Terminology question

Why do people call double cardan joints CV joints. I thought it was just on the forums. But, last night I was on Tom Wood's site and noticed they call them CV joints as well. Does that mean that Double Cardan Joints are a true constant velocity? Tried to find an article about them but couldn't.

Thanks,
Brian

coastie124 said:

Why do people call double cardan joints CV joints. I thought it was just on the forums. But, last night I was on Tom Wood's site and noticed they call them CV joints as well. Does that mean that Double Cardan Joints are a true constant velocity? Tried to find an article about them but couldn't.

Thanks,
Brian

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A Cardan joint (single cross) has two issues to watch out for -
1) There's one at each end of the shaft. The axes have to be lined up damn near exactly (within a quarter of a degree of angle or so, I think,) or you get vibrations.

2) The operating angle of the shaft at each end of the joint has to be damn near exactly the same (within a degree or so, I think,) or you get vibrations.

The vibrations are because the single Cardan joint doesn't turn at a "constant velocity" - it speeds up and slows down slightly as it turns. When you've got a joint at each end, and they're in phase, with the same operating angle; these changes in speed cancel each other out (so they won't vibrate.)

The "constant velocity," or "double Cardan" joint was designed to handle this. There is a centreing ball in between the two crosses that will force them to operate at like (but opposite) angles and cancel each other out. Note that the driveshaft now would extend along the axis of the pinion gear, rather than at an angle to it, during normal operation (the single joint is there to allow for articulation.)

What you're thinking of is a Rzeppa (pronounced "Cheppa") joint as used on FWD and RWD/IRS. This is commonly miscalled a "constant velocity" joint. That's what it does - but that's not its proper name.

There are two types of FWD & RWD/IRS CV joints - a "true" Rzeppa joint (typically with six large ball bearings and a captive cage,) and a "tripot" joint (with three semisperical rollers running on spuds with roller bearings.)

So, there are three types of drivetrain articulation joint in use:

Cardan/Double Cardan (the "cross" joint we all know and love)
Rzeppa (usually the outer joints on FWD or RWD/IRS)
Tripot (usually the inner joints on FWD or RWD/IRS)

Applying "constant velocity" to the "double Cardan" joint is somewhat correct - as both the Rzeppa and Tripot came after. However, "constant velocity" is more a description of function than of actual design.

Make some more sense now?

Brian,

"CV Joint" is a class, not a specific instance.

A Cardan joint, named for Cardano, an ancient philosopher/physician and inventor of the gimbal, is the same as what you know as a U-joint. A U-joint has a velocity profile that is eliptical (or sinusoidal) when the drive shafts are not in-line. By making a double cardan joint (two U-joints in the same housing), the velocity profile of the two joints adds 180 degrees out of phase. This makes a nearly constant velocity power transfer...thus the name "constant velocity" joint.

The Birfield joint is common in off-road vehicles because it is robust, and common on Toyota and Suzuki axles. That's what you think of as a "true CV" joint, eh?

There are several other configurations that qualify as CV joints: the Tripod, the Rzepppa, and the Double Offset come to mind.

Of all, the double Cardan is the most robust, but it's not compact...and it's quite imperfect when it comes to vibration.

Bill

I think you both missed my point. I fully understand what a CV joint is. Maybe I was a little unclear on my question. I was simply asking if the double cardan joint was a true CV joint. I pull from what your saying that it is somewhat a true CV joint. Being that both sides counteract each other.

coastie124 said:

I think you both missed my point. I fully understand what a CV joint is. Maybe I was a little unclear on my question. I was simply asking if the double cardan joint was a true CV joint. I pull from what your saying that it is somewhat a true CV joint. Being that both sides counteract each other.

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There's no such thing as a "true" CV joint operating through an angle, especially if it's any sort of rigid. So, anything that operates as a CV joint, at best, only mimics "constant velocity" operation through some sort of mechanical shenanigan.

The closest thing I can think of that can honestly be called a "constant velocity" joint would be mating splines in shafts that are coaxial. The phrase "constant velocity joint" is a misnomer. But for a true rigid CV joint for transmitting any meaningful amount of power, for the moment, hasn't happened (of which I am aware.)

"I know what you are saying 5-90, this might make him feel better


Geez all this info and you are still confused...... they are all constant velocity joints. Its like this

constant velocity= candy bars

double cardan= reeses BP cups
tripod= kit kat
Rzeppa= butterfinger

Now your getting rude. There are true constant velocity joints. There are even true constant velocity "Universal Joint", (reeses BP cups) so you understand. They are known as Thompson Couplings. Two U Joints positioned coaxial in the joint. I don't know why you have to try and treat me like a moron. I come to you with a question and you act like I'm some tard who doesn't know his on thumb from a bearing cap. Pornstar did people treated you like a freakin tard for trying to eliminate your vaccum resivoir? No they helped you decide what it controlled, if it was needed for you, and other options. I do appreciate the info 5-90 and crawl daddy, and I hope I didn't come off abrasive. I'll admit that I'm a newbie to the cherokees. I just don't know if you got what I was asking at first.

