Spray painting machine with variable speed rotary arm

Abstract: Machine to spray paints and the like onto objects moving on a belt conveyor (T), comprising a plurality of spray guns (3) movable at variable speed along a circular path lying on a plane parallel to the surface of the conveyor, and having at least two sections substantially transversal to the moving forward direction of the conveyor. The spray guns (3) are all carried by a single support (2) mounted at the end of a rotary arm (1), to which there are associated means (5 to 8) to keep the angular position of said support (2), during its rotary movement, unvaried in respect of the moving forward direction of the conveyor (T). The variable speed motion is imparted on the support (2) for the spray guns (3) by drive means (11 to 15), with a substantially sinusoidal law, so as to cause said guns (3) to travel at maximum speed in correspondence of the path sections where they are inactive.

Description: The present invention concerns a machine to apply liquid products - particularly paints - by spraying, onto objects of any type, such as manufactured wooden, plastic and leather articles, or the like, while they are moving on a belt conveyor. More particularly, the present invention concerns a spray painting machine, wherein the spray guns are movable and are carried by a single support rotating at variable speed along a circular trajectory according to the preamble of claim 1. The known spray painting machines with movable spray guns can be substantially divided into three types, according to the motion imparted on the guns W, and precisely: a reciprocating motion, along a path perpendicular to the axis of the conveyor belt onto which move the objects to be treated (oscillating spray painting machines); a continuous motion along an oval path, the minor axis of which is parallel to and overlaps the axis of the belt conveyor (carrousel spray painting machines); a continuous, constant speed motion along a circular path, the centre of which lies on the axis of the belt conveyor (turntable spray painting machines). Such known types of spray painting machines are diagrammatically illustrated in figs. 1A, 1B and 1C, respectively. They all have different limits and drawbacks which shall be briefly pointed out hereunder. In the oscillating type, the continuous reciprocating motion of the gun units determines strict limits due to inertia of the moving masses; the speed - and thus the productivity of the machine - cannot therefore be increased beyond specific levels. Furthermore, at each motion reversal, the guns return at once on the area which has just been sprayed, thereby running into the paint dust which the appropriate suction devices have not yet been able to remove from the spray painting area. This causes a fouling of the guns and hampers the regular removal of paint dust, which can thus deposit onto the painted objects, spoiling the quality of the spray paint. In this type of machines, the working time of the guns - namely, the length of time during which the guns are positioned on the work area - is about 45-50% of the time required for the guns to travel along the whole path. In the carrousel type, the kinematic problem is solved in a far more satisfactory way, since the only inertias - besides those, of scarce importance, determined by the constant bending of the pipes and electric cables - are the ones deriving from the mass of the spray guns in the sudden changes of direction, at the inlet and outlet of the curves of the oval path. Also the problem of paint dust is more easily solved, since the gun paths in a forward and backward direction are spaced to an extent such as to allow a sufficient separation between the dust flows moving in opposite directions. This problem however arises again - even though to a reduced extent - on each single going and return path, which the guns follow at great speed. On the other hand, the carrousel sprayers are rather complicated machines and require a toilsome and expensive maintenance. They must in fact be provided with a central supply system, in the form of a multiple rotating joint, to feed paint and compressed air, as well as to connect the necessary electric circuits to the single guns. A rotating joint of this type is described, for example, in the Utility Model IT-B-184353. Such a device, owing to its complexity, is easily subject to breaking and failures (particularly dangerous in case of electric connections, when spraying paints diluted into flammable solvents), also due to the high pressure imparted on the rotating seals for the compressed air and, above all, for the paint. Furthermore, the pipes feeding compressed air and paint, and the electric cables which connect the rotating joint to the single guns, are subject to continuous bending - since the distance between the guns and the joint varies continuously along their oval path - whereby they have to be periodically replaced in order to avoid dangerous breakdowns, which would