"Plant for the treatment of bodywork"

DESCRIPTION

The present invention relates to a plant for the treatment of bodywork, in particular for motor vehicles and the like and to an operating method.

In the art treatment plants which comprise conveyor lines for transporting a plurality of trolleys, for example in the form of sliding units or skids, each comprising a bodywork to be treated, are known.

For example treatments which involve the immersion of the bodywork in special basins filled with treatment liquid, for example for performing anti-corrosion and electrophoresis pre treatments, are known. These plants have, arranged opposite the basins, immersion systems which for example rotate the bodywork on the trolley so that it can be immersed in the basin and then extracted.

It has been noted that in some points of the plant the trolley may require an additional thrust to overcome particular resistance to the advancing movement. For example, in immersion systems with rotation of the bodywork, the rotating movement may require a temporary increase in the advancing force of the trolley. For example, this may be caused by an imbalance in the weight of the bodywork on the trolley with respect to the axis of rotation.

In particular, immersion systems which obtain the rotational force from the advancing movement itself of the trolley are known. For example, such a rotation system has been proposed in the patent EP3237314. In this document it was also proposed using additional traction systems consisting of a motorized wheel which rests with friction on a longitudinal surface of the trolley and helps the transportation system to push the trolley forwards during rotation of the bodywork.

The use of a friction wheel has, however, a fairly low limit value of the tractional force which the wheel may apply to the trolley and, especially in particularly demanding applications (for example particularly heavy bodywork which must be rotated), the wheel in some conditions may slip on the surface of the trolley. The torque which can be transmitted by the wheel may also decrease significantly owing to the dirt or liquid between the surfaces making contact. Moreover, the wheel may be subject to a relatively high degree of wear and this may result in a further reduction in the traction which can be imparted by the wheel on the trolley and the production of waste matter which may contaminate the bodywork treatment basins. Attempts have also been made to improve the adhesion of the wheel by increasing the force with which it is pushed against the surface of the trolley, but this system worsens further the wear and continues in any case to have a relatively low limit value of the torque which can be transmitted before slipping of the wheel occurs.

The object of the present invention is to provide a plant and a method which solves the aforementioned problems of the prior art.

In view of this object the idea which has occurred, according to the invention, is to provide a plant for the treatment of bodywork, comprising at least one trolley designed to support a bodywork to be treated, a conveyor line for transporting the trolley along the plant and with a first system for moving the trolleys along the conveying line, characterized in that the trolley comprises a traction rack extending in the direction of movement of the trolley along the conveying line and, in at least one zone of the plant, there is a toothed traction wheel which is driven by a motor controlled by a motor control system and which is adapted to engage in the traction rack upon passage of the trolley into this zone so as to move the trolley along a portion of the conveying line under the control of the motor control system and provide a second movement system for the trolleys, there being also sensors designed to detect the angular position of the traction wheel and the approaching movement of a trolley towards this wheel and connected to the motor control system of the toothed wheel so as to cause the engagement between the traction rack and the toothed traction wheel upon arrival of the trolley.

Still according to the invention the idea which has occurred is to provide a method for providing an additional traction to trolleys for transporting bodywork in zones along a line for conveying the trolleys in a plant for treating such bodywork, comprising the steps of providing on the trolleys a traction rack extending in the direction of movement of the trolleys along the conveying line, arranging in said zones along the conveying line a toothed traction wheel which is driven by a motor controlled by a motor control system and which is able to engage in the traction rack when the trolley passes into said zone so as to move the trolley along a portion of the conveying line under the control of the motor control system, arranging sensors designed to detect the angular position of the traction wheel and the approach movement of a trolley towards said wheel and connected to the motor control system of the toothed traction wheel, and the further steps of detecting the angular position of the toothed traction wheel and, when the toothed traction wheel is not engaged in the traction rack, driving the motor so as to bring this wheel into a given position suitable for subsequent engagement with the traction rack of an inbound trolley, detecting the arrival of a trolley and starting the motor with a predetermined acceleration ramp when the front end of the traction rack approaches the toothed traction wheel until the toothed traction wheel and the traction rack are engaged and driving the motor so as to continue the rotation of the toothed traction wheel for a pushing action of the trolley along the conveying line by means of the traction rack engaged in the toothed traction wheel.

