referring to fig1 there is shown a centralizer 10 which is generally identified by reference numeral 1 . the centralizer 1 comprises two annular bands 2 and 3 which are spaced apart by six members 4 to 9 . each member 4 to 9 is inclined at an angle of from between 20 ° to 60 ° and preferably from about 30 ° to 45 ° to the longitudinal axis of the centralizer 1 . the members 4 to 9 are substantially the same , accordingly reference will hereinafter be made to member 4 only . member 4 is hollow , being generally u - shape in cross - section , and has skirt portions 10 , 11 which extend radially inwardly and finish flush with the radially inner surface of the two annular bands 2 and 3 . the member 4 is formed from a thin sheet of steel which is about 3 mm in thickness . the member 4 is hollow . the member 4 is also tapered both radially and circumferentially at each end to facilitate movement of the centralizer 1 in the borehole . member 4 is provided with cylindrical rollers 12 , 13 , 14 . each cylindrical roller 12 , 13 , 14 is free to rotate about an axis which is generally perpendicular to the longitudinal axis of the centralizer 1 ( see fig2 ). the cylindrical rollers 12 , 13 , 14 project from the member 4 by approximately 3 mm to 5 mm . the ends of each axle 15 , 16 are located in holes 17 , 18 , 19 in flanges 20 , 21 , 22 , 23 . the flanges 20 , 21 , 22 , 23 are formed by pressing windows 24 , 25 , 26 in the member into the hollow . the cylindrical rollers are made of steel but could be made from a zinc alloy , ptfe , an elastomer , bronze , brass or other bearing material . the cylindrical rollers 12 , 13 14 are approximately 15 - 20 mm in diameter and approximately 15 to 20 mm in length . the clearance between the outer periphery of each roller and its associated window is sufficient to inhibit typical particles wedging between the two and inhibiting rotation of the roller . early tests suggest that the clearance should be at least 1 mm , preferably at least 2 mm , more preferably at least 3 mm and advantageously at least 4 mm . in use , the centralizer 1 is slid over a tubular ( not shown ). the centralizer 1 may be secured by stop collars ( not shown ) placed above and below the centralizer or allowed to slide along the tubular between boxes . a plurality of centralizers 1 are arranged on a string of tubulars , which may form a liner , which is then lowered through previously set casing and into an open , typically near horizontal , bore hole . the cylindrical rollers 12 , 13 , 14 roll along the inner surface of the liner and set casing and subsequently along the surface of the borehole . once the liner has reached its destination , which may be up to 20 km from the well head , the liner may be hung and set in a conventional manner . occasionally , part of the borehole will collapse when running liners . the usual procedure when this occurs is to withdraw the liner , make good the problem as necessary and reinstate the liner . although the liner can usually be withdrawn without too much difficulty the forces imposed on traditional centralizers often result in their disintegration with the result that broken parts of centralizers remain in the borehole . this is most undesirable . applicants pct publication number wo 96 / 09459 addresses this problem by providing a centralizer with members which have sufficient strength to centralize the liner but which will collapse if with - drawn through a relatively rigid constriction . the underlying principle is that it is better to replace a permanently deformed centralizer at the surface rather than to leave parts of a disintegrated centralizer in the borehole . typically , the members should substantially completely collapse when subjected to a lateral load of from 5 to 15 tonnes with 11 tonnes being currently used for design purposes for most occasions . the cylindrical rollers 12 , 13 , 14 are of small diameter , so that significant collapse of the members 4 to 9 is possible . the cylindrical rollers 12 , 13 14 may also be designed to collapse under a similar lateral load , ie of the order of from 5 to 15 tonnes .