US2660404A - Control for pneumatic cable-tool drills - Google Patents

Description

Nov. 24, 1953' J. H, MEIER CONTROL. FOR: PNEUMATIC CABLE-TOOL DRILLS Filed April 10, 1948 5 Sheets-Sheet l JOHANN HANs MEIER,

INVENToR,

ATTOENEA Nov. 24, 1953 J, H, MEER 2,660,404

CONTROL FOR PNEUMATIC CABLE-TOOL DRILLS Filed April l0, 1948 5 Sheets-Sheet 2 JOHANN HANS /vrz/lz'k,`

INVENTOR,

BY MQ 96 MM ATTORNEY Nov. 24, 1953 J. H. MEIER 2,660,404

CONTROL FOR PNEUMATICI CABLE-TOOL DRILLS Filed April 10, 1948' 5 Sheets-Sheet 3 JOHANN HANS 4E/ER IN V EN TOR,

A W W ATTORNEY NOV. 24, 1953 v J, H, MEER 2,660,404

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BY WW ATTORNEY.

O V JOHANN HANS Mania,

Patented Nov. 24, 1953 UNITED STATES PATENT @dd-i OFFICE CONTROL Foa PNEUMATIC CABLE-Toor. DRILLs Application April 10, 1948, Serial No. 20,277

(Cl. Z55-10) 10 Claims.

My invention relates to new and useful improvements in the operating controls and valve mechanism of pneumatic-type cable-tool drills.

In such drills, a rope passes from a winch drum on the main frame, around a sheave at or near the rear end of the machine, thence under another sheave at or near the front end f the machine, behind the foot of a mast, thence over a crown sheave near the upper end of the mast, and thence downward into the hole which is being drilled in the ground. At the lower end of this rope is a string of percussion drilling-tools, terminating in a bit. The drilltools at the end of the rope are reciprocated up and down by oscillation of the crown sheave which is slidably mounted in the mast and actuated by a pneumatic cylinder-piston assembly.

A principal object of my invention is to provide a valve mechanism for controlling the intake and exhaust of air (or gas) to and from the piston chamber such that:

(1) The length of stroke or" the piston which actuates the crown sheave is adjustable to suit diierent drilling conditions.

(2) Exhaust of the air from the chamber begins automatically at the ktop of the stroke regardless of the length at which the stroke is set, and regardless of the exact actual length of any given stroke.

(3) Exhaust of air from the chamber ceases automatically at a predetermined point near the lower end of the down stroke shortly before the bottom of the stroke is reached, regardless of the actual or setlength of the stroke.

(4) Intake of air into the chamber occurs automatically at the lower end of the down stroke a predetermined distance after exhaust ceases and before the bottom of the stroke is reached, regardless of the actual or set length of the stroke.

(5) Intake of air into the chamber automatically ceases whenever the piston falls a predetermined distance below the normal lower end of its stroke.

A further object of my invention is to control piston speed by controlled air intake or exhaust.

In addition to my principal objects, above stated, I have worked out a number of novel and useful details, which will be readily evident as the description progresses.

My invention consists in the novel parts and in the combination and arrangement thereof, which are defined in the appended claims, and

of which two embodiments are exemplified in the piston-sheave assembly.

Figure 5 is an enlarged front elevation, partly in section, of the lower section of the mast of my drill.

Figure 6 is an enlarged front View, partly in section, of the commutator assembly that controls the valve mechanisms of my drill.

Figure '7 is a section, taken along the line 'l-i of Figure 6, showing the reversing switch A for my intake-exhaust valve control.

Figure 3 is a chart showing the operation of each commutator switch and valve of my first embodiment throughout the piston cycle.

Figure 9 is a wiring diagram showing the electrical circuit for controlling the valve mechanisms of my rst embodiment (commutators shown in starting position) Figure 10 is a chart showing the operation of each commutator switch and valve of my second embodiment throughout the piston cycle.