In order to illustrate more clearly the innovative principles of the present invention and its advantages compared to the prior art, an example of embodiment applying these principles will be described below with the aid of the accompanying drawings. In the drawings:

- Figure 1 shows a schematic side elevation view of a part of a plant designed according to the invention;

- Figure 2 shows a schematic view of the plant cross-sectioned along the line II-II of Figure 1 ;

- Figure 3 shows a schematic view, on a larger scale, of the plant cross-sectioned along the line III-III of Figure 1 ;

- Figure 4 shows a schematic view similar to the cross-section of Figure 3 and with a further device of a plant according to the invention;

- Figures 5, 6 and 7 show two different operating steps along the plant according to the invention;

- Figure 8 shows in schematic form a part of a plant according to the invention during various possible operating steps.

With reference to the figures, Figure 1 shows in schematic form a first example of a part of a plant, denoted overall by 10, applying the principles of the present invention.

The plant 10 comprises at least one means of transport for a bodywork, which is here generally referred to as a trolley 11 (for example it may be a sliding unit or skid or an actual wheeled trolley) intended to support a bodywork 12 to be treated. Usually the trolleys will consist of a plurality and will travel along the plant in sequence so as to each transport a bodywork to be treated into the various treatment stations.

The plant 10 also comprises a conveying line 13 for the sequential transportation of the trolleys along the plant.

As will be clarified below, the plant comprises at least one first system for moving the trolleys along the conveying line.

The conveying line may comprise, for example, travelways, arranged parallel, for supporting side elements 14 of the trolley, such as runners and/or wheels, and the advancing movement may be produced for example by a series of motorized rollers on which the trolleys rest and/or by other known motorized movement means, as may be easily imagined by the person skilled in the art.

As can be clearly seen in Figure 2, the trolleys may comprise, for example, lateral support elements 14 formed for example on one side by of a lateral runner 14a and on the other side by one or more wheels 14b. The runner 14a may rest on one side of the conveying line 13, for example formed with rollers 13 a, while the wheel 14b may rest on a sliding surface or track 13b.

Those rollers of the conveying line which may be motorized may be all connected to a single motor, for example by means of a chain system, as may be easily imagined by the person skilled in the art.

Other known systems for normal transportation of the trolleys along the line 13 may in any case be used, as may be easily imagined by the person skilled in the art on the basis of the description provided here.

Different sections of the conveying system may also be motorized separately and/or use different systems for moving the trolleys in order to obtain for example asynchronous transportation of trolleys which may thus be moved at different speeds and/or stop along the various sections.

The plant 10 also comprises at least one known process liquid basin 15 inside which the bodywork must be immersed, for example for an anti-corrosion and electrophoresis pre treatment. Below reference will be made to a basin, but it is understood that the basins may consist of a plurality along the plant, also depending on the number and type of treatments required, as will be evident to the person skilled in the art.

The trolley 11 described here advantageously comprises a base part 16 for sliding along the conveyor line and a part 17 which supports the bodywork and is rotatable about an axis 18 of a rotatable shaft 19 supported by the base part. Preferably, the axis 18 may be transverse to the direction of movement, even though other arrangements (for example a longitudinal or inclined arrangement) may be used.

With reference to Figure 2, the base frame of the trolley may comprise, for example on one side, the runner 14a, a central crossbeam which comprises or is formed by the rotatable shaft 19 and its corresponding support elements on the frame, and an opposite support point at the other end of the shaft formed for example by a wheel 14b. The base part may thus have advantageously a generally T-shaped form in plan view. Alternatively, the base part could in any case have an H-shaped frame with longitudinal runners on both sides. In any case, the lateral support points of the base part 16 of the trolley on the conveying system 13 are advantageously situated on the outside of the sides of the basins 15.

