The invention relates to a piston construction for an axial bearing of a drilling machine comprising a frame, a percussion piston mounted in the frame, a shank located in the axial extension of said percussion piston, and a plurality of pistons mounted in axial cylinder spaces provided around the shank in the frame, said pistons being arranged to push the shank towards the front end of the drilling machine as a result of a pressure medium acting on the back surface of the pistons .

Finnish Patent No. 84 701 discloses a rock drill in which the shank is supported by an axial bearing. The axial bearing has been formed by drilling, in the drill frame, a plurality of cylinder spaces which are parallel to the axis of the drilling machine and each of which is provided with a separate cylindrical piston. Behind the pistons, the cylinder spaces are connected to pressure medium channels to allow the pressure of the pressure medium to push the pistons towards the front end of the drilling machine. The forward travel of the pistons is limited. Between the pistons and the shank is mounted a support ring, on which the pistons act. When all the pistons press the support ring simultaneously, their combined propulsive force in the forward direction is greater than the feed force of the drilling machine. Since the shank bears on the intermediate ring, it is at an optimal percussion point in view of the operation of the percussion machinery.

In an axial bearing arrangement of this kind, the pistons also serve as a bearing surface positioned against the intermediate ring. In order to be

sufficiently wear-resistant and stress-tolerant, the entire pistons must be made of hard metal, which makes them rather expensive. Another problem is that since only one edge of each piston bears on the stoppers, the pistons are sometimes unsymmetrically supported: an oblique force is thus exerted on them, which sets high requirements for the durability and lubrication of the cylinder space in the frame.

The object of the present invention is to provide a piston construction which allows the advantages of such an axial construction to be utilized to a high degree. The piston construction of the invention is characterized by comprising an actual piston portion to be mounted in a cylinder space provided in the frame, and a bearing portion to be mounted between the piston portion and the shank coaxially with the piston portion.

An essential feature of the invention is that the piston construction consists of two separate portions. One is the actual piston portion, which can be sealed with seals against the inner surface of the cylinder space so that substantially no pressure medium can leak past the piston. The other is a separate, preferably cylindrical hard-metal bearing portion which is coaxial with the piston portion. The hard metal portion is wear-resistant and stress-tolerant at the same time that it loads the piston portion centrally; this reduces unnecessary wear, wherefore less expensive materials can be used in the piston portion. In the following, the invention will be described in greater detail with reference to the accompanying figure, which is a schematic view of the front end of a drilling machine provided with an axial bearing and the piston construction of the invention. The figure shows part of the frame 1 of a

drilling machine. A percussion piston 2 moves within the frame 1 in a manner known per se. The operation of the percussion piston 2 is generally known and self-evident to one skilled in the art, wherefore it will not be described herein. In front of the percussion piston 2, coaxially with it, there is a shank 3, on which the percussion piston strikes causing thus a drill bit (not shown) , provided as an extension to the shank, to strike on rock so as to break it. The shank 3 is rotated normally by a rotation motor known per se; this is generally known and obvious to one skilled in the art, wherefore it will not be described more closely herein. Such operation of a drilling machine and an axial bearing provided therein are described in detail in Finnish Patent No. 84 701, which is incorporated herein by reference.

The frame 1 comprises drill holes which are parallel to the axis of the percussion piston and form cylindrical cylinder spaces 4. All cylinder spaces are connected to a pressure medium source, for instance with a ring channel or in a corresponding manner, so that pressurized pressure medium can be supplied to the cylinder spaces 4. Each cylinder space 4 comprises a piston portion 5 and one or more annular seals 6 between the piston portion and the inner surface of the cylinder space. In front of the piston portion, to the left in the figure, there is a cylindrical hard-metal bearing portion 7 coaxial with the piston portion. To allow all bearing portions located in front of the piston portions to be kept in place and in correct positions with respect to each other, the drilling machine is provided with a guide ring 8 comprising a cylindrical space for each bearing portion 7. The guide ring 8 guides the bearing portion 7 constantly to the right position, i.e. so that it is coaxial with the piston portion 5. The

front edge of the guide ring 8, i.e. the edge on the left in the figure, is provided with a preferably annular shoulder 8a which extends inwards to the cylindrical space of the bearing portion 7. The shoulder 8a allows the bearing portion 7 to travel forward up to the surface of the shoulder 8a, but no further than that. A support sleeve 9 is mounted between the bearing portions 7 and the shank 3. The bearing portions 7 press against one end (on the right in the figure) of the support sleeve 9, whereas the other end (on the left in the figure) extends towards the shank 3. In addition, the arrangement may comprise a separate discharge channel 10 which leads out of the space around the piston portion 5, thus preventing any pressure medium that has flown past the seals 6 from flowing to the shank. The discharge of the pressure medium can be implemented in many different ways.

When a pressure medium is supplied behind the piston portions 5, the pistons push the bearing portions 7 forward. The combined force generated by the pressure medium acting behind all the pistons exceeds the feeding force of the drilling machine and pushes the shank to the optimal percussion position. Finnish Patent No. 84 701 discloses a solution comprising pistons with at least partly different travel. This is achieved by providing the guide ring 8 with a recess 8b for some of the piston portions: this allows them to travel deeper, whereby they push the corresponding bearing portion 7 further so that it supports the shank over a longer distance in the axial direction.

In the specification above and in the drawings, the invention has been described merely by way of example, and it is not limited to this example in any way. An essential feature is that the piston construction comprises a separate piston portion and a

separate bearing portion, which are coaxial with each other; the bearing portion can be made of a more wear- resistant material and, correspondingly, the piston portion can be made of a less expensive material and be properly sealed against the inner surface of the cylinder space by means of separate seals. Pressure medium can also be supplied under different pressures behind piston portions with different travel, for instance by connecting piston portions 5 with the same travel to one pressure medium channel, and piston portions with different travel to other pressure medium channels. It is possible to use either one or more seals between the piston portion 5 and the inner surface of the cylinder space 4, and the seals can be mounted in grooves provided either on the piston portions or on the inner surface of the cylinder space, depending on the manufacturing technique. If necessary, the guide ring 8 can also be replaced by guide holes in the frame, for instance because of the manufacturing technique. Furthermore, the discharge channel 10 can be used for removing pressure medium that has flowed through the clearances when there is only one seal between the piston portion 5 and the inner surface of the cylinder space 4. Piston portions with different travel can be connected to completely separate pressure medium channels or to the same pressure medium channel; pressure medium may also be supplied to them from the same channel through separate pressure regulators. The cylinder spaces of the pistons may surround the shank either symmetrically on the same circumference or in another way.