involve a particularly difficult operation due to the complexity and high number of components having to be replaced. Furthermore, in this second type of machines, the working time of the guns is slightly shorter than that of the oscillating sprayers, as it is equal to about 40-45% of the time required for the guns to travel along the whole oval path. The turntable type of spray painting machines can generally be compared to the carrousel ones, whereby they suffer from all the above drawbacks connected not only with the presence of the central rotating joint, but also with the problem of paint dust; nevertheless, they form an improvement over the same from the kinematic point of view. In fact, this type of machine has no problems of inertia, either for what concerns the guns, since their path is perfectly circular and thus undergoes no sudden changes of direction, or for what concerns the pipes and electric cables which connect the guns to the rotating joint, since these are of fixed length and can thus be permanently anchored to the spokes of the turntable, with undoubted advantages also from the point of view of replacement frequency. This type of sprayer is instead far less efficient for what concerns the uniformity of the painting operation. In fact, since the working path consists of a circle arc covered at constant speed, the transversal component of the guns motion speed varies depending on the angle sine formed by the tangent to the circular path with the axis of the belt conveyor supporting the objects to be painted. Thus, in particular, said transversal speed is considerably reduced at the sides of the conveyor, while being very high at its centre. In this way, the amount of paint being sprayed is greater on the sides of the conveyor than in the centre, which makes it impossible to spread a uniform layer of paint on all the objects being treated. Further negative effects are determined - in the conventional turntable spray painting machines - by the continuous angular variation of the spray jets in respect of the conveyor axis. Said effects particularly arise when painting the vertical edges of panels which, according to their angular lie in respect of the longitudinal axis of the conveyor, will be covered with paint in different ways from the point of view of quality. To partly overcome these inconveniences, turntable sprayers have been proposed, wherein mechanical devices are provided to continuously vary the angular position of the guns in respect of the turntable, so as to keep said position substantially constant in respect of the conveyor axis. Such machines - wherein the actual working time of the guns is anyhow less than 50% compared to the full cycle time - have however proved to be complicated and scarcely reliable, and have met with no success on the market. Machines of this type are described, for example, in WO 91/05615 and in FR-2.172.726. In particular, the sprayer of FR-2.172.726 provides a support for one or more spray guns, mounted at the end of an arm rotating at constant speed to move along a circular path lying on a plane parallel to the conveyor surface, a gearwheel and chain drive, associated to said arm, keeping the angular position of the support unvaried in respect of the moving forward direction of the conveyor during the rotary motion of said arm. Even this solution is however not resolutive, in that - besides adopting complicated structural arrangements due to the location of the support for the rotary unit (which is positioned in the work area) - it provides for working times which are still very low in percentage, owing to the constant travelling speed of the guns both through the work areas and through the dead areas. The dead areas in fact exceed the work areas, also due to the considerable length of the rotary arm adopted for the sprayer of this patent. The object of the present invention is to therefore supply a new type of spray painting machine, which still provides all the kinematic advantages of the known turntable sprayers, but which also allows to reduce the times in which the spray guns are positioned out of the work area (dead times) with respect to those in which they operate in said area. According to the present invention, said object is reached by means of a machine to spray paints and the like onto objects moving on a belt conveyor - of the type comprising a plurality of spray guns movable along a circular path lying on a plane parallel to the surface of the conveyor, and having at least two sections substantially transversal to the moving direction of the conveyor, said spray guns being carried by a single support movable along said path, and means being provided to keep the angular position of said support, during its rotary movement, unvaried in respect of the moving direction of the conveyor (namely, in respect of its longitudinal axis) - characterized in that, the movement of the support for the spray guns is controlled by drive means apt to impart thereon a variable speed, with