(57) An apparatus for movably mounting a work head (ll,l2) on a structure.of circular
cross-section. The apparatus comprises an annulus defined by two or more members (l,2,3)
which are engagable co-operably and movable between an open position which allows
the annulus to be fitted onto the structure and a closed position in which the annulus
surrounds the structure. The apparatus is located on the structure by locating means
(6) comprising three or more support legs (6) mounted on the annulus at spaced circumferential
locations. At least some of its legs (6) are movable in a radial direction relative
to the annulus by actuating means to locate the annulus on the structure. Each supporting
leg (6) is provided with drive means (8) which are controllable independently for
moving the annulus along the structure in a plane normal to the axial centre line
of the structure. The work head (ll,l2) is mounted on the annulus for circumferential
movement with respect to the structure.
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