Directional Control For Rock Drill Feed Support

Abstract

A direction sensing unit and control circuit for prepositioning a rock drill feed support to provide for drilling a series of parallel holes. The direction sensing unit is mounted on the feed support and includes a pivotally mounted weight responsive to gravitational force acting thereon to actuate a plurality of pneumatic valves to provide pressure signals. The control circuit includes pneumatically operated control valves which, in response to receiving pneumatic pressure signals from the sensing unit, are operable to valve hydraulic fluid to and from hydraulic positioning cylinders connected to the feed support.

Description

BACKGROUND OF THE INVENTION

In the art of rock excavation it is often desirable to make cuts in the earth along predetermined planar surfaces defining the limits of the excavated area. For example, in open pit mining and road right of way excavation it is desirable to make cuts in the earth to provide a sloped surface which will be stable and therefore safe and maintenance free. To this end a technique known in the art as presplitting or preshearing has developed wherein a series of closely spaced parallel holes are drilled to a predetermined depth in the plane of the desired surface. These presplit holes are loaded with blasting charges and the resulting blasting operation provides a cleanly sheared cut in the earth having a substantially smooth and fissure free surface.

In presplit drilling it is particularly important that the closely spaced blast holes are drilled parallel and in the same plane to assure that a smooth cleavage is provided upon blasting. Overbreakage and rough surfaces resulting from misaligned holes are costly in terms of extra rock removal required and the development of fissures in the rock surface. In the latter case entry of moisture into the fissures results in erosion and further breakage of rock due to freezing and thawing processes.

Heretofore in the drilling of presplit blast holes with portable rock drilling apparatus the positioning of the rock drill feed support to provide parallel coplanar holes has been a time consuming and tedious operation. Moreover, the accuracy of "sighting in" the directional attitude of the rock drill centerline by manually operating the valves controlling the feed support positioning motors has been so limited that only relatively shallow depths of presplit cleavages could be produced from drilling a series of holes. Accordingly, many somewhat deep cuts in the earth have required repeated cycles of drilling, blasting and excavating to reach the required depth of cut.

Devices are known in the art of rock drill apparatus which are operable to provide for positioning a drill feed support to drill a pattern of parallel holes. Examples of such devices are disclosed in U.S. Pat. Nos. 3,374,975 to H. Bronder, and 3,481,409 to B. A. Westerlund. Whereas the parallel positioning systems disclosed in the above mentioned patents are suitable for use with a particular type of drilling apparatus for drilling a pattern of holes in a tunnel face or the like they are not readily adaptable to be used with conventional mobile drilling apparatus having elongated drill feed supports which are connected to a support by means of a connection providing for pivotal movement about two axes.

Moreover, prior art directional control systems for rock drill feed supports are subject to error, or example, due to leakage of hydraulic fluid from series connected positioning motors or wear in the mechanical linkage connections for certain types of parallel motion mechanisms.

SUMMARY OF THE INVENTION

The present invention provides for portable rock drilling apparatus including a movable feed support automatically positionable to provide for drilling a series of parallel blast holes or the like. With the rock drilling apparatus of the present invention a series of blast holes may be drilled at a predetermined angle with respect to a plumb line, said holes being virtually parallel and in a predetermined plane.

More specifically, the present invention provides a directional control system for a rock drill feed support which is operable to provide for substantially automatic positioning of the feed support to be properly aligned for drilling a series of spaced parallel holes such as used for presplit cleaving of rock faces. With the directional control system of the present invention otherwise conventional portable rock drilling apparatus having swingable boom mounted feed supports may be more rapidly and accurately positioned for drilling parallel coplanar holes in the ground.

There is also provided in the present invention a direction sensing unit responsive to gravitational force to provide pneumatic signals to a pressure fluid control circuit which in turn is operable to control a plurality of pressure fluid positioning motors. The completely pressure fluid operated directional control system of the present invention is operable to utilize pressure fluid normally supplied for use otherwise on portable rock drilling apparatus.

