Source of information: Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK
The penetration of a drilling tool into a hard medium under periodicimpac t action is analyzed and the simulation model presented. This is further development of previously investigated model of a self-excited percussive-rotary drilling system.
The system used the stick–slip phenomenon to generate an impact action superimposed on the drilling process. A phenomenological visco-elasto-plastic model of the media is used and the system response is studied numerically, first as a forced vibration and second as a result of a selfexcited vibro-impact process.
Relief of the main drive has been obtained and an increase in the rate of penetration is observed with increased impact intensity and hardening of the medium. Results of the preliminary drilling experiment with superimposed dynamica ction have shown an improvement in the rate of penetration.
Vibro-impact mechanisms have been studied and widely used in engineering in the former Soviet Union since the post-war period.
The major research work was concerned with finding adequate parameters (maximum impact force) for the development of new machines and intensification of the penetration due to the use of different mechanisms of excitation of vibration and impacts. Basic models of machines were developed including the interaction with the medium during the penetration.
This gradually brought about the formation of the general theory of vibro-impact systems.
Early models of vibro-impact penetration of a tool into a medium were introduced by Tsaplin, who assumed an instantaneous impact force and the velocities before and after impact were related through a restitution coefficient. Later, Tsaplin introduced a depth dependency by gradually increasing the mass of the driven element as the pile moves into the medium.
The vibration and vibro-impact methods of penetration into different media owed their practical widespread to the works of Barkan, Savinov and Luskin and Tseitlin et al..
The initial theoretical analysis of the vibro-penetration process was made by Neimark and Blekhman.
Two models of the indenter–footing resistance were mainly used to reflect the primary experimental data.
In the purely plasticmodel, the resistance was presented as weightless plug held in the borehole by the permanent force.
The advance of the plug is possible when the sum of the applied forces exceeds the resistance force. Under these circumstances the plug followed to the movement of the indenter.
An elasto-plastic model, known as Prandtl model, took into consideration the elasticity of the medium by adding an ideal spring between the indenter and plug.
In this model, the movement of the plug is only possible when the elastic force of the spring exceeds the resistance of the medium. More complex rheological models of the medium were also used.
In percussive-rotary drilling the tool moves into the medium by an impact–scratching action and inherently removes off cuttings.
This permits use of a simplified model to estimate the ROP due to impact. The model below takes into consideration the frontal resistance of the medium as an elasto-plastic process and viscous dissipation during the vibration of the bit.
The dynamicresponse of the model has been studied in Ref. [1].
This is a self-excited and self-synchronising system where, due to the dry friction, the bit displays a stick–slip motion.
The drive represents the entire drilling system with a total mass greatly exceeding the mass of the drill bit.
Therefore, it is considered that the drive moves with a velocity v; which is not affected by the activity of the bit.
The purpose of it is to decouple the vibration of the bit from the drill string.
This is achieved with the help of a special mechanism, which is represented by the springs, the dashpot and the lever.
There exist different techniques of implementing this decoupling mechanism. Hydraulic actuators, cams, system of levers, specially designed springs or mechanically reciprocating devices could be used for this purpose.
The dry friction at the interface induces a stick–slip motion, which excites the striker to impart blows onto the bit.
The periodic vibro-impact motion of the system was obtained numerically where the striker impacts on the bit and both the striker and the bit synchronise their periodic motions.