Patent Publication Number: US-2023158763-A1

Title: Method for building tyres and transfer device of an apparatus for building tyres for vehicle wheels

Description:
TECHNICAL FIELD OF THE INVENTION 
     The object of the present invention is a method for building tyres and a transfer device of an apparatus for building tyres for vehicle wheels. 
     The present invention is situated in the scope of processes and apparatuses for building tyres for vehicle wheels. 
     In particular, the present invention is situated in the scope of methods and devices adapted, during the building of a tyre, to control and verify the correct arrangement and the correct assembly on a drum of components intended to form the green tyre. 
     A tyre for vehicle wheels generally comprises a carcass structure comprising at least one carcass ply having end flaps engaged with respective anchoring annular structures. In radially external position with respect to the carcass structure, a belt structure is associated thereto, comprising one or more belt layers, situated in radial superimposition on each other and with respect to the carcass ply, having textile or metallic reinforcement cords with cross orientation and/or substantially parallel to the circumferential extension direction of the tyre. In radially external position with respect to the belt structure, a tread band is applied, it too made of elastomeric material like other constituent semi-finished products of the tyre. The assembly of at least said belt structure and of said tread band form the crown structure of the tyre. Respective sidewalls made of elastomeric material are also applied on the lateral surfaces of the carcass structure, each extended from one of the lateral edges of the tread band up to the respective anchoring annular structure to the beads. In the tyres of “tubeless” type, the carcass ply is internally covered by a layer of elastomeric material, preferably with butyl base, normally termed “liner” having optimal characteristics of air impermeability and extended from one of the beads to the other. 
     The production cycles of a tyre provide for a building process in which the various structural components of the tyre itself are made and/or assembled on one or more drums. 
     The built green tyres are transferred into a moulding and vulcanisation line where a process of moulding and vulcanisation is actuated that is adapted to define the structure of the tyre according to a desired geometry and tread design. 
     Definitions 
     With the term “elastomeric material” it is intended to indicate a composition comprising at least one elastomeric polymer and at least one reinforcement filler. Preferably, such composition also comprises additives such as, for example, a cross-linking agent and/or a plasticising agent. Due to the presence of the cross-linking agent, through heating, such material can be cross-linked, so as to form the final manufactured product. 
     By “component” or “structural component” of a tyre it is intended any one portion thereof capable of performing its own function or a part thereof. The following are for example components of the tyre: the liner, the under-liner, the sidewall inserts, the bead cores, the filler inserts, the anti-abrasive element, the sidewalls, the carcass ply/plies, the belt layer(s), the tread band, the underlayer of the tread band, the under-belt inserts etc., or a part thereof. 
     By “tyre being processed” it is intended at least one component or structural component of the tyre deposited on a drum. 
     The terms “radial” and “axial” and the expressions “radially internal/external” and “axially internal/external” are used by making reference respectively to a direction perpendicular and to a direction parallel to a rotation axis of the tyre/tyre being processed, and/or of a drum. 
     A plane is defined “radial” when it comprises the rotation axis of the tyre/tyre being processed and/or of a drum. 
     The term “symmetry plane of the tyre/tyre being processed” indicates the symmetry plane orthogonal to the rotation axis of the tyre/tyre being processed. 
     The terms “circumferential” and “circumferentially” are instead used by making reference to the direction of the annular extension of the tyre/tyre being processed. 
     By “centre of a tyre/tyre being processed” it is intended the intersection point between the rotation axis and the symmetry plane of the tyre/tyre being processed. 
     By “middle line plane of a drum” it is intended the plane orthogonal to the rotation axis of the drum and which divides the drum into two halves. 
     By “centre of a drum” it is intended the intersection point between the rotation axis and the middle line plane of the drum. 
     By “longitudinal axis of a transfer device” it is intended the straight axis of the cylinder inscribed between the gripping surfaces of the transfer device at least in gripping conditions. 
     With regard to the transfer device, a plane is defined “radial” when it comprises the aforesaid longitudinal axis. 
     By “middle line plane of a transfer device” it is intended the plane orthogonal to the longitudinal axis of the transfer device and which divides the transfer device in half. 
