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。
The kelly bushing (KB) has an inside diameter profile that matches that of the kelly, usually square or hexagonal.
方钻杆用于传递转盘或者方钻杆补心的旋转运动到钻柱上去，并允许钻柱在上提或下放的过程中旋转。
The kelly is used to transmit rotary motion from the rotary table or kelly bushing to the drillstring, while allowing the drillstring to be lowered or raised during rotation.
从方钻杆补心(KB)、钻台(DF)、底座或者地面标高(GL)到井中测量点或者井底的实际井筒长度。
The actual wellbore length from the kelly bushing (KB), derrick floor (DF), foundation or ground level (GL), to a point in the well or bottom of the well.
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