US3642076A - Impulse-reaction propulsion cycle for mole - Google Patents

Abstract

This disclosure describes a propulsion cycle for a subterranean burrowing device. This cycle is characterized by the impact of an oscillating hammer upon an interior anvil, and by the return of the hammer to its starting position by force of a reaction piston acting on the hammer. While the reaction piston does work on the hammer, the opposite force thus produced on the mole nose causes a further soil penetration. The hammer eventually comes to rest and is returned to the anvil by a unidirectional constant bias force. The cycle is inherently self-timed.

Description

PATENTEDFE'B 15 |972 SHEET 1 AUF 3 h Qmv i l Mw.

JC. COVNE /Nl/ENTORS AR SM/TH ATTORNEY vUnited States Patent Coyne et al.

[451 Feb. l5, 1972 [54] IMPULSE-REACTION PROPULSION CYCLE FOR MOLE [72] Inventors: `James Christopher Coyne, New Providence; Arnold Ray Smith, Chester,

both of NJ.

[73] Assignee: Bell Telephone Laboratories, Incorporated,

Murray Hill, NJ.

[22] Filed: July 17, 1970 [21] Appl. No.; 55,818

[52] U.S. Cl ..173/91, 173/119, 173/135 [51] ..B25d 9/00 [58] Field ofSearch ..173/119,120,134,135,91;

[56] References Cited UNITED STATES PATENTS 2,748,750 6/1956 Altschuler ..173/119 3,465,834 9/1969 Southworth, Jr. ..173/91 3,137,483 6/1964 Zinkiewicz ....173/91 3,407,884 10/1968 Zygmunt et al 173/91 Primary Examiner-J ames A. Leppink Attorney-R. J. Guenther and Edwin B. Cave [571 ABSTRACT This disclosure describes a propulsion cycle for a subterranean burrowing device. This cycle is characterized by the impact of an'oscillating hammer upon an interior anvil, and by the return of the hammer to its starting position by force of a reaction piston acting on the hammer. While the reaction piston does work on the hammer, the opposite force thus produced on the mole nose causes a further soil penetration. The hammer eventually comes to rest and is returned to the anvil by a unidirectional constant bias force. The cycle is inherently self-timed.

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SHEET 2 0F 3 umm o 1 lMPULSE-REACTION PROPULSION CYCLE FOR MOLE FIELD OF THE INVENTION This invention relates to subsoil penetrators, or moles"; and more specifically to a mole propulsion system.

BACKGROUND OF THE INVENTION The mole is a guided subsoil missile designed to form tunnels for the placement of utility services such as telephone distribution cable or service wire.

A typical mole propulsion system is described, for example, in H. Southworth, Jr. U.S. Pat. No. 3,465,834, issued Sept. 9, 1969. Within the moles cylindrical shell, a hammer is mounted to shuttle back and forth, impacting against an anvil at the missile's nose for forward motion, and at a back anvil for the backing mode. The hammer is hydraulically powered through hoses dragged along behind and connecting to surface service equipment. Each impact moves the mole through soil a small amount depending on hydraulic pressure, pressurized area of the hammer, hammer mass, mole body mass, soil properties, impact velocity, and the mechanical properties of hammer, anvil and mole shell.

The propulsion cycle of this typical example begins with the application of hydraulic pressure to the hammer when situated at the rear end of its travel. The mole shell reacts against the tunnel walls with a force which is equal and opposite to the hydraulic force on the hammer and which must be less than the backward slip resistance of the tunnel. This force accelerates the hammer over its full stroke. At impact, energy and momentum of the hammer are transferred to the mole body causing the mole to advance. The hammer is returned to its initial position by switching the hydraulic pressure from behind the hammer to infront of the hammer at the instant of impact. When the hammer was traveled at least half way back to its initial position, the hydraulic pressure is switched again to the rear end of the hammer.