     By “centre of the transfer device” it is intended the intersection point between the longitudinal axis and the middle line plane of the transfer device. 
     By “longitudinal centring” of a tyre being processed with respect to a transfer device it is intended the correspondence between the centre of the tyre being processed and the centre of the transfer device. 
     By “longitudinal centring” of a drum with respect to a transfer device it is intended the correspondence between the centre of the drum and the centre of the transfer device. 
     By “coaxiality” between a tyre being processed and a transfer device it is intended that the longitudinal axis of the transfer device coincides with the rotation axis of the tyre being processed, i.e. said axes are not tilted with respect to each other and/or laterally offset with respect to each other. 
     By “coaxiality” between a drum and a transfer device it is intended that the longitudinal axis of the transfer device coincides with the rotation axis of the drum, i.e. said axes are not tilted with respect to each other and/or laterally offset with respect to each other. 
     Generally it follows that a transfer device and a tyre being processed (or the transfer device and a drum) can be coaxial and longitudinally centred, coaxial but not longitudinally centred, longitudinally centred but not coaxial. 
     State of the Art 
     The document WO2009128046, in the name of the same Applicant, illustrates an assembly station in which a carcass sleeve and an external sleeve made in respective building lines are mutually coupled. The assembly station integrates engagement devices alternatively couplable with an auxiliary drum carrying an external sleeve and with a building drum carrying a carcass sleeve. A gripping unit picks up the external sleeve from the auxiliary drum coupled to the engagement devices, in order to position it around the carcass sleeve carried by the building drum. Shaping devices operatively couplable with the building drum cause a radial expansion of the carcass sleeve so as to couple it to the external sleeve retained by the gripping unit. 
     The document JP2012236392A illustrates a transfer device used for picking up a tread ring from a forming drum and transporting it into an application position, in which such tread ring is applied to the exterior of a base element of a green tyre carried by a shaping drum. The transfer device allows evaluating non-alignments between said transfer device and the shaping drum through a first laser distance sensor and a second laser distance sensor mounted on a support ring of the transfer device. The first and the second sensor measure the distance, respectively, along X and along Y, from a support shaft of the shaping drum. 
     SUMMARY 
     The Applicant has perceived the need to improve the quality of the produced tyres and to ensure greater conformity thereof with the design specifications, in particular of the tyres whose components are obtained through application of semi-finished products on one or more drums carried and moved by movement devices with multiple degrees of freedom configured for moving the drums in the three-dimensional space, such as for example multi-axis robots, preferably but not necessarily anthropomorphic robots. 
     The Applicant has in fact observed that such known movement devices with multiple degrees of freedom are sometimes not able to ensure the precision and repeatability of positioning of the tyre being processed, necessary for optimising the quality of the built tyres and hence their performances. 
     The Applicant has in particular observed that such movement devices with multiple degrees of freedom do not allow ensuring the abovementioned precision and repeatability of positioning between elements of the tyre during the building. 
     The Applicant has observed that such errors of positioning generate defects and non-uniformities on the built tyres. The errors of positioning can also generate performance variability between one tyre and the next. 
     The defects, non-uniformities and the variability are often not visible on the finished tyre but can only be seen through dynamic tests. 
     Such defects, non-uniformities and variability finally affect the performances that the produced tyres are able to offer. 
     The Applicant has perceived that the abovementioned defects and non-uniformities derive in particular from errors of centring between a transfer device and the tyre being processed, in which the transfer device is configured for engaging and retaining the tyre being processed at a radially external portion thereof, for the purpose of picking up the tyre being processed from a drum on which said tyre being processed is arranged or for the purpose of associating a tyre being processed with a drum, it too carrying a tyre being processed. 
     The Applicant has observed that various factors can intervene that compromise the mutual positioning of the transfer device and of the tyre being processed which are, especially but not exclusively, ascribable to the abovementioned movement devices. Among the causes leading to the abovementioned positioning variability, the following can be mentioned: resetting when there is a failure of the apparatus; incorrect alignment activities during implementation of the apparatus; intrinsic variability of the instruments; structural yielding and/or wear of the components of the apparatus. 
