooth as silk and easy to turn. Bearings don't break in, they wear out. Some to soon because they were not properly installed.

Don
Old 05-17-2009, 09:20 PM
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Default RE: Saito 82a Ceramic Bearing Failure

Good points Don.

On seating the bearings. When the components are hot / cold the bearings will simply fall out of the case until the temperature of the case / bearings normalize. Given that the case is 350 degrees it was difficult to keep the bearings seated. In retrospect I'm thinking that this is what happened to the rear bearing. When the temps normalized the bearing was not seated for the entire 360 degrees.

My concern is if the rear of the crank is given a whack the total of that force will be on the small race that is seated on the crank - not the outside race that is incorrectly seated in the case. How about using the backside of a socket whose outside diameter matches the outside diameter of the bearing? Obviously, the front bearing can be seated with a standard bearing driver (just using a small hammer).

Thanks again for the input - this hobby has so darn many learning opportunities!!!!!!!!!!!!!

PS - I should probably disassemble both the Saito 91 and Saito 100 and re-seat the bearings!

Old 05-17-2009, 09:23 PM
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Default RE: Saito 82a Ceramic Bearing Failure

The model engine is a simple device but fit and technique can work you over on a rebuild.
Old 05-17-2009, 09:30 PM
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Default RE: Saito 82a Ceramic Bearing Failure

w8ye - one of our club's very experienced members has told me many times that there is a "2nd year curse". It will bite the builder. It will bite the flier. It will bite at the most inappropriate times. Engines, fuel systems, covering, electrical components, whatever ... So I guess that I'm just paying my dues.

O'Shanasy has one rule - Murphy was an optimist.
Old 05-17-2009, 09:33 PM
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Default RE: Saito 82a Ceramic Bearing Failure

It has taken some 75 yrs of trial and error to get the model engine up to the point where it is now
Old 05-18-2009, 09:55 AM
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Default RE: Saito 82a Ceramic Bearing Failure

I replace a lot of bearings for people. I've been doing engine work for about 15 years and picked up a few things along the way. One thing you can try is to install the bearingsas you described then install the drive hub, collet, prop and prop nut. Torque them down as you normally would and then reheat the case with a torch. This will allow the bearings to reseat with the preload that they are normally under. This will usually cure a bearing that is a little tight because it didn't seat correctly.

I know a lot of people who have tried ceramic bearings in boat and plane motors. Some of these guys were national record holders and knew much more about motors than the average modeler. I'd say 99% of them had the type of failure that you experienced. This isn't isolated to RC-Bearings products.

I use Pauls stainless sets whenever they are available. I've had really good luck with them and they are priced very reasonably.
Old 05-18-2009, 10:53 AM
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Default RE: Saito 82a Ceramic Bearing Failure

Chris - thanks! That sounds like a great suggestion. I'll apply the technique on my Saito 91 as the Saito 100 has already been disassembled.
Old 05-18-2009, 11:43 AM
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mike early
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Default RE: Saito 82a Ceramic Bearing Failure

I too, am done with ceramic bearings.  Quite the headache.

I still have one set in operation, but it's only a matter of time before I regret that one too...
Old 05-18-2009, 02:48 PM
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Default RE: Saito 82a Ceramic Bearing Failure

Mike, what kind of problems have raised their heads?

Btw, I have just re-installed the ceramic bearings in my Saito 100. After initially installing cold bearings in a hot case and letting them cool I inserted the crank with the drive flange and collet. I then pulled the crank home and seated the collet by mounting and tightening the prop. At this point all I have is the case, pair of bearings and crank / collet-flange assembly with the prop. I then reheated the case around the areas of the bearing seats. After cooling, the remainder of the parts were installed and valve clearance checked.

Hope this results in a successful installation.

Gotta admit, I have installed a bunch of stainless bearings in 2-strokes and 1 set in a Saito with no problems. Just wondering if the installation problems that I'm experiencing are systemic to ceramic bearings.

Cheers,
Old 05-18-2009, 08:07 PM
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Default RE: Saito 82a Ceramic Bearing Failure

The Jett web site has the best info for bearing R&R in my opinion.

Much like ChrisAttebury and SeamusG do:

 After the crank and prop drive is reinstalled , I put a spacer and a wavy washer where the prop would be, than a prop nut. I tighten just to put a tiny bit of pre-load on the assembly, NEVER TO PRESS IT TOGETHER!  It is than put in the oven for 10 minutes at 300* than take it out to cool. This way there is no chance of the bearing races creeping away from the seats/shoulders as it cools. With no con rod on, the crank should spin loose with no noise. All it take is a tiny bit of lint or hair to mess things up.

I too, am done with ceramic bearings.
Old 05-19-2009, 09:01 PM
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Default RE: Saito 82a Ceramic Bearing Failure

How do I remove the tapered collet and the drive flange on my Saito 150? They seem to be VERY tight. I'm trying to replace the bearings in it.

Tom
Old 05-19-2009, 09:06 PM
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Default RE: Saito 82a Ceramic Bearing Failure

It takes a gear puller


And something to grip the hub with like this bearing seperator


Get them at Harbor Frieght
http://www.harborfreight.com/cpi/cta...emnumber=93980
http://www.harborfreight.com/cpi/cta...emnumber=30305

Old 05-19-2009, 09:16 PM
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Default RE: Saito 82a Ceramic Bearing Failure

So are you saying you can't just use a press on the front of the crankshaft? If I need those tools, I'll probably send it back to Horizon.

Tom
Old 05-19-2009, 09:17 PM
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Default RE: Saito 82a Ceramic Bearing Failure

You can also use a battery terminal puller. You have to heat the drive washer with a torch though.
Old 05-19-2009, 10:05 PM
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Default RE: Saito 82a Ceramic Bearing Failure

just rebuilt a Mag 91 that has the taper hub setup. I found a large flat washer with a hole the size of the small diameter and just cut a slot in it to fit the grove. You have to file an angle on the edge to mate the angle on the hub. Just a ffew minutes work when you find the right washer. You can then use a cheap 3 jaw puller and pop the hub right off.

The first photo shows the finished product and the second shows the hub. Notice the taper on the back side, The tool will need to be filed to fit that taper. Interestingly, I fit the tool to the hub on my Mag 91 four stroke, The hub in the photo is a brand new replacement hub for the same engine, and the tool would have to be filed down more to fit it.

Don

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Old 05-19-2009, 10:38 PM
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Default RE: Saito 82a Ceramic Bearing Failure

Thanks for the ideas. I managed to get it out with a gear puller. I had to grind the hooks to fit in the groove.

Tom
Old 05-20-2009, 09:14 PM
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SeamusG
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Default RE: Saito 82a Ceramic Bearing Failure

I ordered and received a set of stainless steel bearings from rc-bearings.com. I slipped the large bearing over the crank. I placed both the front bearing and the crank / rear bearing assembly in the freezer. After a couple of hours I fit the front bearing into a room-temperature case - smooth and easy. I then inserted the crank / rear bearing into the back of the case. Same deal - smooth and easy - seated beautifully.

In retrospect ...

Ceramic bearings were an absolute pain in the arse to install. They subsequently failed after 6 flights.
Stainless bearings (at 1/3 the price) were a breeze to install. Will they fail? I'll let you know.






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