Secondary load path detection

1. A nut arrangement for a screw actuator, comprising:

a primary nut for providing a primary load path;

a secondary nut for providing a secondary load path;

a flexible coupling to link the primary nut to the secondary nut during normal operation when the primary load path is active, wherein the flexible coupling provides a bias that allows relative axial movement between the primary and secondary nuts; and

a sensor to detect relative axial movement between the primary and secondary nuts,

wherein the sensor is used as part of a wear detection arrangement to monitor a change in backlash between the primary and secondary nuts;

wherein an interface ring is provided to link the secondary nut to the primary nut and the interface ring is seated with respect to the primary or secondary nut by the flexible coupling;

wherein the interface ring is coupled to the secondary nut with a mechanism which allows the interface ring and the secondary nut to be displaced with respect to each other to test the flexible coupling; and

wherein the mechanism comprises a mount provided on the interface ring for a bolt, a flange provided on the secondary nut and a bolt, the bolt having a tail end portion which extends through the flange of the secondary nut and into the mount, and wherein a spring is provided towards a head end portion of the bolt to bias the head end portion of the bolt away from the flange.

2. The nut arrangement according to claim 1 , wherein the flexible coupling comprises a sprung detent which engages a recess.

3. The nut arrangement according to claim 2 , wherein the sprung detent is located in the primary nut.

4. The nut arrangement of claim 2 wherein the sprung detent is a ball-spring detent.

5. The nut arrangement of claim 4 , wherein the recess is configured to deflect a ball of the ball-spring detent against a spring bias when permitting relative axial movement between the primary and secondary nuts.

6. The nut arrangement according to claim 1 , wherein the secondary nut comprises a device configured to absorb vibration or high acceleration transmitted into the secondary nut.

7. The nut arrangement of claim 6 , wherein the device comprises a guide ring.

8. The nut arrangement of claim 7 , wherein the guide ring is comprised of PTFE.

9. The nut arrangement according to claim 1 , wherein a claw coupling is provided between the primary nut and the secondary nut to prevent relative rotation of the primary nut and secondary nut.

10. The nut arrangement according to claim 1 , wherein a failsafe connection comprising a transfer member is provided between the primary and secondary nuts to transfer load to the secondary load path in the event the flexible coupling decouples.

11. The nut arrangement according to claim 10 wherein the transfer member comprises a hole for endoscopic inspection of the primary and secondary nut.

12. The nut arrangement according to claim 10 , wherein the transfer member comprises a pivot ring adapted to allow a lever to be placed through the transfer member and pivoted to test the flexible coupling for wear or jamming.

13. The nut arrangement of claim 10 , wherein a protrusion extends from the secondary nut and projects into a hole provided in the transfer member connected to or providing part of the primary nut for limiting relative axial movement between the primary nut and secondary nut.

14. The nut arrangement of claim 13 , wherein the transfer member substantially encloses the secondary nut.