a substantially sinusoidal law, between a minimum speed in correspondence of the path sections of the guns substantially transversal to the longitudinal axis of the conveyor (work areas) and a maximum speed in correspondence of the path sections of the guns substantially parallel to said longitudinal axis (dead areas). Further characteristics and advantages of the spray painting machine according to the present invention will anyhow be more evident from the following detailed description of some preferred embodiments thereof, given by way of example and illustrated on the accompanying drawings, in which: Figs. 1A, 1B, 1C, are diagrammatic plan views of the prior art sprayers discussed in the introductory part of the present description; Figs. 2 and 3 are diagrammatic plan and, respectively, elevation views, showing the mechanical structure of the spray painting machine according to the invention; Fig. 4 is a diagrammatic view of the drive means imparting a variable speed on the machine according to the invention; and Figs. 5 and 6 are graphs outlining the law of the travelling speeds imparted by the drive means of fig. 4. As shown in figs. 2 and 3, the spray painting machine according to the invention comprises a single rotary arm 1, at the free end of which there is mounted a support 2 for a set of spray guns 3 (eight guns in this case). Said spray painting machine is positioned above a belt conveyor T - along which slide the objects to be painted - so that the rotation axis of the arm 1 intersects the axis S of the conveyor T. As shown more clearly in fig. 3, the arm 1 is fixedly connected to the lower end of a sleeve 4, which is mounted rotating on a fixed shaft 5 upwardly projecting from the arm 1. To the bottom end of the shaft 5 there is fixed a gearwheel 6, meshing with a toothed driving belt or chain 7, the other end of which meshes with a gearwheel 8 keyed onto a shaft 2a carrying the support 2 for the set of spray guns 3. The shaft 2a is mounted, freely rotating, onto the arm 1, and the gear ratio between the gearwheels 6 and 8 is of 1:1. This mechanical arrangement, known per se, imparts from the top a rotary motion on the arm 1. Furthermore, thanks to said arrangement, when the arm 1 is caused to rotate - by way of the control system acting on the sleeve 4, illustrated hereinafter - the support 2 keeps unvaried its angular position in respect of the axis of the conveyor T (as illustrated in fig. 2 by the various positionings of the set of spray guns 3). The aforedescribed spray painting machine is hence provided with a single set of guns 3, which keeps its angular position unaltered while the arm 1 is rotating, and which can thus be fed by a stiff pantograph rod 9 - diagrammatically illustrated by a dashed line in figs. 2 and 3 - or by an articulated cable chain, thereby fully eliminating the need for a rotating joint. The rod 9, or chain, is connected at one end to the fixed structure of the machine and, at the other end, to the support 2 for the set of spray guns 3 (to any part of said support but, preferably, to its centre, in correspondence of the shaft 2A). According to the invention, this spray painting machine is provided with a system to control the arm 1, such as to determine its rotation at a speed varying with a sinusoidal (or substantially sinusoidal) law, and not at a constant speed as in the case of the known type turntable machines. This variable rotation speed of the arm 1 can be obtained by different mechanical, as well as electronic, techniques of known type, and it allows to accomplish the important advantages described hereinafter. According to a possible, currently preferred, mechanical solution - shown in fig. 4 - said control system comprises two identical gears 11 and 12, having their respective centres in 11a and 12a, but being mounted rotating on eccentric shafts, in correspondence of A and B. In this way, during rotation, said gears continue to oscillate, while keeping constantly meshed in correspondence of C. On the shaft of the gear 11, in correspondence of A, there is keyed a main drive motor (not shown) rotating at constant speed; thanks to the fact that the two gears 11 and 12 continue to oscillate, their meshing ratio evidently varies, as it is determined by the ratio between the distances AC/BC. As the gear 11 rotates at constant speed, together with the motor, the speed of the gear 12 varies continuously with a substantially sinusoidal law. The motion being transmitted to the sleeve 4 of the arm 1 is hence derived from the rotation axis (in B) of the gear 12, through a pair of gearwheels 13, 14, and respective drive belt 15, with a 1:2 gear ratio. The arm 1 thus rotates at a speed with a double sinusoidal law for each cycle, as clearly shown in the graph of fig. 5. This solution allows to obtain the following considerable advantages which are apparent from the graph of fig. 6: the arm 1 rotates at maximum speed, Vmax, in correspondence of the lateral dead areas of transfer of the guns; this