Figure 11 is a wiring diagram, showing the electrical circuit for controlling the valve mechanisms o my second embodiment (commutatore shown in starting position) Turning now to Figure 1, we see that Il is the main frame of a drill, supported by creeping traction I2.

The frame li supports a mast Id (called derrick in the art), a motor i5, a winch I6, an air compressor Il, and associated driving connections, not numbered.

From the winch I6, a rope I8 runs around spooling sheave i9, thence under fairlead sheave 2li, thence over crown sheave 2i at the top of the mast l, thence down to the tools 22.

Turning now to Figure 2, We see that crown sheave 2| rotates about pin 23 mounted in crown-sheave frame 2li which is slidably mounted for vertical movement within upper section 25a of the mast and is rigidly connected to the 3 ilid of piston rod 26 of cylinder-piston assembly Cylinder-piston assembly 27 consists of cylinder 2c, rigidly mounted within section 25h of mast H, and main piston 29 to which rod 29 is connected. The supply of air in lower chamber 36 is controlled through intake valve 52 and exhaust valve 33. Air pressure is supplied to intake valve 32 through main supply line 35 from air compressor Il'. Upper chamber 3| is maintained normally at near atmospheric pressure by vent t. The area of this vent is preferably restricted to provide throttling or buffer action if main piston 29 over-speeds because of too light tools or breakage of the tool line. Vent 34 is located in the wall of cylinder 23 below the head of the cylinder, so that a dash-pot is formed to act as a buer in case the main piston 29 overtravels at the top of the stroke.

To dissipate the energy of the tools when dropping freely from full stroke without striking the bottom of the well, a pneumatic shock absorber is preferably employed at the lower end or" the cylinder 28. This shock absorber consists of two pistons 35h, in tandem and preferably of the same diameter as the `main piston 29, supported by air pressure from main supply line 35. Rubber bumper 3l between main piston 29 and upper shock-absorber piston 39a provides resilience required to accelerate the shock-absorber pistons. The details of this shock absorber form no part o my present invention.

intake and exhaust valves 32, 33 (Figure e) are preferably conventional magnet-controlled valves with solenoids so arranged that the valves are open when the solenoids are not energized (this insures shut-down of machine in case of failure of the electric circuit). They are controlled by the following described automatic mechanisi As crown-sheave-frame 25 moves up and down, it reciprocates drive chain 3B (Figure 2), which in turn drives sprocket 39, worm gear 55, worm wheel 5|, and valve-control shaft 42. Sprocket 39, worm gear si) and worm wheel 5| are of such size that shaft l2 will turn less than 350 degrees in one stroke. In the preferred arrangement, the shaft 52 turns approximately 180 degrees in a maximum normal stroke of the main piston, about degrees additional rotation at one end of the cycle being allowed for cvertravel at the top of the stroke, and about 90 degrees at the other end for overtravel at the bottom of the stroke. Valve control shaft d2 drives commutator shaft d3 of the commutator assembly 44 (Figures 5 and 6) which is carried within lower section 25h of mast lli.

The commutator assembly ifi (Figure 6) controls the electrical circuits which operate the intake and exhaust valves and the feed mechanism (hereinafter described). It consists o1" a main body @5 in which commutator shaft i3 is journalled. Keyed to commutator shaft d3 are commutatore d5 (main line), 41 (intake-exhaust), 48 (exhaust), i9 (cut-oir), 59 (over-travel), and 5| (feed), and friction plate 52 (reverse). A portion of the periphery of each commutator is provided with a commutator bar of conducting material, the peripheral length of each bar being determined by the portion of the cycle during which it is desired to pass current through (i. e., to close) the commutator switch. The remainder of the periphery of each commutator is of nonconducting material. The commutator bars M6, |41, |48, |59, |59 and |5| are mounted respectively on commutators 46, 41, 48, 49, 50 and 5|,

