Coal Age

JAN 2016

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

Contents of this Issue

Navigation

Page 24 of 51

January 2016 www.coalage.com 23 Once the feed pressure increases due to a drilling load, the bypass relief valve opens, dumping off the bypassed oil to the tank port. This slows the drilling feed rate to a constant speed based on the amount of oil through the flow control. The final cartridge in the feed package is the high- pressure relief, which sets the maximum feed pressure. It is important to understand that the feed package is intended to control the drilling process by establishing the feed rate with- out the operator "feathering" the controls. The high-pressure relief in the feed package valve should be set so that there is never enough thrust force to bend the drill steel. If the feed package is properly adjusted, the feed pressure that is observed in the system will not always be at the maximum setting. Instead, the actual feed pressure will be something less, depending on the strata being drilled at the given feed rate and rota- tion speed. If the feed rate is increased to the point where the pressure reaches the maximum high relief setting, the drill will begin to slow down because the high relief valve will be open. This is the point where the feed rate must slow until drilling of the hard strata band is complete, at which time the feed rate will again increase to the level set by the flow control cartridge valve. Modifying Drilling Parameters Reducing the rotation speed reduces the risk of steel whip. It is also likely that most drills have the potential to rotate at faster speeds than what is required to drill effi- ciently in typical roof conditions. In fact, while drilling in harder material, it is neces- sary to reduce rotation speeds to allow the bit to bite into the material instead of skid- ding and wearing out the bit. The excess rotation speeds could produce excess wear on bits and smaller particle sizes for dust collection systems. For these reasons, one significant modifi- cation for this study was to increase the size of the rotation motor displacement to per- manently reduce the drilling speed and increase the drilling torque. The 1.5-in. rota- tion motor is a very common arrangement. Over the years, the 2-in. rotation motor has been an optional motor that was available, primarily selected by mines that were drilling very hard materials. For this study, the 1.5-in. motors were replaced with 2-in. motors. Thrust force was the next parameter that was reviewed and optimized as part of the study. Excessive thrust on a drill steel will bend the steel leading to buckling, produc- ing an unsafe condition for anyone in the immediate area. Excessive thrust is a signifi- cant contributor to premature bit wear and does little to improve the cycle time of the drilling operation in all but the hardest roof strata. A dull bit will slow the feed rate and cause more dust fines as the material is pulverized instead of being removed in larger pieces due to proper bit penetration. It is possible that the holes drilled under excessive thrust loads are not as straight because the steel is bowing during the drilling operation. This complicates bolt insertion and makes removing the steel from the hole more diffi- cult because the steel can easily hang in a bowed hole. Feed rates were a third primary target of the study. The goal concerning feed rates was, first, to ensure that the feed package was controlling the feed rate. This provides a more consistent feed rate by putting the control on the feed package valve instead of the operator. Next, the feed rate would be fine-tuned to provide the fastest possible feed rate without bending the steel or clog- ging the air flow through the steel. Clogged steels slow the drilling process and can cre- ate a blowback of dust, which is a burst of dust backward out of the hole. This burst of dust exposes the operator to respirable dust. The idea was not necessarily to improve feed rates directly, but to optimize the feed rates at the appropriate level as dictated by the strata, drill steel and bit type. Overall, cycle time reduction would then be realized due to the other benefits, such as reduction in steel clogging, drill stalling and time spent changing bits. The drill torque is set by the high-volume relief valve and the size of the hydraulic motor. For this study, most of the torque improvements were made by increasing the motor size. In all cases, the high-volume relief valve that controls the torque was checked and adjusted to the factory recom- mendations. However, the setting was not modified to study the effect of more or less available torque on the drilling process. The amount of torque available was simply increased by increasing the displacement of the rotation motors. If a particular mine had an issue of stalling after the larger motors were installed then the high-volume relief valve that controls the maximum torque would have been adjusted to give more torque. Implementing the Change J.H. Fletcher worked with several coal mines to make the desired changes to their pro- duction machines, determine the effect that the changes had on the drill operation, and draw conclusions about the relative success of the changes when implemented as a package. During a production shift, the machines were observed to find the current status, including the current pressure set- tings, the drilling methods being used, the type of strata and bits being used, the cur- rent drilling feed rate, bit life, noise levels, and drilling cycle times. Once a baseline was established, changes were made to the machine as soon as possible, preferably during the next maintenance shift. The rotation motors were changed out from the 1-1/2 in. to the 2-in. gear motors, and the feed package settings were adjusted back to the factory recommendations. This provided a good starting point for tuning the machine. Following these modifica- tions, the feed rates were more accurately tuned, and the pressures were further adjusted to obtain the optimum drilling rate. Operators were reoriented with the machine to make sure they understood how the feed package should be used. Finally, the same parameters that were studied during the initial survey were recorded after the changes had been imple- mented. The machines were again moni- tored for one production shift. The results were shared with the operators and then with the mine management. Once accepted by the mine, the changes were implement- ed on a broader basis at other machines in the same mine. While the improvements were only monitored for one production shift, it should be noted that many of the mines chose to keep and maintain the changes, and are still realizing the improve- ments. Machine-specific Results There were 12 mines that participated in this testing during 2014, and more machines scheduled for 2015. Following are the specific results from three of the machines operating in Central Appalachia. Mine A was operating RRII arm-feed machines. They were installing 6-ft resin bolts supplemented by 14-ft cable bolts. The material being drilled was soft shale for the first 3 to 4 ft and hard shale and sand- stone for the remainder of the drilling. The setup included TL-2 drill steels and Kennametal SV119 bits. During the initial review, the maximum thrust pressure settings were at 1,500 psi a n d 2 , 2 0 0 p s i f o r t h e l e f t a n d r i g h t booms, respectively. This meant that one r o o f b o l t i n g c o n t i n u e d

Articles in this issue

Links on this page

Archives of this issue

view archives of Coal Age - JAN 2016