Linear actuators inclusive of an electric motor and a transmission assembly which converts the rotary motion given to the rotor of the electric motor into a linear motion are known. In some cases recirculating ball screws are used for this purpose, but they have been found to be disadvantageous both from the cost point of view and because of their great weight. As is known, recirculating ball screws are composed of a large number of parts requiring special costly machining and treatments (such as thread rolling, hardening and grinding) in order to accurately produce the balls and the parts forming the rolling tracks for the balls (the screw and nut). Admissible tolerances are very low and therefore the number of rejects is high. The operations of assembling recirculating ball screws are also time-consuming.

It is an object of the present invention to provide a linear actuator that addresses in the first place the problem of reducing the number of components and he associated cost of manufacture and assembly. One particular object of the present invention is to provide a linear electromechanical actuator capable of producing a large axial force, particularly but not exclusively for operating a friction clutch in a motor vehicle.

These and other objects and advantages, which will be understood more fully in the course of the text, are achieved according to the invention with an electromechanical actuator having the features defined in the appended claims.

The structural and functional features of a preferred but not limiting embodiment of the actuator according to the invention will now be described, reference being made to the appended drawings, in which : Figure 1 is a perspective view of an actuator according to the invention, in which some parts have been removed to show internal constructional details; Figure 2 is a schematic perspective view with some parts cut away of the actuator according to the invention in an axially retracted rest condition; Figure 3 is an exploded perspective view of the actuator of Figure 2; Figure 4 is a view in longitudinal axial section through the actuator according to the invention in a first or axially retracted rest condition; Figure 5 is a view in longitudinal axial section through the actuator of Figure 4 in a second, axially extended active condition; Figure 6 is a perspective view of a discoidal plate component of the actuator according to the invention; Figure 7 is a view in axial longitudinal section through the component of Figure 6; Figure 8 is a perspective view of a piston component of the actuator according to the invention; Figure 9 is a plan view from above of the component of Figure 8 ; Figure 10 is a view in axial longitudinal section taken on the line X-X in Figure 9; Figure 11 is a view in axial longitudinal section taken on the line XI-XI in Figure 9; and Figure 12 is a perspective view of a bar element with which the actuator according to the invention is provided.

Referring initially to Figures. 1,2 and 3, a linear electromechanical actuator according to the invention is indicated as a whole by the number 10. To talk in practical terms (although this should not of course be interpreted as limiting the scope of the invention), the actuator 10 is ideally suited, for example, to operating a friction clutch in a motor vehicle.

The actuator 10 comprises an electric motor 11 whose rotor turns a shaft (not shown) which, by means of a speed- reduction unit comprising, in this example, an epicyclic gear system bearing the general reference 12, turns the rotatable discoidal plate 20 illustrated separately in Figures 6 and 7.

The plate 20 has a central hole 21 with a radial recess 22 to take a lateral projection (not shown) on a rotary output member 13 of the epicyclic gear system 12. The plate 20 forms two radially projecting peripheral arcuate segments 23 which define axial abutment surfaces 24,25, the function of which will be explained later. The face 27, referred to herein as the axially inward face of the plate 20, contains a plurality, in this example three, hemispherical seats 26 angularly equally spaced around the central longitudinal axis x of, the plate 20.

Throughout this description and in the claims which follow, terms and expressions indicating positions and orientations such as"radial","axial"and"longitudinal"refer to the central axis x of the actuator 10.

The reference 30 denotes a casing of basically cylindrical tubular form attached to a stationary flanged element 31 with axial holes 32 for mounting the actuator on a motor vehicle.

A bearing 40, preferably a thrust bearing, is interposed axially between the stationary flanged element 31 and the rotatable discoidal plate 20.

An axially movable piston member, bearing the reference 50 and illustrated separately in greater detail in Figures 8 to 11, is generally cylindrical and has an axially inward transverse surface 51 in which are formed two or more angularly equidistant hemispherical seats 52 equal in number to the hemispherical seats 26 formed in the rotatable discoidal plate 20.

