Investigation and experience in the use of penumatic punches have revealed that the precision of borehole drilling depends, in particular, on the geometric parameters of the body of the punch which determine its rigidity, and on the yield of the soil. The influence of these parameters on the trajectory of the pneumatic punch will be assessed by means of the deviation of its tip from a straight course after encountering an obstacle.In representing the process of formation of a borehole by compaction, the soil is usually regarded as an elasticoplastic medium [1] ; for small deformations, its mechanical properties can be characterized by the yield coefficient s0 , kgf/cm 3 [2].In our scheme of calculations (Fig. 1) the pneumatic punch is regarded as a right circular cylinder, the stiffness of which is determined by its diameter d, wall thickness 5, and length l (we will not consider the rigidities of the components inside the casing). The soil surrounding the punch is regarded as an elastic base exerting a bidirectional influence on the movements of the punch. Thus, under the action of a force P imitating an obstacle, this cylinder (the punch) will deform like a beam on an elastic base.The deviation Y is then given bywhere Y0 is the displacement of the tip of the punch due to force P, 00 is the angle of deviation of the longitudinal axis of the nose of the punch, and X is the distance over which we determine the deviation Y, measured from the point of application of force P.With this scheme of calculations [2], the values of Y0 and 00 are given by the expressions Yo :~dPc~__DB .
When tunnels are drilled for underground communications (such as pipelines, cables, etc.), without digging a trench, and in other excavation work, self-propelled penumatic punches are widely applied in the USSR and abroad [i]. These are simple, efficient, reliable, and convenient machines. They are especially effective in limited-access environments, such as in cities. Modern pneumatic punches have a reversal mechanism permitting reveral of cutting closed-face tunnels, in encountering obstructions, or when the machine deviates from the course.The reversal mechanism is a major component of a pneumatic punch. The quality of this mechanism is crucial for the operation of the machine.. Studies towards improvement of the reversal mechanism are essential. There are currently scores of patented reversal mechanisms with diverse principles of operation and design. A comparison of existing mechanisms would be appropriate, because it helps in selection of those which offer the greatest promise for new pne,-,stic punch designs.The reversal mechanisms are classified in Table i. The classification is based on main features. We believe that such features include the number of communication channels with the pneumatic punch, the method of how instructions are fed to change the operational mode, and the method for feeding instructions to prevent the mechanism from spontaneous reversal. Under these classifications, all reversal mechanisms are broken down into 12 groups, each assigned a conventional number. The mechanisms within a group differ solely by the kinematic scheme and the design.Our classification evaluates the reversal mechanisms in terms of convenience of punch operation.The mechanisms falling into groups 9, I0, Ii, and 12 make use of auxiliary communication lines in the form of a hose or cord (steel cord, rope, etc.). They have the inherent flaw of complicating the punch maintenance because of the presence of these additional components.Mechanisms of group 8 are also inconvenient in operation. Here, the operator, when starting the machine, does not know in which mode'("forward" or "reverse") it is set when it will start operating as compressed air is fed through it. This presents hazared to operator safety.Mechanisms of group 5 are not likely to find a practical application.In these, switching the machine from direct to reverse mode or vice versa involves increasing the pressure in the air supply main. This is done by connecting the punch to a compressed air cylinder. Enough pressure for activating the reversal can only he provided by a large volume of air from the cylinder, which implies using a large-capacity cylinder or frequent cylinder replacement. Both alternatives are inconvenient.In operating the punches with reversal mechanisms of group 6, major difficulties arise when starting the penetration of the ground. Obviously, these machines cannot be started in the normal way, i.e., with a lower air pressure in the main (because of the specifics of the reversal mechanism, the forward mode occurs only at a pressure cl...
