Figure 29-10. A bit showing off-center wear. 



per second, or less. In bits having jet water courses, the nozzles are positioned so 

 that the jet stream is directed onto the bottom of the hole; the resulting tur- 

 bulence tends to keep the cones clean. The velocities used through jet bits are 

 much higher, usually 200 feet per second or more. The type of wear shown in 

 Figure 29-9 occurs only on those bits having conventional water courses and is 

 caused by excessive velocity of the flushing fluid, particularly if abrasive muds 

 are used. This condition can be alleviated by the use of jet bits where the fluid 

 stream impinges on the bottom of the hole instead of on the cones. If the per- 

 formance does not justify the small additional cost of jet bits, this trouble can 

 be eliminated by reducing the velocity of the fluid streams either through the 

 reduction of the circulated volume or by increasing the effective area of the 

 water courses. Normally if the velocity of the flushing fluid is less than 100 feet 

 per second through conventional water courses, erosion such as illustrated here 

 does not occur. 



650 



