A universal joint assembly (12) includes a coupling hub member (26) and a female coupling member (24). The female coupling member (24) includes a receptacle (28) at one end for receiving a pivot end (30) of the coupling hub member (26). The joint (12) includes a central bearing structure (34) interposed between the coupling hub member (26) and female coupling member (24) for enabling the coupling members to pivot up to a maximum pivot angle with respect to each other about a coupling axis. The universal joint (12) also includes a plurality of torque transmitting rollers (50), each received in a different roller slot and spaced apart about the periphery of the coupling hub member (26) near its pivot end (30). The roller slots are formed on both the coupling hub member (26) and female coupling member (24) and enable the torque transmitting rollers (50) to transmit torque between the coupling members while rolling with respect to the roller surfaces as the coupling members pivot about the coupling axis. The universal joint (12) also includes a spherical central bearing structure and a spherical seal structure (60) associated with the coupling hub member (26). Also, each universal joint (12) employed in a down-hole mud motor is oriented in the mud motor with the receptacle (28) of the female coupling member (24) facing downwardly away from the direction of fluid flow through the motor.
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