Specialised roof support carriers began gaining popularity following the introduction of the Bucyrus Model 601 Vers-A-Trac® at Island Creek’s VP3 mine in July, 1978. As longwall systems have become larger and heavier, the Vers-A-Trac line has grown to accommodate them. Today, the Bucyrus Vers-A-Trac 650, the largest battery roof support carrier in the Vers-A-Trac family, is one of the most powerful roof support carriers available on the market. Since making its debut at MINExpo 2004, the VT650 (to be seen on the contents page of IM, August 2009) has grown to a population of 42 machines in the US and 22 in China. It can be configured as DC, AC or Diesel to best suit the needs of the customer’s application. All VT650 models are capable of carrying 45 t and feature a tri-sectional frame design with dual motors located near the axles and an oscillation bearing that’s 180% stronger than that of the VT636-2. The VT650 is able to rotate its rear frame independently from the load half of the machine, giving it extra stability even when handling loads equal to the empty vehicle weight. This ensures that the rear wheels maintain contact with the roadway even under severe conditions.In addition to the tri-sectional frame design, all VT650s share the same tilt/lift feature which aids in the accurate placement of payloads, 100o articulation for greater manoeuvrability and a chassis made from high-strength steel that is 50% thicker than comparative machines. The chassis also incorporates bends where applicable for improved strength and reduced indeterminate stresses introduced by welding.The ergonomic design of the operator’s compartment is another key feature shared by all Bucyrus Vers-A-Tracs. Operator comfort is paramount for maintaining a safe working environment. Controls are logically placed in easy-to-reach positions and they are easily operated from the padded, adjustable seat. The frame is contoured with the top sloping from the center to the outside, promoting visibility around the vehicle’s perimeter. The decks are contoured in the reverse manner, tapering from the center upwards to the outside to ensure maximum ground clearance when negotiating confined areas on the longwall face.Bucyrus prides itself on giving customers options to ensure the best fit for their operations. The newest option when choosing a Vers-A-Trac 650 is the diesel-powered VT650D. Introduced in April 2009, this provides the same features as the battery-powered VT650s with the added power of a Cummins 8.3 179 kW diesel power pack.Converting the highly successful battery-powered VT650 to a diesel machine presented some challenges for engineering. According to Jim Coe, Senior Engineer with Bucyrus, “dealing with unknown centres of gravity for the power pack provided a grey area when it came to knowing exactly what the weight distribution would ultimately be on the completed vehicle.” The placement of the centre of gravity in relationship to the weight of the vehicle relates directly to load handling capability and overall power-to-weight relationships. “The idea is to maintain as light of a vehicle weight as possible and use the length of the unit to counter balance the load,” said Coe. Since the design goal of the VT650D was to retain the load handling characteristics and maneuvering capabilities of the battery-powered VT650s, the length and width as well as the wheel base were easily defined. The weight distribution challenge was overcome by increasing the weight of the chassis and using ballast. Another challenge was packaging the transmission and power pack along with the associated components within the envelope while still providing operator visibility. Routing the driveline from the engine to the transmission for distribution to the axles was difficult due to the machine’s tri sectional chassis and 100o articulation configuration, but the final drive train design will ensure a long service life for all associated components. Despite the challenges, the Bucyrus engineering team was able to create a diesel roof support carrier that fit the bill.Another recent addition is the AC version of the battery-powered VT650 which made its debut at MINExpo 2008. The VT650AC features the Bucyrus HiPAC 10 control system which is an innovative variable frequency inverter drive that allows costly DC motors to be replaced by higher power, more efficient AC motors. Traction power is increased by 100% and hydraulic power by nearly 300%. These increases, in turn, nearly double loaded traction speed and grade-ability and improve hydraulic performance.The HiPAC 10 drive provides regenerative braking by allowing energy that is required to decelerate the vehicle to supplement the battery capacity. This feature reduces the duty cycle of mechanical brakes, increasing brake life.The VT650AC also features an upgraded operator’s compartment. All machine parameters are now visible at a glance on the large display that is also capable of providing fault analysis and data logging.The Vers-A-Trac 650 series offers distinct advantages over the competition. One of these advantages is that VT650s are able to remove most shearers, head drives and tail drives without first disassembling them, saving an operation approximately one full day on a longwall move. The VT650AC, which has a maximum speed of 6.5 km/h, has the advantage of speed and manoeuvrability over cable and crawler type roof support carriers which have a maximum speed of about 1.5 km/h. Based on the assumption of a 50% duty cycle, the theoretical minimum benefit to an operation is equal to one full day on the move. With an average US longwall production of 650 short tons per hour and a $60 per ton market price, one day saved on a non-productive longwall move equates to a gain of $936,000.Giving customers the option of battery or diesel-powered Vers-A-Trac 650s allows them to select the best fit for their operations. Battery-powered Vers-A-Tracs are the best choice for mines where noise and ventilation requirements are a concern as these machines do not negatively impact the working environment with emissions or noise. A quieter machine allows workers to hear each other more easily, making for a safer work environment. The diesel-powered VT650D is the best option when higher ground speeds are necessary to move long distances or when steep grades or poor ground conditions are present.