Actuator lower attachment

1. A screw actuator having a screw shaft and a nut arrangement, the nut arrangement comprising:

a primary nut;

a first secondary nut having a first thread;

a second secondary nut having a second thread; and

a first attachment means;

wherein the first nut, first secondary nut, and second secondary nut are connected to the first attachment means with a first, initial, axial spacing between the first and second secondary nuts; and

wherein the first secondary nut is mounted to the first attachment means via bearings that allow the first secondary nut to rotate relative to the first attachment; and

wherein the first secondary nut is connected to the second secondary nut via a screw thread connection having a pitch lower than a pitch of a screw thread of the screw actuator shaft; and

wherein, when the primary nut fails under load, the threads of first and second secondary nuts load against the screw thread of the screw shaft which causes the first secondary nut to rotate relative to the second secondary nut and, as a result of the screw thread connection, the first and second secondary nuts move from the first axial spacing between the first and second secondary nuts to a second, different, axial spacing between the first and second secondary nuts.

2. The screw actuator according to claim 1 , wherein the primary nut is a ball-screw nut and wherein the first and second secondary nuts are screw nuts.

3. The screw actuator according to claim 1 , wherein there is no backlash between the primary nut and the first attachment means, and wherein there is no backlash between the first primary nut and the first attachment means.

4. The screw actuator according to claim 1 , wherein the second secondary nut is held against rotation by the first attachment means.

5. A method of reducing backlash in a nut arrangement of a screw actuator, the method comprising:

mounting the nut arrangement of any preceding claim onto a screw shaft of an actuator, wherein the first and second secondary nuts have a first, initial, axial spacing;

when the primary nut fails, loading the first and second secondary nuts against a thread of the screw shaft; and

rotating the first secondary nut relative to the second secondary nut such that, due to the screw connection, the first and second secondary nuts move to a second, different, axial spacing.

6. A method of manufacturing a screw actuator having a screw shaft and a nut arrangement, the method comprising:

providing a nut arrangement comprising:

a primary nut;

a first secondary nut having a first thread;

a second secondary nut having a second thread; and

a first attachment means;

connecting the first nut, first secondary nut, and second secondary nut to the first attachment means with a first, initial, axial spacing between the first and second secondary nuts; and

mounting the first secondary nut to the first attachment means via bearings that allow the first secondary nut to rotate relative to the first attachment; and

connecting the first secondary nut to the second secondary nut via a screw thread connection having a pitch lower than a pitch of a screw thread of the screw actuator shaft; and

wherein, when the primary nut fails under load, the threads of first and second secondary nuts load against the screw thread of the screw shaft which causes the first secondary nut to rotate relative to the second secondary nut and, as a result of the screw thread connection, the first and second secondary nuts move from the first axial spacing between the first and second secondary nuts to a second, different, axial spacing between the first and second secondary nuts.