Although the directional control system of the present invention is disclosed in combination with a rock drill apparatus used primarily for drilling on the surface of the earth the directional control system may be advantageously used with rock drill units suited for underground mining and tunneling excavation as well.

With the directional control system of the present invention in combination with a portable rock drill apparatus a series of spaced parallel blast holes may be drilled faster and more accurately aligned than with heretofore known equipment

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation of a mobile rock drill rig embodying the directional control system of the present invention.

FIG. 2 is a detail plan view of the boom pivot of the rock drill rig of FIG. 1.

FIG. 3 is an elevation of the directional sensing unit mounting.

FIG. 4 is a plan view of the directional sensing unit mounting. FIG. 5 is a plan view of the rock drill rig of FIG. 1 showing the drilling pattern of a series of presplit blast holes.

FIG. 6 is a sketch of a typical cross section of a roadway excavation having presplit sloped sides.

FIG. 7 is a center longitudinal section view of the directional sensing unit taken along the line 7--7 of FIG. 4.

FIG. 8 is a section taken along the line 8--8 of FIG. 7.

FIG. 9 is a schematic of the directional sensing unit and control circuit of the present invention.

FIG. 10 is a detail view of the feed support mounting.

FIG. 11 is a detail taken along the line 11--11 of FIG. 7

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 2 a mobile rock drill apparatus or rig is illustrated and generally designated by the numeral 10. The rock drill rig 10 includes a crawler undercarriage 12 including a pair of crawler assemblies 14 pivotally connected to a frame 16. The frame 16 is adapted to support an elongated movable support or boom 18 by means of a bracket 20 which is pivotally connected to the frame 16 for movements about a substantially vertical pivot axis 22 and includes a clevis 24 for pivotal attachment of the boom to the bracket 20. The boom 18 is operable to be elevated about the pivot connection 24 by means of a hydraulic cylinder 26 and to be swung about the pivot axis 22 of the bracket 20 by a hydraulic cylinder 28.

The boom 18 is of a telescopic type already known in the art of rock drill apparatus and includes a portion 30 which may be extended with respect to the portion 32. The distal end of the boom portion 30 includes a positioner mechanism 34 pivotally connected thereto. A hydraulic positioning motor in the form of the cylinder 36 is provided for pivoting the positioner 34 about a first axis 38. The positioner comprises a bracket 40 which is adapted to have mounted thereon an elongated support 42 for reversibly feeding a percussion rock drill motor 44 therealong. The bracket 40 is pivotally mounted with respect to the positioner 34 to be pivoted about a second axis 46 which is perpendicular to the first pivot axis 38. Pivotal movement of the bracket 40 with respect to the positioner, about the axis 46, is accomplished by means of the fluid operated positioning motor 48 comprising a hydraulic cylinder attached to the bracket at 50 and having a piston rod pivotally connected to the positioner at 52, FIG. 10.

The feed support 42 includes a fluid operated feed motor 54 mounted thereon and operable to reversibly feed the drill motor 44 along the feed support. The drill motor is operable to deliver percussive blows to an elongated drill rod 56 which has a bit 58 attached to one end and is chucked in the drill motor at its opposite end. The drill rod 56 is held in desired alignment with respect to the feed support by a centralizer 60.

The rock drill rig 10 hereinbefore described is of a type generally well known in the art of earth drilling apparatus and is adapted to provide for drilling blast holes in a wide range of directions. Such rigs are provided with fluid operated motors such as the motors 61, FIGS. 1 and 5, whereby they may be moved about from one work location to another. It is conventional to provide such rigs with self contained power plants for providing a source of pressure fluid or in the case of the exemplary rig 10 a portable compressor or pump, not shown, is connected to the rig by suitable, flexible conduits. Conversion of pressure fluid energy from pneumatic to hydraulic form or vice versa may be accomplished by suitable motor-pump units on board the rig.

Referring to FIGS. 1, 3, 4 and 5, the rock drill rig 10 is adapted to include a control system for positioning the feed support 42 to provide a predetermined direction of the longitudinal axis of the drill rod 56, FIG. 1. This predetermined direction is defined with respect to a reference which, with the directional control system of the present invention, comprises a plumb line 64 or a straight line through the gravitational center of the earth.

