I am finding different opinion, but I can not find a mathematical demonstration about this topic. for example here is described as non homokinetic: Oldham coupling vs Universal joint
here is homokinetic: Book
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$\begingroup$ Yr book link is in Italian and further is blocked for reading. $\endgroup$ – Carl Witthoft Mar 10 '17 at 16:15
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It's kinda trivial, the middle disk is homokinetically and rotationally coupled to the outer disks. Because the slot and tab in the connections are always parallel (not counting play in the fit).
