BORING RAM

Abstract: In self propelled reversible boring ram, selection of forward/reverse boring modes is determined by the position of a control piston (3). A concentric supply pipe to a source of working fluid pressure is connected to coaxial fluid inputs (9,10). Working fluid pressure for an impacting piston (2) (hammer) is supplied through the input (10). For forward mode operation pressure from a coil spring (7) and reactive force on the piston (3) is overcome by working fluid pressure on servo pistons (5, 6 and 8). Restricting fluid pressure on the pistons (5 and 6) allows the piston (3) to move to reverse mode operating position.

Description: The present invention relates to a boring ram and in particular to a control mechanism for such a ram. In GB-A-2111565 (assigned to British Telecommunications public limited company) a self propelled reversible boring ram is disclosed. Boring rams of this kind are now widely used for boring holes under footways and roadways to facilitate the provision of service ducts to carry telecommunication and power cables for example. One problem associated with boring rams is that of providing a simple control mechanism to the device to effect reversing for example. Another problem is that of a piston in the ram reaching equilibrium. It is one object of the present invention to provide a simple control mechanism for effecting switching of the direction of travel of a boring ram. According to the present invention there is provided a boring ram of the kind comprising a tubular body which contains a reciprocal hammer and a pneumatically balanced control member for supplying working fluid to the hammer characterised in that the ram includes first and second working fluid inputs, the first working fluid input supplying working fluid to the hammer and the second working fluid input supplying working fluid to control means responsive to changes in fluid pressure to effect changes in the direction of travel of the ram, the control means having spring means which brings the control mean to a first operating direction, working fluid pressure from the second input acting on a piston "means of the control means to select operation in an opposed direction". Preferably the working fluid inputs are concentric and the two inputs may be connectable by way of a control valve to a common fluid pressure source. For the avoidance of doubt the term working fluid includes, but is not limited to compressed air. A boring ram in accordance with the invention will now be described by way of example only with reference to the accompanying drawings of which:- Figure 1 is a cross sectional view of the ram in forward working mode; and Figure 2 is a cross sectional view of the ram in reverse working mode. Referring to the drawing, the boring ram which is of the kind sometimes called a self propelled percussion boring ram comprises a tubular body 1 which has a boring head 12 attached at one end, the boring head shown comprising a chisel point. The tubular body 1 has an internal cavity containing a hammer 2 which has a main cylinder 14 connected by way of control ports 15 to the internal cavity. The cylinder 14 contains a control piston 3, the position of which determines whether the hammer 2 strikes at the forward end (Figure 1) or at the rearward end (Figure 2) of its reciprocations. A shaft 16 extends from the rear of the control piston 3 and is slideably mounted between the walls of the internal cavity. The shaft 16 carries three servo pistons 5, 6 and 8, the purpose of which is hereinafter described and is biassed by a coil spring 7 towards the reverse operating position. A full explanation of the working of the reciprocating hammer is given in GB-A-2111565 accordingly only abbreviated description is given herein. In use in the forward mode referring specifically to Figure 1, working fluid (eg compressed air) is supplied at equal pressure to coaxial inputs 9 and 10. A control valve 11, as shown may be used to commence the operation. Working fluid from the inlet 10 enters a tube 4 in the control piston shaft 16 causing the hammer 2 to oscillate, the hammer 2 moving forward due to the air pressure until ports 15 are open at which point the pressure in the forward chamber rises. This pressure rise causes the piston to move towards the rear (after impaction) which once ports 15 clear the piston 3 allow the pressure to be released through the internal cavity to an exit port 17. The air pressure also creates a rearward force on the control piston 3 which combines with the coil spring 7 to tend to switch the ram to the reverse operating mode. This force is counteracted by pressure from the fluid input 10 acting on the servo piston 8 and by pressure from the fluid input 9 acting on the servo pistons 5 and 6 thereby firmly holding the control piston 3 in the forward operating position. Referring now to figure 2, reverse operation is achieved by turning the valve 11 to cut off the fluid pressure to the input 9. Since no forward pressure now acts on the pistons 5 and 6, the coil spring 7 and reaction forces on the piston 3 will cause the control piston to move to the rearward operating position whereby the hammer) 2 drives the ram backwards towards the exit of the channel it has formed. It will be noted that switching the operative mode does not require the main working fluid supply pressure to be turned off there is therefore less chance of the hammer 2 reaching equilibrium and failing to restart when the supply is returned, one of the problems which could occur in prior art devices which required the working fluid supply to be switched off during the changeover. However, if the hammer 2 for any reason reaches a state of equilibrium then by switching the control valve 11 between forward and reverse operation jitter of the control piston 3 may be used to disturb the equilibrium causing the hammer 2 to restart.

Claims: 1. A boring ram comprising a tubular body (1) which contains a reciprocal hammer (2) and pneumatically balanced control member (11) for supplying working fluid to the hammer the ram including first (10) and second (9) working fluid inputs, the first working fluid inputs (10) supplying working fluid to the hammer (2) and the second working fluid input (9), supplying working fluid to the control means (3, 4) responsive to changes in fluid pressure to affect changes in the direction of the ram, the control means is biased to a first operating direction by spring means (7) and working fluid pressure acts on at least one piston means (5, 6) of the control means to select an opposed working direction, characterised in that the control means (3, 4) is biased towards a reverse operating direction and piston means (8) is acted on by fluid pressure from the first working fluid input (10) to assist selection of a forward operating direction. 2. A boring ram as claimed in claim 1 further characterised in that the first (10) and second (9) working fluid inputs are concentric. 3. A boring ram as claimed in claim 2 further characterised in that, in use, the concentric fluid supply line is connected at one end to the first and second working fluid inputs and at the post end by way of a control valve (11) to a common source of working fluid pressure. 4. A boring ram as claimed in claim 3 further characterised in that the control valve (11) is movable between a first position at which fluid pressure is supplied to the first and second working fluid inputs and a second position at which fluid pressure is supplied to the first working fluid input only. 5. A boring ram according to any preceding claim characterised in that cyclic application of fluid pressure and reduced fluid pressure to the control piston (3) is arranged to affect disturbance of equilibrium of the reciprocal hammer (2).

Inventors: HEMMINGS, KENNETH LEWIS
Applicants: BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1989
Main classification: E21B4
IPC codes: E21B4