Referring to FIGS. 1, 3 and 4, the directional control system of the present invention includes direction sensing means comprising a sensing unit mounted on the feed support 42 and generally designated by the numeral 66. The sensing unit 66 is characterized by a substantially spherically shaped housing 68 which is mounted on a bracket 70 attached to the feed support and having a hinged portion 72. The hinged portion of the bracket 70 includes a mounting plate 74 attached thereto and cooperable with the plate 76 to movably clamp the sensing unit 66. The plate 76 is secured to the plate 74 by bolt and nut assemblies 78.

As shown in FIG. 4 and 5 the hinged portion 72 is movable with respect to the feed support 42 in response to the actuation of the double acting hydraulic cylinder 80 having its rod end attached to the plate 74. The opposite end of the cylinder 80 is pivotally connected to the mounting bracket at 82. The hydraulic cylinder 80 is interconnected to a hydraulic cylinder 84, as shown schematically in FIG. 4, by conduits 86 and 88. The hydraulic cylinder 84 is pivotally connected to the frame 16 at one end and at its rod end is pivotally connected to the pivot bracket 20 which supports the boom 18, FIG. 2. The cylinders 80 and 84 are arranged at their respective locations and interconnected by the conduits 86 and 88 such that displacement of fluid from either end of the cylinder 84 in response to pivotal movement of the bracket 20 will result in fluid being forced into the corresponding end of the cylinder 80 to produce equal movement of the cylinder 80 and, accordingly, angular movement of the hinged portion 72 about its pivot substantially equal to the angular movement of the boom 18 about the pivot axis 22 of the bracket 20. Such arrangements of fluid cylinders are believed to be generally well known in the art of fluid power servomechanisms and the purpose of such an arrangement in regard to the directional control system of the present invention is to provide means for controlling the position of the sensing unit 66 with respect to a reference as will be explained further herein. The conduits 86 and 88 interconnecting the cylinders 80 and 84 are also flow connected to conduits 90 and 92 which in turn are connected to a three position normally closed valve 94 represented schematically in FIG. 4 in accordance with U.S.A. Standard fluid power symbols. The valve 94 is in communication with a source of hydraulic fluid, not shown. The valve 94 is operable to communicate pressure fluid to either end of the cylinder 80 to effect angular positioning of the hinged portion 72 of the bracket 70 independently of any movement of the boom pivot bracket 20. In the arrangement of the rig 10, as shown in FIGS. 1 and 2, this independent positioning of the bracket portion 72 is possible due to the fact that fluid pressure acting on the cylinder 84 is not sufficient to overcome the holding force of the boom swing cylinder 28.

Referring to FIGS. 7, 8 and 11, the sensing unit 66 as previously stated, includes a spherically shaped housing 68 having a hollow interior 96 and a threaded portion 98 which is adapted to receive a base 100. The base 100 is adapted to include an elongated rectangular tube 102 extending downwardly therefrom. The base 100 further includes a plurality of four cylindrical bores 104 spaced equidistant one from the other along first and second centerlines 106 and 108 which intersect at right angles. Situated in the bores 104 are cylindrical seats 110 containing ball valve elements 114a, 114b, 114c, and 114d. The ball valve elements are operable to be seated over the ends of passages 115 in the seat elements 110 to prevent fluid flow through the passages into the hollow interior 96.

The sensing unit 66 also includes means responsive to gravitational force comprising a weight 116 pivotally mounted above the valve elements 114a, 114b, 114c, and 114d on a pedestal 118 having a substantially spherical shape except for the flat surface portion 120. The pedestal is supported in the socket 122 in the base 100, the socket 112 being located at the intersection of the centerlines 106 and 108. The pedestal 118 as well as the seat elements 110 are retained on the base 100 by a plate 124 suitably secured to the base by threaded fasteners 126.

