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Universal joint

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Universal joint

Old 18th Sep 2018, 14:47
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Universal joint

you guys did so well on that last test, I decided to give you another opportunity to excel.

It it seems to me that the teetering head is a poorly designed universal joint. Yes it is cheap and easy, and the range of motion is less than 10 degrees. And it works.

But considering efficiancy and binding; has anyone ever used a constant velocity joint in place of the universal?

Let the education begin

thanks
bill
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Old 18th Sep 2018, 16:59
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Yep I can see how a double hooks or a true CV joint would be a floppy mess until up to speed anyway.

I can feel myself getting smarter here by the minute.

Thanks

Bill
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Old 18th Sep 2018, 22:48
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Originally Posted by SansAnhedral View Post
A CV drive would be used on a multi-bladed stiff-in-plane gimballed rotor. To wit, the only craft with this arrangement are tiltrotors.
......or an AS350/H125.
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Old 19th Sep 2018, 16:07
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Originally Posted by heliduck View Post
......or an AS350/H125.
AS350 certainly is not stiff in plane nor gimballed with Starflex.

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Old 19th Sep 2018, 16:09
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Originally Posted by discap View Post
Yep I can see how a double hooks or a true CV joint would be a floppy mess until up to speed anyway.

I can feel myself getting smarter here by the minute.

Thanks

Bill
Production CV drives on tiltrotors also use what is referred to as a hub spring, which is a very large spherical elastomeric that acts as a flapping centering "spring" and also what transmits the thrust to the mast in most cases.
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Old 19th Sep 2018, 22:45
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The OP never asked for information on gimballed or stiff in plane engineering, he/she asked if anyone had used a constant velocity joint instead of a universal. The star flex arrangement is a constant velocity joint.
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Old 20th Sep 2018, 04:04
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Yes heli duck Cv vs universal particularly in a 2 bladed system.

Its “he” by the way.

Bill
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Old 20th Sep 2018, 12:18
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The star flex arrangement is a constant velocity joint.
The angle between the drive shaft and the rotor hub doesn't change - how is that a CV joint? Only the blades and the plates change angle with respect to the MR shaft, not the hub.
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Old 20th Sep 2018, 12:40
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Just some Wiki help so we can all understand the uncommon drive options not found on helicopters very much. I didn't realize there were so many

https://en.wikipedia.org/wiki/Constant-velocity_joint

When referring a starflex as a CV joint, I can see the loose connection, much the same as the OP referred to a teetering head as a Universal joint. Both are generalizing the function/connection.
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Old 20th Sep 2018, 17:17
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As I said, no change in angle between driving and driven elements - ergo not a CV joint
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Old 20th Sep 2018, 18:44
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Originally Posted by heliduck View Post
The OP never asked for information on gimballed or stiff in plane engineering, he/she asked if anyone had used a constant velocity joint instead of a universal. The star flex arrangement is a constant velocity joint.
Except you don't use a constant velocity joint, or any angular joint at all, unless you have a stiff-in-plane gimballed rotor. In all other cases the hub is rigidly fixed to the mast and the angular changes for the rotor thrust vector are performed through discrete hinges or flexures along the blade axes.

Originally Posted by [email protected] View Post
As I said, no change in angle between driving and driven elements - ergo not a CV joint
Agreed, this is precisely the point. There is no angular joint between the rotor hub and the mast whatsoever with starflex, constant velocity or not.

Nobody would consider a wheel bolted to the end of a straight rear drive axle on a truck to be a "constant velocity joint".
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Old 20th Sep 2018, 22:47
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Originally Posted by [email protected] View Post
The angle between the drive shaft and the rotor hub doesn't change - how is that a CV joint? Only the blades and the plates change angle with respect to the MR shaft, not the hub.
if the velocity of the blades don’t change (it remains constant) despite the angle between them & the mast changing then it is a constant velocity joint, hence the name.
To elaborate I’ll refer back to the OP’s comparison of a universal joint & use a truck as an example - although it might appear that the drive shaft on a truck rotates at a constant speed, this is only the case when both drive shafts either side of the universal joint are inline. When the rear axle moves & the drive shafts are no longer inline, the velocity of the driveshaft rotation fluctuates twice/revolution as the centre cross of the universal joint moves. This fluctuation is minute at small driveshaft misalignment angles & increases as the misalignment increases.
The terminology of “constant velocity” refers to the driven portion of a power train, the “joint” refers to the ability of the driven portion to operate out of alignment with the driving section.
The disc does not remain aligned with the mast & the velocity of the blades remain constant, makes sense to me so how is it not a CV joint?
Back to original question, I’m not aware of any CV arrangements for 2 bladed Helicopters. Maybe the teetering head doesn’t create the blade velocity fluctuations like a universal joint would so it’s not worth the engineering costs to change it? If the teetering head doesn’t create velocity fluctuations then by definition it is a constant velocity joint & most of this discussion is irrelevant. I would have thought that an elastomeric head would have been an option, are there any around?
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Old 21st Sep 2018, 07:09
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The blades are attached to the hub by the plates and the elastomeric bearings and they do move with respect to the mast BUT, the hub is what is directly driven by the mast and that angle doesn't change.
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Old 24th Sep 2018, 15:21
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Originally Posted by [email protected] View Post
the hub is what is directly driven by the mast and that angle doesn't change.
Spot on again. Our anatine friend just doesn't get it.
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Old 24th Sep 2018, 20:05
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someone pulled out a thesauraus.....
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Old 24th Sep 2018, 22:55
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Originally Posted by GrayHorizonsHeli View Post
someone pulled out a thesauraus.....
Ornithology/helicopters - similar hobbies in some respects. Usually prefer they dont mix.
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