and are shown in Figure 9 in their proper rela-v tive angular positions. Mounted on rings within the main body d5 are corresponding brush holders 56 (main line), 57 (intake-exhaust), 58 (exhaust), 59 (cut-off), 69 (overtravel), and 6| (feed), and terminal segments 62a, 62e (reverse). Brush holders 59 and 69 and terminal segment 62e are connected by conductor wires (not shown in Figure 6, but shown schematically in Figure 9) to the intake valve 32. Brush holder 58 and terminal segment 62e are similarly connected to the exhaust valve 33; and brush holder 6| to the feed mechanism (see Figure 9). Brush holder 59 (cut-off) is mounted so that its angular position relative to the other holders can be adjusted manually by cut-oit chain 53 which is operated by hand lever :'54 and drives the cut-oli ring 19 in which cut-off brush holder 59 is mounted. Similarly the relative angular positions of brush holders 56, 5l, 58, 50 and 6| can be adjusted by means of set screws 66, 6l, 68, i9, and 1| respectively which nx the angular positions of rings 16, T1, '18, 80, and 8| respectively, in which the brush holders are mounted. Commutator bars |41, |48, M9, |59, and |5| are connected to shaft 43 in a conventional manner by running conductor wires Sl, 95, 99, |99, and |61 from the commutator bars to their adjacent spacers 81, 89, 89, and 9| respectively. Shoe 53 is connected to brush holder 5i of commutator fil by conductor wire |51, and is insulate-d from shaft (i3 by nonconducting friction plate 52 on which it is mounted.

Figure 8 shows schematically the action of each commutator switch and the intake and exhaust valves for all positions of the main piston for both down and up strokes.

Figure 9 shows the wiring diagram of the commutator assembly (commutatore shown in starting position), the valves and feed mechanism which it controls being represented schematically.

(l) Main-Zine commutator 46.-This conducts current from the main line to the commutator shaft 43 and thence through the spacers to the commutator bar of each commutator for all positions of rotation of the shaft. Thus I avoid passing current through the bearings of the shaft.

(2) Intake-exhaust commutator 47.-()n the greater part of the down stroke, this commutator keeps closed the circuit from the main line to the intake valve 52, thereby keeping valve 32 closed. When the main piston 29 descends to a point B shortly above normal starting position S (from rest) at the lower end of the stroke, commutator :il opens the circuit from the main line to the intake valve 32, thereby opening valve 32 and thus building up pressure for the upward stroke. Opening the intake valve before the upward stroke actually starts, prevents lag in pressure build-up that would occur if the valve were opened exactly at the bottom of the stroke, causes fast reversal of the piston assembly, and limits overtravel of the piston assembly after tool impact for fast takeup of the rope |B.

On the up stroke, this commutator is switched automatically from the intake to the exhaust control circuit, by the reversing switch B2b-62e (hereinafter described), and hence operates to keep closed the circuit from the main line to the exhaust valve 33, thereby holding that valve closed after the main piston 29 rises above the above-described intake opening point B, and taking over control of the exhaust-valve circuit from the exhaust commutator 43 (hereinafter described).

(3) Exhaust commutatorASF-On the greater part of the downward stroke, this commutator keeps open the circuit from the main line to the exhaust valve 33, thereby keeping valve 33 open. When the main piston 29 is at a point A a predetermined distance above the intake point B, commutator switch 48 closes the circuit from the main line to the exhaust valve 33, thereby closing valve 33, and holds it closed until the intake point B is reached again. Between points B and A on the upward stroke both commutator switches fil and 48 are closed, commutator switch 4l having been cut into the exhaust-valve circuit by reversal switch B2b-62e at point B, so that between points B and A exhaust-valve 33 is held closed by current that flows through both commutator switches M and 8 which are in parallel. When point A is reached on the upward stroke commutator switch i3 opens, so that thereafter on the upward stroke only commutator switch il is closed and holds exhaust-valve 33 closed.