In the above-described propulsion design, conversion .efficiency of hydraulic energy at the pump to impact energy is typically about 30 percent. The conversion efficiency of impact energy to soil penetration is also typically about 30 percent. The overall efficiency thus is less than l percent. The consequences of this include high oil temperatures, severe shock and vibrations, reduced penetration speed, and an increased probability of stalling in rocky soil.

More specifically, for practical purposes the fraction of kinetic energy in the impacting hammer which is converted into soil penetration in average and weak strength soils is about equal to the hammer mass divided by the mole body mass. The remaining energy is lost in shell vibrations which, in addition to wasting energy, creates fatigue and shock problems.

Accordingly, one object of the invention is to increase the penetration efficiency of a mole.

Another object of the invention is to reduce the shell vibration, particularly the fatigue and shock aspects.

A further object of the invention is to simplify the hydraulics associated with timing and hammer drive-in armole.

ln hard rocky soils the penetration efficiency decreases until the point is reached where all the impact energy goes into elastically straining .the mole and the soil. The force on the soil never reaches the soils yield force and no penetration occurs. Under these hard rocky soil conditions, it is desirable that the hammer have a large impact velocity to reduce the possibility of stalling.

Accordingly a further object ofthe invention is to match the impact velocity of the hammer to the soil hardness, that is, to automatically providea high impact velocity (full stroke) for hard soil conditions and a slower impact velocity (partial stroke) for average and weak soil conditions.

SUMMARY OF THE INVENTION y In the present invention, a constant unidirectional bias force is applied to the hammer, tending to accelerate it toward the front anvil. On strikingthe anvil,'the hammer triggers a sudden release of stored energy, which is directed to forcing the hammer and anvil apart. Thus, the `hammer is propelled backward against the constant bias force; but the moles nose is given an added forward impetus into the soil.

In a particular inventive embodiment, the force is developed hydraulically through the release from an accumulator of hydraulic fluid under substantial pressure. The fluid acts upon a reaction piston mounted in axial relation to the hammer. The pressurized area of the reaction piston is substantially greater than the pressurized area of the hammer. The piston thus propels the hammer back against the bias force. This reaction kick augments the forward impulse occasioned by the hammer-anvil impact. l

The above systems potential for higher penetration efficiency arises because of its higher total energy transfer per cycle. That is, the work done on the soil per cycle is the sum of the energy transfer by the impacting hammer and the work done on the mole body (propelling through soil) by the hydraulics during the reaction piston stroke.

Also of substantial advantage Ais the fact that the control of the hammer cycle.y is inherently adaptive to differing soil conditions. The hydraulic work done by the reaction piston is automatically proportioned between returning the hammer and propelling the mole into soil such that hammer impact velocity on the front anvil increases with increasing soil strength.

A further advantage of this cycle is that the rcontrol of the hammer cycle (i.e., control of hammer stroke) is virtually unaffected by system pressure. Thus, pump pressure can be changed without any substantial change in hammer stroke. This is because both the hydraulic force on the reaction piston and the bias force on the hammer are derived from the same pressure source.

A still further inventive advantage is that the mole can be reversed merely by switching the hydraulic supply and return at the surface control station.

DESCRIPTION OF THE DRAWING FIG. l is a schematic sketch of a typical mole operating environment;

FIG. 2 is a sectional side view of the impulse reaction drive of the invention;

FIGS. 3 and 4 are schematic diagrams of the drive reversing mechanism; and

FIG. `5 is a graph depicting the inventive cycles performance under various soil conditions.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT*C FIG. l depicts a mole l0 in operation in the soil. Typically, the mole consists of a body ll and anarticulated tail section 12 joined to guidance and powering equipment on the surface throughout the umbilical cord 13 that includes hydraulic hoses and power connections.

