Driveline vibration in low rpm - Page 2 - PeachParts Mercedes-Benz Forum

Mxfrank · #16 09-04-2015, 11:20 PM

Quote:

Originally Posted by leathermang

Ok... some little corrections..... I do not think an actual u joint can avoid that speed up and slow down in turning.... that is why they are not called ' constant velocity ' joints.... which are made very differently.

Correcting your correction. A u-joint can run at constant velocity if it's installed with zero offset between input and output shaft. If this wasn't the case, the single u-joint in your Mercedes would be driving you crazy with vibration. In a typical non-Mercedes installation, installing a pair of u-joints out of phase allows the input and output shaft to be offset: since the speed variation of one joint is exactly the opposite of the other, the wobbles cancel out. A CV joint can be considered as a pair of u-joints in one assembly, either literally as in a double cardan joint or nested as in a Mercedes rear axle CV joint.

If a u-joint runs at zero offset, lubricant is distributed just fine. But what happens is that the rollers always track over the same path in the races. As a result, the races develop a typical wear pattern called false brinnelling. This is not a problem unless the joint later needs to compensate for a misalignment as would happen with worn mounts. In this case, the joint could become noisy, as the rollers track over the groove worn in the races.

leathermang · #17 09-04-2015, 11:33 PM

Quote:

Originally Posted by Mxfrank

......If a u-joint runs at zero offset, lubricant is distributed just fine. But what happens is that the rollers always track over the same path in the races. As a result, the races develop a typical wear pattern called false brinnelling. This is not a problem unless the joint later needs to compensate for a misalignment as would happen with worn mounts. In this case, the joint could become noisy, as the rollers track over the groove worn in the races.

I think that the excessive wear you describe is the evidence of the lubrication NOT being ' OK ' if there is not some offset... by definition...

https://books.google.com/books?id=cs0ljZEC-VoC&pg=PA101&lpg=PA101&dq=u+joint+need+for+offset+lubrication&source=bl&ots=vMBxZVXxno&sig=Gk4hJf3gTQo2nx_Zxf1uo7Om5xg&hl=en&sa=X&ved=0CEgQ6AEwCGoVChMIrbTOze7exwIVTJ-ACh130Q7M#v=onepage&q=u%20joint%20need%20for%20offset%20lubrication&f=false

barry12345 · #18 09-07-2015, 06:51 PM

A semi seized u joint might act the way you describe. Easier for the driveshaft to move than the joint at low speed. Joint should be checked for freedom of movement. A little notch is normal as this joint turns almost always in an identical offset orbit.

I suspect you have to at least partially remove the driveshaft to check it. Might be wrong there.