The present invention relates to a solar tracking device.

Plants for the production of concentrated solar power (so-called CSP) contain a multitude of mirrors which reflect the sunlight against a raised boiler. To ensure that during the hours of daylight the sun's rays are constantly reflected towards the boiler, each mirror is equipped with a tracking device. A mechanism of the prior art for moving the mirror is for example described in CN 201269283. A worm screw engages on a toothed wheel integral with a swivelling turret on which the mirror is placed. Control of the rotation of the worm screw thereby enables angular positioning of the mirror. These systems have drawbacks.

The worm screw engages with few teeth of the toothed wheel, therefore becoming rapidly worn and unable to transmit much torque. Since for reasons of ease of installation around the raised boiler the tracker must enable 360° rotation of the mirror, and since in any case in normal conditions daily travel may reach even 180°, the number of work cycles of the worm screw is extremely high, leading to wear and loss of precision. In fact, systems of this type should have a working life of over 10 years without requiring maintenance.

The main object of the present invention is to improve this state of the art. Another object is to obtain an accurate device which is reliable over time. These and are objects are achieved by means of a solar tracking device for making a mirror track the position of the sun, comprising: - a fixed part integral with the ground and a part which is movable in relation to the fixed part on which the mirror may be mounted;

- a shaft or elongated element having a first threaded section and a second threaded section, the second threaded section having the direction of the thread opposite to the first;

- two counter-threaded elements (female screws) engaged respectively with the first and the second threaded section and respectively integral with the fixed and movable part;

- means for axially moving the shaft in relation to the two counter-threaded elements, so that the axial shift of the shaft induces a screwing of the two counter-threaded elements onto the respective threaded sections and thereby a rotation of the movable part in relation to the fixed part.

The worm screw and toothed wheel coupling of the prior art is thereby eliminated and replaced with a screw/counter-threaded element coupling. A greater contact surface of the thread and counter- thread ensues, to the advantage of minor wear, greater transferrable torque and precision. The shaft or elongated element may in general have the threads either on the outer surface and/or be hollow and have them internally, depending on the construction variant. Correspondingly, the elements engaged on its threads will generally be externally threaded female screw members or pins, or combinations of these, or equivalents. The means of axially shifting the shaft may be joined to the fixed part or the swivelling part, or mounted on one of them. Preferably, for the sake of construction simplicity the first and second elements may be female screw members, which screw onto externally threaded sections of the shaft. To improve positioning precision, one or each counter-threaded element may comprise means of annulling the play on the respective threaded section of the shaft it engages on, one advantageous embodiment of which comprises two bushes threaded internally in a corresponding manner to the threaded section of the shaft on which they are engaged, the bushes being positioned one beside the other and blocked in a reciprocal angular opposition so as to annul play on the underlying thread. The principle of the lock nut is here applied.

One simple but effective embodiment for the moving means may comprise a third threaded section of the shaft; a third counter- threaded element (female screw) which is engaged swivelly with the third section; and means of setting the third element in rotation. By rotating the third counter-threaded element the shaft is made to translate axially. In general the means for placing the third element in rotation may be integral with the fixed part or the swivelling part or mounted on one of them.

Two variations of this configuration may be: (i) the third element is constrained so as to be unable to shift along the shaft axis while moving along the third threaded section, or (ii) the third element is axially movable so as to be able to shift along the axis of, and with respect to, the shaft while moving along the third threaded section of the shaft. The third element too, for the sake of constructional simplicity, is preferably made as a female screw member, in particular as an externally toothed conical crown and the means for placing in rotation comprise a conic pinion, with axis orthogonal to the shaft, which is engaged on the teeth of the crown and is driven by a motor. Or the third element may be an externally toothed crown and the means for placing in rotation comprise a toothed cup engaged on the teeth, where the cup is coaxial to the shaft and is driven by a motor aligned with the shaft. In another variation, the third element is not a female screw member but is a second shaft coaxial to the first having a thread engaged with the third threaded section, and the means for placing in rotation comprise a toothed crown integral with the second shaft able to receive rotatory movement from a pinion, with axis orthogonal to the second shaft, which is engaged on the teeth of the crown and is driven by a motor. Or the second shaft may be driven by a motor coaxial to the first shaft, where preferably the second shaft is the output member of the motor.

To ensure the reversibility of the positioning mechanism, it is advantageous for the third threaded section to have a sufficiently small pitch (e.g.M42x2) to prevent the rotation of the third female screw element caused by the torque impressed by the shaft. The pitch should be chosen experimentally or depending on the load to overcome on the shaft.

To maximise the reduction ratio one or each threaded section may have a thread at 45° to the shaft axis. In turn the shaft maybe hollow and open at both ends so as to allow the passage of cables or ducts inside it and rationalise the general architecture of the device.

To further improve positioning precision, the device may have means for annulling the play of the third element on the third threaded section, of which one embodiment may be to mount the crown on two female screw members in forced grip on the third threaded section.

The advantages of the invention will be clearer from the following description of a preferred embodiment, illustrated in the appended drawings wherein:

fig. l shows a lateral view of a device according to the invention; fig.2 shows a cross-section according to the plane II— II ;

fig.3 shows a shaft of the device in fig.l ;

fig.4 shows a pinion of the device in fig. l ;

fig.5 shows a toothed crown of the device in fig. l ;

fig.6 shows a construction variant of the device in fig.l in vertical cross-section;

fig.7 shows a construction variant of the device in fig.l in vertical cross-section;

fig.8 shows a construction variant of the device in fig.l in vertical cross-section.

