Rotasjonsbegrensende anordning for bruk i en styrbar boreanordning

In an apparatus (20) for use in a borehole of a type comprising a rotatable shaft (24) and a housing (46) for rotatably supporting the shaft therein, a rotation restraining device (252) is associated with the housing (46) for restraining its rotation. The device (252) includes a plurality of rollers (254), each having an axis of rotation substantially perpendicular to a longitudinal axis of the housing (46) and being oriented such that it is capable of rolling about its axis of rotation in response to a force exerted thereon substantially in the direction of the longitudinal axis, wherein at least two of the rollers (254) are spaced about a circumference of the housing (46) and axially along the housing (46) so that the rollers (254) are staggered axially along the housing (46). Alternatively, the device (252) includes a plurality of pistons (268), wherein at least two of the pistons (268) are spaced about a circumference of the housing (46) and axially along the housing (46) so that the pistons (268) are staggered axially along the housing (46).