The present invention relates to a boring ram and in particular to a control mechanism for such a ram.

In United Kingdom Patent No 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 main to a first operating direction, working fluid pressure from the second fluid input acting a piston menas 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 UK Patent No 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 impacting piston 2 to oscillate, the piston 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 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 impacting piston (reciprocating 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 piston 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 impacting piston 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 piston 2 to restart.