Patent Description:
Actuators are used in many fields to control the movement and/or positioning of a surface or component. Various types of actuator including hydraulic actuators, electrohydraulic actuators, electromechanical actuators and electrical actuators are known. Typically, an actuator will include a moveable shaft that is connected to the surface or component to be moved, as well as being connected to a drive. Conventionally, the drive will cause rotation of a shaft at an appropriate speed and by a commanded degree of rotation. In a rotary actuator, the shaft rotation will be transferred to rotation of a component or surface (e.g. an aircraft wing flap or panel) connected to the shaft. In linear actuators, the rotation of the shaft is converted to linear motion of an actuator member e.g. a screw shaft which, in turn, causes linear motion of a connected component or surface. In either case, particularly in high load systems, it is desirable to provide an overtravel stop mechanism that prevents excessive rotational or linear motion of the actuator parts. Such stops are often crucial to prevent inappropriate positioning and/or structural damage.

Various types of stop mechanism are known, including stops that prevent overtravel by the engagement of stop teeth and overtravel systems that use electrical switches as limits. The stops are generally set based on the permissible range of movement of the controlled surface or component and will take into account its weight and size and possible speed of movement as well as the actuator stroke. In some cases, however, features of the surface/component or the assembly may mean that the surface/component commences movement some time after the start of the actuator stroke, in which case, the stops may kick in too soon or too late unless they are adjusted to take this into account. Such adjustment is known as rigging. For mechanical stops such as teeth or claw stops, this adjustment requires the overtravel assembly to be disassembled from the drive and shaft assembly which is cost and time intensive. Actuator overstop mechanisms are disclosed in <CIT>, which shows an actuator assembly comprising a drive assembly, an overtravel stop mechanism comprising a screw shaft, a nut threadedly mounted to the outer thread of the screw shaft and a first stop fixed to and radially extending from the screw shaft to define the axial limit of movement of the nut relative to the screw shaft.

There is, therefore, a need for an actuator and overtravel stop assembly that allows for simplified rigging adjustment.

Accordingly, there is provided an actuator assembly comprising: a compliant actuator shaft having an axis and a first end and a second end; a drive assembly provided at the first end of the compliant actuator shaft to cause rotation of the compliant actuator shaft about the axis; an overtravel stop mechanism comprising: a screw shaft, coaxial with the compliant actuator shaft, and mounted to and arranged to rotate with the second end of the compliant actuator shaft, the screw shaft having an outer thread extending in the axial direction; a nut threadedly mounted to the outer thread of the screw shaft and arranged to move axially relative to the screw shaft as the screw shaft rotates; a first stop fixed to and radially extending from the screw shaft to define the axial limit of movement of the nut relative to the screw shaft, in a first axial direction, as the screw shaft rotates, by abutment of the nut against the first stop; and wherein the first end of the compliant actuator shaft is provided with a first number n of splines for spline engagement with the drive assembly and the second end of the compliant actuator shaft is provided with a second number of splines, the second number being one spline more than the first number of splines, for spline engagement with the screw shaft.

Examples according to the description will now be described with reference to the drawings. It should be noticed, that these are only examples, and variations are possible within the scope of the claims.

An assembly according to this invention will now be described in more detail, by way of example.

<FIG> show part of an actuator assembly <NUM> including the overtravel stop assembly <NUM>. The assembly includes a shaft <NUM> defining an axis A and having a first end 12a in contact with a drive assembly <NUM>, such that the shaft <NUM> is rotated by the drive assembly <NUM> about its axis A, and a second end 12b in contact with a load (not shown) to be moved by the actuator assembly.

In the example shown, the shaft <NUM> is driven by a drive assembly <NUM> comprising a gear system <NUM> that, in turn, is connected to a transmission system <NUM> driven by e.g. a motor (not shown). In this example, the gear system is a reduction gear box <NUM> to rotate the shaft <NUM>, connected to the gear box, in response to, but at lower rotational speed, than the motor and transmission system. The shaft <NUM> is connected to the gear system <NUM> by a first spline arrangement <NUM> between the first end 12a of the shaft and the gear system <NUM> such that, when the splines <NUM> around the first end 12a of the shaft <NUM> engage with the gear of the gear system <NUM>, rotation of the gear system causes corresponding rotation of the shaft <NUM>.