allows to increase the productive time; the arm 1 rotates at a decreasing speed, with a sinusoidal law, as it moves from the sides to the centre of the conveyor, as clearly shown in fig. 5 wherein the spraying areas - namely, where the guns are working - are marked by P and include, for example, the sectors of ± 45° about the axis S of the conveyor T. In this way, the transversal component, Vt, of the arm peripheral speed is substantially constant and equal to the minimum speed, Vmin, at which the arm rotates at the centre of the conveyor T; one thereby obtains a perfectly uniform thickness of the layer of paint being sprayed by the set of guns 3. As stated heretofore, said result can be accomplished in an equally satisfactory manner by adopting an electronic feedback control system. A computer detects the angular position of the arm 1, through a de-coder, and determines the rotation speed of the driving motor - according to the preset speed required in that specific angular position - by varying in known manner its electric feeding characteristics. The spray painting machine according to the present invention provides furthermore - thanks to its structural shape - other advantages which are difficult to obtain simultaneously, such as the elimination of the rotating joint, a constant movement with no inertias, the possibility to maintain a constant angle between the spray jets of paint and the axis S of the conveyor T (namely, an arc-shaped painting trajectory, but with the guns always inclined by the same angle in respect of the conveyor axis), and the possibility to carry out an improved and more complete removal by suction of the dust produced by the paint which has failed to deposit on the objects being treated (since the set of guns moves back over the painted areas only after a full cycle, and thus after a time interval which is definitely longer than that taken by the guns in the solutions of known technique. It is thus evident that this longer time interval allows to remove the paint dust in a more efficient way). It is anyhow understood that the invention is not limited to the particular embodiments described heretofore - which are merely examples, not meant to limit its scope - but that a number of modifications can be introduced, all within reach of a technician skilled in the art, without thereby departing from the scope of the invention itself as defined by the appended claims.

Claims: Machine to spray paints and the like onto objects moving on a belt conveyor (T) - of the type comprising a plurality of spray guns (3) movable along a circular path lying on a plane parallel to the surface of the conveyor (T), and having at least two sections substantially transversal to the moving direction of the conveyor (T), said spray guns (3) being carried by a single support (2) movable along said path, and means (5,6,7,8) being provided to keep the angular position of said support (2), during its rotary movement, unvaried with respect to the moving direction of the conveyor (T) - characterized in that, the movement of the support (2) for the spray guns (3) is controlled by drive means (11,12,13,14,15) apt to impart thereon a variable speed, with a substantially sinusoidal law, between a minimum speed in correspondence of the path sections of the guns (3) substantially transversal to the longitudinal axis of the conveyor (T), and a maximum speed in correspondence of the path sections of the guns (3) substantially parallel to said longitudinal axis. Spray painting machine as in claim 1), wherein said drive means apt to impart a variable speed on the support comprise at least two constantly meshing eccentric gears (11,12) - a driving gear (11) and, respectively, a driven gear (12) - the motion of the driven gear (12) being then transmitted to said support (2) with a 1:2 gear ratio. Spray painting machine as in claim 1), wherein said drive means apt to impart a variable speed on the support (2) comprise at least a computer which detects, through a de-coder, the angular position of said support (2) for the spray guns (3) and determines, according to said position, the rotation speed of the driving motor, thereby varying its electric feeding characteristics. Spray painting machine as in claim 1), wherein the support (2) for the spray guns (3) is mounted revolving on one end of a rotary arm (1), the rotary movement being imparted to said arm (1) from the top, at the other end, about an axis intersecting the longitudinal axis of the conveyor (T). Spray painting machine as in claim 1), wherein paint, compressed air and electric energy are fed to the spray guns (3) through continuous pipes and cables, fixed to a stiff pantograph rod or to an articulated cable chain, an end of which is connected to said support (2) while the other end is connected to the fixed structure of the machine.

Inventors: RAGGI GIANNI, RAGGI, GIANNI
Applicants: CEFLA COOP, CEFLA SOC. COOP. A R.L.
Application year: 1993
Main classification: B05B13
IPC codes: B05B13