Within the mole l0, a hammer 20 is centrally mounted upon a ported shaft 21 for `fore yancl aft movement within a cylinder 22. Hammer 20 shuttles between a rear position at which its back surface 20a is coincident with the plane I9 seen edge-on in FIG. 2; and a forward position where its front surface 20b impacts upon the anvil surface 26 or mole body. Plane 19 is slightly forward of a rear anvil 19a, and represents the plane at which, for hardest soil conditions, the hammer must come to rest for forward penetration of the mole.

An interior chamber 23 is bounded at one end by rings 17 fixed to shaft 2l, and at the far end by a forward piston surface 18 having a working surface area denoted A. High pressure hydraulic fluid from supply line l5 enters chamber 23 via port 16. The high .pressure in chamber 23 acts against the piston surface I8, biasing the hammer 20 in the direction of arrow 14. Of course, the length of chamber 23 changes as `the hammer moves.

Reaction piston 27 is also centrally mounted upon shaft 2l and is movable within the chamber 28 substantially between the position shown in FIG. 2 and the chamber end 28a. The working surface area of piston 27, denoted B is greater than the working or pressurized area A of hammer 20. In one typical embodiment B/A=l0. The ratio B/A may vary from to 20, however, within the teaching of the invention.

Forward of piston 27 a poppet valve 35 is mounted on shaft 2l. A four-way spool valve 29 is ported as shown schematically in FIG. 2 where the valve is in its unoperated position. In this mode, valve 29 connects high-pressure hydraulic fluid from supply line l5, via the reservoir 32 for poppet valve 35, and via duct 34, to the chamber 33 of reaction piston 27. At this time, valve 29 alsoconnects the return-line chamber 30 with reaction piston chamber 28 via passage 31. Under these conditions` the supply line pressure in reservoir 32 maintains poppet valve 35 snugly in its seat as shown in FIG. 2. A spring 35a biasing poppet valve 35 establishes the extent of opening of that valve.

Propelled by the unidirectional hydraulic bias force in chamber 23, the hammer moves forward. As it is about to strike the anvil 26, it depresses a plunger 36, causing increased hydraulic pressure in chamber 29a which operates spool valve 29. Chamber 28 is thus switched from return pressure to high pressure and the pressure in chamber 28 and reservoir 32 is equalized. By virtue of the greater area of the nose of poppet valve 35 upon which the high pressure is acting, however, poppet valve 35 is cracked open. When this occurs, high-pressure fluid rushes from the reservoir 32 into chamber 28, thus driv-` ing reaction piston 27 rearward against hammer 20 which is thereby accelerated rapidly away from the anvil 26.

To accomplish this motion, it is necessary to provide an accumulator 44 for a source of hydraulic energy close to the point of use. Accumulator 44 is contiguous with supply liney 15 and reservoir 32. Although shown only schematically in FIG. 2, accumulator 44 in practice is an air-pressurized bladder contained in an annulus.

The reaction to the accelerating force on hammer 20 acts on the missile nose 39, forcing the latter further into the soil for the duration of the time that reaction piston 27 is accelerating the hammer 20. This force, in conjunction with the impulse of the hammer-anvil impact, propels the missile forward.

ln its operated position, subsequent to hammer-anvil impact, spool valve 29 is latched by high-pressure hydraulic fluid applied through duct 34 and latching passage 31a, both of which are of small diameter and low flow capacity. When reaction piston 27 has traveled the prescribed distance, it uncovers a low-pressure port 40 to the return line 4l. The resulting sudden pressure drop in chamber 28 first reverses the force on poppet valve 35, causing it to close. Secondly, the pressure drop unlatches spool valve 29 by dropping the pressure in chamber 29a through latching passage 31a. Valve 29 is returned by bias spring 43 to its unoperated position shown in FIG. 2. This occasions areswitching of pressures, the high pressure going once again to return chamber 33. This position of valve 29 insures that chamber 28 remains at return pressure level such that poppet 35 remains closed. The pressure acting upon the face 42 returns the reaction piston 27 to its initial position.