The weight 116 also includes a dome portion 128 operable to be engaged by a spring biased piston lock 130 movable in the stepped bore 132 in the housing 68. The piston 130 is movable in the bore 132, in response to pressure fluid being admitted to the interior 96, against the bias of the spring 134 to allow the weight 116 to be free to tilt with respect to the centerline 136 about the spherical pedestal 118. The centerline 136 passes through the intersection of the centerlines 106 and 108 and is perpendicular to the centerlines 106 and 108. The piston 130 includes an O-ring 138 forming a seal between the piston and the bore 132. In response to pressure fluid acting on the face 140, the piston is movable upward in the bore 132 until the O-ring 138 passes the lower edge of the V-shaped notches 142, FIG. 11, allowing pressure fluid to vent from the interior 96 through clearance provided between the piston 130 and the bore 132 and out through the notches 142. The spring 134 is of a predetermined size to permit the piston to move upward a sufficient amount to release its holding force on the weight 116 and allow the escape of pressure fluid from the interior 96. Pressure fluid is supplied to the interior of the housing 68 through conduits 144a, 144b, 144c, and 144d in communication with the valves 144a, 144b, 144c, and 144d, respectively.

The weight 116 is retained in the position shown in FIG. 7 by the piston lock 130 to prevent unwanted movement of the weight when the direction sensing unit 66 is not in use. With pressure fluid supplied through the conduits to the passages 115 in the valve seats 110, the ball valve elements will be raised off the seats to engage the surface 146 on the weight and allow the flow of pressure fluid into the interior 96. Pressure fluid acting on the piston 130 will cause the piston to move upward unlocking the weight 116. When the direction sensing unit is oriented such that the resultant force of gravity on the weight 116 acts along the centerline 136, which comprises a reference axis for aligning the sensing unit so that the centerline 136 is coincident with a plumb line, the weight will be balanced to remain in the position shown in FIG. 7. However, if the sensing unit is tilted in any direction the resultant force of gravity on the weight will cause the weight to tilt to engage one or possibly two adjacent ball valves forcing the valves against their respective seats and closing off the flow of pressure fluid through the respective passages. The shutoff of fluid flow produces a signal in the form of a pressure increase in the respective conduits leading to the closed valves. This pressure increase is used as a control signal to effect the positioning of the feed support 42 to assume a predetermined direction in accordance with the operation of the control system of the present invention. A small passage 149 is provided in the housing 68 to vent pressure fluid trapped in the interior 96.

The control system of the present invention includes a pressure fluid control circuit illustrated schematically in FIG. 9 substantially in accordance with U.S.A. Standard fluid power symbols. The control circuit of FIG. 9 includes a pressure fluid supply line 150, preferably using compressed air as the pressure fluid, having a shutoff valve 152 interposed therein. The supply line 150 is connected to each of the conduits 144a, 144b, 144c, and 144d for supplying pressure fluid to the respective valves 114a, 114b, 114c, and 114d. Flow restrictors 154 are provided for limiting the flow of fluid to each valve. The control system also includes a hydraulic control circuit including the positioning motor 36 having conduits 156 and 158 leading to opposite ends thereof and the positioning motor 48 having conduits 160 and 162 leading to its opposite ends. The conduits leading to the positioning motors 36 and 48 are operable to be connected to a source of hydraulic pressure fluid such as the pump 164 and to a reservoir 166 depending on the position of control valves 168 and 170 interposed in the circuit. The control valves 168 and 170 represented schematically in FIG. 9 are of a type which may be actuated by pressure fluid or manually to positions a and b and are normally in the blocked position c in response to balanced actuating forces or no actuating force. As shown in FIG. 9 valves 114a and 114b are respectively connected to the position a and b actuators of control valve 168 for controlling the operation of positioning motor 48 and valves 114d and 114c are respectively connected to the position a and b actuators of control valve 170 for controlling the operation of positioning motor 36. The control valves 168 and 170 and the control circuit shutoff valve 152 may be suitably located on the feed support 42 or at an operator control station on the undercarriage 12.

