MOLE WITH DECOUPLED NOSE AND BODY
United States Patent 3797586
The penetrating forward nose of a Mole is structurally decoupled from the body in this invention to reduce fatigue and shock effects on the body. The nose and body are connected through a compliant sleeve in one embodiment. In a second, the nose is slidably mounted on the body outer shell, and subanvil means are interposed between the hammer and nose.
US Patent References:
Pneumatic well drill
Yungling - July 1932 - 1866335
Pneumatic well drill
Kitching - October 1936 - 2033527
VEHICLE MOUNTED SONIC SHEARING DEVICE HAVING PROPULSION AIDING MEANS
Bodine - April 1969 - 3437381
Two-way ground burrowing device
Zygmunt et al. - October 1968 - 3407884
Method of driving pile shells
Dahren - April 1933 - 1906136
Pneumatic well drill
Yungling - July 1932 - 1866335
Pneumatic well drill
Kitching - October 1936 - 2033527
VEHICLE MOUNTED SONIC SHEARING DEVICE HAVING PROPULSION AIDING MEANS
Bodine - April 1969 - 3437381
Two-way ground burrowing device
Zygmunt et al. - October 1968 - 3407884
Method of driving pile shells
Dahren - April 1933 - 1906136
Inventors:
Coyne, James Christopher (New Providence, NJ)
Mccoy, Robert Gordon (Whippany, NJ)
Smith, Arnold Ray (Chester, NJ)
Mccoy, Robert Gordon (Whippany, NJ)
Smith, Arnold Ray (Chester, NJ)
Application Number:
05/209814
Publication Date:
03/19/1974
Filing Date:
12/20/1971
View Patent Images:
Export Citation:
Assignee:
Bell Telephone Laboratories, Incorporated (Murray Hill, NJ)
Primary Class:
Other Classes:
175/19, 175/56, 173/162.100
International Classes:
B25D17/08; B25D17/24; E21B7/26; B25D17/00; E21B7/00; E21B1/06
Field of Search:
175/19,20,21,22,23,56,92 173/91,132,133,139 279/19.7 299/37
Primary Examiner:
Purser, Ernest R.
Attorney, Agent or Firm:
Graves C. E.
Claims:
We claim
1. An earth-penetrating device comprising:
2. Apparatus pursuant to claim 1, wherein said interior surface of said nose and said exterior surface of said body taper linearly from a minimum diameter at the forward end to a maximum diameter at the back end.
1. An earth-penetrating device comprising:
2. Apparatus pursuant to claim 1, wherein said interior surface of said nose and said exterior surface of said body taper linearly from a minimum diameter at the forward end to a maximum diameter at the back end.
Description:
FIELD OF THE INVENTION
This invention relates to earth penetrators; and particularly those of the self-propelled missilelike type commonly known as Moles.
BACKGROUND OF THE INVENTION
In U.S. Pat. No. 3,465,834 of H. Southworth, Jr. issued Sept. 9, 1969, and assigned to applicants' assignee, there is described a linear impact propulsion system in which a hammer traveling on a central axial shaft within the Mole impacts upon a stationary forward anvil rigidly attached to the Mole body. Kinetic energy from the hammer is transferred on impact to the anvil and Mole body directly.
Analysis has indicated that a substantial percentage--almost two-thirds under some conditions--of the hammer impact energy is dissipated as unnecessary shock and vibration losses in the Mole body. In addition to the penetration loss this represents, excessive stress also causes material fatigue, particularly in the so-called shell and hardware attached thereto.
Although high stress levels in the shell are to be avoided, it is also true that the shell must be stressed to some extent in order to overcome tunnel friction and drag.
Accordingly, one object of the invention is to increase the penetration efficiency of a Mole driven by a linear impacting propulsion system.
A second inventive object is to reduce the stress and shock magnitudes and vibrational energy losses within a Mole driven by linear impacting means.
A third inventive object is to control the stress level in the Mole shell within a range acceptable to overcome tunnel friction on the one hand but avoid stresses leading to fatigue on the other.
A further inventive object is to improve the Mole capability for starting into the ground.
Another inventive object is to reduce impact modulation of the detector coil signals.
An important further inventive object is to avoid seriously foreshortening the Mole life by exposure to excessive stresses.
A more specific object is to separate the hammer energy into two adaptable portions--an impacting portion to generate the high peak forces necessary to drive the nose into the soil and thereby form the tunnel; and a much smaller damped impact portion to propel the Mole body through the tunnel.