     The Applicant observes that the laser sensors illustrated in the document JP2012236392A are unable to detect, with the necessary precision, the coaxiality between the transfer device thereof and the shaping drum thereof and more precisely they are not at all able to detect the longitudinal centring between the transfer device thereof and the shaping drum thereof. 
     The Applicant further observes that the document JP2012236392A does not even show the need for an extremely precise control system capable of detecting coaxiality and longitudinal centring, since the drums thereof substantially can only rotate with respect to the fixed devices that support them and the transfer device illustrated in the aforesaid document can only translate along a fixed rail. 
     The Applicant has perceived that in order to solve the abovementioned problems, the transfer device can be used as a reference for checking the position of the tyres being processed and/or of the drums on which the tyres being processed are to be deposited or from which the tyres being processed are to be picked up. 
     The Applicant has finally found that the transfer device can be used for checking the longitudinal centring between the tyre being processed and the transfer device. 
     According to a first aspect, the present invention regards a method for building tyres for vehicle wheels. 
     Preferably, provision is made for arranging a tyre being processed carried by a drum in a radially internal position with respect to gripping elements of a transfer device; the gripping elements having gripping surfaces directed radially towards a longitudinal axis of the transfer device; the drum being supported by a movement device with at least two degrees of freedom. 
     Preferably, provision is made for detecting, through a measurement device, a longitudinal shift, along a direction parallel to said longitudinal axis of the transfer device, between a centre of the transfer device and a centre of the tyre being processed. 
     The Applicant deems that the present invention also allows remedying the intrinsic imprecisions connected to the use of movement devices with multiple degrees of freedom, in order to obtain tyres which precisely reflect the design specifications. 
     The Applicant also deems that the present invention allows controlling possible variations and drifts that can occur during production of a batch of tyres, so as to limit them and/or oppose them. 
     In accordance with a second aspect, the present invention regards a transfer device of an apparatus for building tyres for vehicle wheels. Preferably the following are provided: an annular support structure; gripping elements arranged as a ring on the annular support structure and having gripping surfaces directed radially towards a longitudinal axis of the transfer device. 
     Preferably, the gripping elements are configured for being arranged around a drum carrying a tyre being processed. 
     Preferably, the drum is carried by a movement device with at least two degrees of freedom. 
     Preferably, a measurement device is provided, mounted on the annular support structure. 
     Preferably, the measurement device is configured for detecting a longitudinal shift, along a direction parallel to the longitudinal axis of the transfer device, between a centre of the transfer device and a centre of the drum. 
     According to a further aspect, the present invention also regards an apparatus for building tyres for vehicle wheels. 
     Preferably, a forming drum is provided, configured for bringing a carcass structure. 
     Preferably, an auxiliary drum is provided, configured for bringing a crown structure. 
     Preferably, a movement device is provided with at least two degrees of freedom configured for supporting the forming drum. 
     Preferably, an auxiliary movement device is provided with at least two degrees of freedom configured for supporting the auxiliary drum. Preferably, a transfer device is provided in accordance with the aforesaid second aspect. 
     Preferably, the movement device is movable between a first position in which the forming drum is in a radially internal position with respect to the gripping elements of the transfer device and a second position in which the forming drum is outside the transfer device. 
     Preferably, the auxiliary movement device is movable between a first position in which the auxiliary drum is in a radially internal position with respect to the gripping elements of the transfer device and a second position in which the auxiliary drum is outside the transfer device. 
     The Applicant further deems that the present invention allows avoiding having to very frequently re-align the apparatus, with consequent waste of time to the detriment of productivity and in any case without having the certainty that every single produced tyre was obtained with the best possible alignment. 
