Universal Joints for Dummies

universal joint

Sometimes, the staffer who submits content for a DotD nomination has to put quite a bit of effort into the reason for the nomination. Perhaps the bogosity is subtle; perhaps the errors require some expertise to uncover. On the other hand, we've had at least a few candidates who could easily nominate themselves based on their obvious failure to understand what they're saying. Today's nominee, eHowian Elizabeth Punke, falls into the latter category. We base that on her post at ItStillRuns.com, "Types of Universal Joints."

We'll be honest about our lack of knowledge here, something that apparently precludes our ever writing for Leaf Group: we have no idea what Punke was talking about in her descriptions of (what she claims are) three different types of universal joints: the Hooke, ring and trunnion, and Bendix-Weiss. More to the point, we can't figure out what her descriptions mean...

On the other hand, several staffers are quite familiar with universal joints as used in automotive driveshafts (and socket wrenches), and we know that her daffynition...

"A universal joint, otherwise called a u-joint, is found in many automotive applications, as well as in other mechanics. For example, a u-joint is used in vehicles between the drive train bar and the transmission or axle where the bars meet at a right angle."

...is about as bogus as they come. Those "bars" Elizabeth mentions? There's the driveshaft "bar" that meets the axle "bars" at a right angle, but you know what, Elizabeth? There's no universal joint at that point! No, that's the differential, you blithering idiot, and that's where you find a ring and pinion joint, which allows for a right-angle transfer of the rotation of the driveshaft.

Punke blew that completely: a universal joint's purpose is to allow for the transfer of rotational motion between two rods – in this case, two halves of the driveshaft – whose axes do not always remain at the same angle. In the case of a vehicle's driveshaft, the difference in angles is typically small, but large enough to deform the shaft in a worst-case scenario.

The truth is that the OQ probably wanted to know the difference between single- and double-cardan U joints and, perhaps some mention of the Spicer and Thompson variations of double-cardan joints. Did Punke answer that question? No... and she also completely hosed the basic definition of a universal joint. For her lack of effort, this mommy-blogger is hereby named our Dumbass of the Day.

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