No full-text available
To read the full-text of this research,
you can request a copy directly from the author.
Computer Modeling and Simulation of Percussive Drilling of Rock
Abstract
As percussive drills are largely built up of rods which are subjected to axial impact at moderate velocities, elastic wave propagation in such rods, and associated phenomena, are fundamental and theory of elastic waves in rods is presented. As the designs of percussive drills are generally such that 1-D theory is adequate for engineering purposes, the presentation is limited to such a theory. Numerical examples are given. It is pointed out that models of existing or conceivable percussive drills can be considered to be built up of members which may or may not coincide with components of the real drills. Different types of such members are segment members, interface members and end members. Finally, some features of simulation programs developed, tested and used by the author and his coworkers are presented. -from Author
... The mechanics of percussive drilling have been addressed in the literature [2][3][4][5][6] through analytical, numerical, and experimental studies aimed to clarify energy conversion, transfer, and efficiency. The interaction between the worked material and the drill bit depends on the force penetration curve, which allows the development of numerical models [7] to predict the efficiency of the percussive drilling. The force-penetration curve is generally defined by one [8] or two strain gauges [7,9] mounted at sections far from the impact region. ...
... The interaction between the worked material and the drill bit depends on the force penetration curve, which allows the development of numerical models [7] to predict the efficiency of the percussive drilling. The force-penetration curve is generally defined by one [8] or two strain gauges [7,9] mounted at sections far from the impact region. The interaction between the drill bit and the worked material has also been modeled with lumped parameters modeling three-dimensional finite element methods [10][11][12][13][14]. ...
This work describes the results of a test campaign aimed to measure the propagation of longitudinal, torsional, and flexural stress waves on a drill bit during percussive rock drilling. Although the stress wave propagation during percussive drilling has been extensively modeled and studied in the literature, its experimental characterization is poorly documented and generally limited to the detection of the longitudinal stress waves. The activity was performed under continuous drilling while varying three parameters, the type of concrete, the operator feeding force, and the drilling hammer rotational speed. It was found that axial stress wave frequencies and spectral amplitudes depend on the investigated parameters. Moreover, a relevant coupling between axial and torsional vibrations was evidenced, while negligible contribution was found from the bending modes. A finite element model of the drill bit and percussive element was developed to simulate the impact and the coupling between axial and torsional vibrations. A strong correlation was found between computed and measured axial stress spectra, but additional studies are required to achieve a satisfactory agreement between the measured and the simulated torque vibrations.
... La solución de este sistema de ecuaciones para barras con superficies laterales libres es compleja [3].. Sin embargo, se puede describir la propagación de esfuerzos en la dirección axial de una barra de manera más sencilla si se considera que en todo instante de tiempo cada sección de la barra permanece plana y está sometida a una distribución de esfuerzos uniforme. Esta aproximación resulta acertada para describir el impacto longitudinal entre barras como así lo demuestran los trabajos de Lundberg [5], Hustrulid y Fairhurst [6] y Elías y Chiang [7]. El método numérico basado en el principio de impulso y cantidad de movimiento, es una alternativa para modelar y simular el impacto entre múltiples cuerpos elásticos. ...
... El método numérico basado en el principio de impulso y cantidad de movimiento, es una alternativa para modelar y simular el impacto entre múltiples cuerpos elásticos. Este método consiste en resolver la ecuación de la onda por tramos empleando el principio de impulso y cantidad de movimiento, según una metodología propuesta por Lundberg [5] y modificada por Chiang y Elías [8]. Este método es muy sencillo de implementar y resulta particularmente útil para caracterizar la operación de herramientas de perforación percusiva. ...
... A numerical method has been developed by the author to estimate the impact stresses in taut cables subject to longitudinal impacts, which is based in the Principle of Impulse-Momentum[5] [6]. The method has been applied earlier to simulate the impact between elastic bodies, in particular in rock drilling impact tools, and is similar in nature to other methods published in the scientific literature [7] [8]. ...
A skyline carriage prototype for timber harvesting in steep terrain has been developed that incorporates low cost automation capabilities in order to help increase productivity and to reduce the risk of accidents. The main improvements are the replacement of the heavy and expensive hydraulic actuation system for a pneumatic actuation system and the addition of cable slack pulling capability. The skyline carriage is operated entirely by remote control and computer logic is used to a check for consistency of the operator commands.
... In the context of percussive drilling it is important to understand the soil-tool interaction. In the case of percussive drilling of rocks, the interaction between the drill rod and the rock, before rock breakage, can be considered as linear elastic (Lundberg (1993)). ...
Planetary drilling is a vital task in the challenge for space exploration. Drilling and planetary soil/rock sample acquisition provides information about history of past events, minerals and chemical composition of the soil/rock of the planetary body, available resources for future manned missions and the mechanical behavior of the planetary soil/rock. Several types of drilling devices have been proposed for lunar, Mars, and planetary subsurface exploration. However, these devices have limitations (e.g. heavy equipment and need for large axial force) that need to be addressed in order to be feasible for extraterrestrial bodies’ exploration. A special type of ultrasonic percussive drill have been proposed by Honeybee Spacecraft Mechanisms Corporation and NASA Jet Propulsion Laboratory (JPL) to address the limitations of current drilling devices. In this study, the mechanical system of the ultrasonic percussive drill and its interaction with the supporting medium is studied. The percussive mechanism consists of an ultrasonic horn, a free mass, and the drill rod. Special attention is given to the impact between the free mass and the drill rod, including the effects of structural damping, the supporting medium of the rod, plastic deformation in the contact area, and repeated impacts of the free mass on the drill rod. A general methodology to analyze the impact of the free mass on the drill rod, analogous to the longitudinal impact of a mass on a rod, is developed. The methodology involves uncoupling the nonlinear problem by determining the response of each body independently under the contact force to find the local indentation, then using Hertz force-indentation relation to find the contact force. This method is applied using mode superposition method and finite element technique for various support conditions of the rod (e.g. rigid, elastic, free, viscoelastic). Additionally, a model to account for drill rod penetration into the supporting medium due to impact of the free mass is also presented. It was found that for an undamped rod, the support condition of the rod does not affect the contact force if the contact ends before the arrival of the reflected wave to the point of contact. It was also observed that the contact force due to impact, for a given support condition, increases with increasing damping of the rod. Moreover, a modified Hertz equation is introduced to include plastic deformation on the rod due to impact of the free mass. The study was performed for identical repetitive impacts and it was found that the largest plastic deformation occurs in the first impact, with additional plastic deformation decreasing with the increasing number of impacts. The dynamic response of the overall percussive drill was investigated with a finite element model, including the interaction of the free mass with the ultrasonic horn and the drill rod. A numerical example indicated that the dynamic response of the ultrasonic drill is directly affected by the supporting medium. It was found that system with fixed support experienced a higher frequency of oscillation of the free mass and higher impact force compared to the system with the elastic support and the model accounting for penetration into the supporting medium.
