US1831437A

US1831437A - Drill control system

Info

Publication number: US1831437A
Application number: US330508A
Authority: US
Inventors: John E Brantly
Original assignee: DRILLING AND EXPLORATING COMPA
Current assignee: DRILLING AND EXPLORATING COMPA; DRILLING AND EXPLORATING COMPANY Inc
Priority date: 1929-01-05
Filing date: 1929-01-05
Publication date: 1931-11-10
Anticipated expiration: 1948-11-10

Description

Nov. 10, 1931. J. E. BRANTLY 1,831,437

DRILL coNTRoL SYSTEM Filed Jan. 5. 1929 2 Sheets-Sheet l Nov. l0, 1931. J. E. BRANTLY DRILL CONTROL SYSTEM 2 Sheets-Sheet 2 Filed Jan. 5, 1929y "Patented Nov. l10, 1 931 UNITED STATES Joan n. BRANTLY, or BIRMINGHAM ALABAMA, `Ass'rerton. 'ro D BILLING Am) nxv rLoaATIoN coMPANY,1Nc., or novnn, DELAWARE, A conronATIoN or DELAWARE' DRILL CONTROL SYSTEM Application mea January s, 1929. serial No. aaasos.

My inventionrelates to a method of and apparatus for feed control, and more particularly to a hydraulic feed control for rotary drillin or boring apparatus, such as utilized 5 in dril ing oil, gas or Water wells in the earth.

In accordance with my invention, a control is effected in response to increase beyond a predetermined value in pressure on the drill ing bit, independently of the torque on the o drill plpe, whereby descent of the drill is halted until the pressure on the bit has decreased to Within its normal range; and further, the rate of feed of the drill is continuous during normal pressureon the bit and 5 is independent of the torque on or exerted by the drill pipe.

. Further in accordance with my invention, the weight of the drill shaft or pipe to which bit or' boring tool is secured is borne par- ZO tially by feed control apparatus for regulating the rate of descent of the drill, and the aforesaid feed control apparatus apportions, for a given length 0f drill pipe, the weight of the pipe between the control apparatus and the bit rest-ing on the bottom of the bore.

Further in accordance with my invention, the feed control apparatus comprises one or more hydraulic cylinders having inletand discharge valves controlling the substantially which a piston or pistons operatively associated with the drill pipe is or are ada ted to reciprocate; and more particularly, t e discharge valves communicate with a valve for regulating the rate of discharge of fluid from the cylinder or cylinders and with a second valve in series therewith'for regulating the 'pressure at which discharge of fluid from the cylinder or cylinders Will take place.

Further and more particularly in accordance with my invention, the` entire Weight of the drill pipe is adapted to be connected, through suitable mechanism, to the aforesaid piston or pistons which operate as a hy- 5 draulic governor or brake against the weight of said drill pipe, automatically to support a predetermined part of the Weight of said drill pipe, the remaining weight resting on the drill bit 'in the bottom of the bore, in accordance with the pressure at which the presnon-compressible motive or control fluid, in,l

operating mec anism for a drill.

plan View of feed control and Fig. 2 1s an elevational view, partly in sec tion, of a hydraulic feed control apparatus.

Fig. 3 is an'enlarged detailed View, partly in section, of apparatus shown in Fig. 2.

In boring holes or wells to great depths, as in drilling for petroleum, etc., very serious problems have been encountered due to the great Weight of the drill shaft, which generally comprises a string of drill rods, and also to the fact that as the drill descends the rate of drilling will vary to a certain extent, de-

pending upon the varying resistance of the subterranean strata encountered. In other words, as the drill descends it will tend to drill rather rapidly through soft shales, etc. While its rate will be considerably dimimshed in drilling through hard rock and equivalent formations. Accordingly, for a fixed drilling rate, the pressure on the bottom of the bore will tend to vary to a'considerable extent in the absence of control apparatus, and in particular will increase to an undesired value when the drill is slowed up, as by hard rock formations, unless the rate of feed is concurrently a-nd c rre'spondingly diminished.

