Abstract: A tool for use downhole has an elongate body, arms movably mounted on the body and means for controlling movement of the arms between an operational position and a retracted position. Movement of the arms is controlled by a motor and a transmission, according to one aspect of the invention the arms being suspended independently of one another and in a second aspect of the invention, movement of the arms being controllable remotely from the tool.

Abstract: A tool for use downhole has means (18, 19, 20, 44) for engaging materials surrounding the hole. The engagement means (18, 19, 20, 44) are operated by hydraulic pressure controlled by a hydraulic circuit (10). A hydraulic power module (11) is provided for varying hydraulic pressure in the hydraulic circuit (10) to effect movement of the engagement means (18, 19, 20, 44). In case of failure, a dumping assembly (17) is provided to allow relief of hydraulic pressure in the circuit (10). The dumping assembly (17) includes a dump chamber (62) of variable volume, a dump valve (60) and a non-return valve (61); the hydraulic circuit being capable of being re-set downhole after dumping, there being no need to bring the tool to the surface.