A downhole drilling apparatus which includes a progressive cavity drive train having a progressive cavity device and a coupling for converting the complex motion of the rotor into simple rotation. The coupling is constructed along the lines of a double Hooke's joint. Each joint includes two transversely arranged pins which pivot within a torsion member. The pins are supported for rotation with a bore formed in the torsion member on a plurality of rollers. An elastomer encases the rollers and fills the remaining spaces between the pin and torsion members such that all sliding surfaces within the coupling are perfectly isolated from the surrounding environment and thereby hermetically sealed.