A still further inventive object is to improve penetration in rocky or very hard soil by increasing the maximum force which the Mole can produce at its nose.
SUMMARY OF THE INVENTION
Pursuant to the general concept of the present invention, the foregoing and further objects are achieved by mechanically and physically separating the Mole nose from the Mole body. The "decoupled" nose can move to a controlled extent axially with respect to the Mole body. The nose is impacted either directly or indirectly by the traveling hammer.
In a particular inventive embodiment, the Mole body and nose although physically distinct are compliantly coupled through a cylindrical elastomer sleeve working in shear. In one embodiment workable with a Mole of about 3 inches outside diameter, the compliant sleeve is about 15 inches long with a wall thickness of 1/8 inch and extends over most of the Mole body length. Normally the compliant sleeve never contains all the impact energy at any one time.
In a second inventive embodiment, the Mole nose is free to slide axially on the outside surface of a shell. Impacting surfaces are provided within the Mole--one on the free sliding nose and the other on the Mole body. The linear impacting hammer first contacts a free sliding anvil structure, which transmits the impact first to the nose and then to the body. Advantageously, the nose is a lightweight structure that decelerates quickly in soil when the impact force on it is removed. Thus, pursuant to this aspect of the invention, the nose is the first member impacted each cycle of the hammer.
The Mole body advantageously travels some distance after the impact force on it is removed. This condition of overtravel pursuant to this aspect of the invention is caused by the relatively large mass and small tunnel resistance of the body as compared to the nose and is what assures that the nose will be the first member impacted on the next cycle.
From the standpoint of improving penetration efficiency, the common principle behind all the decoupling schemes hereinafter to be described is that of spreading the hammer impact impulse into the soil over a longer time period. Energy that will otherwise become vibrational energy, can be utilized for penetration; and energy that already has become vibrational energy can be recaptured for the same purpose. It can be demonstrated that penetration must necessarily increase if the impulse is spread out over a longer time.
The decoupling schemes perform three useful independent functions. Each acts as a shock isolator to limit the peak stresses in the Mole body and also dampens the vibration. Secondly, each permits relatively unimpeded motion of the nose with respect to the body, thereby increasing the stall force. That is, decoupling allows the Mole to exert a larger maximum force on soil at the nose, which is useful in hard soil or rocky soil conditions. Thirdly, decoupling permits a more efficient utilization of energy, i.e., it either reduces the fraction of energy in the vibrational mode or provides means for vibrational energy to perform useful penetration work.
The invention and its further objects, features, and advantages will be more readily discerned from a reading of the detailed descriptions to follow of illustrative embodiments thereof.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic side view in partial cutaway of a first embodiment;
FIG. 2 is a schematic side view of a second embodiment;
FIG. 3 is a graph;
FIGS. 4 and 5 are partial sectional views showing inventive design details of the FIG. 1 embodiment; and
FIGS. 6-8 are partial sectional views showing three related further inventive decoupling schemes.
DETAILED DESCRIPTIONS OF ILLUSTRATIVE EMBODIEMTNS EMBODIMENTS
In a first illustrative embodiment pictured in FIG. 1, the Mole designated generally as 10 consists of a body 11 with a propulsion section 12. A hammer 13 is mounted for linear motion along a central shaft or axis 14. The hammer 13 is driven by hydraulic or pneumatic propulsion means (not shown) which places imbalanced forces alternately on each end of the hammer 13 at prescribed times to cause reciprocation. Since the forcing mechanism forms no part of the present invention, its details are omitted, but a further discussion of one suitable such system may be found in the aforementioned U.S. Pat. No. 3,465,834 of H. Southworth, Jr. which to the extent relevant is hereby incorporated by reference.
An elongated nose 15 including a cone section 16 and a rock breaker 17 is mounted concentrically around the propulsion section 12 of body 11, by means of a compliant sleeve 18. The sleeve 18 is in effect a parallel spring-damper system that can safely absorb the entire impact energy of hammer 13, if necessary.
Since the impact energy is of the order 1,000 inch pounds and since it is desirable that the force transmitted through the spring to the body should not exceed 30,000 pounds, a spring constant (stiffness) of approximately 500,000 pounds per inch and maximum spring displacement of 0.060 inch are appropriate design parameters. Spring displacement in this case is the maximum that could possibly occur, since it corresponds to a situation where all the impact energy resides in the spring.