The result of excesslve pressure on the 'bit in thebottom of the bore may cause overloading of the drilling motors, involving possible damage to the drilling equipment, and if the formation through which the' drill is passing is soft, may also cause balling up of the bit, due to the fact that the excessive drilling rate andpressure for that particular formation prevents the loosened mate- VA'et y rial from bein properly removed from the bottom of the ore. l Automatic regulation of the rate of boring 1n accordance with the pressure exerted bv 5 the boring tool upon the bottom of the well has previously been attained to a certain extent by differential feed control mechanism,

of this method of feed control is that the drilling pressure must have increased tov such value that the consequent overload on the drilling motor causes it to slow up before the control may be effected. ln other words. there is a time lag between the occurrence of excessive drilling pressure and the control effected to correct the same which might prove disastrous even to the point of twisting off the drill pipe before the excessive pressure was relieved.

It follows, therefore, that the distinctive. feature of the differential lcontrol method` outlined above is'that the control is effected only in an indirect sense by the pressure upon the drilling bit. In other Words, the control is effected substantially by the increased undesirable feature since itis often necessary to maintain a uniform rate offeed notwithstanding a substantial increase in torque on the drill pipe. vFor example, in drilling- 40 throufh tough clays or gumbo it is' desirable t ata uniform rate of drilling be maintained, but since the torque on the drill ipe is increased substantially while dril ing through this material, it is apparent that con- .trol apparatus of the differential type would slow up the drilling considerably' .Another method of feed control utilizes hydraulic means for lowering the drill pipe in short steps'at a predetermined rate. T ie disadvantages of this method are at once apparent, since the operation is neither continuous nor does it automatically control the pressure on the drilling bit. Furthermore, the drilling necessarily progresses at a slower rate due to the fact that a new adjustment of the'hydraulic feed control with respeotto the drill pipe must be made for each step of its descent.

My invention comprehends apparatus for v 00 maintaining the pressure on the bit substantially constant at all timesindependently of the 'torque on the drill pipe and to this end a certain part of the weight of the drill pipe i-s borne directly or indirectly by thefeed con- ,05 trol apparatus in accordance with a predetertorque on the drill pi e. This, in itself, is an value, it follows that the feed control apparatus is not supporting its share of the weight of the drill pipe. In response to this decrease in weight upon the control apparatus, further descent of the drill is halted until the control apparatus again picks up its normal share of the drill pipe weight.

It is clear therefore, that as long as the weight on the bit does not exceed a predetermined amount, the drilling will proceed at a substantially uniform rate're ardless of the variations in torque on the drill pipe.

Referring to Fig. 1, a source of power M, comprising a driving motor of any suitable type, asy electric, gas or steam, is suitably mounted upon base structure 1 and is adapted to be energized froml a source of supply (not shown). Motor M comprises a shaft 2 having mounted thereon a sprocket wheel or gear 3 connected to a companion sprocket wheel 4 by the sprocket chain 5. 'Wheel 4 is keyed or otherwise suitably secured to a line shaft 6 having supporting bearings 7, 8 and 9. A Y

sprocket wheel or gear l0 is freely mounted upon shaft 6 and is in positive connection with a sprocket wheel 11 secured to a shaft 12 through the sprocket chaini 13. The shaft 12 is rotatably mounted Within bearings it and 15, and has mounted thereon a `pinion (not shown) for engaging a cooperating ring gear (not shown) for effecting rotation of the rotary table 16. The construction ot rotary tables being well known, it is only necessary to state that it engages the drill-pipe (not shown) extending centrally thereof to eitectv rotation of the same for drilling purposes. Operative connection between line shaft and wheel 10 is effected by throwing a clutch 17, which coo crates with shaft 6, into engagement` wit 1 a cooperating member 17a mounted on the hub of wheel 10. Clutch 17, together with its co-operating member. may comprise what isknown as a-clutch quill unit, and when closed, as illustrated, affords a direction driving connectionl between motor M and rotary table 16. Or clutch unit 17-17a may com rise any suitable type of jaw or friction c utch.