Two diametrically opposite axial slots 53 are formed in the lateral surface of the piston member 50. The slots 53 slidably engage with two corresponding guide elements 33 integral with the casing 30 so as to guide the piston member 50 axially and non-rotatably along the casing 30. In the illustrated example of an embodiment, the guide elements 33 are formed by a pair of keys mounted in diametrically opposite positions and projecting radially inwards from the tubular casing 30. At the bottom of the slots 53, or in an axially more inward position, the piston member 50 forms radial bearing surfaces 54 for corresponding elastic elements 60, such as helical springs. Each spring 60 is interposed axially between a wall 54 and an opposing element such as a screw 34 fastened to the casing 30.

The numbers 35 and 36 denote stop screws carried by the casing 30 to prevent the discoidal plate 20 from turning past two separate predetermined angular positions, as will be explained later.

The number 70 denotes rigid bars, of which there are three in this example, with ball joint portions at opposite ends 71, 72 which at least partly fit into the hemispherical seats 26, 52 formed in the discoidal plate 20 and in the piston member 50, respectively. One of these bars 70 is illustrated separately in Figure 12.

The actuator according to the invention works as follows.

Starting from the passive condition illustrated in Figure 4, the piston member 50 is in an axially retracted position towards the inside of the casing 30, and the three bars 70 are inclined and askew relative to the central longitudinal axis x of the actuator. The rotatable discoidal plate 20 is in a first or passive angular position in which its axial abutment surface 24 is in contact with the first stop screw 35. The piston member 50 is in its axially retracted position that is at the end of its inward stroke.

When the electric motor 11 is turned on, the epicyclic gear system 12 turns the discoidal plate 20 in the direction indicated schematically by the arrow A in Figure 2, until it reaches a new stop position in which the other axial abutment surface 25 is against the second stop screw 36.

Because the angular position of the ends 72 of the bars 70 is fixed on the (non-rotatable) piston member 50, the rotation of the discoidal plate 20 causes, through the bars 70, the axial translation of the piston member 50, which is guided along the keys 33 until it reaches the axially extended position illustrated in Figure 5. In this position the bars 70 are lined up so as to be parallel to the longitudinal central axis x and push against the plate 20, preventing any undesired backward movement or retraction of the member 50 in the direction of the interior of the casing 30.

In performing a combined movement of rotation both about the central axis x and in a plane orthogonal to this axis, the bars 70 straighten out and convert the rotary motion of the plate 20 into a linear motion of translation of the piston member 50.

As can be appreciated, with the configuration described above, when the plate 20 rotates, the bars 70 are capable of transmitting to the piston member 50 a large axial force which increases exponentially in strength to reach a peak in the position of maximum extension of the piston member.

Clearly, the total axial force exerted by the piston member 50 can be varied according to requirements by varying the length of the bars 70 and increasing or decreasing the diameter of the theoretical circumferences on which the hemispherical seats 26,52 lie.

The springs 60 tend to return the piston member 50 to its axially retracted position and co-operate to keep the rounded ends 71,72 of the bars 70 correctly positioned in their hemispherical seats, even when there are no external opposing forces acting on the piston member 50.

Owing to the configuration described above, the axial movement of the piston member 50 is irreversible, unless the electric motor is activated in such a way as to turn the plate 20 in the opposite direction to that described above and so pulls the piston member 50 back into the axially retracted position illustrated in Figure 4. In this return movement, the piston member 50 is assisted, as noted, by the springs 60.

In an alternative embodiment (not shown), the springs can be dispensed with, provided another means is provided to keep the ends of the bars in a stable position on the piston member and on the rotatable plate. For example, the rounded seats may be made in the form of essentially spherical undercut seats into which the ends of the bars would be snap- fitted.

It is intended that the invention is not limited to the embodiment described and illustrated herein, which is to be considered as an illustrative embodiment of the actuator; instead, the invention is susceptible of modifications to the shapes and arrangements of parts, constructional and operational details. For instance, the number and arrangement of the bars may vary, and in all cases they will preferably be arranged uniformly about the longitudinal central axis in order to give the piston member a central combined thrust in line with the longitudinal central axis of the actuator.

In another different embodiment, though less preferred (and not illustrated), the bars 70 may be jointed to the piston member 50 and to the rotatable discoidal plate 20 in a different manner to that described and illustrated, for example by replacing the ball-joint couplings with joints of some other type capable of allowing the combined rotary movements described above.