The sleeve 18 advantageously extends for substantially the entire length of the propulsion section 12 and has at least enough thickness to keep the maximum shear strain under 0.5. Such a thickness should be twice the maximum spring displacement or about 1/8 inch. Additionally, the sleeve 18 must be of sufficient length so that the shear stress does not exceed a safe working limit of either the elastomer material or the bond strength of the elastomer to the metal shell. Typically this value is about 200 psi. The numbers in the above analysis follow from a Mole geometry of approximately three inches in diameter and a maximum of 1,000 inch pounds linear impacting energy.
Advantageously, the material of sleeve 18 is an elastomer poured or injected into the zone between the body and nose, and allowed to cure in place. The surfaces of contact are treated so as to perfect a bond between the elastomer and the surfaces.
The Mole interior is protected from dirt and other particle entry by the bonding of the elastomer sleeve to the body 11 and the nose 15. In FIG. 1, a small gap indicated by the legend is left between the body 11 and the nose 15; but it is seen that no significant amount of dirt can be entrapped therein. The gap can also be filled with elastomer, if desired. FIG. 4 illustrates an alternative structure for further avoiding dirt entrapment, by rounding off the corner 40 of the Mole body 11 and the adjacent end 41 of the nose 15.
Because of the manner in which load is applied to the compliant sleeve 18 following impact of the hammer on nose 15, it is advantageous to taper the inside of skirt 15a of nose 15 in a backwardly direction at a taper of about 1/4 inch per foot of length, as shown in FIG. 5. Concurrently, the outside of the adjacent skirt 11a of Mole body 11 advantageously is tapered to the same extent in a forwardly direction. The advantage of this design is to save weight and space by reducing the thickness of the lightly loaded front end of the skirt 11a and rear end of the skirt 15a.
In the first inventive embodiment just described, the total impulse into soil is spread out over a longer time because of the oscillatory motion of the nose with respect to the Mole body, causing the nose to separate slightly from the soil for a portion of each oscillation. Since the total impulse is a constant, equal to the linear momentum of the impacting hammer; and since the impulse is transmitted into the soil at the nose only when the nose is in contact with soil; and since the soil's penetration resistance at the nose is not significantly different whether the impulse is one continuous impulse or a series of shorter time-duration impulses, it follows that the total impulse must necessarily be spread out over a longer time period thereby producing a greater soil penetration.
It can be shown that this separation and recontacting of the nose with the soil produces a mathematical "non-linearity" in the differential equations which describe the physical situation and thereby provide a means for energy in the vibrational mode to do useful penetration work. In general, vibrational energy can produce no net work on the mass center of a system unless there are nonlinearities in the system. This more efficient utilization of energy increases the Mole's penetration for both hard and soft soils, although it is more effective in hard soil conditions. Furthermore, the use of a massive hammer (large ratio of hammer mass to body mass) decreases the maximum amount of vibrational energy that can develop. For this reason it is desirable to also use a tungsten alloy hammer while designing the body to be as light as possible.
Advantageously, the motion of the nose with respect to the body must be of sufficient amplitude to ensure this separation and recontacting situation. This will be the case if the nose is a lightweight structure compared with the body.
Pursuant to a second inventive embodiment depicted in FIG. 2, the Mole 10 (its forward section only being shown) consists of elements numbered and defined with respect to FIG. 1, and in addition comprises a free-moving anvil 30 and the other elements now to be described.
The anvil 30 is mounted for axial movement along shaft 14. Anvil 30 includes a forward impacting surface 31 that impacts with the nose 15 interior; and a rear impacting surface 32 that is contacted by the advancing hammer 13. The nose 15 is slidably mounted on a forward shoulder 33 of the Mole body 11. The extent of rearward movement of nose 15 is blocked by the end 34 of shoulder 33; and the extent of forward movement of nose 15 is limited by the stop 38 at the nose 15 aft end acting against the shoulder 39 of the body 11.
Pursuant to this aspect of the invention, the moving anvil 30 is free to move from a rear position limited by contact of the axial shaft 14 end with the surface 35; and a forward movement limited by contact of the impacting annulus 36 with the body stops 37 of the Mole body 11.
Pursuant to this second inventive embodiment, the hammer 13 impacts on the traveling anvil 30, which in turn impacts first upon the Mole nose 15 and then the Mole body 11 through contact with body stops 37. Since the nose is light and therefore will be decelerated quickly by soil forces whenever the impact forces are removed, the nose 15 will be the first member impacted each cycle.