... BLINDHEIM 1979: 287, SANDVIC 1978. LUNDBERG (1973LUNDBERG ( , 1993 sieht den Bohrvorgang als Problem des Energietransfers zwischen Bohrhammer (Krafterzeugung), Bohrstange (Kraftübertragung) und Bohrkrone (Schnittstelle Maschine-Gebirge = Lösearbeit). Untersuchungen an Tunnelbohrmaschinen haben gezeigt, daß nur ein Bruchteil der aufgewendeten Energie (2 -3%) in Zerspanungsarbeit umgesetzt wird. ...
(German Version below)
Drillability in Hard Rock Tunneling by Drilling And Blasting
Usually the main subject in preliminary site investigations prior to tunnelling projects is the prediction of tunnel stability. During the last years in conventional drill- and blast tunnelling, problems have occurred also connected with the accurate prediction of drillability in hard rock. The drillability is not only decisive for the wear of tools and equipment but is - along with the drilling velocity - a standard factor for the progress of excavation works. The estimation of drillability in predicted rock conditions might bear an extensive risk of costs. Therefore an improved prediction of drilling velocity and bit wear would be desirable.
The drillability of a rock mass is determined by various geological and mechanical parameters. In this report some major correlations of specific rock properties as well as geological factors with measured bit wear and drilling velocity are shown. Apart from conventional mechanical rock properties (unconfined compressive and tensile strength, Young's modulus) a new property for toughness referring to drillability has been introduced: the specific destruction work WZ. This new property makes it possible to understand better the connection between drilling velocity and the main mechanical rock character. For monitoring and evaluation of the point load test a new statistical method has been introduced. As well as mechanical rock properties the influences of geological parameters on drillability were the main topic of the study. As the principal results of this dissertation can be recorded:
1. Drilling bit wear
- The bit wear increases with the equivalent quartz content. The equivalent quartz content builds the main property for the content of wear-relevant minerals.
- A correlation between bit consumption and technical machine parameters (power of percussive drill hammer, shape of drilling bits) has not been proved.
- For various groups of rock types different connections with the equivalent quartz content could be detected. Qualitative and quantitative bit wear is also dependent on the interlocking of microstructures or the quality of the cementation (e.g. porosity of sandstones). In hydrothermal decomposed rock and sandstone a connection with pore volume could be found. Even porosity is an indicator for decomposition or quality of the grain-to-grain strength in sandstones.
2. Drilling velocity
- First of all, drilling progress is dependent on technical machine parameters (power of percussive drill hammer, shape of drilling bits).
- In general, drilling velocity decreases with rising mechanical rock properties (unconfined compressive strength, Young´s modulus, specific destruction work, tensile strength and rock density).
- Only three of the six treated rock properties - destruction work, unconfined compressive strength,
Young´s modulus, tensile strength, the relation of compressive/tensile strength ("toughness") and rock density showed a significant correlation with drilling rates: the destruction work can be noted as a highly significant, the unconfined compressive strength and the tensile strength can merely be de-scribed as significant parameters.
- Besides, drilling velocity is dependent on a whole lot of geological parameters. Those principal para-meters include jointing of rock mass, orientation of schistosity (rock anisotropy), degree of interlocking of microstructures, porosity and quality of binder in sandstone, degree of hydrothermal decomposition and weathering of rock mass.
Further causes, such as primary stress conditions, fracturing / loosening of rock mass (e.g. in areas of landslides) could not be investigated in this dissertation.
On this knowledge basis a classification of drillability according to bit wear and drilling velocity has been worked out. To be used in the context of preliminary geotechnical site investigations of tunnelling projects a classification has been set up, allowing to predict drilling rates and bit consumption in rock u-sing the main petrographical (equivalent quartz content) and mechanical rock properties (destruction work, unconfined compressive strength) with a sufficient accuracy. Knowledge of these relations and their causes is necessary to make the choice of drilling rigs easier, to estimate the working and drilling progress and - above all - to calculate drilling expenses. For this purpose an inexpensive but practical investigation program has been submitted, which helps to improve the estimation of rock drillability in planning future tunnel projects. Finally some hints are given for carrying out investigations in poor drilling and blasting conditions during running excavation works
Zusammenfassung
Die Bohrbarkeit des Gebirges wird durch unterschiedliche geologische und felsmechanische Parameter bestimmt. In diesem Beitrag werden die wesentlichen Abhängigkeiten zwischen den spezifischen Materialeigenschaften von Gestein und Gebirge und den meßbaren Parametern Bohrkronenverschleiß und Bohrgeschwindigkeit aufgezeigt. Neben den konventionellen felsmechanischen Kennwerten (Druck-, Zugfestigkeit und Elastizitätsmodul) wurde ein neues Maß für die Zähigkeit bezüglich der Bohrbarkeit von Gesteinen eingeführt: die spezifische Zerstörungsarbeit Wz. Die neue Auswertemethode ermöglicht es, den ursächlichen Zusammenhang zwischen der Netto-Bohrgeschwindigkeit und den felsmechanischen Eigenschaften eines Gesteins besser als bisher nachzuvollziehen. Für die Auswertung des Point-Load-Tests wurde ein neues Auswerteverfahren auf statistischer Basis vorgestellt. Neben den felsmechanischen Parametern bilden die Einflüsse der geologischen Faktoren auf die Bohrbarkeit ein wesentliches Arbeitsthema. Als Ergebnisse dieser Arbeit können festgehalten werden:
1. Bohrkronenverschleiß
- Der Bohrkronenverschleiß steigt mit dem äquivalenten Quarzanteil eines Gesteins. Er bildet einen Leitwert für den Gehalt an verschleißrelevanten Mineralen.