The control circuit of FIG. 9 is operable to actuate the positioning motors 36 and 48 to position the feed support 42 so that the drill rod axis 62 assumes a predetermined attitude or direction. With pressure fluid supplied to the conduits 144a, 144b, 144c, and 144d, the valves 114a, 114b, 114c, and 114d will allow fluid to flow into the interior 96 of the housing 68 to actuate the piston lock 130 to release the weight 116 to be free to tilt about the spherical pedestal base 118. If the sensing unit is positioned such that the force of gravity acting on the weight 116 is substantially along the centerline 136 the weight will remain in the position of FIG. 7 and all of the ball valves will be open. The flow restrictors 154 will provide for a uniform reduced pressure in all conduits and with equal pressure on the position a and b actuators of control valves 168 and 170 these valves will remain closed or in position c.

If the direction of the gravitational force acting on the weight 116 should change due to movement of the feed support 42, upon which the sensing unit 66 is mounted, the weight would tilt to engage one of the ball valve elements 114a, 114b, 114c or 114d to block the flow of pressure fluid through its associated conduit. A pressure increase in that conduit would result in the actuation of the associated control valve actuator causing one of the control valves 168 or 170 to valve pressure fluid to the respective positioning motors 48 or 36 thereby effecting positioning of the feed support 42. It is possible that the change in position of the feed support 42 could cause the weight 116 to tilt to engage a combination of one of the valves 114a or 114b on the centerline 108 and one of the valves 114c or 114d on the centerline 106 simultaneously, in which case both control valves 168 and 170 would be actuated to supply pressure fluid to the positioning motors 48 and 36 simultaneously. The surface 120 on the pedestal 118 provides for reduced tendency of the weight to tilt when minor deviations from the true vertical orientation of the centerline 136 are encountered. This does not affect the accuracy of the sensing unit for most applications including the primary uses of the drill rig disclosed.

By way of example of the operation of the control system of the present invention the position of the sensing unit 66 in the mounting bracket 72 on the feed support 42 may be assumed to be that shown in FIG. 3. If the feed support 42 should be swung out of the vertical as indicated by the dotted line position of FIG. 9 the weight 116 would tilt to engage and closed ball valve 114a thereby causing a pressure increase in conduit 144a and actuation of control valve 168 to position a. In position a control valve 168 would supply pressure fluid to the rod end of positioning motor 48 causing the feed support to swing about the pivot axis 46 back to a substantially vertical position whereupon the gravitational force acting on weight 116 would cause the weight to resume the position of FIG. 7 allowing ball valve 114a to open reducing the pressure in conduit 144a and resulting in control valve 168 shifting to the blocked position c. If the feed support 42 were to be swung about the axis 46 in the opposite direction, the weight 116 would tilt to close valve 114b and thereby effect the shifting of control valve 168 to position b to valve pressure fluid to the opposite end of positioning motor 48 until the feed support was again vertical.

In a similar manner if the feed support were pivoted about the positioner pivot axis 38 in one direction or the other, commencement of operation of the control system would result in the weight 116 tilting to engage either ball valve 114c or 114d which would result in the shifting of the control valve 170 to position b or a. This would result in the actuation of the positioning motor 36 to extend or retract its piston rod pivoting the feed support back to the vertical position.

As previously mentioned a desired use of the directional control system of the present invention is to provide for drilling a series of spaced parallel blast holes for presplitting a rock face. FIG. 6 illustrates a schematic cross section of a road right of way 172 comprising the sloping sides 174 and 176 to be formed through the hill 178. The presplit holes 180, FIG. 5, are drilled along the intersection 182 of the plane of the slope 176 with the surface of the hill 178 and are desired to be parallel. The undercarriage 12 is desirably positioned as shown in FIG. 5 with respect to the intersection line 182 and the feed support 42 is positioned so that the drill rod axis 62 is in the plane of the slope 176. The alignment of the drill rod may be initially set by conventional methods such as with the use of a level protractor or clinometer. The feed support 42 is initially positioned to provide the desired angle of the drill rod axis 62 with respect to the vertical or a plumb line 64, FIG. 1, by manually actuating the control valves 168 and 170 to operate the respective positioning motors 48 and 36.