As can be clearly seen in Figure 1 , the top part 17 of the trolley may comprise one or more support elements 17a which extend from the rotatable shaft 19 so as to support and lock the bodywork to the shaft by means of known fixing means, optionally via a support surface or frame 21 (omitted for easier comprehension in Figure 2). Advantageously, in the normal transportation position shown in Figure 1 the support elements are directed upwards above the rotatable shaft 19.

The trolley 11 shown in the figures also comprises a mechanism 22 for the controlled rotation of the shaft 19 which allows the bodywork to be moved between a normal advancing position, as shown in Figure 1, and an overturned position for immersion inside a basin 15, which is shown for example in Figure 7. The plant 10 also comprises a device 32 for actuating the rotation of the shaft 19. This device is advantageously located in each zone along the plant where it is required to perform the controlled rotation of the body about the axis 18 of a trolley.

Advantageously, the rotation mechanism 22 comprises a toothed actuating wheel 23 kinematically connected to the shaft 19, optionally via a transmission assembly 24 known per se with a suitable transmission ratio. Preferably, the transmission between wheel 23 and shaft 19 will be such that the wheel 23 and the shaft 19 rotate in the same direction. However, an opposite direction of rotation may also be used, if required. The transmission ratio may be preferably chosen so as to gear down the number of revolutions in order to reduce the force on the rotation actuation device and on the advancing movement system. Moreover, the transmission ratio may be chosen so as to define a desired speed of rotation in relation to the speed of advancing movement and/or the number of teeth (rollers) of the rack.

As can be clearly seen in the figures, the device 32 comprises advantageously a rotation rack 33 which is arranged along the direction of movement of the trolleys along the conveying line 13.

The rack 33 will have a form suitable for engagement with the toothed wheel 23 so as to rotate the toothed wheel 23 by means of the movement itself of the trolley along the conveying line.

The trolley may advantageously also support a device 27 for locking/unlocking the free rotation of the shaft 10 so that the bodywork does not rotate uncontrollably, for example when the wheel 23 is not engaged with a rack 33.

The device 27 may for example act on the toothed actuating wheel 23 so as to block the rotation thereof. For example it may be of the mechanical type formed by a frame 28 hingeably mounted on the trolley and provided with an engaging element 30 and an actuating shoe 31. In the rest condition, the engaging element 30 is engaged with the wheel 23 so as to lock it, while a suitable operating surface along the trolley conveying path may act on the shoe 31 so as to move the frame 28, thus raising the shoe and disengaging the engaging element 30 from the toothed wheel 23 when required.

In the normal condition for conveying along the line, there is no interference with the shoe 31 and the trolley is able to continue conveying the bodywork in the upper position, while in the zones where the rotation of the bodywork on the trolley is to be released, it is merely required to provide suitable surfaces for operation of the shoe.

In particular, an operating surface may be associated with the device 32 for actuating the rotation of the trolley so that the wheel is released when it must be operated by this device 32. Advantageously, as shown in the figures, the rack may be a roller rack and the wheel 23 may be a wheel which is correspondingly lobed or a petal wheel for engaging sequentially with its lobes between the rollers. This ensures secure, reliable and relatively silent engagement. Moreover, the same rollers of the rack 33 may form the surface for raising the shoe 31. Alternatively, an independent surface for actuating the shoe may be provided in a suitable position, for example parallel to and alongside the rack.

The device 27 for locking/unlocking the wheel 23 ensures also that the wheel 23 comes up against the rotation rack 33 in the correct position for engagement with the teeth without jamming, then allowing rotation when the engagement is secure.

Advantageously the actuating device 32 may also be movable controllably between an operating position engaged in the wheel 23 and a non-operating position where the wheel may pass along the rack without engaging with it and without the locking/unlocking device for the wheel 23 releasing the wheel. The movement of the actuating device 32 may be performed for example using a motorized, hinged, parallelogram mechanism 38 which, when operated, raises or lowers the rack so as to move it between the two operating and non-operating positions, as may now be easily imagined by the person skilled in the art on the basis of Figure 1.