At the Institute of Mining of the Siberian Branch of the Academy of Sciences of the USSR we have invented a new mechanism for drilling boreholes in the ground-the P-4601 pneumatic punch. Th/s is a percussive pneumatic machine which drives itself through the ground.The machine can be used to drive various types of borehole in cohesive soiL In construction engineering it has already found wide recognition as the best method of driving holes up to 250 mm in diameter in soils of categories I, II, and HI during trenchless laying of underground communications. It is especially effective in driving communications under roads and streets, aircraft landing fields, railroad and street-car tracks, and ~n reconstructing underground communications on the sites of existing undertakings, mines, etc. The holes drilled may extend to 50 m or more.The casing of the machine consists of a smooth cylinder with a pointed leading end. Its length is 1500 mm and its diameter 135 ram. Boreholes of diameter 200 and 250 mm can be driven by means of a reaming device attached to the machine.The trailing end has a connecting tube which is connected to a rubber-textile hose which serves as a duct for compressed air from a compressor or from the mair~. Spent air is discharged through holes located round the airsupply connector at the rear end.A piston-striker which moves freely under the action of the compressed air is inside the casing. This moves back and forth at 380-400 cycles per minute. When it moves forward it strikes the front interior wall of the casing and drives it into the ground. The casing moves like a driven pile, compressing the ground before it and round it, and leaving behind a straight hole with smooth consolidated sides. The weight of the pneumatic punch is 80 kG. At 6 atm the compressed air consumption is 3 mS/rnin, The pneumatic punch has successfully passed industrial, factory, and warranty tests by various Ministries, and has been accepted for mass production by the Odessa Construction Finishing Tool
At the Institute of Mining of the siberian Branch, Academy of Sciences of the USSR, we have developed a selfpropelled pneumatic percussive machine (pneumatic borer) designed for drilling boreholes in soil for trenchless laying of underground pipelines and cables. The main advantages of these machines are simplicity of construction, ease of servicing, high productivity, reliability, and long service life. Figure 1 shows a schematic diagram of the pneumatic borer. The frame is made of two components, a casing 1 and an anvil 2. The cavity inside the casing contains striker 3 with front and rear guide rings. The front guide ring has grooves to permit the passage of air. Sleeve 4 which is integral with tail nut 5 is inserted into the tail of the striker. Nut 5 has a window for discharge of spent air. Elastic valve 6 which protects the internal cavity of the machine from obstructions but does not hinder exhaust of spent air is fastened to the tube. Compressed air is fed to the machine by means of rubber-and-fabric hose 7. The rear working chamber B, formed by the walls of the striker and sleeve, is always connected to the air-supply hose via an axial channel in the sleeve. The front chamber A is periodically connected to the rear chamber or to the atmosphere via a window C in the sides of the striker, as the machine operates. Window C is automatically opened and closed as the striker moves relative to the sleeve.When the machine operates, the striker, under the influence of the compressed air, executes reciprocatory motion and strikes the frame. The blows drive the frame into the ground; reverse motion is prevented by friction between the frame and ground. As a result of compaction of the soil, a borehole is formed.Reverse travel, i.e., return of the machine, is achieved by changing the position of the air-distribution sleeve 4 relative to the frame. Axial movement of the sleeve to a given position can be effected by various means, e.g., by means of a pair of screws. In this case the sleeve is moved by rotating the hose. The extreme forward position of the sleeve corresponds to forward motion, and the extreme rear position (shown in Fig. 1 by dashes) to reverse motion. When the sleeve is in the reverse-travel position, the striker inflicts blows on tail nut 5 and the pneumatic borer moves back along the previously-made borehole.The air-distribution system adopted for the pneumatic borer has recommended itself in practice. It is simple to construct, ensures reliable operation and sure starting of the machine, and permits a simple method of reversing the direction of motion. It was used in the IP4603 production-model pneumatic borers. Below we give the results of a study of the operating cycle of the borer, including the features of its air-distribution system.In studying the operating cycle of the borer we made the following simplifications. The movement of the frame due to a single blow is small in comparison with the stroke of the striker; no account is taken of the frictional forces between the striker and frame and ...
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