Abstract: A polished rod clamp includes a clamp housing having a central passage passing through a tapered bore. A wedge lock is arranged in the tapered bore such that a first surface of the wedge lock is in sliding engagement with a tapered surface of the tapered bore and a second surface of the wedge lock grips a rod in the central passage in a lock position. A first reciprocating piston is arranged to apply a first force to the wedge lock that effects a relative sliding motion between the wedge lock and the tapered surface in a first direction corresponding to the lock direction of the wedge lock. A second reciprocating piston is arranged to apply a second force to the wedge lock that effects a relative sliding motion between the wedge lock and the tapered surface in a second direction that is opposite to the first direction.

Abstract: A control system and method utilizing a race between two responses to a changing stimulus are disclosed. An example of a changing stimulus is pressure which increases at a decreasing rate of change during a closed-in period of a drill stem test in an oil or gas well. In a preferred embodiment two components are moved in different directions, but in a net first direction, until the rate of change of the pressure is sufficiently low (e.g., near steady state), at which time the rates of movement of the two components produce net movement in a second direction. In a preferred embodiment, the change in direction of the net movement moves a control valve which communicates a pressure control signal for opening a bypass valve to commence a flow period of the drill stem test.

Abstract: The cable head provides an H.sub.2 S resistant assembly which includes a weak link therein substantially isolated from any stress other than tensile stress induced by the logging cable. The cable head also provides a fishing neck at the top of the tool string after the weak link is severed and the wireline retrieved. Moreover, the logging cable is positively secured to the cable head by a force-fit wedge, and the design thereof ensures that parting of the weak link also provides for a positive mechanical disengagement of the logging cable from the cable head. Finally, an external sinker bar has been provided which mounts externally of the cable head, surrounding the logging cable above the cable head and mechanically interlocking therewith; such sinker bars may be run above the cable head in series, each interlocking with the one below it.