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The present invention relates to a hydraulically driven striking device, comprising
A FRAME,
A STRIKING PISTON STRIKING AGAINST A TOOL AND MOVING IN THE STROKE AND RETURN DIRECTION IN THE FRAME,
A HIGH PRESSURE CIRCUIT AND A LOW PRESSURE CIRCUIT FORMED IN THE FRAME,
A FIRST CYLINDER SPACE SURROUNDING THE STRIKING PISTON AND CONTINUOUSLY COMMUNICATING WITH THE HIGH PRESSURE CIRCUIT,
A SECOND CYLINDER SPACE SURROUNDING THE STRIKING PISTON AND ALTERNATELY COMMUNICATING WITH THE HIGH PRESSURE AND LOW PRESSURE CIRCUIT, AND
A SLEEVE SHAPED CONTROL VALVE POSITIONED IN THE LATTER CYLINDER SPACE AND SURROUNDING THE PISTON, SAID CONTROL VALVE BEING ARRANGED TO CONTROL THE COUPLING OF THE SECOND CYLINDER SPACE TO THE HIGH AND LOW PRESSURE CIRCUIT BY MOVING IN THE CYLINDER SPACE IN THE STROKE AND RETURN DIRECTION OF THE STRIKING PISTON ESSENTIALLY SYNCHRONOUSLY WITH THE STRIKING PISTON, THE HYDRAULIC FLUID THEREBY PUSHING THE CONTROL VALVE IN THE RETURN DIRECTION, WHEREBY, WHEN THE STRIKING PISTON STOPS WHEN STRIKING AGAINST THE TOOL, THE CONTROL VALVE CONTINUES TO MOVE IN THE STROKE DIRECTION, THUS DISCONNECTING THE CONNECTION FROM THE SECOND CYLINDER SPACE TO THE LOW PRESSURE CIRCUIT AND OPENING THE CONNECTION FROM THE SECOND CYLINDER SPACE TO THE HIGH PRESSURE CIRCUIT, AND AN ANNULAR CHAMBER BEING FORMED IN THE WALL OF THE SECOND CYLINDER SPACE AND THE OUTER WALL OF THE CONTROL VALVE BEING PROVIDED WITH A CORRESPONDING PRESSURE ACTION SURFACE.
In striking devices of this type (Finnish Pat. No. 50,307), the control valve is made to move in the stroke direction so that the piston strikes against the control valve after it has finished its return motion. During the entire striking motion, the piston pushes the control valve ahead of itself until the piston strikes against the tool, e.g., a drill, whereafter the control valve continues to move in the stroke direction by itself.
It is the object of the present invention to improve such a kind of striking device further, and, for this purpose, the striking device according to the invention is mainly characterized in that the annular chamber is arranged to be coupled to the high pressure circuit in the initial stage of the striking motion of the striking piston, the control valve thereby obtaining an initial speed lower than the speed of the striking piston in the stroke direction so that the striking piston reaches the control valve and pushes it ahead of itself shortly before striking against the tool. The efficiency of the device is considerably improved because the control valve is given an initial speed in the stroke direction by means of hydraulic fluid. Owing to this between the control valve and the piston at the moment when the piston reaches the control valve is very small whereby the kinetic energy lost by the piston to the control valve is very small. Further, the impact between the piston and the control valve will be more gentle, thereby avoiding possible mechanical damages.
The invention will now be described by way of example with reference to the accompanying drawings wherein:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a longitudinal sectional view with the piston in a position where it strikes against the tool while the control valve is still on the way down; and
FIG. 2 is an enlarged sectional view showing the clearances between the control valve and the walls of the chamber.
The striking device shown in the drawing, preferably a rock drilling machine, essentially comprises a frame 1 and a striking piston 2 located in a cavity in the frame, said valve moving in the stroke and return direction (down and up in the drawing) and striking against a tool shaft 3 inserted in the front end of the frame to drive the tool, e.g., a drill, into the ground or into a rock. The frame is provided with a high pressure circuit 4 and a low pressure circuit 5 for transport of hydraulic fluid. The striking valve is partly surrounded by a rear cylinder space 6 communicating continuously with the high pressure circuit, and a front cylinder space 7 communicating alternately with the high pressure and low pressure circuit.