It can be demonstrated that the largest impact forces by far are transmitted through the nose, thereby providing substantial penetration efficiency. The secondary impact on the body occurs at a lower impact velocity; thus, substantially less stress is produced in the body by the secondary impact. The primary impact of the hammer 13 with the nose 15, of course, produces no significant shock stresses in the body.
The body is a relatively massive structure and, further, is retarded by relatively small soil forces. Because of these conditions, the body 11 will over-travel a relatively large distance as compared to the nose, even after the impact force on it is removed. It can be seen from the foregoing that this set of conditions provides an assured body over-travel that causes a gap delta (δ) shown in FIG. 2 to be opened at the end of each cycle. It follows that pursuant to the invention, the nose 15 will be the first member impacted on the next cycle, as reference to FIG. 2 will illustrate.
Certain analytical aspects of the second inventive embodiment are technically significant. The transmission of impact energy to the soil through the light nose structure is comparatively very efficient. Since most of the soil resistance is at the nose, most of the work to be done is also at the nose. Accordingly, the decoupled nose scheme of the second inventive embodiment matches a high efficiency impact process to that portion of the job requiring the most work.
It can also be shown that after several cycles any initial gap such as δ will progressively increase or decrease until a steady state value is reached. Thereafter, the gap δ remains essentially constant unless the ratio of soil forces changes.
An idea of the increased impact efficiency to be gained from the fully decoupled nose of the second illustrative embodiment is illustrated in FIG. 3. For a 9 lb. nose, a 25 lb. hammer, and a 62 lb. body, a conservative estimate for the efficiency for the fully decoupled nose propulsion scheme is:
e = (.151(1 + F*)/.94 + .205F*) (1)
where F* is the ratio of soil force on the nose to the tunnel friction force on the body. A typical range of values for F* is 3 to 10. In contrast, a similarly obtained conservative estimate for a nose that is structurally connected to the Mole body is given by (H=hammer, B=body, N=nose)
e = m H /m H +m B +m N (2)
The aforementioned conservative estimates are based on a "coefficient of Restitution" equal to zero. This is what is referred to herein as a "least efficient Impact."
Both efficiencies are plotted in FIG. 3 against the ratio F*. Values of F* below 1.5 are not realistic in practice and hence are omitted.
From FIG. 3 it can be seen that the efficiency of a decoupled nose actually increases in normal soils by a factor of about 1.5; and more in harder soils.
In the second inventive embodiment, the total impulse into the soil is spread out over a longer time because of the sequential impacts--first on the nose structure and then on the body; and thereafter a possible third impact on the nose again. Since generally the soil's penetration resistance on the nose is much greater than the tunnel friction on the body, the total impulse consists of first a short time-duration, large magnitude force at the nose, followed by a long time-duration small magnitude force at the body. If the tunnel friction is sufficiently small, the body may travel far enough to impact the nose, thereby producing a third part of the total impulse. This third part of the total impulse is a force which is the sum of both the nose penetration force and the tunnel friction, and it persists for sufficient time to consume all the remaining impulse. The total impulse--which must be a constant equal to the linear-momentum of the impacting hammer--is thus advantageously divided into two or three parts which together must necessarily persist for a longer time than if the total impulse were not so divided. Consequently, according to the common principle of both decoupling schemes, the Mole must penetrate further into soil for each impact of the hammer.
Whereas in the first embodiment vibrational energy is recaptured, in this second embodiment a portion of the vibrational energy which otherwise would be produced is prevented from being so produced, and this energy is utilized advantageously to increase the Mole penetration.
Hydraulic positioning and coupling means (not shown) can obviously accomplish by means of fluid springs and/or viscous coupling the desired decoupling between the Mole nose and body such as described in embodiments of FIGS. 1 and 2.
A variation of the FIG. 2 embodiment is shown in FIG. 6 where like numerals denote like components. The nose 15 in this embodiment is firmly attached to an anvil 50. The rear face annulus 51 of anvil 50 is impacted by hammer 13. The impact energy will be imparted directly to nose 15, or to body 11 through a subanvil 52 contacted by the forward anvil shoulder 53--depending upon the rest position last assumed by nose 15. If subanvil 52 is constructed of hard metal, the operation is substantially the same as in FIG. 2. Advantageously, however, subanvil 52 may be made of a high-modulus elastomer such as polyurethane, in which case the shock-induced body stresses are further reduced. Importantly, the benfit of maximum impact on nose 15 is still realized.
