1. . 
A device fitted with a hinged collar and bowsprings to keep the casing or liner in the center of the wellbore to help ensure efficient placement of a cement sheath around the casing string. If casing strings are cemented off-center, there is a high risk that a channel of drilling fluid or contaminated cement will be left where the casing contacts the formation, creating an imperfect seal.
3. . 
A device that helps to maintain the logging tool in the center of the borehole. On wireline tools such devices typically have three or more flexible bow springs. They may be mounted on the outside surface of the logging tool or else mounted in-line, between two cartridges or sondes. Some measurements, such as acoustic logs, respond better when the tool is centralized, while others, including induction logs, are better when eccentralized.