When the desired direction of the drill rod axis 62 has been predetermined by positioning the feed support 42 the sensing unit 66 is positioned so that the centerline 136, FIG. 7, is coincident with a plumb line. This may be accomplished by releasing the clamping plate 76 and aligning the tubular extension 102 with a suitable indicator such as a clinometer or a plumb line attached to the bracket 72. The pivoted bracket 72 is also aligned so that the centerline 108 of the sensing unit 66 is parallel to the intersection line 182, and therefore also the plane of the slope 176, by actuating the valve 94 to move the cylinder 80. With the sensing unit 66 properly set to have the centerline 136 coincident with a vertical or plumb line, when it is desired to move the feed support from the center position shown in FIG. 5 to drill the additional holes 180 along the line 182, the control valves 168 and 170 are manually actuated along with suitable controls for swinging and extending the telescoping boom portion 30 until the feed support 42 is in the approximate location and attitude for drilling. The sensing unit 66 is then actuated by opening valve 152. If the sensing unit is not in an attitude which places the centerline 136 coincident with the vertical, the weight 116 will tilt to actuate the control circuit as aforedescribed until the positioning motors 36 and 48 have oriented the sensing unit 66 so that the centerline 136 is vertical. Operation of the positioning motors by manually overriding the automatic actuation of the control valves 168 and 170 may be necessary until the bit 58 is spotted over the exact desired hole location. However, the final directional attitude of the feed support to place the drill rod axis 62 in the predetermined direction is assured with the control system disclosed herein. Furthermore, the accuracy of the directional attitude of the feed support for all of the holes 180 enables the drilling of deeper holes and hence the presplitting of deeper cuts without the danger of faulty cleavages or fissures developing in the rock face.

As described in the foregoing paragraphs the sensing unit 66 is aligned so that the centerline 108 is parallel to the intersection line 182 and the plane of the slope 176. This alignment is maintained with sufficient accuracy by the pressure fluid circuit including the cylinders 80 and 84 as the boom is pivoted about the axis 22 and is necessary to assure proper positioning of the feed support by the positioning motor 48. Moreover, in order that the ball valves 114a, 114b, 114c and 114d maintain their proper orientation with respect to the feed support 42 it is necessary to prevent rotation of the sensing unit 66 about the axis or centerline 136. To this end, referring to FIGS. 3 and 4, a slot 186 is formed on the periphery of the housing 68 in a plane through the centerline 136. A cylindrical pin 188 mounted on the bracket portion 72 projects into the slot 186 in a plane perpendicular to the centerline 36 and through the center of the spherical housing 68. The arrangement of the pin 188 and slot 186 provides for movement of the sensing unit in any direction within the envelope of a conical surface of revolution about the centerline 136 as shown in FIG. 3 by the dotted line positions of the tubular extension 102.

It is also not necessary that the rig undercarriage 12 be positioned with respect to the intersection line 182 as illustrated in FIG. 5 to enable the drilling of a series of substantially parallel and coplanar holes. By providing the control valve 94 in the servo circuit 86, 88 the bracket portion 72 may be aligned parallel to the intersection line 182 even though the undercarriage is not oriented as shown in FIG. 5. Some error in the positioning of the bracket 72 in response to pivoting of the boom 18 will be introduced due to the fact that equal linear movement of the piston rods of the cylinders 84 and 80 may not produce equal angular movement of the boom 18 and the bracket portion 72, each with respect to their own pivot axes. However, such errors are of no consequence within the limits of angular movement of the boom 18 about the axis 22 for conventional drill rigs.

It may be appreciated from the foregoing that the directional control system of the present invention may be used to substantially improve the speed and accuracy of drilling a plurality of blast holes not only for presplit or preshearing work but for tunnel face drilling and room and pillar mine excavation as well. Furthermore, the directional control system of the present invention may be advantageously used with virtually any type of drilling apparatus havIng a feed support operable to be pivotally moved about two axes perpendicular to one another.