A device 10 comprises an outer casing 12 formed by an elongated fixed part 14 and a swivelling turret or cap 16. The relative rotation between the two parts 14, 16 is around a longitudinal axis X of the fixed part 14 and is enabled and facilitated by a series of bearings 80 positioned between them. The part 14 is goblet-shaped and contains a rotationally aligned hollow cylindrical sleeve 18 of the part 16. Coaxially and internally to the parts 14, 16 a shaft 20 extends with an axis X, having three threaded sections: a distal section 22, a central section 24 and another distal section 26. The thread has the same structure (at 45°) in the sections 22, 24 but in opposite directions, while the section 26 has a thread with a finer, shallower pitch.

The shaft 20 is coaxial to the sleeve 18 and can move inside it. The female screw of a pair of bushes 32, 34 which are mounted integral with the fixed part 14, engages on the section 22. The female screw of a second pair of bushes 42, 44 which are mounted integral with the swivelling part 16, engages on the section 24. The bushes 32, 34, 42, 44 are therefore constrained to maintain a reciprocal axial position which is fixed along the shaft 20 (and along the axis X). On the section 26 the reciprocal teeth of a toothed crown 50 engage (fig. 5) coupled so as to swivel by means of bearings 82 to the swivelling part 16, but constrained to maintain a fixed position along the shaft 20 and the axis X. A pinion 52 (fig. 4) with axis Y orthogonal to the shaft 20 engages on the teeth of the crown 50. The pinion 52 is mounted so as to swivel in the summit of the part 16 and is connected to a motor (not shown) able to make it rotate in a controlled manner. The final reduction ratio between a revolution of the pinion 52 and a revolution of the turret 16 is for example 1 : 60. The functioning of the device 10 is as follows. When moving of the mirror is desired, the pinion 52 is made to rotate, impressing torque on the toothed crown 50. The latter is forced to screw onto the section 26, but being axially blocked it is the shaft 20 which is forced to translate along the axis X. Such translation in actual fact also occurs by rotation, given that the shaft 20 can only advance linearly by screwing into the bushes 32, 34, 42, 44. Consequently the pinion 52 controls the direction and speed of rotation of the shaft 20. The screwing of the shaft 20 into the bushes 32, 34 thereby implies that it must rotate and translate. The two movements are summed on the bushes 42, 44 which are forced to rotate in relation to the part 14, placing the entire movable part 16, and thereby the mirror, in rotation around the axis X.

Fig.6 shows an assembly variation of the crown 50 on the shaft 20 so as to eliminate the play of the crown 50 and improve precision. A corresponding teeth 60 of the, or the entire, crown 50 is constrained to two female screw bushes 62, 64 engaged on the third thread of the shaft 20. The two bushes 62, 64 are blocked by a key 66 in an angular position wherein both are pressed onto the underlying thread (like the bushes 32, 34 or 42, 44).

Fig. 7 shows a variation 100 of the tracker device regarding the rotation mechanism of the swivelling part. As previously there is an outer casing 112 formed by an elongated fixed part 114 and a turret or cap 116 swivelling around a longitudinal axis XI. Many details have already been described and will not be repeated, the drawing being sufficient. Coaxially and internally to the parts 114, 116 a shaft 120 extends with axis IX, having three threaded sections: a distal section 122, a central section 124 and another internal distal section 126 opposite the first. The thread has the same structure (at 45°) in the sections 122, 124 but opposite directions, while the section 126 has a thread with a finer, shallower pitch. The female screw of a pair of bushes 132, 134 which are mounted integral with the fixed part 114, engages on the section 122. The female screw of a second pair of bushes 142, 144 which are mounted integral with the swivelling part 116, engages on the section 124. As previously, the bushes 132, 134, 142, 144, are constrained to maintain a reciprocal axial position which is fixed along the shaft 120 (and along the axis IX). The thread of a stem or second shaft 190 is engaged on the section 126 which is kept swivelling parallel to the axis IX by bearings 182 but is axially constrained by the interference of the bearings 182 of a abutment 192 and a toothed crown 150. The crown 150 is placed at the end of the shaft 190 and is firmly joined to it, tightened by a ring nut 194. The toothed crown 150 is coupled to a motor pinion 152 with axis 1Y orthogonal to the axis IX.

The functioning of the device 100 is as follows. When shifting of the mirror is desired, the pinion 152 is made to rotate, impressing torque on the toothed crown 150 which transfers it to the shaft 190. The latter screws onto the section 126 and tends to push the shaft 120 along the axis IX. As before, the shaft 120 is forced to screw into the bushes 132, 134, 142, 144 which are forced to rotate in relation to the part 114, placing the entire mobile part 116, and thereby the mirror, in rotation around the axis IX. The shaft 190 may even be the output shaft of a motor to directly drive the first shaft 120.

The member impressing rotatory movement on the shaft 20 or 190 may also be parallel, preferably coaxial, to it. Fig. 8 shows a possible variation 200, where only the details different from those already described or required for explanation are indicated, the rest remaining as before. In the device 200 there is a stem 290, structurally and functionally identical to the stem 190, engaged on a shaft 220 the same as the shaft 120. The stem 290 is mounted on two bearings 282 and axially blocked by a ring nut 250 from which it projects with a tang 299 for connection to a motor (not shown), which by means of an output shaft (not shown) rotating around the axis 2X is able to transfer rotational movement to the stem 290.

Otherwise the device 200 functions in the same way as the previous variations. The motor driving the stem 290 may even be part of it, for example as in the case in which the stem 90 constitutes the output shaft of such motor.