The further variation depicted in FIG. 7 is akin to the FIG. 6 embodiment, except that washer-springs 54 are interposed between nose 15 and the forward end 11a of body 11. Similarly, washer-springs 55 are interposed between the anvil shoulder 53 and the body shoulder 11b. It will be appreciated that, with both sets 54, 55 of washer springs in place, the penetration operation is substantially the same as the embodiment depicted in FIG. 1. With the front set 54 of washer springs removed, the penetration operation is much the same as depicted in FIG. 6.
For the FIG. 7 embodiment, elastomer springs 54a, 55a made, for example, of a urethane polymer can be substituted for the washer sets 54, 55. Such a substitution is depicted in FIG. 8.
Several further advantages of the decoupled nose scheme of the FIGS. 2, 6, 7 and 8 inventive embodiments may be noted. The scheme is adaptive in the sense that the greater impulse is applied to either the nose or the body depending upon whether the instantaneous penetration resistance on one or the other is greater. Further, in rocky soil conditions all the impact forces are directed into the rock thereby reducing the possibility of stalling. The Mole body may not be impacted at all for several cycles at a time thus eliminating during these cycles the body stress that contributes to fatigue. Additionally, in a nonsteering mode of operation, the body is impacted at a reduced velocity, thereby reducing shock stresses in the shell and mounting hardware. Finally, the body need be impacted only as hard as necessary to overcome steering drag and internal friction; and this is achieved in the present invention.
The spirit of the invention is embraced in the scope of the claims to follow.
This invention relates to earth penetrators; and particularly those of the self-propelled missilelike type commonly known as Moles.
BACKGROUND OF THE INVENTION
In U.S. Pat. No. 3,465,834 of H. Southworth, Jr. issued Sept. 9, 1969, and assigned to applicants' assignee, there is described a linear impact propulsion system in which a hammer traveling on a central axial shaft within the Mole impacts upon a stationary forward anvil rigidly attached to the Mole body. Kinetic energy from the hammer is transferred on impact to the anvil and Mole body directly.
Analysis has indicated that a substantial percentage--almost two-thirds under some conditions--of the hammer impact energy is dissipated as unnecessary shock and vibration losses in the Mole body. In addition to the penetration loss this represents, excessive stress also causes material fatigue, particularly in the so-called shell and hardware attached thereto.
Although high stress levels in the shell are to be avoided, it is also true that the shell must be stressed to some extent in order to overcome tunnel friction and drag.
Accordingly, one object of the invention is to increase the penetration efficiency of a Mole driven by a linear impacting propulsion system.
A second inventive object is to reduce the stress and shock magnitudes and vibrational energy losses within a Mole driven by linear impacting means.
A third inventive object is to control the stress level in the Mole shell within a range acceptable to overcome tunnel friction on the one hand but avoid stresses leading to fatigue on the other.
A further inventive object is to improve the Mole capability for starting into the ground.
Another inventive object is to reduce impact modulation of the detector coil signals.
An important further inventive object is to avoid seriously foreshortening the Mole life by exposure to excessive stresses.
A more specific object is to separate the hammer energy into two adaptable portions--an impacting portion to generate the high peak forces necessary to drive the nose into the soil and thereby form the tunnel; and a much smaller damped impact portion to propel the Mole body through the tunnel.
A still further inventive object is to improve penetration in rocky or very hard soil by increasing the maximum force which the Mole can produce at its nose.
SUMMARY OF THE INVENTION
Pursuant to the general concept of the present invention, the foregoing and further objects are achieved by mechanically and physically separating the Mole nose from the Mole body. The "decoupled" nose can move to a controlled extent axially with respect to the Mole body. The nose is impacted either directly or indirectly by the traveling hammer.
In a particular inventive embodiment, the Mole body and nose although physically distinct are compliantly coupled through a cylindrical elastomer sleeve working in shear. In one embodiment workable with a Mole of about 3 inches outside diameter, the compliant sleeve is about 15 inches long with a wall thickness of 1/8 inch and extends over most of the Mole body length. Normally the compliant sleeve never contains all the impact energy at any one time.
In a second inventive embodiment, the Mole nose is free to slide axially on the outside surface of a shell. Impacting surfaces are provided within the Mole--one on the free sliding nose and the other on the Mole body. The linear impacting hammer first contacts a free sliding anvil structure, which transmits the impact first to the nose and then to the body. Advantageously, the nose is a lightweight structure that decelerates quickly in soil when the impact force on it is removed. Thus, pursuant to this aspect of the invention, the nose is the first member impacted each cycle of the hammer.