If it is required for any reason to convey a trolley into the vicinity of an actuating device 32 without causing rotation thereof it is thus sufficient to move the actuating device towards its non-operative position. For example, this may be useful in the case where there are some types of bodywork which must be treated in some basins and other bodywork which must be treated in other basins along the same conveying line or in the case of maintenance activities which for example require the exclusion of one or more basins.

The plant 10 comprises according to the invention an additional traction system 60 for providing, when desired, the trolleys with additional traction compared to the traction provided by the normal system for performing transportation along the line. This additional system therefore provides a second system for moving the trolleys along the conveying line.

In particular, when rotation of a bodywork on the trolley must be performed, the weight itself of the rotating bodywork may result in the need for a thrust for displacing the trolley along the conveying line which is greater than that normally required for simple sliding of the trolley with the bodywork on the trolley in the normal conveying position. This is particularly necessary for example when the rotation of the bodywork is performed by the same forwards movement of the trolley, as in the embodiment described above.

The additional traction system 60 comprises a toothed traction wheel 61 driven by an electric motor 66 and a complementary traction rack 62 arranged on the base body of the trolley and extending parallel to the direction of movement of the trolley so as to engage in the toothed wheel 61 upon passage of the trolley into the zone where the additional traction is required. Advantageously, as can be clearly seen in Figure 2, the rack 62 of the additional traction system may be placed on one side of the trolley, for example above the sliding runner 14a.

As can be seen in the figures, the toothed traction wheel 61 may be for example a wheel with petals or lobes and the rack may be a roller-type rack.

The system 60 also comprises a first sensor 63 for detecting the angular position of the wheel 61 and a second sensor 64 for detecting the approaching movement of the trolley (or in an equivalent manner of its traction rack) towards the wheel 61. The sensors 63 and 64 are connected to a control system or circuit 65 which controls the motor 66 for rotation of the toothed traction wheel 61. The control system 65 and the sensors 63, 64 form a synchronization system which allows the toothed wheel 61 to be engaged in the rack 62 while the trolley advances, avoiding any jamming.

In particular, the control circuit 65 receives the signals from the two sensors 63, 64 and causes rotation of the wheel 61 so as to synchronize electronically the wheel with the traction rack 62 so as to allow them to engage together without jamming, despite the sliding movement of the trolley, before the engagement is activated by the other system for driving the trolley along the conveying line. The control circuit 65 may be a structure which is generally known per se for the systems for actuating an electric motor and the detection and processing of signals from sensors (for example an electronic circuit formed with cabled logic and with a suitably programmed microprocessor system). It will therefore not be further illustrated or described in detail here.

A synchronization method according to the invention, which may be implemented in the control system 65, may envisage also the following steps:

Initially, when the toothed traction wheel 61 is not engaged in the traction rack 62, the angular position of the toothed traction wheel 61 is detected by means of the angular position sensor 63 and the motor 66 is controlled so as to bring the wheel 61 into a given position suitable for subsequent engagement with the traction rack 62 of an incoming trolley. In particular, the wheel may be arranged with a recess between two teeth directed towards the rack so as to receive the first roller or tooth thereof. The angular position sensor may be any known sensor suitable for detecting the correct position of the wheel. For example, it may be an absolute encoder, a proximity sensor which controls a tooth, etc. The sensor may also be incorporated in the motor or consist of the motor itself. For example, an absolute angular control motor may be used.

Obviously it is not necessary to know exactly which recess (or tooth) is in the correct position, but it is sufficient to detect that the wheel is in the desired position suitable for subsequent engagement.

Preferably, once the traction wheel is positioned as a result of the sensor 63, the control circuit 65 stops the motor 66 and the wheel 61 is released (not braked).