- Eine Abhängigkeit der Bohrkronenstandzeit von maschinentechnischen Parametern (Bohrhammerleistung, Bohrkronenform) konnte nicht nachgewiesen werden.
- Für die verschiedenen Gesteinsgruppen können z. T. unterschiedliche Abhängigkeiten vom äquivalenten Quarzanteil festgestellt werden. Die Verschleißcharakteristik hängt außerdem vom Verzahnungsgrad des Mikrogefüges bzw. der Qualität des Bindemittels (z.B. Porosität der Sandsteine) ab. Für hydrothermal zersetzte Gesteine und für Sandsteine konnte ein Zusammenhang mit dem Porenvolumen nachgewiesen werden. Dabei ist die Porosität nur ein Indikator für den Verwitterungsgrad oder die Qualität der Korn-Korn-Bindung in Sandsteinen.
2. Bohrgeschwindigkeit
- Der Bohrfortschritt ist zunächst von maschinentechnischen Parametern (Bohrhammerleistung, Bohrkronenform) abhängig.
- Die Bohrgeschwindigkeit sinkt generell mit steigenden felsmechanischen Kennwerten (einaxiale Druckfestigkeit, Elastizitätsmodul, Zerstörungsarbeit, Spaltzugfestigkeit und Trockenrohdichte).
- Von den sechs Parametern Zerstörungsarbeit, Elastizitätsmodul, einaxiale Druckfestigkeit, Spaltzugfestigkeit, Verhältnis Druck-Zugfestigkeit („Zähigkeit“) und Trockenrohdichte weisen nur drei einen signifikanten Zusammenhang mit der Bohrgeschwindigkeit auf: die Zerstörungsarbeit kann als hoch-signifikanter, die einaxiale Druckfestigkeit und die Spaltzugfestigkeit können lediglich als signifikante Parameter bezeichnet werden.
- Die Bohrgeschwindigkeit ist außerdem von einer Reihe geologischer Faktoren abhängig. Zu diesen Einflussgrößen gehören der Durchtrennungsgrad des Gebirges, die Raumlage der Schieferung (Gesteinsanisotropie), der Verzahnungsgrad des Mikrogefüges, die Porosität und Qualität des Bindemittels von Sandsteinen, der Grad der hydrothermalen Zersetzung und der Verwitterungszustand von Gestein und Gebirge.
Weitere Einflussfaktoren wie die Primärspannungsverhältnisse oder eine eventuelle Auflockerung des Gebirges (z.B. in Talzuschubs-Zonen) konnten im Rahmen dieser Arbeit nicht untersucht werden.
Auf der Basis der ausgewerteten Daten wurde eine Klassifikation der Bohrbarkeit nach Bohrkronenverschleiß und Bohrgeschwindigkeit erarbeitet. Zur Verwendung im Rahmen von geotechnischen Voruntersuchungen zu Tunnelprojekten wurde eine Klassifikation aufgestellt, mit der die Bohrbarkeit von Gesteinen anhand der wichtigsten petrographischen (äquivalenter Quarzanteil) und felsmechanischen Kennwerte (spezifische Zerstörungsarbeit, einaxiale Druckfestigkeit) mit einiger Genauigkeit vorhergesagt werden kann. Die Kenntnis dieser Zusammenhänge und Hintergründe ist notwendig, um die richtige Auswahl der Bohrgeräte zu erleichtern, den Arbeits- und Bohrfortschritt abzuschätzen und vor allem die Bohrkosten zu kalkulieren. Zu diesem Zweck wurde ein Untersuchungsprogramm vorgeschlagen, welches bei künftigen Vorerkundungen für Tunnel- und Stollenprojekte helfen soll, Gestein und Gebirge im Hinblick auf die Bohrbarkeit besser zu erfassen. Hinweise für die Durchführung von Untersuchungen im Zuge der Beweissicherung bei Bohrbarkeitsproblemen während des Tunnelvortriebs schließen diese Studie ab.
A methodology developed to estimate with accuracy the instantaneous specific rock energy using corrected down-the-hole (DTH) drill monitoring data is presented. A specific rock energy profile can be generated for every hole, and thus a drilling site can be mapped for this index. A special data acquisition system was developed to measure and register the following operational variables: penetration rate, torque, hole depth, pull-down force, air pressure, revolutions per minute (rpm) and the hammer percussion frequency, the latter obtained by sound recording and signal processing. The measured data are fed into two simulation models that estimate the power absorbed by the rock through impact, and then the specific rock energy index. The first of these models simulates the thermodynamic cycle of the DTH hammer, rendering the piston kinetic energy at impact, impact velocity as well as impact frequency. The second model is used for stress wave propagation analysis to estimate the effective energy delivered to the rock. Correlations were found between the specific rock energy and penetration rate, and between the specific rock energy and impact frequency, as well as between the penetration rate and applied torque, and between the penetration rate and impact frequency. Nomenclature: A, hole area (m 2); E Impact , impact energy (piston contribution) (J); E Torque , torque energy (J); E Thrust , thrust energy (J); e, piston impact restitution coefficient; F, percussion frequency; F Pull-down , applied pull-down force (weight on bit) (N); F Thrust , thrust force (N); m Piston , piston mass (kg); SRE, specific rock energy (J cm-3); Pow Hammer , hammer power (W); V Impact , piston impact velocity (m s-1); V Penetration , penetration rate (m s-1); Torque, torque applied to the hammer (N m); W Rods-hammer , rods and hammer weight (N); ω, hammer angular velocity (rad s-1); ∆t, time increment; ∆θ, hammer spin angle increment; ∆x, hammer displacement increment; ∆Vo, rock volume increment
Because of variability of the force vs. penetration relationship (FPR) from one blow to another in percussive drilling, and difficulty to predict FPRs under such conditions, use is commonly made of simple FPR models, such as the bilinear one defined by its loading/unloading slopes. Here a biconvex model with an added parameter representing convexity is considered. One aim is to study the effect of convexity on maximal penetration, maximal force and efficiency. Another is to assess, with the biconvex FPR as an example, how well a bilinear FPR can be used to approximate one that is nonlinear. A simple percussive top-hammer drill model is considered, comprising a hammer, a drill rod and a bit with the same characteristic impedance. The maximal penetration is found to decrease and the maximal force to increase with increasing convexity. The efficiency has a maximum for a finite hammer length (incident wave duration), and the highest maximal efficiency is obtained for a linear FPR. With increasing convexity, the maximal efficiency decreases and occurs for shorter hammers (incident waves). The bilinear approximation of a biconvex FPR accurately predicts the position of the maximum in efficiency, even for large convexity, but somewhat overestimates its height and width.