The return stroke of reaction piston 27 takes place while the hammer 20 coasts away from the anvil against its unidirectional bias force present in chamber 23. The kinetic energy imparted to hammer 20 during its acceleration by reaction piston 27 enables the hammer to coast against the biasing force until coming to rest at a point not beyond plane 19. Thereafter, this biasing force propels the hammer 20 back toward the anvil to start the next cycle.

REVERSING As seen in FIG. 2, the rear end of the mole body contains hydraulic apparatus substantially identical to that found in the forward end which has just been described. The components are identified by primed numerals, like numerals corresponding to the components earlier described.

In the rear end hydraulics, a reaction piston 27' is used having much less diameter and working face area than piston 27, since in the reverse mode, less work need be done on the hammer 20. Similarly, the accumulator 44 has a much lower capacity.

Reversal capability requires the valve denoted 50 in the forward end, and 50 in the rear end connected to the hydraulic positions shown as A, B, C, D and A', B', C', D in FIG. 2. Valve 50 is depicted in detail in FIGS. 3 and4 as consisting of a chamber 5l and a plunger 52 with arcuate passages 53, 54. The function of valve 50 is conventional` i.e., to give an output at outlet C of the higher of two pressures present at A and B, regardless of which of the latter positions experiences the higher pressure.

In the embodiment shown in FIG. 2, it is possible to reverse direction by switching the pressure on supply and return lines l5, 4l so that high (supply) pressure occurs in line 4l and low (return) pressure occurs in line l5. This is advantageously done at the surface station. The reversing valves 50, 50' in response operate lto provide the now-high pressure of line 4l to the ducts 34 and 34', just as was present in the'se passages in the forward penetration mode. Similarly, low pressure is provided to the passages 3l, 3l. With high pressure in the line 4l, hammer 20 moves to the left under its bias force, striking the plunger 36' and then the rear anvil 19a. Plunger 36', operated, actuates the hydraulics at the mole rear end, in the manner already described for the front end, causing the piston 27 to work against hammer 20 accelerating it to the right. The resultant reaction kicks the mole rearward. Hammer 20 in this mode is accelerated rearward no farther than plane l9b, so that plunger 36 is not tripped. While this takes place, the front end hydraulics are locked in the mode depicted in FIG. 2 for reversing, which avoids its oscillating. Similarly to the same end, the rear end hydraulics are locked in the mode depicted in FIG. 2 when the mole is in its forward penetration mode.

Analysis shows that a substantial improvement in penetration efficiency is obtained with the foregoing propulsion cycle `over prior propulsion cycles which return the hammer at the same force level as that accelerating the hammer` to impact. The penetration efficiency is here defined as the ratio of work done penetrating soil per cycle to the kinetic energy of the impacting hammer. The analysis shows that the greater the force used in returning the hammer, the greater the improvement in` the penetration efficiency; also the shorter the required stroke of the reaction piston to produce a desired hammer stroke. A

straightforward energy balance, neglecting pressure drops,

gives the following useful relationship.

Return Force Bias Force Thus the advantage of a largearea return piston is evident. Such a large area return piston demands a large hydraulic flow over a brief time, necessitating the presence ofthe accumulator 44 as well as a fast-opening large flow passage poppet valve 35. The accumulator undergoes rapid discharge once each cycle, supplying the energy for the hammer return immediately after each impact, and thereafter is recharged from the supply line during the remaining position of each cycleA The graphs shown in FIG. 5 help demonstrate some of the beneficial properties of this propulsion cycle. The following three assumptions are made in the analysis leading to these graphical results. l The soil presents a purely Coulomb-type resistance to penetration. That is, the soil possesses negligible inertia and compliance. This is a reasonable and valid assumption for the present situation where the soil is deformed far beyond its elastic range each cycle and the anticipated cycle rate is slow, approximately l0 Hz. (2) The impacting hammer experiences insignificant rebound from the anvil. This is an experimentally observed fact, which is a consequence of the massiveness of the hammer relative to the anvil and nose structure of the mole. The major portion of the mole body mass is distributed along a relatively long shell. lt can be readily shown from the theorem of conservation of linear momentum that the fraction of kinetic energy in the impacting hammer transferred to the mole body in a nonvibrating mode is equal to the ratio of hammer mass to mole body mass. This means that in a soil having Coulomb resistance, the penetration efficiency due to the impacting hammer is also equal to this mass ratio. Finally, (3 Pressure drops are neglected. This assumption does not alter the qualitative nature of the analytic results.

