Locking device for locking an axle comprising a given axial direction and a given diameter, for example a fixed rod or a rotating shaft, with a housing (110,111), with a fluid connection provided on one side thereof, with an actuating piston, which can be actuated from a first position into a second position by applying fluid against the force of a return spring, and with a clamping piece, which co-operates with the actuating piston and is arranged in the housing (110,111) on its other side with respect to the fluid connection, for gripping around the axle, the two clamping arms (140,141) of the clamping piece being movable from a release position into a clamping position with respect to the axle, wherein the clamping piece is force-locked to the housing (110,111) in a non-rotatable manner on its side facing away from the fluid connection, wherein the clamping piece is flexurally elastic and is formed in one piece with a through bore for receiving the axle with play in its release position, wherein the clamping arms (140,141) are separated by a gap extending into the through bore, each of the clamping arms (140,141) comprising a respective clamping surface on the outer sides directed away from each other at their two free ends, wherein the clamping piece is embodied to bend when the clamping surfaces of the clamping arms (140,141) move towards one another from the release position into the clamping position with simultaneous reduction of the gap, and wherein between the actuating piston and the clamping piece an abutment ring is provided which is immovably fixed with respect to the housing (110,111), characterized in that the actuating piston comprises, on its side facing away from the fluid connection, a control pin extending into the region of the abutment ring and having a rotationally symmetrical control outer surface, that a control ring guided on the inside of the housing (110,111) is provided between the abutment ring and the clamping piece with an upper control inner surface remote from the clamping piece pointing inwards, and with a lower control inner surface close to the clamping piece pointing inwards, which control ring is acted upon by the force of at least one preload spring away from the clamping piece, and that a cage is provided between the abutment ring and the control ring with steel balls distributed evenly around the circumference, which are supported on the one hand on the abutment ring and on the other hand simultaneously both on the control outer surface of the control pin and on the upper control inner surface of the control ring. wherein the clamping piece is force-locked to the housing (110,111) in a non-rotatable manner on its side facing away from the fluid connection, wherein the clamping piece is flexurally elastic and is formed in one piece with a through bore for receiving the axle with play in its release position, wherein the clamping arms (140,141) are separated by a gap extending into the through bore, each of the clamping arms (140,141) comprising a respective clamping surface on the outer sides directed away from each other at their two free ends, wherein the clamping piece is embodied to bend when the clamping surfaces of the clamping arms (140,141) move towards one another from the release position into the clamping position with simultaneous reduction of the gap, and wherein between the actuating piston and the clamping piece an abutment ring is provided which is immovably fixed with respect to the housing (110,111), characterized in that the actuating piston comprises, on its side facing away from the fluid connection, a control pin extending into the region of the abutment ring and having a rotationally symmetrical control outer surface, that a control ring guided on the inside of the housing (110,111) is provided between the abutment ring and the clamping piece with an upper control inner surface remote from the clamping piece pointing inwards, and with a lower control inner surface close to the clamping piece pointing inwards, which control ring is acted upon by the force of at least one preload spring away from the clamping piece, and that a cage is provided between the abutment ring and the control ring with steel balls distributed evenly around the circumference, which are supported on the one hand on the abutment ring and on the other hand simultaneously both on the control outer surface of the control pin and on the upper control inner surface of the control ring.