A drum shaft 18 is rotatably mounted Within bearings 19 and 20. and has mounted thereon a drum 21 comprising a spool-like member upon which a 'cable (not shown) for supporting the drill pipe may lm nound. Drum 21 is fixed with respect to shaft TS. and is operatively connected to line-' shaft..v 6 through either sprocket wheel 22er 23. procket wheel 22 is adapted to rotate with 24 being of the type previously described,

and is connected to a companion sprocket Y wheel 25 secured to line shaft 6.by the sprocket chain 26. In the same manner, sprocket wheel 23, being of smaller diameter, 1s operatively connected to shaft 18 through. clutch 27, and is connected through sprocket chain 28 to lthe sprocket wheel 29 secured to shaft 6. Accordmgly, by closing either clutch 24 or 27, which in the present instance are shown in open position, it is possible to rotate drum 21 at two different speeds by a motor M to raise or lower the drill pipe.

Drum shaft 18 has also mounted thereon a sprocket wheel 30 adapted to rotate with shaft 18 through clutch mechanism 3l, shown as closed in the lpresent instance. Wheel 30 is connected to a companion sprocket wheel 32 by a sprocket chain 33 for a purpose hereinafter described.

Feedcontrol apparatus for regulating the rate of rotation of drum 21, and consequently the descent of the drill pipe, comprises a base member 34 upon which hydraullc governors 35 and 36 are mounted. A crank shaft 37 mounted Within bea-rings 38 vand 39 carried by the base 34 cooperates with connecting rods 4() to effect reciprocation of the salne upon rotation of the crank shaft.

The above described apparatus may be associated with a derrick, or equivalent struc ture, in a manner Well known in the art.

' Referring now more particularly to Fig.

, 2, wherein feed control apparatus comprising a single hydraulic Vgovernor or brake 35 is shown, connecting rod 40 is secured in a wellknown manner to a cross-head 41 adapted for reciprocation within a cross-head guide 42. Secured to the opposite side of crosshead 41 is a piston rod 42 having mounted thereon piston 43. Piston 43 isadapte'd t0 reciprocate within a cylinder 44 having endplates 45 and 46 suitably secured thereto, the

end plate 46 havingfa bore centrally thereof v in registry with a packing gland47 for receiring piston rod 42. Piston 43 may be of any suitable construction, and in the present instance comprises a plurality of plates 43a, to the periphery of which is secured an annular member 48 of suitable material serving to provide a snug fit between the piston and cooperating cylinder..wal1s.y Piston 43 is maintained in position on rod 42 by clamping nuts 49 threaded to rod 42 and engaging piston 43 on opposite sides thereof. Cylinder 44 is provided with

inlet passages

50 and 51. at opposite ends thereof, said passages adapted to communicate through valve structure, hereinafter described, with a reservoir 52 containing a substantially non-compressi ,ble fluid, aswater, for example.

Reservoir 52 communicates through pipe 53 with the interior of valve housing 54 having mounted therein a pair of intake valves 55l and 56 adapted to move in unison through va common actuating member 57. Valves 55 and 56 may be'of any suitable type, and in the present instance comprise tapered plug members, having their seating surfaces facing each other, mounted upon a common valve stem 58, which is guided for reciprocal motion within the end guides 59 comprising a part of the end closure members or plugs 60, which also serve to provide access to the valve structure.` Valve housing 54 is provided with conical valve seats 55a and 56a for cooperating with valves55 and 56 respectively, and

comprises an inlet passage 54a for admitting fluid past valves 55 and 56 to the

inlet passages

50 and 51. Accordingly, fluid from the supply conduit 53,. which is admitted to passage 54a between valves 55 `and 56, may flow by gravity into cylinder 44 through either

