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HelicopterMain Discussion › Grip Bearing Arrangement
01-27-2013 05:34 AM  4 years agoPost 21
BobOD

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I'm sorry but you are wrong. And your statments, quite frankly, don't make any sense. This is very simple engineering practice. All thrust load is taken up by the thrust bearing and all radial load is taken up by the radial bearings in this arrangement. This is not a new technique, it is well known practice and has been used for probably at least a century now.

The shoulder in the grip, designed to transfer the thrust load to the thrust bearing transfers the thrust load through the thrust bearing, spacer and inner race to the bolt. Simple and straight forward.

If you disagree, that's fine. Objection noted. Let's move on.

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01-27-2013 05:39 AM  4 years agoPost 22
Havoc

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Bob is right. Next.

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01-27-2013 05:45 AM  4 years agoPost 23
BobOD

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Radial, thrust, radial will decrease the angular deflection since the distance between the radials is greater.
Fastflyer20, you are correct. Radial/thrust/radial arrangements allow the 2 radial bearings to be spaced further apart. This will reduce the moment forces created by lift on the blades. Simply put, they have more leverage.

There is one other reason. Anybody see anything else that the radial/radial/thrust may have trouble with?

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01-27-2013 06:02 AM  4 years agoPost 24
Dood

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Wescanson

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radial will decrease the angular deflection since the distance between the radials is greater.
What good are 2 radial bearings spaced further apart when the outermost bearing is trying to rip itself apart?

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01-27-2013 06:05 AM  4 years agoPost 25
Dr.Ben

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Some of the most veteran helicopter desginers in the world use radial/thrust/ radial arrangment. The Raptor G4 series by Taya-san does in both the head and the tail. The Whiplash and Tempest series rotor heads use this arrangement as well. And the Caliber 90/Quest. And Kasama. If the outermost radial bearing is arranged such that the bearing's
ID only is in direct contact with the thrust race and that bearing is a glide fit into the grip, then there is no mechanism for transfer of thrust loads to that radial bearing. If the arrangement were problematic in this way, we would see tremendous tracking problems and issues with radial bearing longevity in 700 class models turning 2200 rpm with 200 g blades.

Ben Minor

Team Synergy Team Futaba Team Kontronik USA
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01-27-2013 06:17 AM  4 years agoPost 26
Havoc

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I should add: If the closer arrangement is more than sufficient to deal with the forces, placing them further apart by trading places with the thrust wouldn't be of much added value. So yes, if relying on the WAG theorem then you would want the thrust bearing in the center. If you know the forces, then you can free up some design ideas or help deal with other constraints. I've used the thrust bearing on the outside when bearing options and shaft requirements called for it.

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01-27-2013 06:19 AM  4 years agoPost 27
BobOD

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the outermost bearing is trying to rip itself apart
You need to explain how that could be happening. You can't just say it will happen because I know it won't. You have to give a clear mechanism of this failure you are concerned about.

It's very simple, the radial bearing is best to be furthest out to handle the moment load (leverage). Leverage is effected by the distance out. Thrust is a straight line force, it doesn't matter how far out the thrust bearing is. How can you argue that, it is pretty simple.

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01-27-2013 06:28 AM  4 years agoPost 28
BobOD

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Havoc, have you done the math? I have. I've made plenty of grips in my time too. It is very significant. By changing places, the moment force can be a lot more. We are talking about leverage here. If you are supporting a load 400mm out on the blade and the radial bearings are spaced 12mm apart versus 18mm apart, the load on those bearings at 12mm spacing will be dramatically higher. You'll need a much bigger bearing.

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01-27-2013 06:33 AM  4 years agoPost 29
ShuRugal

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Killeen, TX

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Hate to break it to you Dood, but you're flat wrong here. In the setup bob showed, the outer race of the radial bearing sees zero axial loading, because the edges of the race are not being pushed on. Bob's illustration is not very clear so i've done up another one that shows the contact points more clearly.

Red denotes points of contact which bear an axial load.
Blue is the thrust bearing
Green is the inner race of the radial bearing
Purple is the outer race
Cyan is the washer
Yellow is the bolt and shaft

Now, the "centrifugal" force of the rotor blade is transferred to the bearing stack through the flat back section of the rotor grip. This surface transfers the entire load into the thrust bearing. From the thrust bearing, the load now passes to the washer which is in contact with the inner race of the radial bearing, but not the outer race. The inner race of the bearing then transfers the load to the bolt head.

Makes sense now?


EDIT: typo, said outer race where i meant inner race at the end. Removed mistaken implication of Havoc.

AMA 700159

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01-27-2013 06:45 AM  4 years agoPost 30
Dr.Ben

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Nice illustration.