The other end 12b of the shaft engages, by means of a second spline arrangement <NUM>, a screw shaft <NUM> mounted axially around the shaft <NUM>. Splines of the second spline arrangement, around the second end of the shaft, engage with an interior 14a of the screw shaft <NUM> such that, when the splines are engaged, rotation of the shaft <NUM> causes corresponding rotation of the screw shaft <NUM>.

The screw shaft <NUM> is provided, along (in the axial direction) its outer surface 14b, with teeth or threads <NUM> that protrude radially outwards.

A nut <NUM>, attached to rods <NUM>, that prevent the rotation of the nut, is mounted around the screw shaft <NUM> but is not rotationally attached to the screw shaft <NUM> or the shaft <NUM>. The nut <NUM> has an interior thread that engages the outer thread <NUM> of the screw shaft <NUM>, so that as the screw shaft <NUM> rotates about axis A, the nut <NUM> translates along the rods <NUM>, and rides along the thread, thus moving axially relative to the shaft <NUM>. The direction of axial movement of the nut <NUM> depends on the direction of rotation of the screw shaft.

For linear actuators, the nut <NUM> and rods <NUM> may be the part of the actuator assembly connected to the load i.e. the surface or component to be moved. Rotation of the shaft by the drive causes linear/axial movement of the nut and corresponding movement of the surface/component. The assembly also includes an overtravel stop assembly <NUM> as will be described further below.

The stop assembly comprises a first stop 40a which is a radial projection from the screw shaft <NUM> at an end position beyond which the actuator stroke should not continue. For a linear actuator, this defines the limit of axial motion of the nut <NUM> based on the limit of desired movement of the surface/component. In the example shown, the first stop 40a is a stop to define the end of axial movement in the retracting direction R of the screw shaft - i.e. retracting axially into the actuator housing <NUM>. In addition, or alternatively, a similar stop 40b may be provided at the other end of the screw shaft <NUM> to limit axial movement in the extending direction E. The first stop 40a is fixed relative to the screw shaft <NUM>.

The nut <NUM> is provided with an interior thread <NUM> that threadingly engages with the thread <NUM> of the screw shaft <NUM>. Thus, the nut travels along the screw shaft <NUM> as the screw shaft rotates. At the desired extent of travel of the nut <NUM>, it abuts against the first stop 40a which prevents further axial movement of the nut <NUM> in that direction.

In typical assemblies, the shafts are designed and arranged, and the stops are set at positions, based on the types, ranges of motion, size and weight of the loads being driven and characteristics of the drive, to ensure that the length of the actuator stroke as defined by the stops corresponds to the desired movement of the surface/component being moved. This assumes that the start of the actuator stroke - i.e. the start of rotation/axial movement - corresponds to the start of movement of the surface/component. Often, the positioning of the actuator and its stroke will need to be adjusted or 'rigged'. Rigging usually requires the actuator assembly to be disconnected from the transmission and from the surface/component and the screw shaft and stops to be disassembled and re-positioned.

Further, in high speed/high load systems, the speed at which the actuator moves results in the stops abutting at high speed as the actuator reaches it end limit(s). This high impact can cause undesired vibrations etc. and can even damage structural parts of the assembly. In existing assemblies, adaptations have been made to reduce this end of stroke impact by. incorporating damping springs in the system or controlling the system to adjust the speed of the actuator before impact. Such solutions, however, typically use separate devices in the system to performing the damping or decelerating function, which results in a larger, heavier and more expensive assembly.

The arrangement of this invention allows a simple rigging adjustment process whereby the start point between the transmission stroke and application to movement of the surface/component can be more easily and accurately calibrated and adjusted. Further, the impact at the stops of the overtravel stop assembly is reduced without the need for additional parts dedicated to this function. This is achieved by virtue of the shaft <NUM> being a compliant shaft having a set of two adjoining splines having one tooth difference in spline count and providing a fine adjustment feature. In other words, there is a single tooth difference between the spline assembly at one end of the shaft <NUM> and the spline assembly at the other end. The shaft <NUM> is designed to introduce a specific stiffness by virtue of its length, cross-sectional area and material properties, which can be tuned within the boundaries of the transmission system inertia during the concept stage of the assembly design to absorb potential yield cases within the transmission system.

