Coal Age Magazine - For more than 100 years, Coal Age has been the magazine that readers can trust for guidance and insight on this important industry.
Issue link: https://coal.epubxp.com/i/629433
pletely out of adjustment, and the operators were setting the feed rate by feathering the joystick. This also results in the pressure staying at the maximum setting with the relief dumping as the hole is drilled. The rotation motors were running at 650 rpm. Drill steel bending was noted on both booms as the operators attempt- ed to control the feed rate as fast as pos- sible. Steels were hanging in the holes, w h i c h r e s u l t e d f r o m t h e h o l e s b e i n g arched instead of straight. The operators w e r e h a v i n g d i f f i c u l t y c o n t r o l l i n g t h e rotation at medium speeds in the hard material. There was some steel clogging, as well. The bit life was low at 3 ft per bit, drilling at an average rate of 1.3 in/s, but the rate was quite variable and operator controlled. Because of the extremely hard condi- tions, this mine was able to achieve signif- icant improvements by changing to the larger rotation motors and tuning the drilling parameters. Immediately, the drill steel bending was eliminated, which pro- vides better operator safety and straighter holes. As a result, the drill tools hung in the hole much less. The maximum thrust pressure reduced slightly to 1,800 psi on both sides, but, because the feed package was set to control the feed rate, the drill ran below the maximum setting during most of the drilling period. The new feed rate was actually improved to 1.6 in/s as controlled by the feed package valve. Bit life saw a substantial improvement to 12 ft per bit, which is a 400% improvement over the initial results. In addition, the hydraulic system was observed to be operating cooler. The noise and dust levels appeared to have been reduced. This mine elected not to incorpo- rate the changes discussed in this paper. The test machine was put back to its origi- nal condition. The mine does see the bene- fit in the changes, but the main deciding factor was based on the resin in use at this mine requiring high spin speeds. Work is under way to sort this out so that these improvements can be implemented. Results Initial Survey: During the initial surveys, it was noted that some drills had high thrust pressure settings well above the recom- mended settings leading to a high likeli- hood of steel bending and tool and steel failures. Other machines had their maxi- mum thrust pressure settings close to Fletcher factory recommendations, but most of these still had the bypass cartridge adjusted incorrectly so that the feed pack- age could not control the feed rate accu- rately. Operators were observed feathering controls in an effort to set the feed rate and installing bolts without use of the two- hand fast feed valve. This demonstrated that the feed package was out of adjust- ment. Several drills were observed to stall frequently under heavy loads with high feed rates, especially in harder material when the operator attempted to slow the drill speed. In softer material, the drill vac- uum system clogged frequently creating blowouts, which is a discharge of dust back out of the hole. Clogging of drill steels also slows down the overall cycle time for installing a bolt. Safety Considerations: The maximum rotation speed was decreased on all of the machines to reduce the potential energy of the steel. High thrust loads can lead to steel failure and whipping. By setting the maximum thrust and con- trolling the feed rate, the risk due to a steel failure was reduced. Bit Life: The results were varied, with some mines experiencing no bit life improvement, while others saw dramatic improvement. Most of the mines did see some improve- ment in bit life. Some of the more significant improvements were noted when the original feed pressures were considerably off during the initial survey. At least two mines went from using one bit per hole to being able to drill several rows on the same bit. Feed Rate: Several mines were able to realize significant increases in feed rates by optimizing the settings and proper use of the feed package. A few machines had no improvement in feed rate or even a slight decrease in feed rate. However, the goal was to obtain the appropriate feed rate and to have that rate controlled by the feed package valve. The overall cycle time was improved for all mines, which resulted from other improvements including reductions in stalling, clog- g i n g , s t e e l h a n g i n g , a n d t i m e s p e n t changing bits. Straighter Holes: By eliminating steel bow- ing, the drilled holes were straighter, leading to a decrease in occurrences of steels hang- ing in the holes. Hanging steels force opera- tors to have to stop and pull the steel out of the hole. It is common to see the operators pecking or hammering on the side of the steel while pulling it out of the bowed holes. This releases respirable dust from the hole as the steel comes out and places the opera- tor in awkward body positions trying to reach out from under the canopy to free the steel. An operator trying to free a stuck steel may be more at risk for injury. Drill Stalling: Stalling of the drill motor, which was observed during the initial sur- veys, was significantly reduced. During the initial studies, stalling was observed rarely in soft conditions, but occurred as much as four to five times per hole in the hardest strata, with an average of two times per drilled hole. After the modifications, stalling was no longer observed on any of the test machines. The improvement can be attributed to the increased torque of the larger motors, as well as setting the feed rates to appropriate levels. During the drilling of very hard materials, the operators would slow the rotation, which created the stall of the motor. Slowing the rotation speed in hard materi- als is necessary in order to allow the bit to cut the material with improved bite. This is essential to obtaining good bit life. With the increased torque and the feed rate properly adjusted, this problem improved. Reduced Clogging: Clogging of the vacu- um system was reduced significantly. In the worst scenarios observed, the system would clog on nearly every hole during the initial January 2016 www.coalage.com 25 r o o f b o l t i n g c o n t i n u e d Table 1 — Typical Data Collected