Drilling machines using the top hammer drilling method incorporate a drilling mechanism whereby the torque and percussive force generated by the hydraulic drifter are transferred to the rock surface through a rod and drill bit. To date, mainstream studies of the hydraulic drifter addressed the behavior of the elastic waves produced by the impact and the drilling characteristics related to rock properties. Meanwhile, studies of design sensitivity analysis and percussion performance improvement for the development of a high-efficiency hydraulic drifter have been insufficient. In this study, to develop a high-efficiency hydraulic drifter, the validity of a model was verified by comparing the test results with a previously constructed analysis model. In addition, using the model, the supply pressure and inlet flow rate, stroke regulator opening, and striking piston’s acting area and mass were selected as the design parameters affecting the percussion performance; through the impact frequency and impact energy, the percussion performance was analyzed. As a result, design values that improve the percussion performance were derived.
The problem of finding the optimal incident wave of given duration that maximizes the efficiency of conversion of wave energy into work in percussive drilling with detachable drill bit is considered. The drill rod is modelled as 1D linearly elastic and the drill bit as a rigid mass. The bit/rock interaction is described by a history-dependent force versus penetration relation with different constant slopes for primary loading and unloading/reloading. A functional expressing the dependence of the efficiency on the shape of an arbitrary incident wave of given duration is derived and maximized. For short incident waves, there is a weak influence of the bit mass on the optimal wave shape which is nearly rectangular. For longer incident waves, there is a strong influence of the bit mass on the optimal wave shape which significantly differs from rectangular. The efficiencies for optimal waves approach those for rectangular waves for short waves. For long waves they approach or assume values which are independent of wave duration but decrease with increasing bit mass. Relative to commonly-used rectangular waves significant increase in efficiency can be achieved through optimization of the wave shape if the wave is not too short. Optimal incident waves can be realized accurately, e.g., by piezoelectric means.
This paper presents a novel dynamical model to analyze the long-term response of a percussive drilling system. This departs from existing approaches that usually consider a single activation and bit/rock interaction cycle for the analysis of the process performance. The proposed model integrates the axial dynamics of an elastic piston and an elastic drill bit, a motion-dependent pressure law to drive the piston, and a generalized bit/rock interaction law representative of the dynamic indentation taking place at the bit/rock interface. It applies to down-the-hole percussive drilling as well as top-hole, with minor modifications. The model does not account for the angular motion or the hole cleaning, however. The model is first formulated mathematically; then, a finite-dimensional approximation is proposed for computations. Numerical analyses of the model response, for a low-size down-the-hole percussive system, follow. The period-1 stationary response for the reference configuration is studied in detail, and parametric analyses assessing the influence on the rate of penetration of the bit/rock interaction parameters, the feed force, and the percussive activation parameters are conducted. These analyses reveal that the multiscale nature of the process is well captured by the model and recover expected trends for the influence of the parameters. They also suggest that a significant increase of the penetration rate can be achieved by increasing the percussive frequency. Copyright © 2015 John Wiley & Sons, Ltd.
Rock drill operations are classified as top hammer drilling (THD), down-the-hole drilling, or rotary drilling. The rock drill in the THD method consists of a percussion drill rig module and a drill bit. The percussion drill rig module consists of a drifter, feed drive, and auto rod changer. In particular, the drifter generates the impact and rotational force for drilling. The purpose of this study was to analyze the hydraulic circuits of the drifter and to develop an analysis tool for analyzing the impact capability. This study also analyzed the capability of the drifter with regard to the penetration rate and varying kinetic energy, which is dependent on rock stiffness, using the developed analysis tool.
A design methodology for Down-The-Hole (DTH) pneumatic hammers used for rock drilling is proposed which renders an optimal design for a given set of constraints. A generic non-linear dynamic model developed by the authors is used to compute the hammer performance. This model consists of a set of sir differential equations plus a set of twenty non-linear polynomial equations. In addition there are parameter range restrictions given by fabrication and operational standard procedures. In any given application, magnitudes such as power, impact energy, frequency, efficiency and mass flow may be sought for optimality. However these magnitudes must be computed by integration after solving the dynamic model over an entire cycle, thus traditional optimization methods for non-linear equations that are based in gradient information are not suitable. Hence a method that uses secant information is used to approximate the gradient of the space of design variables. Several prototypes using this optimization method have been designed and field tested. The results are in agreement with predicted values.
The problem considered is that of finding the optimum wave of given finite duration that maximizes the efficiency of conversion of wave energy into work in percussive drilling with integral drill steel (drill rod with integrated bit). A 1D model is used for the drill rod, and the bit-rock interaction is represented by a piecewise linear force versus penetration relation with different penetration resistances for primary loading and for unloading/reloading. A functional expressing the dependence of the efficiency on the incident wave is derived and maximized. The optimal incident wave has exponential shape with time constant for the growth rate equal to the characteristic response time of the percussive drill system, including the rock. The maximal efficiency increases monotonously with the duration of the optimal wave. It approaches zero for very short waves and unity for very long waves. Optimal waves of short duration are close to rectangular while those of long duration approach the semi-infinite exponential wave derived by Long in the 1960s. Optimal waves of medium or longer duration give significantly higher efficiencies than commonly used rectangular waves of the same duration.
The efficiency of churn drilling was studied with consideration of wave energy radiation into the rock. In this percussive drilling process, a hammer with cutters on its front subjects the rock to direct impact. A 3D elastic finite element model was used for the hammer and the rock, with a nonlinear force versus penetration relationship representing the rock crushing process. Two rock models with the same crushing properties were employed: (i) one perfectly rigid and (ii) one linearly elastic, representing Westerly Granite. Under favourable drilling conditions, with efficiencies higher than 60%, the elastic response of the rock significantly influences the efficiency of the drilling process. However, the radiation of wave energy into the rock, less than 3% of the impact energy, does not play an important role. Under unfavourable drilling conditions, with efficiencies lower than 40%, the elastic response of the rock does not significantly influence the efficiency of the drilling process even though the energy radiated into the rock is greater than 5% of the impact energy.