     The present invention, in at least one of the aforesaid aspects, can have one or more of the preferred characteristics which are described hereinbelow. 
     Preferably, detecting the longitudinal shift comprises: measuring a longitudinal distance, parallel to the longitudinal axis, between a lateral portion of the transfer device and a longitudinal end of the drum and calculating the longitudinal shift starting from said longitudinal distance. This measurement type is relatively simple, since the longitudinal end of the drum projects laterally from the transfer device and can be operatively reached by the measurement device. 
     Preferably, measuring the longitudinal distance comprises: sighting the longitudinal end of the drum through a sensor of the measurement device mounted on the lateral portion of the transfer device. 
     The abovementioned sensor is easily installable on the transfer device, making use of the space on the sides thereof. 
     Preferably, sighting comprises: generating a laminar beam of electromagnetic waves lying in a radial plane of the transfer device and configured for at least partially hitting the longitudinal end of the drum. Preferably, the laminar beam is diametral. 
     Preferably, the laminar beam is a beam of laser light. 
     The laminar beam partly or totally hits the longitudinal end of the drum, hence partly or completely intercepting the surface of the drum. As a function of the portion of the laminar beam intercepted, the sensor through electronics associated therewith is capable of providing a value correlated to a longitudinal position of the drum, and of the tyre being processed arranged thereon, with respect to the transfer device. The measurement is made without bringing mechanical parts in contact and hence is safe as well as precise. 
     Preferably, provision is also made for detecting a relative position between the longitudinal axis of the transfer device and a rotation axis of the tyre being processed. 
     In addition to the longitudinal centring, it is possible to further reduce defects and non-uniformities by checking the coaxiality between the transfer device and the tyre being processed. 
     Preferably, detecting the relative position comprises: detecting a relative position between the longitudinal axis of the transfer device and a rotation axis of the drum. 
     Since the tyre being processed is picked up from the drum or is applied on the drum (with respect to which it is coaxial), it is simpler to check the coaxiality between the transfer device and the drum. 
     Preferably, the relative position between the longitudinal axis of the transfer device and the rotation axis of the drum is detected by measuring radial distances between the transfer device and a radially external surface of the drum. 
     Preferably, said relative position is detected by calculating, starting from said radial distances, a position of the rotation axis of the drum with respect to a reference system integral with the transfer device. Since the dimensions of the drum and those of the transfer device are known, the abovementioned radial distances allow finding the position of the rotation axis of the drum. 
     Preferably, the radial distances are detected at opposite longitudinal ends of the drum. 
     Said opposite longitudinal ends of the drum project laterally from the transfer device and can be used as a target for measuring the radial distances. In this manner, it is possible to detect the positions of the two opposite longitudinal ends of the drum and, since the geometry of the drum is known, obtain the position of the rotation axis thereof with respect to the transfer device. 
     Preferably, the radially external surface of the drum is a deposition surface. 
     In other words, said radially external surface is that surface on which the tyre being processed lies; the radial distances are detected at the radially external surfaces of the opposite longitudinal ends of the drum which project laterally (along a longitudinal direction). 
     Preferably, the radially external surface of the drum is a surface of a shaft projecting longitudinally with respect to the deposition surface. 
     In other words, said radially external surface belongs to a shaft which is part of the drum and which projects laterally (along a longitudinal direction) from the deposition surface. 
     Preferably, the radial distances are measured on a first plane and on a second plane, in which said first plane and second plane are placed on opposite sides with respect to a middle line plane of the transfer device. Preferably, detecting the relative position comprises: calculating, for each of the longitudinal ends, a respective first centre lying on the first plane and a respective second centre lying on the second plane; in which the rotation axis of the drum passes through said first and second centres. 
     Preferably, said first plane and second plane are symmetric with respect to the middle line plane of the transfer device. 
     Since the coordinates are known, in a reference system that is fixed with respect to the transfer device, of the abovementioned first and second centres belonging to the rotation axis of the drum, the position of the rotation axis is also known with respect to the longitudinal axis of the transfer device. The abovementioned two axes can be: coinciding, parallel and spaced from each other, tilted with respect to each other and intersecting, tilted and spaced (oblique). 
