Linear actuator

A linear actuator has a housing, a piston slideable within the housing along a slide axis, a first shaft rotatable about a first axis substantially perpendicular to the slide axis, a second shaft rotatable about a second axis parallel to the slide axis, in which the second shaft and the piston are engaged to convert rotational motion of the second shaft into linear motion of the piston, and in which the first shaft and the second shaft are engaged by a ball-worm gear.

RELATED APPLICATIONS

The present application is based on, and claims priority from, Great Britain Application Number 1001178.1, filed Jan. 26, 2010, the disclosure of which is hereby incorporated by reference herein in its entirety.

The present invention relates to a linear actuator. In particular, the present invention relates to a linear actuator comprising a ball-worm drive.

Known linear actuators are powered in a variety of ways. Hydraulic and pneumatic actuators are common, but suffer the disadvantage that a compressed fluid source is required to power them. Electric actuators are also known, powered from rotary electric motors. These devices are large and often require gearing between the motor and actuator to provide the desired torque at the desired speed.

In aircraft applications, it is often desirable to provide a high torque at a relatively low speed, because the components of an aircraft which require actuation are commonly very heavy and the applications do not demand significant speeds. One example of this is landing gear extension and retraction.

In theory, as the gear ratio increases, the motor can be made smaller because the torque is increased. However, there is a point at which the weight saved by the reduction in motor size is negated by the increasing size of the gearbox. Therefore motors still tend to be large and heavy as the gearbox required to make them any smaller is too large itself to justify the design change.

Increasing the size of the gearbox also increases the number of components that could potentially fail, and also increases the amount of packaging space required by the actuator.

It is an aim of the present invention to provide an improved linear actuator.

According to a first aspect of the invention there is provided a linear actuator comprising a housing, a piston slideable within the housing along a slide axis, a first shaft rotatable about a first axis, a second shaft rotatable about a second axis transverse to the first axis, in which the second shaft and the piston are engaged to convert rotational motion of the second shaft into linear motion of the piston along the slide axis, and in which the first shaft and the second shaft are engaged by a ball-worm gear.

The ball-worm gear provides a compact, simple arrangement with a high gear ratio that only requires small actuators to provide a high torque at a relatively low speed, ideal for aircraft actuation applications.

A ball-worm assembly100used in the present invention is shown inFIG. 1. The assembly comprises a driving shaft assembly102and a driven shaft104. The driving shaft assembly102(also shown inFIG. 1c) comprises a driving shaft106defining a driving axis108. A ball race110is attached to the driving shaft106. The ball race is generally rotationally symmetric and defines a concave outer profile with radius R. A helical ball channel111surrounds the ball race110about the driving axis108. A plurality of bearing balls112sit within the channel and are encased by a housing (not shown) so that they can move only along the path defined by the channel. The housing includes a recirculation system to provide the balls112with the ability to move along the channel in use.

The driven shaft104has an outer radius R and defines a driven axis105. The driven shaft104defines a series of axial ball tracks114on its outer surface. When the race110and the driven shaft114are placed in close proximity, the channel and the tracks114define a path for the balls112.

When the driving shaft is rotated, a circumferential force about driven axis105is applied to the axial ball tracks114of the driven shaft104by the balls112because they move in a helical path around the ball race110. Torque is therefore transmitted through the race110to the balls112and consequently to the driven shaft104. The ball-worm gear100provides a gear reduction from the driving shaft106to the driven shaft104and transfers the drive through 90 degrees from the axis108to the axis105. Therefore a high speed, low torque input from the driving shaft106is geared to a low speed, high torque output at the driven shaft104.

Turning toFIGS. 2 to 5, a linear actuator500is shown. The actuator500comprises a housing502comprising a first mounting formation504. The housing500is generally cylindrical and hollow.

The actuator500comprises a central shaft506rotatably mounted therein on a roller bearing508. The central shaft506is coupled to the second shaft104of the aforementioned ball-worm arrangement.

The driving shaft assembly102is mounted to the exterior of the housing502and the ball-worm gear engages the second shaft104through an aperture510in the housing outer wall. A first motor512and a second motor514are mounted to the housing502and drivingly connected to the shaft106. Therefore upon activation of the motors512,514, the second shaft104and the central shaft506rotate about the axis105.

A roller nut516is assembled on an outer surface of the central shaft506engaged therewith. The roller nut516engages with the central shaft506to form a ball screw joint in known fashion. The roller nut516is connected to a piston518which surrounds and is concentric with the central shaft506. A second mounting formation520is mounted to the end of the piston518. The assembly of the second mounting formation520, piston518and roller nut516is keyed to the housing502such that it can slide within the housing502along a slide axis (coincident with axis105) but cannot rotate relative thereto.

Therefore, as the central shaft506is rotated, the piston506is extended from the housing502and the actuator is extended. Rotation in the opposite direction will retract the piston506.

It will be understood that the rotational-linear joint between the central shaft and piston may be achieved in any known manner, e.g. through a simple threaded connection (acme screw), or roller screw.

Turning toFIG. 5, a landing gear assembly550is shown. The assembly550comprises a landing gear strut552extending to a landing gear wheel mounting formation554. The strut is mounted to rotate about an axis556between a deployed (shown) position in which the strut is vertical and a stowed position in which the strut is horizontal within the aircraft fuselage. Rotation about the axis556is achieved by actuation of the linear actuator500between its extended condition (shown) and a retracted condition.

In the above embodiments, the driven and driving shafts have been arranged perpendicularly. Further, the driven shaft has been arranged parallel to the slide axis. It will be understood that this is not necessary for functioning of the invention, and the shafts need only be transverse—i.e. relatively oriented to the extent that a driving connection can be made between them. This may require the use of extra componentry—for example the mechanical formations of the ball-worm may need to be a different form, or a universal joint may need to be installed between the driven shaft and the landing gear axle.

The piston may be any kind of linearly movable rod.