Transmission of quasi-longitudinal elastic waves through elastic bar transitions between uniform and collinear bar segments is studied by means of three-dimensional (3-D) finite element analysis. The aim is to examine, through an example, to what extent two geometrically different transitions which are transmission-equivalent in a one-dimensional (1-D) context may retain this property under 3-D conditions. The difference between the two transitions is assessed from the difference between 3-D results for reflected waves, while the presence of 3-D effects is judged from the difference between 3-D finite element and 1-D analytical results for transmitted waves. It is found that two smooth and significantly U001different elastic bar transitions which are transmission-equivalent in a 1-D context may retain this property with good approximation under significantly 3-D conditions, well beyond those under which 1-D theory can be used for prediction of wave shapes. This extends the domain of potential applications in areas such as percussive drilling correspondingly.
Transmission of extensional waves through elastic junctions between two uniform and collinear elastic bars with given characteristic impedances was studied with the aim to synthesize all junctions with the transmission properties of a given junction. For a junction which consists of a finite number N of segments with constant characteristic impedances and equal transit times, there are at most two to the power N different such transmission-equivalent (TE) junctions, including the one given. They can be obtained through substitutions which imply inversion in the unit circle of zeroes of a polynomial of degree N which represents the given junction. The TE junctions include one which can be obtained through combined inversion and reversion of the characteristic impedance function of the given junction. If the characteristic impedances of the input and output bars are the same, they also include junctions which can be obtained by either inversion or reversion. Numerical examples are given for cases with N=2,3,4 and 40. Experimental tests were carried out with the four TE junctions obtained with N=3 and with two out of those obtained with N=40. For the same incident waves, there was close agreement between the waves transmitted through any synthesized TE junction and the corresponding given junction, even in the presence of some moderate 3-D effects.
We present a numerical method derived from the Impulse-Linear Momentum Principle that can be used in the design of impact tools for rock drilling. This method allows the prediction of energy transmission to rock, the profiles of stress, displacement and velocity, all of which are important in the dynamic analysis of such tools. Using the Impulse-Linear Momentum Principle in an algorithmic manner, multibody interaction is simplified, and also different load and boundary conditions such as external forces, initial strains, and initial body separations can be directly considered. The accuracy of the method has been contrasted theoretically with both one-dimensional and three-dimensional FEM analysis, as well as experimentally.
A methodology developed to estimate with accuracy the instantaneous specific rock energy using corrected downthe-hole (DTH) drill monitoring data is presented. A specific rock energy profile can be generated for every hole, and thus a drilling site can be mapped for this index. A special data acquisition system was developed to measure and register the following operational variables: penetration rate, torque, hole depth, pull-down force, air pressure, revolutions per minute (rpm) and the hammer percussion frequency, the latter obtained by sound recording and signal processing. The measured data are fed into two simulation models that estimate the power absorbed by the rock through impact, and then the specific rock energy index. The first of these models simulates the thermodynamic cycle of the DTH hammer, rendering the piston kinetic energy at impact, impact velocity as well as impact frequency. The second model is used for stress wave propagation analysis to estimate the effective energy delivered to the rock. Correlations were found between the specific rock energy and penetration rate, and between the specific rock energy and impact frequency, as well as between the penetration rate and applied torque, and between the penetration rate and impact frequency.
In percussive drilling of rock, elastic stress waves are generated in a drill string through repeated axial impacts by the hammer of a rock drill. For holes deeper than a few meters, several drill rods are commonly joined by means of cylindrical coupling sleeves with internal threads which connect drill rods with external threads at their ends. Each coupling sleeve (CS) joint serves to transfer stress wave energy from one drill rod to the next with minimum loss of energy due to reflection and dissipation. This paper deals with the development of an identification procedure for the nonlinear dissipative spring mass (NDSM) model of a CS joint developed by Lundberg et al. Stiffness, friction and mass parameters were determined by minimizing the diffence between simulated and measured responses of the joint, in reflection or transmission, to the same incident stress wave loading. Similar results were obtained as with an existing mixed static and dynamic identification procedure, but with considerably less expenditure of equipment and time. The most reliable results and the smallest deviation between simulated and measured responses were achieved in transmission.
Wave propagation and efficiency in percussive rock drilling with a bent rod is studied theoretically using a combined approach of FFT and matrix method. The bent drill rod is represented by straight uniform elements. The extensional and flexural stress waves in the drill rod are described by the one-dimensional wave equation and by Euler-Bernoulli theory, respectively. The conditions at the bit and rock during loading are represented by a penetration resistance, a lateral stiffness and a rotational stiffness. Numerical results are given for two cases; the first involves a drill rod with a constant radius of curvature through a certain bend angle, and the second a drill rod with a sharp bend defined by the same bend angle. It is found that for slightly bent rods the efficiency is practically the same as for straight rods. Thus, slight bends, which are always present in real drill rods, do not appear to be significant from the point of view of efficiency. Furthermore, it is found that the efficiency can remain high even if the rod is significantly bent. Thus, percussive rock drilling systems with special designs for drilling in curved paths are feasible from the point of view of efficiency.
A three-dimensional (3D) finite element approach for modeling impact as it occurs in impact tools used in rock drilling is presented. The model permits one to simulate the energy transmission to the rock, the bit–rock interaction, and the process of rock fragmentation, all of which are important in the study and evaluation of such tools. The finite elements method (FEM) analysis allows one to simulate the impact in 3D stress–strain problems, to consider linear material properties, and to include post failure fracture propagation. Anisotropic elements have been used to model the rock post failure behavior. Infinite domain elements have been used to characterize boundary conditions far away from bit–rock interaction. The accuracy of the model has been evaluated both theoretically by comparing the result to those obtained with a model based on impulse–momentum principle as well as experimentally.
A methodology to obtain the dynamic force-penetration curve of a rock specimen is presented. For this, a simple portable experimental
setup is used. On one end, a hand-held hammer strikes a slender chisel. On the other end the chisel is in contact with the
rock specimen into which the generated stress wave propagates. Strain gages installed at a selected section of the chisel
are recorded throughout the impact duration. The theoretical signal at the same section is simulated using a numerical method
based on the impulsemomentum principle previously developed by the author. Essentially the shape of the simulated theoretical
response at the strain gage section depends on the impact velocity of the hammer and the slope of the assumed force-penetration
curve. These two parameters are adjusted until the shape of the theoretical signal at the strain gage closely matches the
experimental signal. Due to its simplicity, the proposed methodology has the potential of becoming a standard dynamic test
for obtaining the force-penetration curvs in rocks.
