... , and more suitable for drilling at high rotary speed and high tem-perature working condition.3. New ... series tri-cone rock bits for well drilling: 1, Very soft with low compressive strength, such as soft ... 300~60 r/m.2, Soft with low comprssive strength, such as shale, clay, sadstne, soft limestone, ... 60 r/m.3, Medium soft with hard abrasive treak, such as hard shale, soft limestone, sandstone, etc. ...