The kelly bushing (KB) has an inside diameter profile that matches that of the kelly, usually square or hexagonal.
方钻杆补心的内径通常与方钻杆的外径（通常是四方钻杆或六方钻杆）相匹配。
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The kelly bushing has an inside profile matching the kelly's outside profile (either square or hexagonal), but with slightly larger dimensions so that the kelly can freely move up and down inside.