Four graphs are shown in FIG. 5 plotted against soil strength Fa (the Coulomb resistance of the soil) normalized by the biasing force Pa on the hammer. The results presented in the figure were computed for a ratio of hammer mass to mole body mass of one-third and a ratio of reaction pistonarea to hammer area of l0. Sm, is the maximum hammer stroke, and l is the constant stroke of the return piston.

At high soil strength the penetration of the mole into soil is small as shown by curve 3. Thus the fraction of the output work of the reaction piston going into soil penetration is small. This fraction is just /I, In this illustrative case the reaction piston stroke (l) is l0 percent of the maximum hammer stroke. Since nearly all the output work of the reaction piston goes into imparting kinetic energy to the returning hammer, the hammer stroke shown by curve l is nearly equal to the maximum hammer stroke. Also, for the reason that a small fraction of the output work of the reaction piston goes into soil penetration, the penetration efficiency shown by curve 2 is only slightly greater than that shown in curve 4 for pure impact, which was stated to bev just the mass ratio-in this case, one-third.

As the soil strength becomes less strong, the soil penetration (8) increases and therefore the fraction of the work output (/l) of the reaction piston going into soil penetration increases. Consequently the energy imparted to the returning hammer becomes smaller and the hammer stroke is reduced as shown on curve l. The increased work output of the reaction piston going into soil penetration causes an increase in penetration efficiency as shown on curve 2. Thus in average strength soils, the structure of the mole is not stressed by high velocity impacts more than necessary and still the penetration efficiency is increased. [n still weaker soils, those in which the Coulomb resistance is less than five times the hammer bias force, the penetration per cycle into soil greatly increases as shown on curve 3 and becomes an appreciable part of the hammer stroke. Hence, the distance that the hammer has to travel to return to its initial position is appreciably reduced. Although a very small fraction of the work output of the reaction piston goes into imparting kinetic energy to the hammer, this small amount of energy is nevertheless sufficient to provide a further increase in stroke as shown on curve l. Since nearly all the work output of the reaction piston goes into soil penetration, the efficiency further increases. At such weak soils, the cycle operation is better described as that of statically pushing through soil by virtue of the reaction force ofthe reaction piston, rather than that of impact penetration.

lt is to be understood that the embodiments described herein are merely illustrative of the principles of the invention. Various modifications may be made thereto by persons skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

l. A linear impacting unit for a subsoil penetrator, comprising:

a forward anvil fixed with respect to the penetrator nose,

said anvil defining the forward end of a cavity; a hammer mounted in said cavity for impacting with said anvil; means for maintaining a continuous forward bias on said hammer' a source of hydraulic energy; and

means responsive to hammer impact for applying said energy to said hammer against said bias for a defined distance of hammer return travel.

2. Propulsion apparatus for a subsoil mole, comprising:

an elongated axial cavity in said device bounded by forward and rear anvils and containing an axially shuttling hammer;

means for biasing said hammer continuously toward said forward anvil with a substantially constant first force; and

means for accelerating said hammer away from said first anvil on contact therewith with a second force substantially greater than said first force, the energy imparted to said hammer by said second force being controlled to limit hammer rearward travel to a point short of said second anvil.

3. Apparatus pursuant to claim 2, wherein said biasing means and said accelerating means are both hydraulic and operated from the same pressure source.

