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+--- Thread: Purpose of the U-Joint (/showthread.php?tid=4948)
Purpose of the U-Joint - mxfrank - 09-26-2013
I was recently surprised to discover that there's a u-joint in the middle of Mercedes driveshafts. It's no big secret, I guess, just something I've never messed with.
My question is why is it there? If there's an angle between the tranny and diff, then you would need two u-joints, otherwise the driveshaft would wobble. Where there's one, you would expect a second, 90 degrees out of phase to counteract the wobble. Alternatively, you could have one double Cardan joint.
One u-joint is only wobble free if the transmission and diff are exactly in line. But if they are in line, there's no need for a U-Joint. It could be that the u-joint takes up small angular variations due to the rubber driveline mounts. But I thought that's what the flex disks did. You would probably have some wheel hop on hard acceleration if the joint isn't phase paired with something. And yet is all seems to work.
RE: Purpose of the U-Joint - hooblah - 09-26-2013
Cant help you there mate, but two terms I've come across when talking about propshafts are cardan and spicer. What do these mean?
RE: Purpose of the U-Joint - mxfrank - 09-26-2013
(09-26-2013, 03:32 PM)hooblah Cant help you there mate, but two terms I've come across when talking about propshafts are cardan and spicer. What do these mean?
Cardan is the european name for U-Joint, after the Italian inventor Cardano. A double Cardan joint is like a pair of u-joints back to back, it's a simple sort of CV joint.
Spicer joint is the English name for u-joint. Spicer is Hardy Spicer. Formerly a UK maker of driveline hardware, last I looked it was owned by Tremec, which is a Mexican maker of transmission components.
And just to round out your vocabulary, the CV joints in Mercedes axles are properly called Rzeppa joints, after the Ford engineer who invented them.
RE: Purpose of the U-Joint - willbhere4u - 09-27-2013
Older BMW's have a similar kind of setup on there drive shafts also my guess is to compensate for engine torquing when the engine and transmission rock over in there mounts under load. Bmw and Mercedes of the time have fairly narrow drive shaft tunnels maybe they where worried one long shaft could rock over and hit it under high stress situations.
RE: Purpose of the U-Joint - Druk - 09-27-2013
The rather unfortunate upshot of them using a spicer where there is hardly any movement is that it results in the joint ' false brinelling' and the joint becoming aligned in one plane only. Try it next time you have a Merc shaft out. The centre joint feels like it has a notch position around centre.
http://en.wikipedia.org/wiki/False_brinelling
RE: Purpose of the U-Joint - willbhere4u - 09-27-2013
Subaru have the same problem with there rear shafts and they usually have 3 joints they never move so they always get knotchy near center
RE: Purpose of the U-Joint - mxfrank - 09-27-2013
(09-27-2013, 09:28 AM)willbhere4u Older BMW's have a similar kind of setup on there drive shafts also my guess is to compensate for engine torquing when the engine and transmission rock over in there mounts under load.
This is the only purpose I can think of for it. I guess the rubber flex disks don't provide enough flex...so maybe the question should be why have those? If there's an alignment issue due to mount compliance, then I would think the center U-Joint would cause wheel hop or at least chatter on hard acceleration.
The false brinelling problem is an inevitable consequence of a bearing that has limited movement, makes perfect sense. You also can't get too creative with engine or transmission swaps: this driveline has to be arrow straight in it's nominal position.
Let me turn the question inside out, why wouldn't Mercedes solve this the same way most everyone else does...with a u-joint at either end? That arrangement is proven, and gives a lot more flexibility than this. Plus you'd never have an exploding flex joint.
(09-27-2013, 09:40 AM)willbhere4u Subaru have the same problem with there rear shafts and they usually have 3 joints they never move so they always get knotchy near center
Just took a look at my Subie FSM(it's not true that I have one of everything, but close). The center joint is a CV joint, which may still be subject to false brinelling, but is free of the NVH problems of a single U-joint.