(4) Cut-01j' commutator 49.-This closes the circuit from the main line to the intake valve 32, thereby closing this valve, after the main piston 29 rises to point C a predetermined height (determined by the manual setting of adjustable brush holder 59), and thereby establishes the average length of the stroke, which actually extends beyond the cut-olf point due to overtravel that occurs with normal air expansion in the lower chamber 30.

(5) Overtraoel commutator 5).-This closes the circuit from the main line to the intake valve 32, thereby closing it, when the main piston 29 descends past the intake valve port. This prevents loss of air through vent 3d of the upper chamber, and also prevents build-up of pressure above the piston.

(6) Feed-commutator 51 .-This closes the circuit from the main line to the feed mechanism which, when thus energized, causes winch i@ to feed out a predetermined length of rope it, whenever main piston 29 descends below its normal reversal point D at the lower end of the stroke as the hole grows deeper. The feed mechanism is indicated symbolically in Figure -9, since it forms no part of the present invention, and is fully described in my copending application, Serial No. 613,843, led September 27, 1945, now Patent #2,587,638.

(7) Reversing switch 62ab-c.-Keyed to commutator shaft d3 is a nonconducting friction plate 52. Spring shoe 63, which is preferably, though not necessarily in the form of a ring, has a free fit on friction plate 52, and carries a moveable contact arm B2b. Accordingly, when the coinmutator shaft Q3 rotates in one direction corresponding to the down stroke of main piston 29, arm B2b bears against terminal segment ilia, thereby closing the circuit between the brush holder El of intake-exhaust commutator l'l and the intake valve (see Figure 7); and when the commutator shaft 43 rotates in the other direc-` tion corresponding to the upward stroke of main piston 29, arm 62h bears against terminal segment 62e, thereby closing the circuit between the brush holder 5l' of the intake-exhaust commu- Ato lby virtue ci cut-orf conimutator fle.

6 in the piston cycle, close or open the electrical circuits that control the intake and exhaust valves. Analogously the spring shoe 63 and its contact arm Sith together constitute another shiitable member which, by virtue of the free nt of shoe 53 on friction plate 52, which is driven by the piston, has a connection to the friction plate that constitutes an always-set doub e-acting slip-drive coupling. Accordingly, when the piston moves in either direction, this shiftable member is driven into a position that will close a valve-control circuit, and is maintained in that position by continued movement of the piston in the same direction. A change of velocity of the piston in the form of a reversal of direction of velocity will cause the shiftable member to be drive-n in the opposite direction into a position that will close another valve-control circuit. Accordingly, the word velocity, as used in the claims, includes not the speed but also the direction of the motion of the piston.

With the main piston in starting position (from rest) as shown in Figure 2 (point S in Figure 8) the cycle of operation is then as follows: The main piston being initially at rest below the point A (Figure 8) at which the exhaust valve 33 is closed and below the point B at which the intake valve iii is opened (position of reversalswitch arm 52?) being immaterial), pressure is built up in lower chamber 3@ forcing the piston upward. As soon as upward motion commences, contact between arm d20 and segment 62a is broken, so that closing of the circuit atr intakeexhaust cominutator il cannot close the intake valve 32. Arm 52o then makes contact with segment 62o, thereby closing the circuit to the eX- haust valve through intake-exhaust commutator lil before the circuit to the exhaust-valve through exhaust commutator i3 isfbroken at point A. When cut-off point C is reached, intake Valve 32 is closed by cut-ofi commutator i353. The cut-olf point C has been predetermined by setting brush holder 59, and determines the average length of the stroke. Upward motion of the piston continues for some distance against line pull, and eventually pressure in the lower chamber falls below balancing pressure and at some time thereafter reversal occurs. Upon reversal of piston motion at the top the stroke, contact between arm @2b and segment is broken and exhaust valve 33 opens, the intale valt-e remaining closed The changeover at reversal switch. a-b-c occurs always upon reversal of motion regardless of length of the stroke which varies with cut-off point and drilling conditions. After some further downward motion, contact is established between arm 62h and segment This sets the circuit through intake-exhaust cornmutator il to take over control of intake valve i2 when piston 29 drops to point C where contact through cut-off commutator it is broken. The piston now drops freely with the tools until it passes points A and B. At point A, the exhaust valve 33 is closed and at point E, the intake valve 32 is opened. If, due to downward progress of the tools in the well, piston 29 descends past its normal reversal point to D at the lower end of the stroke, the feed mechanism is energized automatically to feed out more rope. Due to downward momentum, the piston may descend some distance below point D. Such motion is resisted by the emergency shock absorber 36 (above described), and if overtravel occurs past point E (when main piston 29 is entirely below the intake port), the intake Valve 32 automatically closes as a safety measure to prevent possible build-up of, pressure in upper chamber 3| (upper vent 34A being usually of limited capacity). Upon reversal the reversing switch throws the intake-exhaust commutator over into the exhaust line (see Figure 9) and the upward stroke commences as at the start of the cycle.