The features according to the invention that provide the above capabilities can best be seen in the examples shown in Figs. 1C, <FIG> and 2C, with reference to <FIG>.

In <FIG>, the shaft <NUM> is located within the assembly housing <NUM> with its first end 12a in spline engagement with the drive gear system <NUM>. The first end 12a is formed with the first spline arrangement <NUM> having n splines. The compliant shaft <NUM> extends through the assembly and its second end 12b is in spline engagement with the screw shaft <NUM>. The second spline arrangement <NUM> has <NUM> more (n +<NUM>) spline than the first spline arrangement. <FIG> shows the assembly with the actuator in fully retracted position where the nut and the stop 40a are engaged to stop further axial movement of the nut <NUM> in the retraction direction. Rotation of the shaft <NUM> in the opposite direction would cause the nut <NUM> to move axially in the extension direction E. Further stops 40b may be provided to limit axial movement in the extension direction E.

If, for whatever reason, the stroke of the actuator as defined by the stops in their present positions, does not match the required movement of the controlled surface/component, the stops can be reset such that they engage to align with the rigged surface/component position.

To do this, the first end 12a of the compliant shaft <NUM> is disengaged from spline engagement with the drive gear system <NUM> by drawing the first end 12a of the shaft axially out of the drive gear system in the R direction. The first end is extracted from the gear system e.g. by means of a tool such as a thread bar which may be inserted into the end of the compliant shaft to pull it out of the gear system. Because the second end 12b of the shaft <NUM> has n+<NUM> splines, however, when the first end splines have disengaged with the gear system, the second end splines still maintain engagement with the screw shaft <NUM>, as shown in Fig. 2C. As the shaft <NUM> is now no longer in rotational engagement with the gear system and, hence, with the drive assembly, the screw shaft <NUM> can be independently rotated to determine the stroke required for the desired starting position of the surface/component and to adjust the position of the nut <NUM> on the screw shaft, thus adjusting when it will abut against the first stop 40a when the shaft is re-connected to the drive assembly.

Once the stop position has been adjusted as required, the first end 12a of the compliant shaft re-connects with the drive mechanism via the first spline arrangement. The contact accuracy of the adjusted stop 40a will be less than the size of one spline tooth. This accuracy is achieved by calculating the difference between the spline pitch values of n teeth at the first end 12a and n+<NUM> teeth at the second end 12b. The pitch of the teeth of the first spline arrangement <NUM> is equal to <NUM> deg. / n and the pitch of the teeth of the second spline arrangement <NUM> is equal to <NUM> deg. The accuracy of the adjusted contact is equal to half of the difference between the pitch of the first teeth and the pitch of the second teeth.

Claim 1:
An actuator assembly comprising:
a compliant actuator shaft (<NUM>) having an axis (A) and a first end (12a) and a second end (12b);
a drive assembly (<NUM>, <NUM>, <NUM>) provided at the first end of the compliant actuator shaft to cause rotation of the compliant actuator shaft about the axis;
an overtravel stop mechanism (<NUM>) comprising:
a screw shaft (<NUM>), coaxial with the compliant actuator shaft (<NUM>), and mounted to and arranged to rotate with the second end of the compliant actuator shaft, the screw shaft having an outer thread (<NUM>) extending in the axial direction;
a nut (<NUM>) threadedly mounted to the outer thread (<NUM>) of the screw shaft and arranged to move axially relative to the screw shaft as the screw shaft rotates;
a first stop (40a) fixed to and radially extending from the screw shaft to define the axial limit of movement of the nut relative to the screw shaft, in a first axial direction, as the screw shaft rotates, by abutment of the nut (<NUM>) against the first stop (40a); and wherein
the first end (12a) of the compliant actuator shaft (<NUM>) is provided with a first number n of splines (<NUM>) for spline engagement with the drive assembly and the second end (12b) of the compliant actuator shaft is provided with a second number of splines (<NUM>), the second number being one spline more than the first number of splines, for spline engagement with the screw shaft.