In this work a study of impact in Down-the-Hole (DTH) rock drilling is carried out. We present an alternative to a method previously introduced by Lundberg and his co-workers. Our model is formulated in terms of the impulse–momentum principle while Lundberg’s method is based in solving the one-dimensional wave equation. In the case of DTH drilling, the study of the subject becomes easier because the handling of many bodies interacting dynamically is simplified, and different boundary conditions, such as constant body forces, distributed forces and initial strains, can be directly included. The rock–bit interaction is modeled using both a non-linear spring and a variable gap using experimental parameter data obtained by other researchers and by a normalized quasi-static penetration test described in this work. The simulation results are in good agreement with results in previous publications as well as with experimental validation measurements carried out by the authors.
The aim is to investigate the influence of 3D effects on the efficiency of three processes for percussive rock drilling, viz., hammer drilling, down-the-hole drilling and churn drilling, each based on the use of tube-shaped members. A 3D axisymmetric finite element study is carried out for systems with idealised geometries. The efficiencies obtained are compared with formulae obtained from 1D analyses. It is found that the efficiencies based on 3D analyses are generally slightly lower than those based on 1D analyses. The difference is typically about 4% for hammer drilling, of the order of a percent or less for down-the-hole drilling, and practically non-existent for churn drilling. In the case of hammer drilling, the reduction of efficiency due to 3D effects is found to be slightly larger for tubes with relatively thin walls than for tubes with massive cross-sections. The lower efficiency in 3D is partially due to contributions to the kinetic and potential energies from components of velocity and stress which do not promote the performance of work on the rock. It is concluded that from a practical point of view, there is no need of 3D corrections of the 1D results for efficiency except possibly in the case of hammer drilling.
In the percussive deep hole drilling test, the authors have studied on the drilling mechanism to increase the drilling efficiency and drilling speed with the long rod useing Inada Granite as a sample.
The results obtained are as follows:
(1) By measuring the stress distribution of drill rods, the chipping mechanism of rocks was observed clearly.
(2) The chipping resistance of rock termed chipping strength is constant, while the rock is chipping.
(3) The chipping strength was increased instantaneously at the initiatin of the chipping process of rock, as well as in the case of the mechanical properties of steel.
(4) The chipping strength changed proportionaly to the impact velocity of the hammer, but has no relation with the hammer weight and the rod length.
(5) As the hammer weight increased, duration of chipping was prolonged.
(6) The chipping amount of rock were not influenced of the hammer weight, the impact: velocity and the rod length, but has relation only to the impact energy of the hammer.
(7) On the deep hole drilling the elements which reduced the drilling efficiency do not due to increase of the rod length, but to the decreased output of the drilling machine caused by the friction between the rod and cuttings, the other hand it depend on the reduction of transverse rigidity of the rod.
When a force is applied to a body, waves of stress and velocity radiate through the body from the point of application of the force. When these waves strike the boundaries of the body they may be wholly or partially transmitted to surrounding bodies or reflected back into the body, where they may reverberate back and forth within its confines, like sound waves within the body of air in a closed room. By superposition of these radiated and reflected waves the stress and motion of all parts of the body are gradually, and in general discontinuously, brought up to the values ordinarily assumed.
In the case of a small body this whole process takes place so quickly that there is great difficulty in detecting it, and generally it is of little practical importance. But there are cases where it is desirable to study the process in detail, particularly when the dimensions to be dealt with are no longer very small compared to the velocities of such waves, and in consequence the time element is no longer negligible, or where the time of application or variation of applied forces is very small and hence of the same order of magnitude as the aforementioned time element, as in cases of impact or harmonic forces of considerable frequency. In the present paper an attempt is made to consider some of the more important phases of this subject in as simple a manner as possible.
One important source of noise from percussive rock drills is bending vibrations of the drill rod. These vibrations originate from imperfect rotational symmetry in the system. It is possible to improve symmetry at the machine end by means of tight fittings and small tolerances. It is, however, difficult to improve the conditions at the rock end of the rod.This paper presents a theoretical investigation of how much of the energy fed into the system that can be expected to be converted to bending energy in the drill rod. The model considered is simple, an integral steel with the rock response represented by an eccentrically acting viscous damper, but it is believed to give qualitively correct answers.The results indicate that in a typical situation, one can expect about 10% of the incident stress wave energy to be converted to bending. For a system with a detachable drill bit the fraction is probably less. The form of the incident stress wave is not important, although a steep front tends to give more bending. A heavier piston reduces the bending fraction.
Central longitudinal impact of slender bodies has a number of important technical applications. Theoretical treatment of these cases has, however, often been unduly scarce or crude, probably because available methods suitable for engineering applications have not been sufficiently well known. This is the first one of a series of papers intended to fill this gap by presenting theoretical solutions for a number of cases of various types, including hammers and bars of various fundamental forms, impact with elastic and plastic deformation and restraint by solid friction, and by suggesting some applications. This paper reviews known analytical, graphical and numerical methods for one-dimensional treatment of longitudinal impact and introduces a slightly modified version of the graphodynamical method which will be used in the following. Stress pulse measurements made with wire strain gauges on a bar impacted by cylindrical hammers of various diameters and materials are presented and found to agree reasonably well with corresponding theoretical pulse forms. Formulae and diagrams are given for the influence of the ratio of areas and material constants of hammer and bar on force, stress, energy transmission and other important quantities of the type of impact mentioned.
Extension drilling of rock is simulated numerically using a Victor desk-top computer. The hammer, adapter and extension rods are uniform and have the same characteristics impedances. The same is true for the joints and the bit. Two efficiencies are determined as functions of six dimensionless parameters which represent the number of joints, the joint-to-hammer length ratio, the joint-to-rod characteristic impedance ratio, the initial gap between bit and rock and the bit/rock interaction (two parameters). The first efficiency is defined as the ratio of the work performed on the rock to the impact kinetic energy of the hammer. The second is based on the sum of the work performed on the rock and the rebound kinetic energy of the hammer, which is assumed to be fully retrieved. Both efficiencies generally decrease with the number of joints and the joint-to-hammer length ratio. Exceptionally, however, the efficiencies may increase slightly with the number of joints. The difference between the second and first efficiency, which is due to hammer rebound, generally increases when the rock becomes increasingly hard. This dependence on rock hardness is strongest when the number of joints and the joint-to-hammer length ratio are low. In the absence of joints the efficiencies have an oscillatory dependence on a small initial gap. When one or several joints are used, however, the efficiencies essentially decrease with the initial gap.