The Mole body advantageously travels some distance after the impact force on it is removed. This condition of overtravel pursuant to this aspect of the invention is caused by the relatively large mass and small tunnel resistance of the body as compared to the nose and is what assures that the nose will be the first member impacted on the next cycle.
From the standpoint of improving penetration efficiency, the common principle behind all the decoupling schemes hereinafter to be described is that of spreading the hammer impact impulse into the soil over a longer time period. Energy that will otherwise become vibrational energy, can be utilized for penetration; and energy that already has become vibrational energy can be recaptured for the same purpose. It can be demonstrated that penetration must necessarily increase if the impulse is spread out over a longer time.
The decoupling schemes perform three useful independent functions. Each acts as a shock isolator to limit the peak stresses in the Mole body and also dampens the vibration. Secondly, each permits relatively unimpeded motion of the nose with respect to the body, thereby increasing the stall force. That is, decoupling allows the Mole to exert a larger maximum force on soil at the nose, which is useful in hard soil or rocky soil conditions. Thirdly, decoupling permits a more efficient utilization of energy, i.e., it either reduces the fraction of energy in the vibrational mode or provides means for vibrational energy to perform useful penetration work.
The invention and its further objects, features, and advantages will be more readily discerned from a reading of the detailed descriptions to follow of illustrative embodiments thereof.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic side view in partial cutaway of a first embodiment;
FIG. 2 is a schematic side view of a second embodiment;
FIG. 3 is a graph;
FIGS. 4 and 5 are partial sectional views showing inventive design details of the FIG. 1 embodiment; and
FIGS. 6-8 are partial sectional views showing three related further inventive decoupling schemes.
DETAILED DESCRIPTIONS OF ILLUSTRATIVE EMBODIEMTNS EMBODIMENTS
In a first illustrative embodiment pictured in FIG. 1, the Mole designated generally as 10 consists of a body 11 with a propulsion section 12. A hammer 13 is mounted for linear motion along a central shaft or axis 14. The hammer 13 is driven by hydraulic or pneumatic propulsion means (not shown) which places imbalanced forces alternately on each end of the hammer 13 at prescribed times to cause reciprocation. Since the forcing mechanism forms no part of the present invention, its details are omitted, but a further discussion of one suitable such system may be found in the aforementioned U.S. Pat. No. 3,465,834 of H. Southworth, Jr. which to the extent relevant is hereby incorporated by reference.
An elongated nose 15 including a cone section 16 and a rock breaker 17 is mounted concentrically around the propulsion section 12 of body 11, by means of a compliant sleeve 18. The sleeve 18 is in effect a parallel spring-damper system that can safely absorb the entire impact energy of hammer 13, if necessary.
Since the impact energy is of the order 1,000 inch pounds and since it is desirable that the force transmitted through the spring to the body should not exceed 30,000 pounds, a spring constant (stiffness) of approximately 500,000 pounds per inch and maximum spring displacement of 0.060 inch are appropriate design parameters. Spring displacement in this case is the maximum that could possibly occur, since it corresponds to a situation where all the impact energy resides in the spring.
The sleeve 18 advantageously extends for substantially the entire length of the propulsion section 12 and has at least enough thickness to keep the maximum shear strain under 0.5. Such a thickness should be twice the maximum spring displacement or about 1/8 inch. Additionally, the sleeve 18 must be of sufficient length so that the shear stress does not exceed a safe working limit of either the elastomer material or the bond strength of the elastomer to the metal shell. Typically this value is about 200 psi. The numbers in the above analysis follow from a Mole geometry of approximately three inches in diameter and a maximum of 1,000 inch pounds linear impacting energy.
Advantageously, the material of sleeve 18 is an elastomer poured or injected into the zone between the body and nose, and allowed to cure in place. The surfaces of contact are treated so as to perfect a bond between the elastomer and the surfaces.
The Mole interior is protected from dirt and other particle entry by the bonding of the elastomer sleeve to the body 11 and the nose 15. In FIG. 1, a small gap indicated by the legend is left between the body 11 and the nose 15; but it is seen that no significant amount of dirt can be entrapped therein. The gap can also be filled with elastomer, if desired. FIG. 4 illustrates an alternative structure for further avoiding dirt entrapment, by rounding off the corner 40 of the Mole body 11 and the adjacent end 41 of the nose 15.
