Background of the invention and prior art Liquid driven downhole drilling machines of this kind for rock drilling are known for example by EP-0394255-B. The drive fluid is water, possibly thickened by bentonite.

The impact motor in a water-driven downhole drilling machine causes pressure spikes backwards into the drill tube and since as high an impact frequency as possible is wanted, the time between highest and lowest pressure will be in the magnitude of 2 milliseconds or less.

When drilling is interrupted, debris may penetrate into the machine through the channels in the drill bit and cause wear and functional disorder in the subsequent drilling.

Object of invention and brief description of prior art It is an object of the invention to prevent the intrusion of liquid from the borehole into the machine. This object is met by a spring loaded one-way valve for the flow through the impact motor arranged in connection with the downhole drilling machine and comprising a valving element slideable against a guide such that a liquid-filled chamber is formed that has a leakage connection with the liquid downstream of the valving element. Rightly dimensioned, the one-way valve will be too slow to react to the impact motor pulsations but it will close rapidly when the pressure lowers.

Description of the drawings and detailed description of a preferred embodiment Figure 1 shows a rock drilling arrangement according to the invention.

Figure 2 shows enlarged and in a longitudinal section an adapter for a downhole drilling machine shown in figure 1, and the adapter contains a one-way valve shown in its open position.

Figure 3 corresponds to figure 2, but it shows the one-way valve in its closed position.

In figure 1 is shown a rock drilling arrangement with a drill string 11 that has a downhole drilling machine 12 mounted on its forward end. The downhole drilling machine may suitably be of the kind described in the above mentioned EP-0394255-B and it is therefore not described in detail. It has an impact motor with a valve-controlled piston hammer that is reciprocated by pressure fluid in the form of pressure water that is supplied through the drill string and then will escape through the front end of the drill bit 13 between the rock crushing carbide buttons of the drill bit; a so called open system. In a conventional way, the drill string is rotated and a feeding force is applied to it by a feed beam with a rotary motor and a feed motor. This arrangement outside of the hole is not illustrated because it is conventional in the art.

The drill string 11 (the drill tube) is put together of tube-like string elements with conical threads. Between the downhole machine 12 and the drill string 11, an accumulator unit 15 may be inserted that reduces the pressure fluctuations. An adapter 14 between the accumulator unit and the drill string is shown at a larger scale in a longitudinal section as figures 1 and 2. The accumulator unit consists of a thin-walled elastic tube sealingly mounted in the outer tube of the unit so that there will be an air pocket between the outer tube and the elastic inner tube. This elastic tube, for example a tube of fibre-reinforced plastics or titanium, acts as a fast accumulator and reduces the pressure spikes to some extent. This accumulator unit is not necessary but the adapter 14 can be an adapter directly to the machine housing.

The adapter 14 has a conical male thread 20 at its rear end and a conical female thread 21 at its front end. Inside its pressure fluid channel 22, there is a one-way valve that has a valving element 23 guided on a guiding body 24 that is fastened in the adapter housing by a threaded connection 25. The valving element 23 is cylindrical and formed as a cap and it slides with its cylindrical inner surface on a corresponding cylindrical outer surface of a tube-formed cylindrical portion 26 on the guiding body 24. The portion 26 is open backwards. A closed chamber 27 is formed between the valving element 23 and the tube-formed portion 26 and a return spring 28 is in the chamber 27 for biasing the valving element towards closed position.

In figure 2, the valving element is shown in its open position in which it holds side ports 29 in the tube-formed part 26 closed so that the liquid in the closed chamber 27 will prevent the valving element 23 from moving in pace with the pressure fluctuations that have a periodicity that is usually in the order of 50-100 Hz and depend on the variation of the pressure fluid flow through the impact motor during an impact cycle. When the pressure fluid supply to the upper end of the drill tube is interrupted, the spring force will however slowly move the valving element 23 backwards due to liquid leaking into the chamber 27 from the end of the valving element to the side ports through the cylindrical clearance 30. When the end of the valving element then reaches the ports 29, the valving element will accelerate and rapidly reach its position shown in figure 3 in which its bevelled edge 31 closes against the seat 32. The valve can then open unhindered when the pressure fluid is again supplied to the drill tube.

The play that gives the clearance a suitable leakage can be easily found in order to have the valving element not reacting to the pressure fluctuations during an impact cycle but still closing sufficiently fast when the pressure fluid flow is interrupted.

The one-way valve is shown arranged in the supply conduit to the impact motor, that is, upstream of the impact motor. Alternatively, the one-way valve can be in the drill bit, that is, downstream of the impact motor. This arrangement is desirable in jet grouting when a hole is bored and a conduit for cement grout ends above the drilling machine. Cement grout is then forced out under high pressure while the drill string is pulled up. Since this method is used in formations that are not very hard and the impacts therefore need not be very heavy, it will be possible to have the one-way valve in the drill bit that is subjected to impacts.