Claims

What is claimed is:

1. In a rock drill apparatus:

a movable support;

an elongated drill feed support;

pivotal connection means between said feed support and said movable support providing for movement of said feed support with respect to said movable support;

drill motor means including elongated drill rod means having a longitudinal axis, said drill motor means being mounted on said feed support and operable to be reversibly driven therealong;

pressure fluid actuated positioning motor means operably connected to said movable support and said feed support for moving said feed support with respect to said movable support; and,

control means including a direction sensing unit mounted on said feed support for movement therewith and characterized by a weight mounted on said sensing unit for movement with respect to said sensing unit in response to gravitational force acting on said weight, a pressure fluid control circuit for producing pressure fluid signals in response to movement of said weight with respect to said sensing unit, a pressure fluid control circuit including control valves interposed therein and responsive to receiving pressure fluid signals from said first mentioned control circuit for valving pressure fluid to said positioning motor means to move said feed support to maintain said drill rod axis in a predetermined directional attitude in response to movement of said movable support, and fluid operated locking means on said sensing unit for locking said weight to prevent movement of said weight with respect to said sensing unit.

2. In a rock drill apparatus:

a movable support;

an elongated drill feed support;

pivotal connection means between said feed support and said movable support providing for movement of said feed support with respect to said movable support about first and second pivot axes;

drill motor means including elongated drill rod means having a longitudinal axis, said drill motor means being mounted on said feed support and operable to be reversibly driven therealong;

pressure fluid actuated positioning motor means operably connected to said movable support and said feed support for moving said feed support with respect to said movable support about said first and second pivot axes; and,

control means including a direction sensing unit mounted on said feed support for movement therewith and characterized by a weight mounted on said sensing unit for movement with respect to said sensing unit in response to gravitational force acting on said weight, signal producing means for producing signals in response to the movement of said weight with respect to said sensing unit, and a pressure fluid control circuit including control valves interposed therein and responsive to receiving signals from said signal producing means for valving pressure fluid to said positioning motor means to move said feed support about said first and second pivot axes to maintain said drill rod axis in a predetermined directional attitude.

3. The invention set forth in claim 2 wherein:

said first pivot axis is substantially perpendicular to said second pivot axis and said control means is operable to actuate said positioning motor means to pivot said feed support about said first and second pivot axes simultaneously.

4. The invention set forth in claim 2 wherein:

said signal producing means comprises a pressure fluid control circuit including a plurality of pressure fluid valves mounted on said sensing unit and operable to be engaged with and actuated by said weight in response to the movement of said weight to produce a pressure fluid signal for operating said control valves.

5. The invention set forth in claim 4 wherein:

said plurality of valves operable to be engaged by said weight includes at least two valves arranged along a first centerline and at least two valves arranged along a second centerline perpendicular to said first centerline, and said weight is located on said sensing unit with respect to a reference axis substantially perpendicular to said first and second centerlines whereby when said drill rod axis is positioned in said predetermined directional attitude the direction of a resultant gravitational force acting on said weight is substantially coincident with said reference axis.

6. The invention set forth in claim 5 wherein:

said feed support includes a mounting bracket for mounting said sensing unit in a plurality of positions whereby said reference axis may be positioned in a predetermined direction with respect to said drill rod axis.

7. The invention set forth in claim 6 wherein:

said rock drill apparatus includes a frame, said movable support being pivotally mounted on said frame;

said mounting bracket for said sensing unit is pivotally mounted on said feed support; and,

said rock drill apparatus includes means for controlling the position of said sensing unit in response to pivotal movement of said movable support with respect to said frame.

8. The invention set forth in claim 7 wherein:

said means for controlling the position of said sensing unit is operable to position said sensing unit to maintain one of said centerlines in a plane substantially parallel to a plane through said drill rod axis.

9. The invention set forth in claim 2 wherein:

said positioning motor means includes a first pressure fluid actuated positioning motor for pivoting said feed support about said first pivot axis and a second pressure fluid actuated positioning motor for pivoting said feed support about said second pivot axis.