Although I prefer to use power-closing intake and. exhaust valves, as above stated, it is clear that this is not essential, and that power-opening valves may be used with a modied control circuit, as shown in Figure 11. The operation of the commutator switches and valves for this embodiment is shown in Figure 10, the commutator switches being now open (instead of closed) when it is desired to close the valves which they control. It will be noted that in this second embodiment the overtravel commutator is eliminated, since its function can be taken over by the intake commutator which is no longer connected to the reversing switch.

It will be noted that, due to the fact that the intake and exhaust valves in the embodiment of Figure 11 are power-opening instead of powerclosing valves, the circuit diagram of Figure 1l differs from the circuit diagram of Figure 9 in the following respects: (l) the reversing switch 62a'b'-c' is set to close the intake control circuit on the upstroke (instead or on the downstIOke), and is set to close the exhaust control circuit on the downstroke (instead of on the upstroke), (2) the exhaust and cut-oit commutators 48' and 49 are now connected in series with reversing switch 62ab'c (instead of in parallel), and intake-exhaust commutator 41 is in parallel with the reversing switch and cut-off commutator (instead of in series with the reversing switch), and (3) the commutator bars of commutators 41', 48 and 49 are respectively of the same peripheral length as the open-circuit portions of the corresponding commutators G1, 48 and 49 of Figure 9. When this wiring circuit is used, the same end results are produced as in the embodiment of Figure 9, as may be seen by comparison of the valve-closed positions illustrated in Fnfures 8 and 10. Since, in the circuit of Figure ll, neither of the poles of commutator switches 48 and 49 are grounded, commutator bars |48 and |49' are not grounded (as were bars |48 and |49 in Figure 9) ,but make and break a, circuit between pairs of brushes ESL-|58 and 59'| 59' respectively which are shown schematically in Figure 11.

In addition to the above-described valve mechanisms for controlling length of stroke by means 'of variable cut-off, I also employ means to control downward speed of the piston, this means vconsisting of throttle 83 at exhaust port 82 operated by a conventional mechanical remote control (not shown). For control of upward Speed and also length of stroke, I use the air pressure control (not shown) on the air compressor il.

Although I have described my invention operated by compressed air, other gases such as steam can be used satisfactorily.

Having now described and illustrated two forms of my invention, I wish it to be understood that my invention is not to be limited to the specific form or arrangement of parts herein described and shown.

