1. A rotary mechanism comprising a first camming element and a second camming element having its inner face in contact with the outer face of the first camming element, one of the contacting pair of said faces having a cam profile of at least three identical segments, each segment including a first axially-straight, radially-circular surface connected to the adjacent first surface by a second radially-circular surface of a different radius which is tangent to the first surface and may be infinity, the other of the contacting pair of faces having at least three equally spaced rollers in oversize U-shaped sockets permitting radial movement, which sockets are connected by a surface which does not interfere with the cam profile, and the number of rollers differing by at least one from the number of cam segments, each roller being in contact with the cam profile during rotation of one camming element relative to the other when the rotary mechanism is used as a fluid pressure device.

10. A rotary mechanism comprising an outer camming element, an intermediate camming element having its outer face in contact with the inner face of the outer camming element, and an inner camming element having its outer face in contact with the inner face of the intermediate camming element, said inner and outer camming elements having axes which are common when one is not rotating, one of each contacting pair of said faces having a cam profile of at least three identical segments, each including a first axially-straight radially-circular surface connected to the adjacent first surface by a second radially-circular surface of a different radius which is tangent to the first surfaces, the other of each contacting pair of faces having at least three equally spaced rollers in oversize U-shaped sockets permitting radial movement, which sockets are connected by a surface which does not interfere with the adjacent cam profile, the number of rollers differing by one from the number of cam segments in each contacting pair of faces, and the number of cam segments and rollers at one contacting pair of faces being independent of the numbers of cam segments and rollers at the other contacting pair, and each roller being in contact with the cam profile in the contacting face during rotation of the camming elements relative to each other when the rotary mechanism is used as a fluid pressure device.

11. In a fluid motor or pump having at least one inlet, at least one outlet and a three-element rotary displacement mechanism comprising inner and outer elements having a common fixed axis and an intermediate element, the radially outer face of the inner element, the radially inner face of the outer element, and the radially inner and outer faces of the intermediate element each having a face profile of at least three segments which cooperate to form inner and outer expanding and collapsing chambers upon rotation of the intermediate and one other of the three elements, the improvement comprising: one or two inboard plates are arranged to turn with the intermediate element at one or both of its sides, respectively, an outboard plate is attached to the outer element adjacent each inboard plate, each inboard plate has one outer port beyond the minimum radius of each segment of the outer face of the intermediate element and one inner port within the maximum radius of each segment of the inner face of the intermediate element, said outboard plate or plates have two ports aligned with and angularly offset slightly to either side of each port of the inboard plate or plates when said port of the inboard plate is centered on a fully expanded chamber, and where there are two outboard plates, each has one of said two ports which are all angularly offset to the same side, the ports of the outboard plate or plates which are offset to one side are channeled either to a single inlet or in inner and outer sets to a pair of inlets, and the ports of the outboard plate or plates which are offset to the other side are likewise channeled to one or two outlets.