Ben Minor

Team Synergy Team Futaba Team Kontronik USA
Progressive RC

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01-27-2013 07:08 AM  4 years agoPost 31
BobOD

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Indeed! Thanks for that.
I hope you'll stick around because there is an even bigger issue with the rad/rad/thrust arrangement. It's a bit more complex so you're illustration skills would come in very handy.

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01-27-2013 07:29 AM  4 years agoPost 32
BobOD

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Dood, for the record.
I really hope there are no hard feelings here. I have read many a post of yours and have all respect. On top of that, I happen to think you have the absolute best username on this forum. Cracks me up every time.
But, I am quite sure that my information here is absolutely correct. That is no reason for hard feelings...I hope.

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01-27-2013 09:12 AM  4 years agoPost 33
OICU812

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Edson, Alberta, Canada

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Radial / Thrust / Radial

Energy force dynamics, this was one very long discussion and part of my schooling. Several extended tests on very large turbines with rediculous rpm and forces being applied were done and that was the most reliable way to do this. I can not see it being any different on our rc helicopters. NOW... when you have very large diameter bearings and thrusts as some RC helis do this kinda goes without saying it does not make a bunch of difference if any at all... but on the 600 and under class it surely can. As with RC helis in general the main thing is that the bearings themselves are that of best quality which seems to be biggest factor. Align bearings are still well... not good.. This kinda turned into a chest puff game here, but really this is rc helicopters, all the in depth analysis applies more to forces much higher/demanding then what we are putting through. The largest failures we see are not so much from the actual bearing positioning in head rather just crap bearings. IMO....

...Once upon a time there were Nitros, flybars and frequency pins...

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01-27-2013 12:38 PM  4 years agoPost 34
wrongler

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shuRugal...Very nice illustration, easy for the laymen to understand, I may have learned something in this post.

Bill Whittaker

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01-27-2013 01:08 PM  4 years agoPost 35
Havoc

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Hate to break it to you Dood/Havoc, but you're flat wrong here.
Shurugal, I agreed with Bob. This is why I don't post on this crap because nobody reads each others posts and just bitches for the sake of bitching. The novelty of that wears off.
Havoc, have you done the math? I have. I've made plenty of grips in my time too. It is very significant. By changing places, the moment force can be a lot more. We are talking about leverage here. If you are supporting a load 400mm out on the blade and the radial bearings are spaced 12mm apart versus 18mm apart, the load on those bearings at 12mm spacing will be dramatically higher. You'll need a much bigger bearing.
Yes I've done the math but you aren't following me. I am agreeing that having the radial setup further apart and thus the thrust in the center will win a grudge match against the thrust on the outside. But both setups can fly a helicopter perfectly well if designed for their respective setups. Anyway, didn't mean to interrupt the pissing match. Proceed.

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01-27-2013 02:12 PM  4 years agoPost 36
BobOD

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Yes, I agree either can be designed to work. But, you need masively larger bearings and more space with the rad/rad/thrust.

And BTW, I've tested both arrangements...using the same bearings in both. And, they were Align bearings. LOL The difference was huge. Not a little better...HUGE.

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01-27-2013 02:59 PM  4 years agoPost 37
Havoc

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And BTW, I've tested both arrangements...using the same bearings in both. And, they were Align bearings. LOL The difference was huge. Not a little better...HUGE.
I scratch build my scale and turbine mechanics. I don't use hobby bearings or off the shelf hobby parts or use them as a point of reference. I start with my requirements and work out from there. So that dictates where, what, and how my bearings are located.

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01-27-2013 03:03 PM  4 years agoPost 38
BobOD

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I've done the same using high quality bearings as well. My point was to show that clever design made a bigger impact than quality of bearings.

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01-27-2013 03:08 PM  4 years agoPost 39
ShuRugal

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Shurugal, I agreed with Bob. This is why I don't post on this crap because nobody reads each others posts and just bitches for the sake of bitching. The novelty of that wears off.
My apologies in that case, i must have misread something, i'll check again.

Also, an addendum i forgot to add to my previous post: the washer is not needed if the race of the thrust bearing which contacts the radial bearing is a smaller diameter than the the race which contacts the grip. In this instance, the race on the thrust bearing is small enough that it only touches the inner race and not the outer race

AMA 700159

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01-27-2013 03:14 PM  4 years agoPost 40
Havoc

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My apologies in that case, i must have misread something, i'll check again.
Easy to miss:
Bob is right. Next.
My first post to the thread

and when I said "I should add", I meant add as a qualifier to my first post. Not to the statements from the people that followed as they posted while I was writing the second post.

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