The control system then waits for detection of arrival of a trolley and starts the motor 66 with a predetermined acceleration ramp when the front end of the traction rack 62 moves closer towards the toothed traction wheel 61 until the toothed traction wheel and traction rack engage with each other.

The position sensor 64 may advantageously form part of a linear positioning system such as a linear encoder. The position sensor 64 may also be a proximity sensor.

When the trolley arrives, if there should be a small impact between rack and tooth this impact may in any case be absorbed by the mechanical friction of the motor 61 (which will preferably be a gear motor) which at the moment of impact will be advantageously braked and have a low or zero operating torque.

At a given relative position of the rack and the petal wheel (calculated by the system using the sensor 64) which establishes that engagement has now started, the motor 66 may be advantageously activated by means of a gradual torque.

The rising torque curve is advantageously graduated so as to make engagement as soft as possible, preventing excessive acceleration or deceleration which could result in an anomalous force acting on the trolley movement system which moves the trolley towards the engagement point.

After engagement, the motor 66 is controlled by the system 65 so as to continue with rotation of the toothed traction wheel 61 so as to move the trolley along the conveying line by means of the traction rack engaged in the toothed traction wheel 61. The speed of movement of the trolley will therefore be essentially governed by the control system by means of the motor 66. Essentially, the start of the command for rotation of the motor 66 is determined by means of the position sensor 64 which allows the circuit 65 to calculate when the front end of the traction rack 62 comes up against the first tooth of the traction wheel, positioned correctly by means of the sensor 63.

Advantageously, the sensor 64 also allows the detection of the position of the trolley which is at the end of the engaging operation or which is in any case close to the end of engagement of the additional traction system. In this way, at the end of engagement, the motor 66 may be used again to position correctly the wheel 61 by means of the sensor 63 and wait for the arrival of the next trolley.

Obviously, even though the sensor 64 has been described as a single sensor which may detect the position of the trolley which is close to the start of engagement and the position of the trolley which is close to the end of engagement, this sensor 64 may be composed of two sensors which each detect one of the two positions so as to communicate them to the circuit 65, as may now be easily imagined by the person skilled in the art.

Although the system 60 has been defined as an additional traction system, this system 60 must be understood as being a system which provides a greater traction for the movement of the trolley and not necessarily as a system which is in addition to the normal system for moving the trolleys along the conveying line 13. In particular, it may be envisaged that the system which moves the trolley towards the point for engagement between the toothed traction wheel 61 and the traction rack 63 is deactivated once the traction wheel 61 has engaged with the traction rack 52 so that the task of moving the trolley is passed over entirely to the system 60. In the case where the use of the additional traction system is envisaged in order to favour the advancing movement during the operations for rotating the body on the trolley, the relative position of the additional traction system composed of the traction wheel 61 and rack 62 with respect to the rotation mechanism operated by the shaft 19 may be such that the rotation system acts only after engagement of the additional traction system has been completed.

In particular with the rotation mechanism shown in the figures, the wheel 61 is positioned with respect to the rack 33 for performing rotation of the shaft 19 so that engagement of the rotation wheel 23 on the respective actuating rack 33 occurs only after engagement of the additional traction system has been completed. Similarly, advantageously engagement of the additional traction system terminates (namely the wheel 61 is disengaged from the respective rack 62) only after rotation, namely engagement between the wheel 23 and the respective rack 33, has terminated.

Advantageously, it may also be envisaged that, in the zone where engagement of the additional traction system must be performed, movement systems with a motorized friction wheel are provided for the trolleys, these forming or also forming part of a first system for moving the trolleys.

This is for example illustrated by way of example in the form of a single motorized friction traction device 70 in Figure 1. These systems may, however, may be more than one in number, being distributed along the conveying line so that they are able to pass in sequence from one trolley to another trolley along the conveying line (as for example shown schematically in Figure 8). Along the sections where the trolley is moved by these devices 70 and/or by the additional traction system 60, the conveying line 13 may be advantageously of the passive type, namely without a drive system or with the drive system deactivated. For example, along these sections the conveying line 13 may be composed of idle rollers and/or by sliding surfaces.