Iclaim:

l. In a pneumatic cable-tool drill, the combination of: a bit; pneumatic spudding vmeans having a cylinder-piston assembly supporting said bit to raise and drop said bit; a piston and a pressure chamber in said cylinder-piston assembly, said chamber having an intake port and an exhaust port; a valve to open and close said intake port; control means to control the intake valve; a reciprocating element constrained to move with the piston during spudding; driving means actuable by said reciprocating element to alter the setting of the control means; said control means including a member shiftable to open and close said intake valve and a driving connection between said driving means and said shiftable member, said connection operating to set the shiftable member in valve-closing position when the piston reaches a predetermined overtravel position oi said piston beyond the position of normal reversal from bit-lowering to bitraising travel of said piston.

2. In a pneumatic cable-tool drill, the combination of: a bit; pneumatic spudding means having a cylinder-piston assembly supporting said bit to raise and drop said bit; a piston and a pressure chamber in said cylinder-piston assembly, said chamber having an intake port and an exhaust port; control means to open and close said exhaust port; said control means including an electrically-actuated exhaust valve and an electrical circuit with switch means to control said valve; said switch means being responsive to reversal of travel of said piston and including a pair of xed terminal segments, a reciprocating member driven by said piston, a friction-actuated shoe having a friction surface in slipping contact with said reciprocating member, and a contact member mounted on said shoe in such a position that said contact member engages one of said terminal segments thereby closing the exhaust valve circuit when said piston travels in one direction, and engages the other of said terminal segments thereby opening said circuit when said piston travels in the opposite direction.

3. In a control for a pneumatic cable-tool drill having: a bit; pneumatic spudding means having a cylinder-piston assembly supporting said bit to raise and drop said bit; a piston and a 4pressure chamber in said cylinder-piston assembly, said chamber having associated with it a huid-pressure intake port and an exhaust port; an exhaust valve for said exhaust port and means to open and close said exhaust valve; and an intake valve to open and close said intake port; the combination therewith of: means adapted to open and close said intake valve; said last-mentioned means comprising: first, second and third valve control means each having portclosing and port-opening positions to effect closing and opening of said intake valve; rst pistonposition responsive means connected to and 0perated by said piston to actuate said first control means to assume a port-opening position when the piston reaches a position in advance of its normal position of reversal from a bitlowering to a bit-raising stroke; second pistonposition responsive means connected to and operated by said piston to actuate said second control means to assume a port-closing position When the piston reaches a predetermined intake cut-oi position on its bit-raising stroke; pistonvelocity responsive means connected to and operated by said piston to actuate said third control means to assume a port-opening position at said reversal, independent of the piston position at which such reversal occurs; rst interlock means connecting said first and third control means, said rst interlock means providing for dependent operation of said first and third control means with respect to their port-closing function to out off intake of huid-pressure only during the portion of the bit-lowering stroke of the piston ahead of said advance position, and for independent operation of said iirst and third control means with respect to their port-opening function to effect fiuid pressure intake during the portion of the bit-lowering stroke of the piston past its said advance position and during its bit-raising stroke; and second interlock means connecting said second-and third control means,. said second interlock means providing for independent operation of said second and third control means with respect to their portclosing function to cut oi intake of fluid pressure at said cut-off position of the piston on its bit-raising stroke.

4. In a control for a pneumtatic cable-tool drill having: a bit; pneumatic spudding means having a cylinder-piston assembly supporting said bit to raise and drop said bit; a piston and a pressure chamber in said cylinder-piston assembly, said chamber having associated with it a fluid-pressure intake port and an exhaust port; an exhaust valve for said exhaust port and means to open and close said exhaust valve; and an intake valve to open and close said intake port; the combination therewith of: adapted to open and close said intake valve; said last-mentioned means comprising: first, second and third valve control means each having portclosing and port-opening positions to eiect closing and opening of said intake valve; pistonposition responsive means connected to and operated by said piston to actuate said iirst control means to assume a port-opening position when the piston reaches a position in advance of its normal position of reversal from a bitlowering to a bit-raising stroke; second pistonposition responsive means connected to and operated by said piston to actuate said second control means to assume a port-closing position when the piston reaches a predetermined intake cut-01T position on its bit-raising stroke; pistonvelocity responsive means connected to and operated by said piston to actuate said third control means to assume a port-opening position upon said reversal of the piston independent of the piston position at which such reversal occurs; and interlock means connecting said rst and third control means, said interlock means providing for dependent operation of said first and third control means with respect to their port-closing function to cut oi intake of iiuid pressure only during the portion of the bit-lowering stroke of the piston ahead of said advance position, and for independent operation of said iirst and third control means with respect to their port-opening function to effect fluid pressure intake during the portion of the bit-lowering stroke of the piston past its said advance position and during at least a portion of its bitraising stroke.

