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December 9, 2020

Propeller Shaft

A car's propeller shaft, or drive shaft to the Yanks, has a pretty simple job--just transmit rotational motion from the aft end of the gearbox to the front end of the differential.  To accommodate the fact that the gearbox output and the differential input shaft axes are usually not coaxial, a prop shaft normally incorporates a pair of universal joints.  The U joints also permit a little relative motion between the shafts it couples to.  The relative motion also requires a sliding spline joint.

My prop shaft was dirty and rusty, but appeared to be intact.  Though the U joints felt OK, I decided to replace them with ones of known quality and condition.




Disassembling U-joints gracefully is an acquired skill that I don't really have yet.  It wasn't pretty, but they did eventually yield to my brutish efforts.




The smaller parts cleaned up well and didn't reveal any damage that would prevent using them.  They got a nice powder coat.




The main shaft had some rust pitting, but the splines appeared perfect.




There was a deceased seal in the chamber that holds the splines.  It looks like it was cork.




Forensic examination of the seal suggests that it started out at about 3/16" thick.  All I had on hand was some 1/16 cork gasket material, so I cut three rings of appropriate size.
 



So finally, all the parts were ready for reassembly.  U joints were first.




These U joints had grease zerk holes on the cross pieces.  On the spline end of the shaft, there eight ways to install the cross piece in the yokes, and seven of them are wrong.  Only one orientation will end up with the longish zerk fitting nested in a cutout in the spline side yoke.  Otherwise, the zerk fouls the yoke on deflection, and will likely get bent or broken off.




On the other U joint, the odds are a little better.  There are only six wrong orientations.  There is a bevel cut on one side of each shaft yoke arm that gives a little extra clearance for the zerk.




For the spline grease seal, there is a split steel backer ring that has to go in the cap first.  Then the spline seal has to be cut to fit over the splines, and the original had a scarf joint to try to limit the grease leakage.  With my new triple seal, I could stagger the joints for an even better seal.  It's an upgrade!




So now this puppy goes on the shelf to wait it's turn for install.




I should mention here that there is one other more serious way that a propeller shaft can be assembled incorrectly.  When a U joint has an angle between it's input and output axes, as it normally will in use, it will impart a distortion to the rotation.  That is, the output rotation is not a perfect reproduction of the input rotation.  There is a periodic variation of the output angular motion.  The good news is that, under specific conditions, a second U joint will exactly cancel these distortions.  This is why U joints are usually found in pairs.  One of these specific conditions is that the joints be "phased" correctly.  In correct phasing, the driving yoke of one joint must be rotated 90 degrees from the driving yoke of the other joint.  Otherwise, the distortions double instead of canceling, causing driveline vibration.  For those with an unwholesome desire to see some of the math associated with this situation, and some of the other conditions that have to be met, look here.

Further, I was very careful to mark the parts on this shaft so I could reassemble it with everything in the same relative place as original.  By doing this, I feel I obviated the need for balancing the shaft.  If I'm wrong, removing he shaft is not that difficult.

This was a pretty quick, fun project.  It only took a few hours, and the cost was around $40 for the U joints.

Comments to Ed at elhollin1@yahoo.com

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