A method is described for computing the efficiency of conversion of the energy in the stress wave in the drill steel produced by the striker impact into work done by the bit on the rock. This method is applicable to any arbitrary but physically realizable waveform of the stress wave in combination with any arbitrary but physically realizable force-displacement characteristic for the motion of the bit into and out of the rock. The results of representative computations show that fairly good efficiencies (> 50 per cent) are obtained over a fairly wide range of values of a dimensionless parameter that incorporates quantitative measures of the stress waveform, the force-displacement characteristic, and the material properties and geometry of the drill steel. Although efficiency values of 100 per cent are theoretically possible, they may be obtained only under certain special conditions that apparently are not physically realizable with percussive drilling systems as presently conceived. The optimum value of the applied thrust force on a percussive drill is shown to be essentially equal to twice the average rate of transfer of momentum from striker to drill steel. Hence, the maximum benefit with respect to raising the power output of a percussive drill by increasing blow frequency is obtainable only if the thrust force is correspondingly increased.
Control of the impact energy delivered to the bit of a percussive rock drill is determined by the piston impact velocity and the piston geometry. The basic equations describing generation, transmission, and reflection of the stress wave are defined and a computer program is described by means of which pistons may be designed to produce waveforms of desired characteristics.
A nonlinear dissipative spring mass (NDSM) model for a percussive drill rod joint of the coupling sleeve (CS) type is established. Such a joint consists of a cylindrical coupling sleeve with internal thread which connects two drill rods with external threads at their ends. The model disregards wave motion in the coupling sleeve but accounts for axial mobility of the sleeve relative to the rods. This mobility is due to local deformation and slip of the threads. The model is characterized by the mass of the sleeve and by three parameters which represent the coupling between the sleeve and the drill rods through the threads. A static and a dynamic test have been developed for determining the coupling parameters. The model is validated by simulating previous impact tests with a commercial CS joint. Good agreement is obtained between theoretical and experimental results for waves and energies by using values for the two stiffness parameters from the static tests and for the friction parameter from the dynamic tests. Then, the discrepancies for transmitted and dissipated energy are only of the magnitude of the estimated error in the impact tests.
Transmission and dissipation of stress wave energy at a percussive drill rod joint is determined for three joint types, three joint preload levels, three hammer lengths (∼ incident wave lengths) and a range of impact velocities (∼ incident wave amplitudes). The energies of the incident, reflected and transmitted waves are evaluated from measured strains, and then the dissipated energy is determined as the deficit of energy in the two latter waves compared with the first. The accuracy needed is obtained by introducing a compensation factor and determining its value from the requirement of momentum conservation. The experimental results for threaded standard joints are compared with experimental results for threadless dummy joints, made in one piece, and also with theoretical results. The latter are based on one model which represents the joint as a well on a one- dimensional elastic rod (characteristics impedance or CI model) and on another model which represents the joint as a rigid mass between one-dimensional elastic rods (rigid mass or RM model). For the joints 70–100% of the incident wave energy is transmitted and 0–20% is dissipated. The corresponding figures for the dummies are 90–100% and (±)1%, repectively. The latter figure indicates the inaccuracy in the measurement and evaluation procedures. The two models agree fairly well with the dummies. Under certain conditions they also give reasonably accurate predictions for the energy transmitted through a joint. As they sometimes give quite inaccurate results for energy transmission and furthermore fail to predict energy dissipation they need be refined.
An Apple II Plus microcomputer with high resolution graphics is used to simulate stress wave energy transfer to rock in percussive drilling. The drilling model is based on one-dimensional stress wave theory for the rod and bit, and on a nonlinear history-dependent force/penetration relationship for the bit/rock interaction. The programming language is Applesoft Basic. Input data are given for rod, bit, incident wave and bit/rock interaction. Also the position of a gauge is prescribed. During simulation the wave propagation in rod and bit is visualized. Also the normal force at the gauge station is plotted vs time on a simulated oscilloscope display. After simulation the drill-bit efficiency, defined as the ratio of work on rock to incident wave energy, and the maximum and minimum forces at the gauge station, are displayed. For the case of an integral drill steel and a rectangular incident wave, the computed efficiency differs from the known ideal efficiency by only a few tenths of a per cent. The use of the simulation program 'Bit' is illustrated by studying the influence of bit mass and shape on drill-bit effieiency. For the bits chosen mass is shown to be a far more important parameter than shape. The program has uses in research and development as well as in education.
The theoretical mechanics of the percussive drilling of rock are developed from analysis of stress-wave interactions in the drilling system. Particular attention is given to 1.(i) the efficiency of impact energy transfer from drill steel to rock, and2.(ii) the minimum thrust force needed to ensure bit-rock contact at the beginning of each impact.A simple expression is derived from which the penetration rate of a percussion drill can be predicted, provided the amount of impact energy per unit volume of rock broken is known for the appropriate drill bit and rock type, together with certain and operational parameters of the drilling machine.
The nonlinear dissipative spring mass (NDSM) model for a percussive drill rod joint of the coupling sleeve (CS) type has been implemented into a Modula-2 program with the aid of which percussive drilling of rock is simulated. Transmission and dissipation of energy are first studied when a rectangular stress wave, generated through the impact by a uniform hammer, is transmitted through a single joint. The efficiency of energy transmission increases from 81 to 94% and the relative energy dissipation decreases from 8 to 1 or 2% when the length of the hammer varies from relatively short to relatively long. The effect of the joint preload is weak in the range from medium to relatively high preload. The efficiency of the percussive drilling process decreases with the number of joints but depends little on the joint preload. For soft rock, the efficiency increases with hammer length, whereas for medium and hard rock the dependence of efficiency on hammer length is not monotonic. This is because soft rock requires a long incident wave for efficient conversion of energy to work at the bit, whereas the reverse is true for hard rock. It is also found that the efficiency of the percussive drilling process may be considerably underestimated if the effects of each joint on the length and shape of the transmitted wave and of multiple reflections within the drill string are neglected.
