Method and apparatus for running a mechanical roller arm centralizer through restricted well pipe

A roller arm centralizer has two sets of arms 31, 32 which are biassed outwardly by springs 28 and have casing engaging rollers 40 at their junctions 36. A first set of arms 31, 32 carry a lower set of rollers 40 spaced evenly around the centralizer but staggered from each other in the axial direction. Similarly a second set of arms carry an upper set of axially staggered rollers. This arrangement reduces the force necessary to move the centralizer through a casing restriction. A flange 39 on spring housing 26 is located between stops 37, 38 to limit the extension and retraction of the arms. In a second embodiment (not shown) the sets of arms have independent biassing means and in a third embodiment a cam arrangement ensures that the biassing means exerts an outwardly directed force when the arms are fully retracted.