Correct rotation speed has direct effect on both bit life and hammer performance. Proper rotation is essential for long hammerand bit life. some common elements involving rotation speed are:
- Rotation Too Slow - The main purpose of rotating a down hole hammer and bit is to index the carbide button inserts to fresh rock on every impact If the rotation is too slow the buttons may tend to bury themselves and this will result in an erratic rotation. Slow rotation can also result in the recrushing of the rock. also known as regrinding, which results in a rapid carbide wear.
- Rotation Too Fast - increasing the rotation speed will not necessarily increase the penetration rate. It will usually in rapid gage carbide wear due to the high scraping forces ratherthan a crushing force.
Please consider these tips for the optimum performance of the hammer:
- Turning in clockwise direction keepsthe tooljoints tight.
- Rotation indexes the carbide buttons to fracture fresh rock with each impact.
Note: Reverse rotation and impact without rotation may cause too joints to become loose. Loose joints may cause drilling equipment to break free and be lost down the hole.
An operate must learn to have a feel for ﬁnding the proper rotation speed that will delivers optimum penetration rate without sacriﬁcing bit ﬁle. As a starting point an operator can use the following:
R.P.M = 1/2 Penetration rate per hour in feet
R.P.M = 1.6 X Penetration rate per hour in Meters
As an example, if the average penetration rate is 60 feet (18.3 m) per hour. the revolutions per minute should be around 30 (29.3).
Note: This is just a guideline. Many factors need to be taken into account for proper rotation speed uch as ground conditions, formation hardness, abrasiveness. etc.
ROTATION TORQUE REQUIREMENTS CLASSIFIED BY BIT SIZE
|#||Bit Size||Torque Required at Operating RPM|
|1||4" (102 mm) Class||500 ft. lbs. (69 KGM)|
|2||5" (127 mm) Class||650 ft. lbs. (90 KGM)|
|3||6" (152 mm) Class||800 ft. lbs. (111 KGM)|
|4||8" (203 mm) Class||1,500 ft. lbs. (207 KGM)|
|5||1O" (254 mm) Class||2,000 ft. lbs. (277 KGM)|
|6||12" (305 mm) Class||5,000 ft. lbs. (691 KGM)|
|7||20" (508 mm) Class||8,000 ft. lbs.(1106 KGM)|
|8||22" (559 mm) Class||10,000 ft. lbs. (1386 KGM)|
|9||24" (610 mm) Class||12,000 ft. lbs. (1659 KGM)|
|10||30" (762 mm) Class||20,000 ft. lbs. (2765 KGM)|
|11||36" (914 mm) Class||28,000 ft. lbs. (3871 KGM)|
|12||43" (1092 mm) Class||40,700 ft. lbs. (5629 KGM)|
|13||48" (1219 mm) Class||50,688 ft. lbs. (7011 KGM)|
|14||54" (1372 mm) Class||64,152 ft. lbs. (8873 KGM)|
|15||60" (1524 mm) Class||79,200 ft. lbs.(10,954 KGM)|
Note:The above figures are estimated at operating RPM.bit sizes from 4" through 12" (102-305 mm) diameter may require a stall torque factor of three or four times the normal operating range. Bit sizes above 12" (305 mm) in diameter may require a stall torque of one and one half totwo times the normal operating range.