     Preferably, the radial distances are detected through a first group of distance sensors, mounted on a first of two longitudinally opposite lateral portions of the transfer device. 
     Preferably, the radial distances are detected through a second group of distance sensors, mounted on a second of the two longitudinally opposite lateral portions of the transfer device. 
     The abovementioned distance sensors are easily installable on the transfer device by making use of the spaces on the sides thereof. These are also easily reachable, for example in order to be able to perform operations of maintenance, substitution, calibration, etc. 
     Preferably, detecting the radial distances comprises emitting electromagnetic radiation beams and capturing corresponding reflected beams. 
     Preferably, the electromagnetic radiation beams are laser light beams. The measurement is made without bringing mechanical parts in contact and is therefore safe as well as precise. 
     Preferably, the emitted and reflected beams are at least two for each of the two lateral portions. 
     Preferably, the emitted and reflected beams are at least three for each of the two lateral portions. 
     Preferably, the emitted and reflected beams are at least four for each of the two lateral portions. 
     Preferably, the movement device has at least six degrees of freedom. Preferably, the movement device with at least six degrees of freedom is a multi-axis robot. 
     Preferably, the movement device with at least six degrees of freedom is an anthropomorphic robot with at least six axes. 
     Preferably, the movement device is configured for being coupled with a central portion of the drum placed at an axial end of the drum, so as to projectingly support said drum. 
     The multi-axis robots can be programmed for managing a wide range of tyres sizes and hence of drums and of structures of the tyres to be built. 
     Preferably, the tyre being processed is a carcass structure and the drum is a forming drum. 
     The checking of the centring is carried out between the carcass drum which carries the carcass structure and the transfer device that carries the crown structure before associating the carcass structure and the crown structure together, so as to obtain a coaxial centred coupling. Preferably, the tyre being processed is a crown structure and the drum is an auxiliary drum. 
     The checking of the centring is carried out between the auxiliary drum that carries the crown structure and the transfer device before the latter picks up the crown structure from the auxiliary drum, so as to obtain perfectly coaxial centred coupling between the crown structure and the transfer device and then the subsequent coaxial centred coupling between the carcass structure and the crown structure. 
     Preferably, provision is made for generating a first warning signal if the longitudinal shift exceeds a first threshold of longitudinal shift and, preferably, provision is made for generating a first alarm signal if the longitudinal shift exceeds a second threshold of longitudinal shift, greater than the first. 
     Preferably, the first threshold of longitudinal shift is +/−2 mm. Preferably, the second threshold of longitudinal shift is +/−3 mm. Preferably, checking the coaxiality comprises evaluating a non-coaxiality. 
     Preferably, the non-coaxiality is calculated as a function of the position of the first centre with respect to an intersection point of the longitudinal axis of the transfer device with the first plane. 
     Preferably, the non-coaxiality is calculated as a function of the position of the second centre with respect to an intersection point of the longitudinal axis of the transfer device with the second plane. 
     Preferably, the non-coaxiality is a function of a first radius of a circle with centre in the intersection point of the longitudinal axis of the transfer device with the first plane and passing through the first centre. Preferably, the non-coaxiality is a function of a second radius of a circle with centre in the intersection point of the longitudinal axis of the transfer device with the second plane and passing through the second centre. 
     Preferably, provision is made for generating a second warning signal if the first radius and/or the second radius exceeds/exceed a first non-coaxiality threshold and, preferably, for generating a second alarm signal if the first radius and/or the second radius exceeds/exceed a second threshold of non-coaxiality, greater than the first. 
     Preferably, the first threshold of non-coaxiality is +/−1 mm. 
     Preferably, the second threshold of non-coaxiality is +/−2 mm. 
     Preferably, provision is made for: feedback controlling the movement device as a function of the longitudinal shift until the transfer device is longitudinally centred with respect to the tyre being processed, carried by drum. 