Vpassage

50 and 51, depending upon the position of valves 55 and 56. The common valve i' actuator 57 extends through a side of an enlarged portion'61 of the valve housing, and

comprises Van offset portion 57a, the Vend of which is secured in any suitable manner to Valve stem 58 as by the clamping 'nuts 575. The opposite end of actuator 57 is supported and gulded by bearin 62 mounted upon the end of a comparative y rigid rod 63 secured to the cross-head guide 42. A rod 65 secured4 'to cross-head v41 is free to move laterally -within slot 64 formed in the upper wall of the cross-head-guide, during reciprocation of the cross-head, and has mounted at its upper end'a' housing member 66 com risin a combned valve actuator and bu er. ousing 66 cooperates with

stop members

69 and 70 secured to valve actuator 57 for. the purpose of seating valves 55 and 56 in accordance with movement of cross-head 41. y

Referring to Fig. 3, housing 66 comprises a pair of recesses 71 and 72 separated bya wall 73 through which the actuator 57 freely and longitudinally extends.`

Springs

67 and 68 each seat upon wallk73 and exert compressive forces upon a pair of disc-

like members

74 and 75 restrained within recesses 7l and 72 respectively by the inwardly extending annular shoulders 76 and 77. Accordingly, the springs tend to normally maint-

ain discs

74 and 75 in abutting relation to their respective limiting shoulders. Stop

members

69 and 70 are provided with reduced portions or members 69a and 7 0a which .are adapted to engage the resilientlybiased discs 75 and 74 'upon reciprocation of the cross-head to elf'ect reciproca action of valves 55 and 56. Accordingly, shock to the valve actuator 57 is obviated due to the buffer effect of

springs

67 and 68.

. Cylinder 44 is also providedat its opposite ends with a pair of discharge passages 7 81 and 79, each adapted to communicate through normally closed vdischarge .valves 80 and 81 to valve structure-82 adapted to regulate .the

rate of discharge of the fluid from cylinder 44. Discharge valves 80 and 81 are mounted within housings 83 and 84 connected in any suitable manner, as by conduits, with the discharge passages 78 and 7 9, and comprise conical valve seats 80a and 81a for cooperating with the correspondingly shaped valves which are normally biased by spring means 80?) and 815 to closed position. Valve housings 8 3 and 84 communicate Vthrough connections 85 and 86 to the interior of valve housing 82, having mounted therein a rate of dischargeregulating valve 87. Valve 87 is adapted to co-act with conical seat 87a through a threaded valve stem 88 having an adjusting knob 89 secured to its upper end. Rotation of the adjusting knob 89 in either direction eects longitudinal movement of valve 87 with respect to its seat 87 a, and accordingly the valve opening may be adjusted to any desired position, depending upon the rate at which fluid is to be discharged 'from cylinder 44. The valve controlled opening within housing 82 communicates through a pipe connection 90 to a second valve structure 91 adapted to permit flow of fluid therethrough only in response to a predetermined pressure. Disposed intermediate the aforesaid valve structures, and connected to pipe CO, is a pressure gauge 92 for determiningl the extent of adjustment of the pressure actuated valve.

Valve structure 91 comprises a housing 93 which may be cylindrical in shape, having its lower end closed by a plug lmember 94 serving bothV as a valve seat and a'connection for pipe 90. A valve 95 is adapted to seat within a correspondingly shaped valve seat. in

member 94 and is normally biased to seating position by a spring 96 or equivalent, one end ojwhich bears against valve 95 and the other end of which bears against an adjustable abutment 97 whose position along the longitudinal axis of housing 93 may be determined by the adjusting member 98. That is, rotation of member 98 e'ects longitudinal movement of abutment 97 with respect to valve 95 so as to either increase or relieve the tension on spring 96 so that valve 95 will open only when the predetermined s ring tension is overcome. The interior o housing 93 communicates at a point above valve 95 with reservoir 52 through the pipe or conduit 99, thereupon completing a closed system of circulation from and to reservoir 52.