Inside the last-mentioned front cylinder space 7, a sleeve shaped control valve 8 surrounding the piston 2 is located. This control valve controls the pressure in the front cylinder space by moving in the stroke and return direction of the return piston essentially synchronously with the striking piston, thereby closing in its rear (upper) end position the connection or first part means 25 between the high pressure circuit and the cylinder space 7 and opening the connection to the low pressure circuit, and in its front end position opening the connection from the high pressure circuit and closing the connection to the low pressure circuit.
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The rear end of the piston is, moreover, surrounded by a cylindrical chamber 9 communicating continuously with the low pressure circuit.
The motions of the control valve 8 are controlled by three cylindrical chambers 10, 11 and 12 provided in the wall of the cylinder space 7 and by three radial pressure action surfaces such as a first shoulder 26 and the shoulder facing in the opposite direction as shoulder 26 in the outer surface of the control valve.
As appears from the figure, the chamber 11 communicates continuously with the high pressure circuit 4. In this way, an action of force is provided which continuously strives to move the control valve in the return direction. The chamber 10 can be alternately connected to the high pressure space 6 and low pressure chamber 9 through a channel 13, a groove 14 in the frame and a groove 15 in the piston. When the chamber 10 communicates with the high pressure space 6, a force acting in the stroke direction is imposed on the control valve, and when said chamber communicates with the low pressure chamber 9, an action of force is produced in the return direction. The third chamber 12 communicates continuously with the low pressure circuit.
The chambers 10 and 11 are provided with a damping chamber 16 and 17, respectively, both serving to exert a braking effect on the motions of the control valve near the end positions thereof. The damping chamber 16 acts in the return direction and the damping chamber 17 in the stroke direction. The action of these chambers is based on the fact that a radial tolerance as at 22 (shown in FIG. 2) is provided between the control valve and the frame, said tolerance choking the flow of oil in a desired manner. This kind of damping, however, suffers from the disadvantage that it makes it impossible to quickly accelerate the control valve out of the chamber in question. According to a preferred embodiment of the invention, this disadvantage has been eliminated by connecting the damping chamber 16, to accelerate the control valve, through a channel 18 to a groove 19 in the frame which, depending on the position of the piston, can be connected to the high pressure space 6 through a groove 20 in the piston. The control valve can move in the return direction only when the piston is in the rear position, the connection from the chamber 16 to the space 6 thereby being disconnected due to the fact that the cylinder surface of the piston plugs the groove 19.
The acceleration of the control valve in the return direction out of the damping chamber 17 is accomplished because the pressure action surface of the control valve in the space 7 is bigger in the return direction than in the stroke direction. This can be stated also so that the pressure action surface corresponding to the chamber 10 is bigger than the sum of the pressure action surfaces corresponding to chambers 11 and 12.
When the piston 2 moves in the stroke direction, the control valve 8 is accelerated separately nearly to the same speed as the piston. The piston reaches the control valve shortly before it strikes against the tool 3 and pushes the control valve ahead of itself in the stroke direction. This ensures that the control valve obtains a desired minimum speed which is as big as the speed of the piston. The control valve continues to move with this speed in the stroke direction also after the piston has stopped and closes the connection from the space 7 to the low pressure circuit and opens the connection to the high pressure circuit.
The immediate contact or mediate contact through an oil cushion between the piston and the control valve shortly before the piston strikes against the tool ensures an accurate synchronism between the motions of both these organs which is essential for the efficiency of the device. The chamber 11 ensures that the control valve, after having disconnected the connection between the high pressure circuit 4 and cylinder space 7, moves on in the return direction up to the damping chamber 16. Owing to this, the valve can be given exactly the desired speed in the stroke direction and the contact between the valve and piston always takes place at the right point.
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