Device in hydraulic percussion tools

Abstract

A percussion tool is hydraulically operated by a piston whose stroke is controlled by a spring biased valve. A pressure accumulator is connected to the working pressure chamber of a hydraulic cylinder and receives pressure therefrom during the return stroke of the piston while discharging pressure during the working stroke. A piston is displaceably mounted in the pressure accumulator for movement into an end position when the pressure is discharged and a second valve is actuated by the piston in the end position to control the spring biased valve. The invention refers to an improvement in hydraulic percussion tools. Hydraulic percussion tools, for rock drilling, breaking or the like of known construction have a reciprocating working piston in a cylinder, driven by a pressure liquid, usually hydraulic oil, during the reciprocating movement. The pressure liquid is usually supplied from a continuously operating hydraulic pump and is fed pulsewise into the cylinder via valve means at a rate corresponding to the desired percussion frequency of the working piston. The impact energy of the working piston is transferred to an exchangeable tool arranged co-axially with the piston and having the shape of a rock drill, breaker or the like. In percussion tools of this kind it is desirable to enable variation of the piston stroke within comparatively large limits. A given impact frequency answers purely physically to each stroke. It is therefore natural and in practice as good as necessary to let the movement of the working piston regulate the valve means for the pressure pulses to the cylinder. An example of such a solution is described in Swedish Pat. No. 354,029. In this construction the valve means consists of a four-way slide valve having two discharge ducts connected respectively to a cylinder chamber on each side of the working piston, an inlet duct for the pressure oil and an outlet duct for return oil from the cylinder. When the working piston after a return movement has reached a turning point, it coacts with a pressure oil cylinder whereat the slide is set from a first position where the working piston makes a movement towards impact. Return of the slide from this second position to the first position for the working piston return movement is actuated by means of a pressure spring biassed against the opposing pressure caused by the pressure oil cylinder. This opposing pressure which must be reduced to a certain low value in order that the compression spring shall be capable of resetting the slide is determined in its turn by the outflowing speed and pressure of the return oil from the valve means. Regulation, i.e., adapting the slide resetting to a rate corresponding to the impact frequency of the piston (i.e., indirectly to the stroke) is limited in this device to variation of the dynamic pressure from the outgoing return oil. Practical experience has shown that there are certain disadvantages with this construction. For example, the outflowing speed of the return oil is dependent on the impact frequency (stroke). Moreover, variations in the oil viscosity can cause alterations in the flow pressure which, in their turn, effect the resetting of the slide. Adjustment of the slide operation is therefore not so simple and positive as is required in the practical use of a tool of the kind in question. According to the present invention, a device is provided for the regulation of the slide operation which produces a complete and positive resetting of the slide completely independently of the return oil. The invention is applicable to the kind of percussion tool as described in the above mentioned patent, but can also be applied to similar devices which are constructed according to the same principle, i.e. with a reciprocating working piston obtaining pressure pulses via a 4-valve. According to the invention, the necessary opposing pressure reduction for the slide is achieved by means of a special valve means, so that the slide can be moved from the second position to the first position by the pressure spring, the valve means being reset so that the opposing pressure to the slide is momentarily reduced at a definite position of the working piston. An instantaneous readjustment of the slide is thereby obtained at a predetermined and completely unaffected point of time in the cycle, so that a positive slide operation is obtained independent of the impact frequency of the working piston, and identical initial conditions for the working cycle are obtained. The distinguishing features of the invention are defined in the claims.

Dependencies are inferred from the claim text.