The traction device 70 which pushes the additional traction system towards the engagement point may be easily switched off once engagement of the traction system has been completed (i.e. with at least one tooth engaged) so that the traction of the trolley is transferred from the traction device 70 to the additional traction system. The opposite may happen when the action of the additional traction system has terminated, namely when the rack 62 is no longer engaged with the toothed traction wheel 61.

In order to activate and deactivate the traction devices 70 at the right moment, operation thereof is also advantageously controlled by the control circuit 65 which, via the sensor 64, detects the start and the end (or the nearing of the end) of engagement of the additional traction system. Alternatively, the devices 70 may continue to operate so that their action is added to that of the wheel 61 , even though this may complicate the system for controlling the speeds of the two systems such that they are uniform with each other.

As shown for example in Figures 1 and 3, the traction device 70 may comprise a gear motor 71 which operates a wheel 72 which rests on a suitable surface of the passing trolley. Advantageously, the wheel 72 may be pushed against this surface with a suitable pressure by means of a resilient thrusting system 73, for example of the spring type. As can be clearly seen for example in Figure 3, the surface of the trolley on which the wheel 72 rests for movement of the trolley may be a suitable surface 74 parallel and close to the traction rack 62. In order to ensure that the trolley remains on the conveying line without being lifted laterally despite the forces acting on it for conveying and rotation along the plant 10, idle wheels 75 may be provided at intervals in suitable positions, these resting vertically on top of suitable sliding surfaces of the trolley. As shown in Figure 4, these surfaces may be for example the same surface 74 for supporting the motorized wheel 72.

Preferably, the idle wheels 75 may be pushed against the corresponding surfaces of the trolley by means of a resilient thrusting system 76, for example of the spring type, for providing an adequate vertical pressure.

As is clear from the figures, the devices for rotating the bodywork, the tractions wheel 61 with the associated motor, any friction traction devices 70 and any idle wheels 75 may be advantageously all arranged on the same side of the trolleys and preferably outside of the vertical plane of the basins so as to simplify the structure and the maintenance of the plant. In particular, the additional traction devices formed by the wheel 61 and the associated motor 66, the friction traction devices 70 and any idle wheels 75 may be suspended from a fixed frame 77 arranged on one side of the conveying line. This frame 77 may advantageously comprise uprights which are generally in the form of an overturned L and horizontal beams and may also define a space for allowing the rotation wheel 23 and the associated actuating devices 32 to pass through, as is clearly visible in Figures 2, 3 and 4.

The devices 70 may be present only at the inlet, only at the outlet, at both the inlet and outlet, or may be not present at all, depending on the requirements and structure of the conveying system, as may now be easily understood by the person skilled in the art on the basis of the description provided hitherto.

Figures 5, 6 and 7 show by way of example various operating steps in a plant according to the invention. In particular Figure 5 shows a trolley during its approach movement towards the rotation system after the additional traction system has been engaged. This Figure 5 also shows in broken fines the possible existence of a device 70 for pushing the incoming trolley towards the engagement point of the additional movement system and in solid lines a device 70 for pulling the trolley out from the engagement point of the additional movement system. Figure 6 shows the action of the rotation system which rotates the bodywork so as to overturn it and immerse in an underlying basin 15.

As is now clear to the person skilled in the art, the length of the rotation rack may be determined so as to produce any desired angle of rotation, also by combining it with the transmission ratio chosen for the transmission of the movement between the rotation wheel 23 and the shaft 19. For example, the angle of rotation of the shaft 19 formed by sliding of the rotation wheel along the entire rotation rack may be suitably chosen as fractions within the range of 0° to 360°, depending on the requirements of the treatment process and/or the number of teeth (rollers) of the racks. For example, the rotation may be in the region of 180° or in the region of 360°. The angle of rotation may also be chosen so as to keep the bodywork at a small angle when entering and/or exiting the rotation system, depending also on the requirements of the treatment process.