Because of the manner in which load is applied to the compliant sleeve 18 following impact of the hammer on nose 15, it is advantageous to taper the inside of skirt 15a of nose 15 in a backwardly direction at a taper of about 1/4 inch per foot of length, as shown in FIG. 5. Concurrently, the outside of the adjacent skirt 11a of Mole body 11 advantageously is tapered to the same extent in a forwardly direction. The advantage of this design is to save weight and space by reducing the thickness of the lightly loaded front end of the skirt 11a and rear end of the skirt 15a.
In the first inventive embodiment just described, the total impulse into soil is spread out over a longer time because of the oscillatory motion of the nose with respect to the Mole body, causing the nose to separate slightly from the soil for a portion of each oscillation. Since the total impulse is a constant, equal to the linear momentum of the impacting hammer; and since the impulse is transmitted into the soil at the nose only when the nose is in contact with soil; and since the soil's penetration resistance at the nose is not significantly different whether the impulse is one continuous impulse or a series of shorter time-duration impulses, it follows that the total impulse must necessarily be spread out over a longer time period thereby producing a greater soil penetration.
It can be shown that this separation and recontacting of the nose with the soil produces a mathematical "non-linearity" in the differential equations which describe the physical situation and thereby provide a means for energy in the vibrational mode to do useful penetration work. In general, vibrational energy can produce no net work on the mass center of a system unless there are nonlinearities in the system. This more efficient utilization of energy increases the Mole's penetration for both hard and soft soils, although it is more effective in hard soil conditions. Furthermore, the use of a massive hammer (large ratio of hammer mass to body mass) decreases the maximum amount of vibrational energy that can develop. For this reason it is desirable to also use a tungsten alloy hammer while designing the body to be as light as possible.
Advantageously, the motion of the nose with respect to the body must be of sufficient amplitude to ensure this separation and recontacting situation. This will be the case if the nose is a lightweight structure compared with the body.
Pursuant to a second inventive embodiment depicted in FIG. 2, the Mole 10 (its forward section only being shown) consists of elements numbered and defined with respect to FIG. 1, and in addition comprises a free-moving anvil 30 and the other elements now to be described.
The anvil 30 is mounted for axial movement along shaft 14. Anvil 30 includes a forward impacting surface 31 that impacts with the nose 15 interior; and a rear impacting surface 32 that is contacted by the advancing hammer 13. The nose 15 is slidably mounted on a forward shoulder 33 of the Mole body 11. The extent of rearward movement of nose 15 is blocked by the end 34 of shoulder 33; and the extent of forward movement of nose 15 is limited by the stop 38 at the nose 15 aft end acting against the shoulder 39 of the body 11.
Pursuant to this aspect of the invention, the moving anvil 30 is free to move from a rear position limited by contact of the axial shaft 14 end with the surface 35; and a forward movement limited by contact of the impacting annulus 36 with the body stops 37 of the Mole body 11.
Pursuant to this second inventive embodiment, the hammer 13 impacts on the traveling anvil 30, which in turn impacts first upon the Mole nose 15 and then the Mole body 11 through contact with body stops 37. Since the nose is light and therefore will be decelerated quickly by soil forces whenever the impact forces are removed, the nose 15 will be the first member impacted each cycle.
It can be demonstrated that the largest impact forces by far are transmitted through the nose, thereby providing substantial penetration efficiency. The secondary impact on the body occurs at a lower impact velocity; thus, substantially less stress is produced in the body by the secondary impact. The primary impact of the hammer 13 with the nose 15, of course, produces no significant shock stresses in the body.
The body is a relatively massive structure and, further, is retarded by relatively small soil forces. Because of these conditions, the body 11 will over-travel a relatively large distance as compared to the nose, even after the impact force on it is removed. It can be seen from the foregoing that this set of conditions provides an assured body over-travel that causes a gap delta (δ) shown in FIG. 2 to be opened at the end of each cycle. It follows that pursuant to the invention, the nose 15 will be the first member impacted on the next cycle, as reference to FIG. 2 will illustrate.
Certain analytical aspects of the second inventive embodiment are technically significant. The transmission of impact energy to the soil through the light nose structure is comparatively very efficient. Since most of the soil resistance is at the nose, most of the work to be done is also at the nose. Accordingly, the decoupled nose scheme of the second inventive embodiment matches a high efficiency impact process to that portion of the job requiring the most work.
It can also be shown that after several cycles any initial gap such as δ will progressively increase or decrease until a steady state value is reached. Thereafter, the gap δ remains essentially constant unless the ratio of soil forces changes.
