nterlocking pressurizing kelly bar is one type adapted to the hard layer.Its structure is that the outer keys is composited of 3 keys or 6 keys,there are usually 3 sets of or N sets of  concave 

pressurizing parts.

The inner keys are composited of 3 or 6 wear-resistant strip. 

This kelly bar has the advantage of passing pressure , anti-torque capacity better, and is used for hard and soft layer. 
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