An Oldham coupling has three discs, one coupled to the input, one coupled to the output, and a middle
disc that is joined to the first two by tongue and groove. The tongue and groove on one side is
perpendicular to the tongue and groove on the other. The middle disc rotates around its center at the
same speed as the input and output shafts. Its center traces a circular orbit, twice per rotation, around
the midpoint between input and output shafts. Often springs are used to reduce backlash of the
mechanism. An advantage to this type of coupling, as compared to two universal joints, is its compact
size. The coupler is named for John Oldham who invented it in Ireland, in 1820, to solve a paddle
placement problem in a paddle steamer design.