An idea of the increased impact efficiency to be gained from the fully decoupled nose of the second illustrative embodiment is illustrated in FIG. 3. For a 9 lb. nose, a 25 lb. hammer, and a 62 lb. body, a conservative estimate for the efficiency for the fully decoupled nose propulsion scheme is:
e = (.151(1 + F*)/.94 + .205F*) (1)
where F* is the ratio of soil force on the nose to the tunnel friction force on the body. A typical range of values for F* is 3 to 10. In contrast, a similarly obtained conservative estimate for a nose that is structurally connected to the Mole body is given by (H=hammer, B=body, N=nose)
e = m H /m H +m B +m N (2)
The aforementioned conservative estimates are based on a "coefficient of Restitution" equal to zero. This is what is referred to herein as a "least efficient Impact."
Both efficiencies are plotted in FIG. 3 against the ratio F*. Values of F* below 1.5 are not realistic in practice and hence are omitted.
From FIG. 3 it can be seen that the efficiency of a decoupled nose actually increases in normal soils by a factor of about 1.5; and more in harder soils.
In the second inventive embodiment, the total impulse into the soil is spread out over a longer time because of the sequential impacts--first on the nose structure and then on the body; and thereafter a possible third impact on the nose again. Since generally the soil's penetration resistance on the nose is much greater than the tunnel friction on the body, the total impulse consists of first a short time-duration, large magnitude force at the nose, followed by a long time-duration small magnitude force at the body. If the tunnel friction is sufficiently small, the body may travel far enough to impact the nose, thereby producing a third part of the total impulse. This third part of the total impulse is a force which is the sum of both the nose penetration force and the tunnel friction, and it persists for sufficient time to consume all the remaining impulse. The total impulse--which must be a constant equal to the linear-momentum of the impacting hammer--is thus advantageously divided into two or three parts which together must necessarily persist for a longer time than if the total impulse were not so divided. Consequently, according to the common principle of both decoupling schemes, the Mole must penetrate further into soil for each impact of the hammer.
Whereas in the first embodiment vibrational energy is recaptured, in this second embodiment a portion of the vibrational energy which otherwise would be produced is prevented from being so produced, and this energy is utilized advantageously to increase the Mole penetration.
Hydraulic positioning and coupling means (not shown) can obviously accomplish by means of fluid springs and/or viscous coupling the desired decoupling between the Mole nose and body such as described in embodiments of FIGS. 1 and 2.
A variation of the FIG. 2 embodiment is shown in FIG. 6 where like numerals denote like components. The nose 15 in this embodiment is firmly attached to an anvil 50. The rear face annulus 51 of anvil 50 is impacted by hammer 13. The impact energy will be imparted directly to nose 15, or to body 11 through a subanvil 52 contacted by the forward anvil shoulder 53--depending upon the rest position last assumed by nose 15. If subanvil 52 is constructed of hard metal, the operation is substantially the same as in FIG. 2. Advantageously, however, subanvil 52 may be made of a high-modulus elastomer such as polyurethane, in which case the shock-induced body stresses are further reduced. Importantly, the benfit of maximum impact on nose 15 is still realized.
The further variation depicted in FIG. 7 is akin to the FIG. 6 embodiment, except that washer-springs 54 are interposed between nose 15 and the forward end 11a of body 11. Similarly, washer-springs 55 are interposed between the anvil shoulder 53 and the body shoulder 11b. It will be appreciated that, with both sets 54, 55 of washer springs in place, the penetration operation is substantially the same as the embodiment depicted in FIG. 1. With the front set 54 of washer springs removed, the penetration operation is much the same as depicted in FIG. 6.
For the FIG. 7 embodiment, elastomer springs 54a, 55a made, for example, of a urethane polymer can be substituted for the washer sets 54, 55. Such a substitution is depicted in FIG. 8.
Several further advantages of the decoupled nose scheme of the FIGS. 2, 6, 7 and 8 inventive embodiments may be noted. The scheme is adaptive in the sense that the greater impulse is applied to either the nose or the body depending upon whether the instantaneous penetration resistance on one or the other is greater. Further, in rocky soil conditions all the impact forces are directed into the rock thereby reducing the possibility of stalling. The Mole body may not be impacted at all for several cycles at a time thus eliminating during these cycles the body stress that contributes to fatigue. Additionally, in a nonsteering mode of operation, the body is impacted at a reduced velocity, thereby reducing shock stresses in the shell and mounting hardware. Finally, the body need be impacted only as hard as necessary to overcome steering drag and internal friction; and this is achieved in the present invention.
The spirit of the invention is embraced in the scope of the claims to follow.