Coal Age

JAN 2016

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A renewed interest in reducing operating costs, along with the ongoing desire to improve safety, created a perfect environ- ment to take a fresh look at the existing fleet of roof bolting machines to see what could be accomplished. The goal was to take a practical approach to reviewing machine parameters to determine what changes could be made. The emphasis was on changes that would be feasible to roll out to an entire fleet of machines in a short period of time with minimal capital expenditure. The thought process was that significant improvements could be made by simply tun- ing up the machine parameters to provide a more efficient and safer bolting process. The typical Fletcher bolter is equipped with a two-section hydraulic gear pump on each side of the machine (see Figure 1). The pumps are fixed positive displacement; as such, they produce a fixed flow rate at a giv- en rpm. The pump sections are sized to deliver 9 gallons per minute (gpm) and 31 gpm for the low-volume and high-volume flow, respectively. The high-volume pump supplies a fixed displacement gear rotation motor that is coupled to a gear case that rotates the drill steel. Changes to the size of the rotation motor affect the rotation speed and the maximum available torque. The high-vol- ume relief valve controls the pressure. Because torque is a function of the system pressure and the displacement of the hydraulic motor, and the motor is fixed, the relief valve is the device that controls the torque output of the drill. On most drills, the full high volume is available to be delivered to the rotation motor. Some machines are equipped with rotation flow controls to allow the rotation speed to be adjusted for specific conditions, but, in this study, the rotation speed is decreased and the torque is increased by replacing the motor with a larger displacement fixed motor. During drilling, the low-volume pump supplies a feed package control valve, which delivers oil to a directional control valve that supplies the feed cylinder. The feed package control valve is perhaps the most important valve involved in the drilling and bolting process. It is also the valve that is most often misused by bolter operators who desire to put the maximum thrust force at their fin- gertips. When set correctly, the feed package valve (see Figure 2) controls the drilling speed and sets the maximum feed pressure for safety and drilling efficiency. Part of the challenge is to educate mine management on how the feed package valve can be used to improve their safety and performance. The feed package valve is designed so that the operator can throw the handle wide open on the directional control valve, allow- ing the feed package valve to control the drilling feed rate. It contains a pressure compensated flow control that will provide a fixed feed rate for drilling. The feed rate will remain constant regardless of the thrust pressure being delivered. The flow control is adjustable so that the feed rate of the drill can be set for various conditions. When the feed is operated, some oil flows through the flow control, but a high per- centage of the oil is bypassed through the compensator portion of the valve. There is a "bypass" relief cartridge in the valve that prevents the bypassed oil from exiting to the tank until the feed pressure exceeds the relief setting. The result is that, when the bit tip is not under a load, the bypassed oil is routed back into the feed supply to provide the full 9 gpm for medium-speed position- ing in free air. b l a st h o l e d r i l l i n g 22 www.coalage.com January 2016 Figure 1 — A roof bolter drills the top with a Fletcher CHDDR roof drill. r o o f b o lt i n g B Y D A R I N C O T T O N , T I M B U R G E S S A N D T I M M A R T I N Optimizing Drilling Parameters for Improved Bolting Safety and Performance Testing reveals that optimizing drilling parameters for feed pressure, feed rate, rotation speed and drill torque can have an impact on roof drill performance

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