Figure 7 also shows in schematic form a possible design of the plant which allows the bodywork to be overturned and to remain immersed in the overturned position until the next rotation system is reached and the bodywork on the trolley is brought back upright. In this case, in the overturned condition of the bodywork, the action of the additional traction system may no longer be necessary. As can be seen in Figure 7, both engagement of the traction system and engagement of the rotation system may therefore terminate after the overturning operation and the trolley may advance by means of the action of the transportation system formed by the conveying line and/or any devices 70.

In this way, the bodywork may remain immersed inside the basin (designed with a suitable lengthwise dimension) along a predetermined section of the conveying line and for a predetermined duration. A following rotation system with an associated additional traction system such as those described above and shown in Figure 1 may then bring the bodywork out of the basin.

All of this is shown schematically in Figure 8 which shows, in sequence, two rotation systems 10a and 10b of a plant 10 according to the invention, the first one of which turns the bodywork over inside the basin and the second one of which (which may be at any distance from the first one, depending on the bodywork immersion requirements) brings the bodywork back into the normal transportation position.

As can be seen in Figure 8, at the start of a long basin a rotation rack having a length such as to produce a half rotation of the shaft may be provided, so as to overturn completely the bodywork and immerse it completely inside the basin. The trolley may then proceed along the basin with the bodywork immersed. At the end of the basin a second rotation rack may be provided, said rack having a length such as to produce another half rotation of the shaft so as to completely raise the bodywork into the normal advancing position (which is not necessarily horizontal but also depends on the transportation requirements and/or the treatment process and in which the bodywork may also be inclined).

Alternatively, for example in the case of short basins, a single rotation rack may be obviously designed with a length such as to cause the bodywork to perform a full rotation during the advancing movement of the trolley.

If necessary, the trolley may also be stopped when the bodywork is fully immersed, such as to cause it to remain in the liquid for the entire duration needed for the particular treatment.

It is also possible to consider making the bodywork perform several immersion and emersion rotations, by means of a suitable longer rack or several racks in sequence, or perform only partial overturning operations, for example in order to keep the immersed or partially immersed bodywork inclined, so as to allow evacuation of the air bubbles.

Obviously, when necessary the engagement of the additional traction system 60 will also be provided so as to ensure the necessary additional thrusting action for moving the trolleys.

At this point it is clear to the person skilled in the art how it is possible to combine the various parts of the plant according to the invention in order to obtain desired progressions of the bodywork along the line, with great ease and flexibility.

Obviously, the above description of an embodiment applying the innovative principles of the present invention is provided by way of example of these innovative principles and must therefore not be regarded as limiting the scope of the rights claimed herein.

For example, especially along the sections which do not coincide with treatment basins, the transportation system may be of any known type, for example of the type with traction from below using interconnected motorized rollers, etc. It is also possible to easily imagine how the various embodiments and the various devices described may be combined with each other in any number in order to perform treatments which are complex and/or with immersion of a bodywork in successive treatment basins.

As mentioned above, the conveying line may also be designed with systems different from the motorized rollers on which the skids rest and travel. For example, other systems, such as linear motors or friction rollers, etc., may be used, as may be now easily imagined by the person skilled in the art on the basis of the description provided above.

Known systems for causing oscillation of the immersed bodywork in order to facilitate for example the evacuation of air bubbles may also be provided. These known systems may act for example on the shaft 19 during the forwards movement of the trolley with the bodywork immersed and the shaft 19 released. For example the overturning systems and the structure of the trolleys may be different from those shown. For example, as already mentioned above, the trolley may have an Fl-shaped structure in plan view with two side members provided with runners or wheels. The sections of the plant which require the additional traction according to the present invention may also be different from those shown here by way of example, the additional traction system being able to be used along other sections where the force for movement of the trolley requires the additional traction provided by the system 60.