     The invention therefore allows correcting possible errors of mutual positioning (longitudinal centring and/or coaxiality) through a dynamic feedback control on the movement device, in particular even if the latter is a multi-axis anthropomorphic robot. 
     Preferably, provision is made for resetting the reference coordinates of the movement device as a function of errors of mutual positioning (longitudinal centring and/or coaxiality). 
     Preferably, feedback controlling comprises bringing the longitudinal shift below a reference longitudinal shift. 
     Preferably, the reference longitudinal shift is 1 mm. 
     Preferably, feedback controlling comprises: cancelling the longitudinal shift. 
     In this manner, one obtains the longitudinal centring, i.e. the centre of the transfer device substantially coincides with the centre of the drum and of the tyre being processed. 
     Preferably, feedback controlling comprises: making the longitudinal axis of the transfer device coincide with the rotation axis of the tyre being processed. 
     Preferably, feedback controlling comprises: bringing the first radius and/or the second radius below a reference radius. 
     Preferably, the reference radius is 0.5 mm. 
     Preferably, feedback controlling comprises: cancelling the first radius and/or the second radius. 
     In this manner, the coaxiality is obtained, i.e. the rotation axis of the drum and of the tyre being processed substantially coincides with the longitudinal axis of the transfer device. 
     Preferably, provision is made for recording errors of centring (in terms of coaxiality and/or longitudinal centring), relative to subsequent assemblies of tyres, between the transfer device and the drum. Preferably, provision is made for recording the longitudinal shift, the first radius and the second radius relative to subsequent building of tyres. 
     This historical data will serve for the evaluation of the progressive course of the errors and for their correlation with data relative to the uniformity of the produced tyres. 
     Preferably, the measurement device comprises a sensor mounted on the annular support structure; wherein the sensor is configured for measuring a longitudinal distance, parallel to the longitudinal axis, between the lateral portion of the transfer device and a longitudinal end of the drum. 
     Preferably, the sensor is mounted on a lateral portion of the annular support structure. 
     Preferably, the sensor is sensitive to electromagnetic waves, more preferably is sensitive to laser light. 
     Preferably, the sensor comprises an emitter and a receiver situated in diametrically opposite positions of the annular support structure, wherein the sensor is configured for generating a beam of electromagnetic waves, preferably a beam of laser light, extended between the emitter and the receiver. 
     In other words, the sensor is mounted on the transfer device and is preferably fixed with respect to the annular support structure. 
     The sensor is of a type known with the term “laser edge sensor”, e.g. like the laser micrometer IG-028 KEYENCE™. 
     Preferably, the measurement device comprises at least one first group of distance sensors mounted on the annular support structure and configured for measuring radial distances from a radially external surface of the drum. 
     Preferably, the measurement device comprises a first group of distance sensors, mounted on a first of two longitudinally opposite lateral portions of the annular support structure, and a second group of distance sensors, mounted on a second of the two longitudinally opposite lateral portions of the annular support structure. 
     The groups of distance sensors are mounted on the transfer device and are preferably fixed with respect to the annular support structure. Preferably, each group of distance sensors comprises at least two distance sensors, preferably three distance sensors, preferably four distance sensors, angularly spaced from each other. 
     Preferably, the distance sensors of each group are angularly equidistant from each other. For example, there are three distance sensors arranged at 120° from each other, or four at 90°. 
     Preferably, the first and the second group of distance sensors are situated on planes symmetric with respect to a middle line plane of the transfer device. 
     Preferably, the distance sensors are sensitive to electromagnetic waves, preferably to laser light. The sensors are for example of the series LK-G400 of KEYENCE™. 
     Preferably, each of the distance sensors comprises an emitter and a receiver side-by-side each other. 
     Preferably, the auxiliary drum comprises a plurality of sectors consecutively arranged around the rotation axis thereof and defining the deposition surface, in which the distance sensors detect the distance from said sectors. 
     Preferably, the forming drum comprises a shaft configured for being connected by the movement device, in which the distance sensors detect the distance from said shaft. 
     Preferably, the apparatus comprises a control unit operatively connected to the measurement device and to the auxiliary movement device, in which the control unit is programmed for: 
     receiving, from the measurement device, an auxiliary signal relative to a longitudinal shift of the auxiliary drum,
 
