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HomeScaleAircraftScale HeliScale Helicopter Main Discussion › Main shaft bearing not seating properly
03-15-2015 09:43 PM  3 years agoPost 1
ssmith512

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Indianapolis, IN USA

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Got a small problem with the Cobra. The main shaft bearing seats are just ever so slightly over sized on the Roban mechanics. The bearings almost can fall out of the seats by their own weight. The unsupported section of mainshaft above the topmost bearing is pretty long, so the result is the mainshaft has quite a bit of "wobble" to it (the mainshaft has enough leverage to rock the bearings in the seat). Locktite makes a "gap filling" retaining compound, but it is ridiculously expensive. Looking for some ideas.

Thanks in advance.

Steve

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03-15-2015 10:15 PM  3 years agoPost 2
Mojave

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Palos Verdes, Ca. USA

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The tolerance of the OD of the bearing may be on the low side. Have you tried another bearing first?

Take a dial caliper and measure the bore of the housing, then measure the OD of the bearing. Subtract the 2 numbers and divide the remaining number by 2. This will be the actual clearance dimension. Let me know what that number is and I'll give you a better idea of how to fix this. If the final clearance number is .002 or less, then the Loctite is a viable option.
Barry

All helis and planes have an expiration date stamped on them...you only find it after you crash!!

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03-15-2015 10:33 PM  3 years agoPost 3
ssmith512

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Indianapolis, IN USA

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Thanks for the help Barry.

Yes, I have tried another bearing. Same result. The bearing literally falls out of the seat under its own weight (at room temperature) and I can spin the outer race of the bearing while in the seat.

Both bearings measure .943" OD. Bearing seat measures .944-.945" ID. So clearance is .0005" - .001".

Steve

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03-15-2015 11:11 PM  3 years agoPost 4
no1gsxrman

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Baltimore, Md

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if you can I would use a flanged bearing and drill and tap the main bearing supports to hold the bearings.

Put the woman and children to bed it's time to go fly!

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03-15-2015 11:35 PM  3 years agoPost 5
Mojave

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OK, that's very small clearance. There should be a slight interference (press) fit (.0002-.0003)between the bearing shell and the bore. Is the bearing block plastic or aluminum? I would contact Roban and request a new set of bearings, or a new bearing block.

If you don't want to do that, I can let you in on a couple of old machinists tricks, but I need to know what material the bearing block is made from. Also, if you choose to use these tricks, it's at your own risk...
Barry

All helis and planes have an expiration date stamped on them...you only find it after you crash!!

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03-15-2015 11:46 PM  3 years agoPost 6
treotraveler

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San Diego, Calif

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Hey Steve...

Contact Cliff or Ed at RCA about a new pair of main shaft bearings. When I bought my Roban 700 Cobra, Cliff gave me 2 replacement bearings. The replacement bearings are suppose to be proper sized replacements...

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03-16-2015 12:02 AM  3 years agoPost 7
ssmith512

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Indianapolis, IN USA

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teotraveler - the kit was an "updated" kit and came with "upgraded" mainshaft bearings and new ball links (both were packaged loose from the mechanics in the box and labeled "UPGRADE", so I am pretty sure I have the newest stuff

Barry,

The bearing block is black anodized aluminum. The bearing is a standard 6901Z bearing. I am interested in learning the "tricks". I will still probably contact Roban though.

Thanks!

Steve

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03-16-2015 01:59 AM  3 years agoPost 8
Mojave

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OK, here ya go on the machinist cheater tricks. There are 2 ways to tighten the bearing bore. The second method is easier to get the bearing started in the bore.

1) You take a ball bearing that is approximately 20% larger than the ID of the hole. Seat the bearing in the hole of the bearing block, then compress it with moderate pressure in a bench vise(the vise pushes the bearing against the lip of the bore). You do this just enough to put a small "compressed" chamfer on the lip of the bearing bore. This pushes a small amount of material into the center of the bore, making it a few thousands smaller then it was when you started. Then put a little locktite around the ID of the bore and press the bearing in place. Pressing the bearing in place will displace the small amount of material and lock it in place.

2) You take a spring loaded center punch and make a few small dimples inside the ID of the bearing bore, equally spaced around the diameter and approximately in the center of the bore. This leaves the ID of the bore untouched at the top, which allows you to get the bearing into the bore, then you apply a little locktite and press it past the dimples.
Barry

All helis and planes have an expiration date stamped on them...you only find it after you crash!!

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03-16-2015 02:38 AM  3 years agoPost 9
ssmith512

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Indianapolis, IN USA

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Thanks Barry!!!

Those are easy enough to do. Hammers and ball bearings and bench vises, what could go wrong!! LOL!

I appreciate the help!!

Steve

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03-16-2015 03:43 PM  3 years agoPost 10
Mojave

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LOL, it's a little unconventional... Machinists will do just about anything to save a part.
Barry

All helis and planes have an expiration date stamped on them...you only find it after you crash!!

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03-19-2015 12:44 AM  3 years agoPost 11
ssmith512

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Indianapolis, IN USA

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Success!!!

Major thanks to my new bestest, mostest favorite bud and BFF, Barry!

I used a combo of the center punch method and for the fun of it, I added some hysol for extra holding power.

Steve

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03-19-2015 01:29 AM  3 years agoPost 12
payne1967

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uk

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a proper machinist would attempt to knurl the hardend bearing
then give up get the centre punch out and the bottle of loctite

www.alcesterhelicopterclub.bmfa.org

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03-19-2015 01:56 AM  3 years agoPost 13
ssmith512

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Indianapolis, IN USA

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a proper machinist would attempt to knurl the hardend bearing
then give up get the centre punch out and the bottle of loctite
There is little about me that is "proper" and when I went for the bottle,it wasnt a bottle of locktite that I got out of the cubbard. LOL!

Steve

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03-19-2015 03:03 AM  3 years agoPost 14
Mojave

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Palos Verdes, Ca. USA

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Lol You're very welcome. Over 30 years in aerospace fabrication and I believe I have seen just about every trick in the book...
Barry

All helis and planes have an expiration date stamped on them...you only find it after you crash!!

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