U-joint Phasing Photos,
© Copyright, 2014, R. Fleischer
The front driving yoke is in phase with the final driven yoke. The two yokes between the center cross-pieces inside the yokes must be aligned. Below is another sketch showing the same thing.
You do not want these forces to be ADDITIVE (nor subtractive either!, which amounts to the same thing ...think about it!) with any others. If additive or subtractive (the forces are not constant, remember!), ...the vibration/movements so-caused will be prominent, and rpm sensitive. You could have what feels like constant high frequency vibration at high driveshaft rpm, and cyclical vibrations at low driveshaft rpm. These vibrations can have interactions with other vibrations of the vehicle, including tires, gears rotating, engine torque pulses ...just about anything that moves. You could have the driveshaft assembly exhibit various vibrations at several different rpm!
There are reasons that yoke type driveshafts are to be assembled per these sketches. In the explanation below I have taken certain liberties with the descriptions, otherwise it would be even harder to understand. For some, you may not be able to picture these effects in your mind.
If you have a driveshaft with a U-joint in your hands, rotate the shaft and flange. As the angle goes from zero to considerable, you will see, and feel, what I am discussing here.
If the driving yokes and driven yokes of any particular U-joint (Universal Joint) are not assembled in the proper manner, there is no problem if the angle between driving and driven yokes is zero.
However, as the angle increases from zero, at any one joint, the two mating parts do not move at the same speed with relation to each other. While the average speed is the same ...as the joint rotates, one yoke is always moving at a different speed than the other, if there is ANY angle besides zero. This creates a constantly changing loading and an actual loss of power transmitted ...the loss is converted by the forces primarily but not exclusively into heat in the U-joint.
There is what is called a Constant Velocity U-joint that can eliminate much of the forces of rotation, compared to the yoke and cross-piece design in the above sketches. Those CV joints can be quite expensive and generally need substantial size for the power to be transmitted, and have some special problems of their own, such as lubrication requirements, sensitivity to foreign matter, ETC. There is a type of cross-piece U-joint that has fewer problems than the ones in the illustration, and is capable of a somewhat larger angle before prominent problems occur. This type is actually a SORT-OF joint and a half. The only place you are likely to see one of these is in the short front driveshaft of 4 wheel drive light trucks, such as pickup trucks. GM used this type of design successfully. I don't have a sketch, probably should make one, for the nerdy curiosity-minded folks here.
Anton Largiader's website has a couple of pertinent articles. First, this one: http://largiader.com/gs/shaft.html.
BMW has made motorcycles with both one and two universal joints. The Paralever uses two, the Monolever and TwinShock models both use one. Various things have caused BMW to use damping and other methods on various driveshaft assemblies. This article you are reading does not get into this subject; I am sure many brains are already clouding-over.
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Last check/edit: Saturday, October 01, 2016