Simple models representing churn drilling, down-the-hole drilling and hammer drilling are established and analyzed with respect to the efficiencies (work done on rock/ impact energy) of these percussive methods. Hammers and rod are treated according to one- dimensional stress wave theory, whereas the bits are treated as rigid masses. The bit-rock interaction is represented by a force-displacement relationship which is linear during loading as well as unloading. Physical insight and a clear overall picture are emphasized.For churn drilling an efficiency close to 100 per cent is attainable, and for down-the-hole and hammer drilling the maximum efficiencies are slightly higher than 90 per cent. For hammer drilling this means that the maximum efficiency is shown to be about 10 per cent higher than according to previous theories in which the bit mass has not been considered. This maximum efficiency appears to be obtainable with standard equipment for normal rocks.The efficiency in down-the-hole drilling is generally greater than or equal to the efficiency in hammer drilling, while the efficiency in churn drilling may be higher or lower than the efficiencies of the two other methods. The efficiencies are all quite insensitive to variations in rock-drilling parameters.Stress waves predicted theoretically in hammer drilling agree well with measured stress waves for different bits.
The performance of two different percussive rock drills has been studied theoretically by simulating the drilling process. The two drills studied correspond to an idealized drill and a more complex, prototype drill. The efficiency, the maximum stress (maximum tensile stress) and the minimum stress (maximum compressive stress) have been determined during the simulations. Also, the stability of the drill string with respect to buckling has been estimated as a function of a constant applied thrust. Two cases of applied thrust were simulated. They correspond to a constant applied thrust and an applied thrust with time dependent harmonic variations. The results show, as expected, that for low frequency variations in thrust, the performance of the rock drill is almost the same as for a constant applied thrust. In practical drilling the applied thrust is in general almost constant during drilling in homogeneous rocks. It was therefore assumed in the remainder of this study that the applied thrust can be treated as a constant force. The results obtained for a constant applied thrust may be summarized as follows: the magnitude of applied thrust selected will be a compromise between a number of contrasting requirements such as: (i) high efficiency, (ii) low stress levels in the drill string, (iii) stability of the drill string with respect to buckling, and (iv) small wear of the drill bit.
A solution has been formulated, which allows one to analyse all the major performance characteristics in a chain of impacting elastic ‘rods’. By allowing one end of the elastic chain to impact against a surface which resists penetration according to a specified penetration law it is possible to analyse the impact system of a variety of mechanical devices such as percussive tools and squeeze film dampers. Virtually, unlimited variety in the number and geometry of the impacting rods, and virtually unlimited complexity in the form of the reaction of the terminal face is permitted. A computer program for treating one or two elastic rods has been developed. The program prints out all important stresses as well as the stress history at several selected points. It also prints out the depth of penetration, the energy transferred from rod to rod, the overall energy transfer into the receiver, rebound velocities and times of separation between rods and between rod and 4receiver. Agreement with exact solutions and experiments is demonstrated.
The efficiency of a percussive process for fragmentation of rock and similar materials has been studied experimentally. The percussive system comprised a cylindrical hammer and a cylindrical bit with the same characteristic impedance. The bit was terminated with a wedge. In front of the wedge there was a heavy block of concrete. The length of the bit and the initial gap between bit and concrete were varied systematically. The force versis penetration relationship and the work of fragmentation were determined in each test using a new technique based on measurement of strains at two cross-sections of the bit. Each test was simulated individually using a previously developed one-dimensional model. The results of simulations and experimental tests were found to agree well.
Churn drilling, down-the-hole drilling, hammer drilling, and related drilling and breaking processes are simulated by means of a microcomputer. The simulation programs are written in Apple Pascal for the Apple II family of computers. Accounts are given of the simulation programs and their theoretical basis, and results are presented from two different simulation studies. The first study concerns the efficiency of a simple percussive system which consists of a cylindrical hammer and a cylindrical bit, both with the same characteristic impedance. Sharp maxima are obtained when the bit-to-hammer length ratio is equal to unity or zero. Also it is shown that the efficiency is generally an oscillating function of the initial gap between bit and rock. If the bit is longer than the hammer, this function is periodic. The second simulation study concerns a commercial percussive drill, namely, Atlas Copco's COP 1038 HD. Stresses in the drill, efficiency, coefficient of restitution of the hammer, and force acting on the rock are determined. For both simulation studies, comparisons are made with exact theoretical results.
An analytical model has been developed that describes the motion of the bit of a pneumatic jackhammer and the forces exerted at its tip during penetration of a target. This model assumes that the energy is transmitted in the form of a one-dimensional longitudinal wave without inclusion of the effect of lateral inertia. The piston and the bit can be of arbitrary shape, but are approximated here by uniform stepped sections.
A computer program was developed to provide numerical results for the operation of the jackhammer/target system. The analysis includes the pressures that act on both the top and bottom of the piston and additional thrusts that are applied by the operator, an external thrust device, due to the weight of the jackhammer, or as the result of a combination of these effects. The program includes (1) the incident wave for an arbitrarily shaped piston/bit system, (2) the force history at any position of this unit, (3) the force profile along the bit at any instant, (4) the indentation history of the jackhammer bit when the force-indentation relation for the bit/target combination is knowna priori, (5) the rebound velocity of the piston, and (6) the energy transfer from the piston to the target. Numerical examples indicating the results of the calculations are presented and compared with data from corresponding experiments that involved the penetration of an Ingersoll-Rand jackhammer with five different tip shapes into Sierra granite. Both the force histories and indentation histories at the bit tip were measured; correspondence with predicted values was very good.
The effects of variations in air pressure and applied thrust as well as the character of the force-indentation relation on the energy transferred to the target are also discussed.
Caption title. From Transactions of the American society of mechanical engineers, 1930. Applied mechanics. Thesis (PH. D.)--University of Michigan, 1930.
Der Schlagablauf in Kolben und Stange beim Schlagenden Bohren
- Arndt
Digital machine computations of the stress waves produced by striker impacts in percussion drilling machines
- Simon
Untersuchung über die Energieübertragung beim Schlagforgang im Blickfeld des schlagenden Bohrens
- Arndt
On longitudinal impact I–III
- Fischer
Graphical analysis of impact of elastic bars
- De Juhasz