Is the Thompson Coupling rather like a Double Cardan? It sounds like it might be - I may have to look it up. (That's why I said there was no "true CV joint that I know of...:laugh3: )

I'm not an engineer (yet,) I don't even play one on TV - and I was sure I'd make it through school before the accident - but I'm not so sure now. Gotta let my brain finish rewiring itself before I can decide that... I can't even keep up with the progress in the field of general engineering - just the Automotive stuff. I'll probably end up looking up Thompsons anyhow - but I'd appreciate any information/links you have to hand...

Yeah I'm not too sure on their strength ratings. I haven't seen any test at least. They claim it is just as strong due to it not having load bearing sliding surfaces. http://www.cvcoupling.com/ I just watched the video on their website and I especially like the part in the end when they show what happens if the driveshaft lets go. It's pretty bulky looking just like a double cardan. Personally I think I'll just use the Double Cardan.

Don't know if it's relevant or not, but something to file away in the old gray matter. The reason double Cardons work, is because they bring the total of the number of joints to three on a drive shaft or axle. The odd number is what makes them work.
I don't think the type of joint is as important as the odd number of joints. I've seen various configurations, such as a Cardon (U-joint), flexible bushing, Cardon joint (three joints).
Like 5-90 said, some of the so called CV joints are some sort of engineering trick, to bring the total to an odd number of joints on a shaft. Or I'm guessing, making it so the shaft rotates on four plains at any given moment.

8Mud said:

The reason double Cardons work, is because they bring the total of the number of joints to three on a drive shaft or axle.

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Unless I'm mistaken, that is not correct.

You take two elipse orbits and are coupling them out of phase. They cancel each other and the velocity curve going in is more correctly repesented on the output,

The third u-joint is used to account for rotation of the pinion. You could just as well sub that with a CV joint of whatever flavor.

Zuki-Ron said:

Unless I'm mistaken, that is not correct.

You take two elipse orbits and are coupling them out of phase. They cancel each other and the velocity curve going in is more correctly repesented on the output,

The third u-joint is used to account for rotation of the pinion. You could just as well sub that with a CV joint of whatever flavor.

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Could be, it has been a long time since I did that course. But thinking for a moment, a standard drive shaft has one Cardon joint out of phase with the other, on each end.
A double Carden setup, has a shaft (yoke), U-joint, driveshaft, U-joint, connection (actually a short shaft), U-joint, shaft (yoke).
The course I took was about 200 hundred hours and was devoted mostly to differential physics. I imagine U-joints would require a course of there own.
I seem to remember the number of joints reduces the drive angel, the less the drive angel, the less the angular velocity differences, from 180 degrees (flat).
The instructor did mention the odd number of joints being a factor in vibration control (or maybe it was the even number of shafts). In the end they all have to add up to 180 degrees or a flat plain. And the angle of the U-joint less than 90 degrees .

8Mud said:

Could be, it has been a long time since I did that course. But thinking for a moment, a standard drive shaft has one Cardon joint out of phase with the other, on each end.
A double Carden setup, has a shaft (yoke), U-joint, driveshaft, U-joint, connection (actually a short shaft), U-joint, shaft (yoke).
The course I took was about 200 hundred hours and was devoted mostly to differential physics. I imagine U-joints would require a course of there own.
I seem to remember the number of joints reduces the drive angel, the less the drive angel, the less the angular velocity differences, from 180 degrees (flat).
The instructor did mention the odd number of joints being a factor in vibration control (or maybe it was the even number of shafts). In the end they all have to add up to 180 degrees or a flat plain. And the angle of the U-joint less than 90 degrees .

Click to expand...

Yeah I feel main effect the double cardan joint provides is decreasing driveline angles. Two joint in a driveshaft both have 15 degree angles. On that same setup with three joints you'd have 10 degree angles. Like ZUKI-RON said you could put any other type of joint at the other end. They even make driveshafts with double cardan on both sides. Though I'm not sure how well that would work. To me the double cardan joint is more like the driveshafts you see on some longer trucks. They'll have a cardan joint some where in the middle and decrease the driveline angles. The connection peice between the two u-joints on the double cardan is more or less a second driveshaft, it's just short. Though these are just the thoughts of an unedgemucated Arkansan.:rof: