1.  Field of the Invention
This invention relates to a movable or "walking" mine roof support and bolter system and, more particularly, to a plurality of vertically extensible props for supporting spaced apart, transverse roof engaging truss members above a mining machine as it dislodges material from a mine face in which the props are movable in both linear and arcuate directions of travel to turn the roof support system around corners as it follows the mining machine during the mining operation. 
2.  Description of the Prior Art
It is known in underground mining operations to use walking roof supports to provide a mobile tunnel for supporting the mine roof over a continuous mining machine and to advance with the machine.  Examples of known walking roof support systems are disclosed in U.S. Pat.  Nos.  2,795,936; 3,621,661 and 4,143,991.  The walking roof support disclosed in U.S. Pat.  No.  2,795,936 includes a pair of laterally spaced elongated roof engaging beams positioned in spaced relation to extend transversely across the mine passageway.  Each beam is supported at its end portions by vertically extensible props.  The props are positioned adjacent the mine walls, one behind the other.  Tandem pairs of props are connected by extensible struts.  Each prop includes a floor engaging member in the form of a skid-type foot to be dragged along the mine floor.  The struts are angularly movable relative to the props to facilitate turning the roof support system around a corner in a mine passageway. 
In the execution of a turn, the strut at one end is locked by a brake mechanism so that extension of the opposite strut angularly moves the opposite prop and the end of the roof beam.  A turn is executed with the leading beam lowered from engagement with the mine roof and the trailing beam engaging the roof.  The lowered beam is swung through an angle while the other beam remains stationary.  After the leading beam is moved, it is raised to engage the mine roof and the trailing beam is lowered.  The strut that was extended is now retracted to swing the trailing beam to a position behind the leading beam.  Both the leading and trailing beams are walked or advanced straight forward a short distance.  The swinging movement of the beams is then repeated several times until the roof support completes the turn around the corner and is ready to move ahead in a new straight line direction. 
With the above described arrangement, a plurality of roof support beams are arranged in spaced apart pairs one behind another to form a tunnel of a preselected length to provide temporary roof support above the mining machine.  The pairs of beams are independently movable to form the tunnel.  Thus, while some of the pairs of beams of the tunnel advance in a straight line direction, other pairs of beams may be turning a corner.  This occurs because all the props, struts and beams are not connected to form an integral roof support system.  As a result, the movement of the pairs of beams must be individually controlled, particularly when executing a turn around a corner in the mine passageway. 
U.S. Pat.  No.  3,621,661 discloses a walking roof support that includes two units, one straddling the other.  Each unit includes a pair of spaced apart girders that extend in the direction of travel of the roof support.  In use, the girders are positioned adjacent the opposite walls or ribs of the mine passageway.  The ends of the girders are mounted on double acting vertically extensible props.  A plurality of roof engaging beams are mounted on the girders and extend transversely across the mine passageway.  The transverse beams of the first unit overlap the girders of the second unit, and the transverse beams of the second unit overlap the girders of the first unit. 
With the above arrangement to advance, for example the first unit relative to the second unit, the props of the first unit are raised out of contact with the mine floor and the beams thereof are lowered onto the girders of the second unit which is supported on the mine floor.  The non-supporting unit is then forwardly advanced by operation of rams to a preselected position and reset into engagement with the mine roof and floor.  The other unit which was supporting is moved to a non-supporting position and is advanced in the same manner on the other unit which is supporting.  In this manner, each unit is progressively advanced as the mining machine operates beneath the girders and beams.  Provision is also made for pivotally connecting arms to the props to carry a bar for transversely mounting a drilling machine.  Pivoting the arms on the props raises and lowers the bar that carries the drilling machine. 
U.S. Pat.  No.  4,143,991 discloses a mine roof support system in which a plurality of transversely extending roof engaging beams are supported by vertical props connected to one another by horizontally positioned rams.  The props raise and lower the beams into and out of engagement with the mine roof.  Selected extension and retraction of the rams are operable to both advance all of the beams as a single unit in a straight line path through the mine passageway above the mining machine or through an arcuate path to turn the roof support around a corner to move with the mining machine from one mine entry into another mine entry. 
As above discussed, U.S. Pat.  No.  3,621,661 discloses mounting a drilling machine on a walking roof support.  It is also well known as disclosed in U.S. Pat.  Nos.  3,892,100; 3,995,905; 4,079,792 and Re.  28,556 to utilize drilling machines in combination with movable roof support systems.  U.S. Pat.  No.  3,892,100 discloses a roof support that is carried on a mining machine to provide temporary roof support above the mining machine.  The temporary roof support includes roof engaging beams supported by telescopic jacks.  To permit the mining machine to advance, the beams are lowered from contact with the mine roof and the jacks are raised from contact with the mine floor. 
A similar arrangement for providing temporary roof support above a mining machine is disclosed in U.S. Pat.  No.  3,995,905.  Not only does the mining machine include temporary roof supports, but also includes roof bolters spaced on opposite sides of the machine for pivotal movement to drill bolt holes and install roof bolts in the mine roof for permanent roof support.  Thus as the mining machine is operating to dislodge material from the mine face, the temporary roof support system supports the mine roof as roof bolts are progressively installed in the mine roof as the mining machine advances. 
U.S. Pat.  Nos.  4,079,792 and Re.  28,556 are examples of known types of mobile vehicles used in underground mining operations for providing temporary roof support as bolt holes are drilled and roof bolts are installed in a mine roof for permanent roof support. 
While it is known to provide walking roof support systems for temporary roof support over a continuous mining machine as material is dislodged from the mine face and to also drill holes and install roof bolts in the mine roof adjacent the mine face as the mining operation is carried out, there is need for a movable roof support system that is readily adaptable for changing the direction of movement of the roof support as it moves with the mining machine, particularly in executing a right angle turn around a corner from one passageway to another.  Not only must the roof support system be easily moved in order to follow the path of the mining machine, but also be adaptable to support one or more roof drills for the installation of roof bolts above the roof support system.