During a steering movement, therefore, the high pressure from the pump (14) can always be applied to the pressure blocking groove (42 or 43) of the pressurised HP groove (24 or 25).

According to the invention, these pressure blocking grooves are connected to the fronts of a drilling (51) via channels (48 and 50).

Therefore 'pressure blocking grooves' (42 and 43) are to be fitted between the HP and NP grooves.

The at least one locking piston is slidably contained within the locking bore and is actuated by pressure in the locking bore to apply a locking force on the pin.

The locking state of the compression hook is secured by a trigger element positioned in a locking position.

The detent element can be used as a stop or thrust piece.

Pressure forces acting on the connecting element (42) are absorbed by a mechanical retainer (52) of the contact pressure device (51).

The other side of the lock sleeve is exposed to a chamber (36) under atmospheric or low pressure.

The pressure separation is realized by means of a cut-off valve (V4).

Applying 360 degrees of reduced pressure helps avoid blockage.

Disclosed are a clamping unit and a hydraulic control shaft that is embodied therewith.

When the pressure is released pressure springs (6) mounted on the tubular plungers return the blocking arms to their initial position.

If no blockage is present between the reduced pressure source and the tissue site, the differential pressure is limited to a second maximum differential pressure.

The temperature of the water will help breakup the blockage.

The pressure by the water will displace the blockage.