4. Propulsion apparatus for a missilelike subsoil burrowing device, comprising:

an axial cavity within said device bounded by end walls comprising forward and rear anvils;

a hammer mounted for fore-and-aft movement between said anvils and comprising an interior chamber defined by a fixed wall and a forward working surface of area A;

separate hydraulic supply-and-return lines connected between said device and a remote hydraulic source;

means connecting said supply line and said chamber to bias said hammer continuously toward said forward anvil;

a reaction piston having a working surface of area B where B A, mounted in a second chamber for fore-and-aft movement of said working surface B from said forward anvil into said cavity;

a hydraulic accumulator connected to said supply line and to said second chamber;

means for discharging said accumulator into said second chamber after impact of said hammer on said forward anvil, said reaction piston accelerating said hammer rearwardly, and thus said device forwardly, the work thereby done on said hammer being controlled to bring said hammer to rest before striking said rear anvil.

S. Apparatus pursuant to claim 4, further comprising a third interior chamber defined between said fixed wall and a rear working surface within said hammer;

a second reaction piston mounted in a fourth chamber for fore-and-aft movement from said rear anvil into said cavity;

means for feeding hydraulic supply pressure into said third chamber thereby to bias said hammer into impacting contact with said rear anvil rather than said forward anvil; and

means operative after said impacting contact for accelerating said second reaction piston against said hammer thus to drive the latter forwardly, and said device rearwardly.

6. Apparatus pursuant to claim 4 where the ratio B/A is in the range 5 to 20.

Claims (6)

1. A linear impacting unit for a subsoil penetrator, comprising: a forward anvil fixed with respect to the penetrator nose, said anvil defining the forward end of a cavity; a hammer mounted in said cavity for impacting with said anvil; means for maintaining a continuous forward bias on said hammer; a source of hydraulic energy; and means responsive to hammer impact for applying said energy to said hammer against said bias for a defined distance of hammer return travel.

2. Propulsion apparatus for a subsoil mole, comprising: an elongated axial cavity in said device bounded by forward and rear anvils and containing an axially shuttling hammer; means for biasing said hammer continuously toward said forward anvil with a substantially constant first force; and means for accelerating said hammer away from said first anvil on contact therewith with a second force substantially greater than said first force, the energy imparted to said hammer by said second force being controlled to limit hammer rearward travel to a point short of said second anvil.

3. Apparatus pursuant to claim 2, wherein said biasing means and said accelerating means are both hydraulic and operated from the same pressure source.

4. Propulsion apparatus for a missilelike subsoil burrowing device, comprising: an axial cavity within said device bounded by end walls comprising forward and rear anvils; a hammer mounted for fore-and-aft movement between said anvils and comprising an interior chamber defined by a fixed wall and a forward working surface of area A; separate hydraulic supply-and-return lines connected between said device and a remote hydraulic source; means connecting said supply line and said chamber to bias said hammer continuously toward said forward anvil; a reaction piston having a working surface of area B where B>A, mounted in a second chamber for fore-and-aft movement of said working surface B from said forward anvil into said cavity; a hydraulic accumulator connected to said supply line and to said second chamber; means for discharging said accumulator into said second chamber after impact of said hammer on said forward anvil, said reaction piston accelerating said hammer rearwardly, and thus said device forwardly, the work thereby done on said hammer being controlled to bring said hammer to rest before striking said rear anvil.

5. Apparatus pursuant to claim 4, further comprising a third interior chamber defined between said fixed wall and a rear working surface within said hammer; a second reaction piston mounted in a fourth chamber for fore-and-aft movement from said rear anvil into said cavity; means for feeding hydraulic supply pressure into said third chamber thereby to bias said hammer into impacting contact with said rear anvil rather than said forward anvil; and means operatIve after said impacting contact for accelerating said second reaction piston against said hammer thus to drive the latter forwardly, and said device rearwardly.

6. Apparatus pursuant to claim 4 where the ratio B/A is in the range 5 to 20.

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