The operation is as follows:

Assuming the apparatus to be in the'positions illustrated in Fig. 1, the drill pipe is lowered into the well to a point just above the bottom of the bore and is heldin such position by a brake (not shown) cooperating with drum 21. The driving or drilling motor M meanwhile `is rotating the drill pipe through the rotary table 16, sinceY clutch 17 is closed to eect rotation of sprocket wheel 10 with the line shaft. With clutch 31 closed as shown, the entire weight of the drill pipe is adapted to be supported through drum 21,

.Weight of the drill pipe may be readil calculated from known data, it is possi le to adjust the tension of the pressure discharge valve spring 96 so that the pressure upon valve 95, which is proportional to the weight of the drill pipe, is not sufficient to lift the valve from its seat. VThe discharge regulating valve 87 engages its seat 87a to close the discharge passages leading from cylinder 44. Accordingly,`piston 43, acting against a substantially non-compressible. fluid, is maintained motionless, since escape of fluid from cylinder 44 is prevented, and the weight of the entire drill pipe is carried by the controlling or governing apparatus, or more properly by the derrick associated therewith. Valve 87 is now opened tothe desired extent for regulating the rate of flow of fluid from cylinder 44, so that the entire weight of thel drill pipe is now representative of the reading of pressure gauge 92. The tension of valve spring 96 is now relieved by member 98 permitting passage of fluid past the valve until the pressure gauge 92 indicates a value representative of the desired proportion of the weight of the drill pipe which is to be supported by the controll apparatus. This weight of the drill pipe, transmitted to pisr ton 43 through crankshaft 37 in the manner above described, tends to elect reciprocation of the same to force or pump the non-compressible fluid through the outlet passages and past valves 87 and 95. Assuming now that the weight of the drill pipe tends to rotate sprocket wheel 32 in a clockwise direction, piston 43 will tend to move toward the right in cylinder 44, thereupon forcing the fluid between the piston and end Wall 46 through outlet ypassage 79, past discharge valve 81 which readily opens to permit passage of the fluid, lthrough connection 86, past valve 87 to the pressure regulating valve 95.

As piston 43 moves toward the right, the

crosshead 41 moving in the same direction, moves the member 66 with respect to the valve actuator 57, but does not effect movement of the same until the piston is near the end of its structure, at which time portion 7 0a of the stop member 70 is engaged by the spring biased disc to move actuator 57 a short distance towards the right. Valve 56, which during the above described movement of pis'- ton 43 was in closed position to prevent escape of the fluid through the inlet passage, now moves to open position, piston 43 being at the limiting position of its stroke, and valve 55 which was formerly opened to permit passage of fluid from reservoir 52, into the left side of the cylinder asthe piston progressed toward the right, is now in closed position, so that upon reversal of movement of piston 43 'exhaust of fluid from the cylinder may taket lace onl through the discharge passage V8, etc. Vith the inlet valves in the position above described, i. e., valve 'closed andvalvc 56 open,'movement,of piston 43 towards the left forces the Huid by way of discharge assage 78 past valves 87 and 95. vDur-in t e greater par-t of this movement, mem er 66 "is freely moving towards the left with respect to actuator 57 and .does not come into engagement with stop member 69 until it has approximately reached the -position illustrated in Fig. 2,"at which time actuator 57 is moved to close valve 56 and open valve 55 to permit reciprocation of the sls'ton in the opposite direction. Passage of uid to cylinder 44 through the inlet valves into the space behind the piston is aided by the suction created by piston 43 in that part of the cylinder. As long as the weight of the drill pi e ltendin to rotate sprocket wheel 32 in a cloc wise irection, is maintained at a -predetermined value, the piston 43 will continue to reciprocate in the manner of a pump acting a ainst a highA resistance, and a continuous circulation of the working fluid is maintained through valves 87 and 95. In other words, the draulic control apparatus by analogy mayv e likenedto a heavy load which may be moved only upon ap llcation of a predetermined force. If the fiorce falls below thisv value, thev load remains stationary, and remlairiis so until suilicient force `is again' app 1e u n Since the rate of descent of the drill is effected solely by' a portion of its own weight acting upon the iston 43, and also by the adjustment of va ve 87, it'may happen that this rate of feed will be too rapid for certain rock formations encountered. In this event, the descent of the drill pipe. throughout its upper section would be morera idthan the movement of the bit through t e more resistant formation, with the result that the bit would be crowded by the greatly increased weight of the drill'pipes bearing thereon. Simultaneous with increase in pressure upon the bit, a decrease in'weight on the control apparatus occurs, with the result that valve 95 closes due to the decreased pressure thereon, and prevents reciprocation of piston 43. Accordingly, descent ofthe drill is halted until the bit has drilledsufliciently into the formation to p'ermit the control apparatus to again assume its apportioned share. of the` weight ofthe drill pipe, afterwhich the normal drilling rate isresumed. Ifcontinuous resistance to such formations` is encountered, the discharge valve 87 may be adjusted to further restrict theA outlet passage, thereby establishing a. slower rateof descent of the ythe power required to effect the control is drill in accordance with the higher resistance caused by the more restricted discharge passage. V It will of course be a parent that the length of drill pipe cannotlb definitely as the. drilling progressesv for a given setting ofthe pressure discharge valve, since the bit pressure would increase to abnormal values. By wayof example however, it has been found that for a normal bit ressure of about four tons, vabout one t ousand pounds of drill pipe may be added before chalmging the setting of the pressure discharge va ve. f

