| 摘要 |
<p>A rotatable pressure seal (53) between a relatively rotatable shaft (21) and body structure (10) comprises two annular sealing discs (55,57) concentric with the shaft, one disc (55) being mounted on the body structure and the other disc (57) being carried on the shaft, the discs having engaging sealing faces (56,58) formed of polycrystalline diamond or other superhard material. The pressure seal is particularly for use in a modulated bias unit, for controlling the direction of drilling of a rotary drill bit when drilling boreholes in subsurface formations. The bias unit comprises a number of hydraulic actuators (13) spaced apart around the periphery of the unit, and a selector control valve (15) modulates the fluid pressure supplied to each actuator in synchronism with rotation of the drill bit so that, as the drill bit rotates, a thrust member (28) of each actuator is displaced outwardly at the same selected rotational position so as to bias the drill bit laterally and thus control the direction of drilling. The selector control valve (15) is located within a cavity (16) in the body structure and is operated by a shaft (21) which extends into the cavity through a pressure seal (53) according to the invention. <IMAGE></p> |