5. In a control for a pneumatic cable-tool drill having: a bit; pneumatic spudding means having a cylinder-piston assembly supporting said bit to raise and drop said bit; a piston and a pressure chamber in said cylinder-piston assembly, said chamber having associated with it a fluid-pressure intake port and an exhaust port; an intake valve for said intake port and means to open and close said intake valve; and an ex- `haust valve to open and close said exhaust port;

means the combination therewith of; means adapted to open and close said exhaust valve; said lastmentioned means comprising: first and second valve control means each having port-closing and port-opening positions to eiect closing and opening of said exhaust valve; piston-position responsive means connected to and operated by said piston to actuate said iirst control means to assume a port-closing position when the piston reaches a position in advance of its normal position of reversal from a bit-lowering to a bit-raising stroke; piston-velocity responsive means connected to and operated by said piston to actuate said second control means to assume a port-opening setting when the piston reverses from a bit-raising to a bit-lowering stroke independent of the piston position at which such reversal occurs; and interlock means connecting said first and second control means, said interlock means providing for dependent operation of said first and second control means with respect to their port-opening function to exhaust said fluid pressure only during the bit-lowering stroke of the piston ahead of said advance position, and for independent operation of said rst and second control means with respect to their portclosing function to prevent fluid-pressure exhaust during the bit-lowering stroke of the piston past its said advance position and during its bit-raising stroke.

6. A pneumatic cable-tool drill according to claim 5, further characterized by the fact that the piston-velocity responsive means includes: a first member shiftable into port-opening and port-closing positions to actuate the second control means into corresponding positions; a double-acting drive connection between the piston and said first member; and a second member having a xed support, and engaged by and having a constraining connection with said rst member when the first member assumes one of its positions; one of said connections being a slipping connection so arranged that said first member assumes a port-closing position in response to motion of the piston in one of its directions of travel and a port-opening position in `response to motion of the piston in the other of its directions of travel.

'7. A pneumatic cable-tool drill according to claim 5, further characterized by the fact that the exhaust valve is an electrically-actuated valve having an electrical circuit adapted to openv and close said exhaust port; that said pistonvelocity responsive means includes a reciprocating element constrained to move with the pistonv during spudding, driving means actuable by said reciprocating element, and a friction-actuated shoe having a friction surface in slipping contact with said driving means; and that the second control means includes a switch in said circuit, said switch having a circuit-closing position and a circuit-opening position and being actuable by said shoe to assume a position to close said circuit when the piston travels in one direction and to open said circuit when the piston travels in the opposite direction.

8. A pneumatic cable-tool drill according to claim Li, further characterized by the fact that the intake valve is an electrically-actuated power-closing valve having an electrical circuit adapted to open and close said intake port, and lthat the interlock between the first and third control means is a series electrical circuit.

9. A pneumatic cable-tool drill according to claim 5, further characterized by the fact that References Cited in the le 0f this patent UNITED STATES PATENTS Number Name Date Storer July 17, 1883 Ongley June 2, 1896 Engberg Apr. 27, 1897 Swan Apr. l1, 1903 Klay Oct. l0, 1905 Kellum Sept. 17, 1912 Farquhar Feb. 24, 1925 Pearsall Feb. 2l, 1933 Peyton Nov.- 5, 1940

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