Lower attachment for trimmable horizontal stabiliser actuator

1. A lower attachment system for a trimmable horizontal stabiliser actuator comprising:

a screwshaft forming a part of or coupled to a main screw of the actuator, the screwshaft having an axial direction;

a primary ballnut disposed on the screwshaft and forming a part of a primary load path of the lower attachment system;

a lower secondary nut disposed on the screwshaft axially adjacent the primary ballnut;

an upper secondary nut disposed on the screwshaft adjacent the lower secondary nut and on the opposite side thereof to the primary ballnut; and

a secondary connection arranged for axial and rotational movement and coupled to each of the lower secondary nut and the upper secondary nut;

wherein the secondary connection is arranged to be loaded when the secondary load path is loaded and the secondary load path also includes the upper and lower secondary nuts; and

wherein the couplings between the secondary connection and the upper and lower secondary nuts are arranged such that during normal use, when the primary ballnut transmits the loading of the actuator along the primary load path, then there is no relative rotational movement of the secondary connection, the upper secondary nut and the lower secondary nut, and such that when the secondary connection is subject to an axial load resulting from load being transferred to the secondary load path, the upper secondary nut and lower secondary nut are compelled to rotate relative to one another thereby locking the threads of the secondary nuts with the screwshaft;

wherein one of the secondary nuts is coupled to the secondary connection via an anti-rotation device having a portion thereof constrained within a slot and that is held in position in the slot by a ramp;

wherein the slot is shaped such that when the portion of the anti-rotation device is able to pass the ramp then the secondary nut and the secondary connection can move rotationally relative to one another thereby locking the threads of the two nuts with the screwshaft

wherein the portion of the anti-rotation device is held against the ramp by a resilient device such that sufficient rotational force between the secondary nut and the secondary connection will cause the portion of the anti-rotation device to pass along the ramp, depressing the resilient device and passing beyond the ramp and further along the slot to achieve the relative rotational movement of the upper and lower secondary nuts.

2. A lower attachment system as claimed in claim 1 , wherein the anti-rotation device prevents relative rotational movement when no axial load is applied to the secondary connection, but which permits relative rotational movement when an axial load above a threshold level is applied to the secondary connection.

3. A lower attachment system as claimed in claim 1 , wherein the slot is formed in the surface of one of the secondary connection or the upper secondary nut; and wherein the resilient device is held within an axial bore in the other of the secondary connection or the secondary upper secondary nut.

4. A lower attachment system as claimed in claim 3 , wherein the portion of the anti-rotation device is a pin and wherein the slot has a first portion extending in the axial direction of the screwshaft and a second portion joining at an angle to an end of the first portion, the second portion extending in a circumferential direction, and wherein the ramp is formed on the second portion and acts to keep a pin of the anti-rotation device the pin in the second portion until a sufficient rotational force is applied to cause the pin to pass by the ramp and to move along the remainder of the second portion, to the angle and into the first portion.

5. A lower attachment system as claimed in claim 1 , wherein only one of the upper secondary nut and the lower secondary nut moves rotationally relative to the secondary connection.

6. A lower attachment system as claimed in claim 1 , wherein the system comprises a biasing mechanism disposed between the ballnut and the lower secondary nut; and

wherein the biasing mechanism is arranged to hold the lower secondary nut in such a position that its threads are disposed centrally between the threads of the screwshaft when no axial load is applied to the lower secondary nut.

7. A lower attachment system as claimed in claim 6 , wherein, when the axial load arising from failure of the primary load path is applied to the secondary connection, the coupling between the secondary connection and the lower secondary nut transfers an axial load to the lower secondary nut, this axial load acting against the force applied by the biasing mechanism and hence moving the threads of the lower secondary nut so that its threads are no longer disposed centrally between the threads of the screwshaft such that the threads of the lower secondary nut bear on the screwshaft threads when the axial load is transferred from the secondary connection.

8. A lower attachment system as claimed in claim 6 , the system comprising a thrust bearing between the upper and lower secondary nuts.

