Background art In a locking device designed to lock a joint between a first arm and a sec- ond arm constituting, for instance, a supporting arm for a drilling screw driver, power-wrench, fixture or the like where the aim is to hold the arms fixed in relation to each other and arranged, for instance, to be able to absorb forces arising upon activation of a drilling screw driver or power-wrench. Conventional locking devices comprise various mechanically co-operating friction means such as a cone cou- pling or other similar clamping devices. Common to these are that they are locked manually by means of levers or the like and that they have a relatively long overall length and therefore encroach on the available space.

Description of the invention The object of the present invention is to provide a locking device for the purposes mentioned above, that does not have the drawbacks of the known lock- ing devices. The object is achieved with a locking device arranged in a cylindrical bearing in a housing where the bearing comprises a sleeve with a membrane that communicates with a space located between the sleeve and the housing, and containing a first pressure medium. This space communicates with a primary pis- ton arrangement via the first pressure medium. The primary piston arrangement is also in communication with a secondary piston arrangement that in turn commu- nicates with a pressure source for a second pressure medium. The shaft can be locked to prevent turning or axial displacement in relation to the housing when in- creased pressure is applied to the second pressure medium, which in turn causes increased pressure in the first pressure medium and an elastic decrease in di- ameter in the membrane of the sleeve. The first pressure medium preferably con- sists of a liquid and the second pressure consists of air.

The primary piston arrangement comprises at least one piston, arranged to move parallel to the longitudinal direction of the shaft and the secondary piston arrangement comprises at least one first secondary piston in connection with the primary piston or pistons in the primary piston arrangement. The first secondary piston is rigidly connected to at least one second secondary piston running in a second secondary cylinder, the secondary cylinders communicating with a pres- sure source via pipes or tubes.

In a certain embodiment of the invention two secondary cylinders are ar- ranged along one and the same centre line and are supported by a common pis- ton rod. However, in different embodiment, the secondary cylinders may be ar- ranged annularly on a piston sleeve running on the outside of the housing, its bot- tom supporting the hydraulic primary pistons. In this embodiment the secondary cylinders are formed by a casing that encompasses the piston sleeve and in- cludes a partition wall.

The constructional length of the locking device in accordance with the in- vention can be made extremely short and space-saving, while still providing satis- factory locking between shaft and housing, i. e. between the parts surrounding the locking device, e. g. arms in a drilling screw driver. Furthermore, thanks to the pis- ton arrangement included in the invention, the active piston area that is subjected to pressure from the pressure medium can be made larger than the piston area of a single piston running in a single cylinder, thereby enabling greater forces to be developed within a given cylinder diameter.

Additional features and characteristics of the invention are revealed in the following description with reference to the drawings and in the claims.

Description related to the drawings The invention is described in the following, referring to one embodiment by way of example, and with reference to the accompanying drawings.

Figure 1 shows a piston arrangement in accordance with the invention, ar- ranged in a hydraulic locking device to lock a joint between two arms.

Figure 2 shows a longitudinal section through the hydraulic locking device ac- cording to Figure 1, including the piston arrangement.

Figure 1 shows a locking device 1 arranged to lock a joint between a first arm 2 and a second arm 3 that may constitute a supporting arm for a drilling screw driver, power-wrench, fixture or the like, where the purpose is to keep the arms fixed relative each other, and arranged, for instance, to be able to absorb forces arising upon activation of a drilling screw driver or power-wrench. The joint between the arms 2,3 comprises a shaft pin 4 arranged rigidly on the second arm 3 and about which the first arm 2 can turn in a cylindrical recess 2.1 arranged therein. The shaft pin 4 is journalled in a housing 5 in the locking device 1, which housing is arranged rigidly on the first arm 2 via a flange 6 and having a sliding bearing in the form of a sleeve 5.1 in the housing 5, simultaneously acting as a hydraulic membrane.

A piston arrangement 8 for compressed air is arranged to influence a hy- draulic piston arrangement 7 having two hydraulic pistons 7.1 running in cylinder cavities 7.2 filled with hydraulic liquid and located parallel with the longitudinal di- rection of the shaft pin 4. At the inner end of each cylinder cavity 7.2 a channel 7.3 is arranged, connecting each cylinder cavity 7.2 with a space 7.4 located be- tween the housing 5 and the membrane in the sleeve 5.1. The piston arrange- ment 8 for compressed air comprises a first piston 8.1 running in a first com- pressed air cylinder 8.2 and in connection with the two hydraulic pistons 7.1 ar- ranged on the lower side of the first piston 8.1. A second piston 8.3 is rigidly con- nected to the first piston 8.1 and runs in a second compressed air cylinder 8.4.

The two compressed air cylinders 8.2,8.4 have the same cylinder diameter, are situated along the same centre line as the shaft pin 4 and the sleeve 5.1 and are separated from each other by a partition wall 8.5 having an opening for a piston rod 8.6 that connects the two pistons 8.1,8.3. The second compressed air cylin- der 8.4 is covered by a cylinder head 8.7. The space under the lower side of the pistons 8.1,8.3 communicates with the open air through an evacuation opening, not shown in Figure 2. Compressed air is supplied to the two cylinders 8.2,8.3 through pipes or tubes 8.8,8.9.

The construction of the piston arrangement 8 for compressed air can be seen in Figure 2, where the partition wall 8.5 is larger in diameter than the first cyl- inder 8.2, and rests against an annular rim of this cylinder. The partition wall 8.5 is retained by an annular cylinder sleeve 8.10 having the same outer diameter as the partition wall 8.5 and the same inner diameter as the first cylinder 8.2. The cyl-

inder head 8.7 abuts the cylindrical sleeve 8.10 and is held in place by means of a resilient locking ring 8.11. The pistons 8.1,8.3, the partition wall 8.5 and the cylin- der head 8.7 are provided with seals 8.12 arranged around the outer border of each member.

When the compressed air cylinders 8.2,8.4 are not under pressure, the pistons 8.1,8.3 in them are kept displaced to a rear position by means of springs 9 arranged between the housing 5 and the lower side of the first compressed air piston 8.1. When the compressed air cylinders are pressurised the pistons 8.1, 8.3 are pressed towards the sleeve 5.1 with the shaft pin 4 inserted, whereupon the hydraulic pistons 7.1 press hydraulic fluid under high pressure into the space 7.4 adjacent to the membrane in the sleeve 5.1. The pressure increase in the hy- draulic fluid causes the membrane to resiliently press against the shaft pin 4 and lock this firmly in the sleeve 5.1. The joint between the arms 2,3 is thus prevented from turning.

In the locking device 1 shown in Figure 3 the piston arrangement 8 for compressed air is arranged annularly, surrounding the housing 5 with the sleeve 5.1 and its membrane. The pistons 8.1,8.3 are arranged annularly on a piston sleeve 8.13 running on the outside of the housing 5 and having a bottom 8.14 that supports the hydraulic pistons 7.1. The cylinders 8.2,8.4 are formed by a casing 8.15 that encompasses the piston sleeve 8.13 and inclues a partition wall 8.5.

Compressed air is supplied to the two cylinders 8.2,8.4 through pipes 8.8,8.9 as in the previous example. Neither the securing of the housing 5 on the first arm 2, nor the components in the piston sleeve 8.13 and casing 8.15 are shown in Figure 3.