It will be apparent that Weight vof the drill ipe carried by the bit,'and the rate `of drill eed are not necessarily interdependent as inthe case of the differential feed control above referred to. That is, for a given setting of the pressure regulating valve 95, which arbitrarily a portlons the Weight of the drill pipe toI gerne by the bit and-con-` trol apparatus, there may be different adjustments of valve 87 for permittingdiffer-en .rates of fe'ed or descent ofthe drill. In the same manner, valve 87 ma be adjusted for a given rate of feed, and t e pressure' regulating vvalve 95 may be tensione'd to permit different amounts of the weight of the drill pipe to be'borne by the bit. It is therefore within the control ofthe operator in lchar e of the drilling 'o eration to independently adjust the rate-o feed of the {drill as well as theV pressure .which the bit'exerts upon the bottom of the bore.

A further im rtant consideration is that supplied entirely b a portion of the weight of the drill pipe. in other words,no auxiliary motors or other power equipment' are necessary 'to effect the control, as contrasted with the 'differential method wherein a re'gu- -lating motor is additionally required.

e increased inj.

fico,

By my invention, it is possible to prevent i vdamage tothe drill pipe, as by twist-off, by

determining in advance substantially the exact pressure brought to bear upon the bit. When this pressure is exceeded, the hydrau- I lic feed control immediately stops further descent of the drill pipe, thereupon permitting the same to rotate within the bore until the bit has entirely cleared itself, and is again operating at its normal pressure. Furthermore, since from. past experience it is possible to roughly estimate the .proper drilling rate :throu h dilferent'rock formations, a given rate o feed may accordingly be established by regulation of' the discharge valve, thepressure valve being also adjusted to provide adequate workingv pressure ontken bit, so that more footage 'of well, i. e. hightime is very costly and the comparatively Aer average rate of feed, is obtainedthanby case of sudden overload thereon, in orderl thatvthe drill pipe may not twist od, or as in the case of the step-by-step method of feed which requires constant attendance and supervision.

Although I have described the operation of but one hydraulic control cylinder for regulating the rate of descent of the drill, and the extent of pressure exerted by the bit on the bottom of the bore, it shall be understoodv that two or more hydraulic control cylinders of the same ty e may be readily employed if desired, depending of course on the o erating requirements. In such an event, the ifferent pistons are operatively connected to a crankshaft inthe mannerillustrated in Figl, andthe discharge passages of each cylinder connected to valves 87 and 95, which valves are common to the multiple cylinder arrangement and inde endently regulate the rate of :flow

of uid rom the cylinders and the pressure at which uid may be discharged therefrom.'