9. A method of using a lower attachment system for a trimmable horizontal stabiliser actuator to avoid backlash in a secondary load path, the lower attachment comprising:

a screwshaft forming a part of or coupled to a main screw of the actuator, the screwshaft having an axial direction;

a primary ballnut disposed on the screwshaft and forming a part of a primary load path of the lower attachment system;

a lower secondary nut disposed on the screwshaft axially adjacent the primary ballnut;

an upper secondary nut disposed on the screwshaft adjacent the lower secondary nut and on the opposite side thereof to the primary ballnut; and

a secondary connection coupled to each of the lower secondary nut and the upper secondary nut;

the method comprising:

arranging the secondary connection for axial and rotational movement;

arranging the secondary connection to be loaded when the secondary load path is loaded, wherein the secondary load path also includes the upper and lower secondary nuts;

arranging the couplings between the secondary connection and the upper and lower secondary nuts such that during normal use, when the primary ballnut transmits the loading of the actuator along the primary load path, then there is no relative rotational movement of the secondary connection, the upper secondary nut and the lower secondary nut;

when the secondary connection is subject to an axial load resulting from load being transferred to the secondary load path, rotating the upper secondary nut and lower secondary nut relative to one another in order to lock the threads of the secondary nuts with the screwshaft; and

providing a biasing mechanism disposed between the ballnut and the lower secondary nut, and using the biasing mechanism to hold the lower secondary nut in such a position that its threads are disposed centrally between the threads of the screwshaft when no axial load is applied to the lower secondary nut.

10. A method as claimed in claim 9 , comprising using the coupling between the secondary connection and the lower secondary nut to transfer an axial load to the lower secondary nut when the axial load arising from failure of the primary load path is applied to the secondary connection, this axial load acting against the force applied by the biasing mechanism and moving the threads of the lower secondary nut so that its threads are no longer disposed centrally between the threads of the screwshaft.

11. A method of using a lower attachment system for a trimmable horizontal stabiliser actuator to avoid backlash in a secondary load path, the lower attachment comprising:

a screwshaft forming a part of or coupled to a main screw of the actuator, the screwshaft having an axial direction;

a primary ballnut disposed on the screwshaft and forming a part of a primary load path of the lower attachment system;

a lower secondary nut disposed on the screwshaft axially adjacent the primary ballnut;

an upper secondary nut disposed on the screwshaft adjacent the lower secondary nut and on the opposite side thereof to the primary ballnut; and

a secondary connection coupled to each of the lower secondary nut and the upper secondary nut;

the method comprising:

arranging the secondary connection for axial and rotational movement;

arranging the secondary connection to be loaded when the secondary load path is loaded, wherein the secondary load path also includes the upper and lower secondary nuts;

arranging the couplings between the secondary connection and the upper and lower secondary nuts such that during normal use, when the primary ballnut transmits the loading of the actuator along the primary load path, then there is no relative rotational movement of the secondary connection, the upper secondary nut and the lower secondary nut;

when the secondary connection is subject to an axial load resulting from load being transferred to the secondary load path, rotating the upper secondary nut and lower secondary nut relative to one another in order to lock the threads of the secondary nuts with the screwshaft; and

providing a coupling between one of the secondary nuts and the secondary connection via a pin constrained within a slot and held in position in the slot by a ramp;

wherein the slot is shaped such that when the pin is able to pass the ramp then the nut and secondary connection can move rotationally relative to one another thereby locking the threads of the two nuts with the screwshaft; wherein

the method comprises:

holding the pin against the ramp by a resilient device such that sufficient rotational force will cause the pin to pass along the ramp, depressing the resilient device and passing beyond the ramp and further along the slot to achieve the rotational movement;

forming the slot so that it has a first portion extending in the axial direction of the screwshaft with a second portion joining at an angle to an end of the first portion, the second portion extending in a circumferential direction, and

providing the ramp on the second portion of the slot so that it acts to keep the pin in the second portion until a sufficient rotational force is applied, at which point the pin will pass by the ramp and be free to move along the remainder of the second portion, to the angle and into the first portion.