The o eration of the pistons in the multiple cylin er arrangement may be either synchronous, as where the connecting rods are connected to the same throw of the crankshaft, or in timed sequence, as in the operation of internal combustion engines, for example, although the latterarrangement is preferred.

It shall be urther understood that my invention is not limited to the specific' structure herein described, but comprehends in general a hydraulic governor or governors wherein substantially continuous though` alternate, movement of a member is resisted by anoncompressible fluid, bhe release of said fluid being subject to a variable resistance.

It shall still further-be understood that the term drill pipe as used in the above specification and appended claims is-intended to comprehend means for transmitting rotative movement from the drill actuating means to the bit, including both drill rod and pipe.

What I claim 1s:

l. In brake apparatus for restraining ciated with said discharge.l passages for'permitting escape of fluidl -from said cylinder only in response to predetermined force exerted thereon by said piston, valve structure on one side of said piston and to cut olf com-- munication between said supply and the cylinder on the other side of said piston, means movable with said piston and operable substantially u on completion of the piston stroke in eit er direction to reverse the position of said valve means, a discharge connection, check valves biased to closed position and providing respectively for liquid discharge into said connection from opposite ends of said cylinder, each of said checkvalves disposed to open with the pressure in said cylinder, valve means disposed in said discharge connection and operable when closed to o pose liquid How therein, and means yiel ingly holding said last-named valve means in closed position.

3. A brakinvr system for controlling the descent of a load comprising a cylinder adapted to receive a substantially non-compressible fluid, a pistonreciprocated in said cylinder by descent of said load, valve structure in the path of iiuid discharged from said cylinder adjustable to predetermine the maximum rate of descent of said load, and. pressure responsive valve structure in said path preventing discharge from said cylinder and descent of. said load when said load is less' than a redetermined magnitude.

4. A rakin descent of a load comprising a cylinder adapted to receive a substantially non-compressii ble fluid and having inlet and discharge passages at opposite ends thereof, a piston reciprocated in said .cylinder by descent of said load, valve structure in the path of discharge of iiuid from said cylinder which closes to revent movement of said piston when the i orce exerted thereon by said load is less than a predetermined minimum, valves controllin admission of fluid to said cylinder throu said inlet passages, and structure mova le in timed relation to said piston for actuating said inlet valves substantially `at the end of the piston stroke to open the valve at the end of the cylinder adjacent the piston and to close the inlet valve at the opposite end of the cylinder.

5. A braking system for controlling the rate of descent of a rotary drill comprising a cylinder adapted to receive al substantially system for controlling the' non-compressible fluid, a piston reciprocated in said-cylinder by descent of said drill, valve structure in the path of fluid discharged from said cylinder adjustable to predetermine the maximum rate of descent of said drill, and pressure-responsive valve structure in said path preventing discharge from said cylinder and descent of the drill when the force exertedv byf said drill on said piston is less than' a predetermined minimum. y

6. A braking system for controlling the pressure. at the cutting endl of rotary drill apparatus comprising a cylinder adapted to receive a substantially non-compressible fluidl` a piston reciprocated in said cylinder-by descent of said drill, a-valve in the path of Huid discharged from said cylinder and responsive to the pressure thereof for closing said path to stop descent of thedrill when said pressure exceeds a predetermined maximum, and a second valve in said path to control the rate of descent of said drill 'independent ly of said ressure. 4

7. A braiiingsystem for controlling the descent of a load comprising a cylinder l Y adapted to receive a substantially non-compressible iuid, a piston reciprocable in said cylinder to act on fluid therein for both directions of movement, means connecting said piston to said load for effecting reciprocation thereof upon descent ofthe load, and valve structure adjustable to permit discharge of fluid from said cylinder at a desired rate to 'eii'ect continuous descent of :the load at a constant and desired rate.- l JOHN E. BRANTLY.