calculating, from said auxiliary signal, a first error of longitudinal centring of the auxiliary drum with respect to the transfer device,
 
feedback controlling the position of the auxiliary movement device, so as to longitudinally centre the auxiliary drum with respect to the transfer device.
 
     Preferably, the control unit is operatively connected to the movement device and is programmed for: 
     receiving, from the measurement device, a signal relative to a longitudinal shift of the forming drum,
 
calculating, from said signal, a second error of longitudinal centring of the forming drum with respect to the transfer device,
 
feedback controlling the position of the movement device, so as to longitudinally centre the forming drum with respect to the transfer device.
 
     Preferably, the control unit is programmed for: 
     receiving, from the measurement device, distance signals relative to radial distances from the auxiliary drum,
 
calculating from said distance signals the position of the rotation axis of the auxiliary drum,
 
feedback controlling the position of the auxiliary movement device, so as to render the auxiliary drum and the transfer device coaxial.
 
     Preferably, the control unit is programmed for: 
     receiving, from the measurement device, distance signals relative to radial distances from the forming drum,
 
calculating from said distance signals the position of the rotation axis of the forming drum,
 
feedback controlling the position of the movement device, so as to render the forming drum and the transfer device coaxial.
 
     Further characteristics and advantages will be clearer from the detailed description of preferred but not exclusive embodiments of a method for building tyres and of a transfer device according to the present invention. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Such description will be set forth hereinbelow with reference to the enclosed drawings, provided only as a non-limiting example, in which: 
         FIG.  1    schematically shows an assembly station of a plant for manufacturing tyres for vehicle wheels; 
         FIG.  2    is a side view of a transfer device belonging to the assembly station of  FIG.  1   ; 
         FIG.  3    is a frontal and partially sectional view of the transfer device of  FIG.  2    associated with an auxiliary drum in a first position; 
         FIG.  4    is a frontal and partially sectional view of the transfer device of  FIG.  2    associated with an auxiliary drum in a second position; 
         FIG.  5    is a side view of the transfer device associated with the auxiliary drum in the second position of  FIG.  4   ; 
         FIG.  6    is a radial half-section of a tyre assembled in the assembly station of  FIG.  1   . 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG.  1   , reference number  1  overall indicates an assembly station of an apparatus for building green tyres in turn part of a plant, not illustrated in its entirety, for making tyres for vehicle wheels. 
     A tyre  2 , made in said plant and assembled in the assembly station  1 , is illustrated in  FIG.  6    and essentially comprises a carcass structure  14  having two carcass plies  4   a ,  4   b . An impermeable layer of elastomeric material or so-called liner  5  is applied inside the carcass ply/plies  4   a ,  4   b . Two anchoring annular structures  6 , each comprising a so-called bead core  6   a  carrying an elastomeric filler  6   b  in radially external position, are engaged with respective end flaps of the carcass ply/plies  4   a ,  4   b . The anchoring annular structures  6  are integrated in proximity to zones normally identified with the name “beads”  7 , at which the engagement between the tyre  2  and a respective mounting rim usually occurs. A belt structure  8  comprising belt layers  8   a ,  8   b  is circumferentially applied around the carcass plies  4   a ,  4   b , and a tread band  9  is circumferentially superimposed on the belt structure  8 . The belt structure  8  can be associated with so-called “under-belt inserts”  10  each situated between the carcass plies  4   a ,  4   b  and one of the axially opposite terminal edges of the belt structure  8 . Two sidewalls  11 , each extended from the corresponding bead  7  to a corresponding lateral edge of the tread band  9 , are applied in laterally opposite positions on the carcass plies  4   a ,  4   b . The portion of each sidewall  11  proximal to the lateral edge of the tread band  9  is known as the shoulder of the tyre. The tyre  2  has a middle line plane “M” ( FIG.  6   ) equidistant from the respective beads  7  and perpendicular to the main rotation axis “X-X” thereof, when the tyre is operating. 
     The plant, not illustrated in detail, comprises a tyre production line  2  formed by the apparatus for building green tyres and by at least one moulding and vulcanisation unit operatively arranged downstream of the building apparatus. 
     In one embodiment, the apparatus for building green tyres comprises a carcass building line, at which forming drums  12  are moved between different semi-finished product dispensing stations arranged to form, on each forming drum  12 , a carcass structure  14  comprising the carcass plies  4   a ,  4   b , the liner  5 , the anchoring annular structures  6  and possibly at least one part of the sidewalls  11 . 
     Simultaneously, in a crown building line, one or more auxiliary drums  13  are sequentially moved between different work stations arranged to form, on each auxiliary drum  13 , a crown structure  15 , comprising at least the belt structure  8 , the tread band  9 , and possibly at least one part of the sidewalls  11 . 
     In the abovementioned assembly station  1 , the crown structure  15  is coupled to the carcass structure  14 . 
     The assembly station  1  comprises a transfer device  16  comprising a base  17  fixed on the ground and an annular support structure  18  mounted on the base  17 . The annular support structure  18  carries, at a radially internal portion thereof, a plurality of gripping elements  19  arranged as a ring and having gripping surfaces  20  directed radially towards a longitudinal axis “Z-Z” of the annular support structure  18  and of the transfer device  16 . The longitudinal axis “Z-Z” is preferably horizontal. 
     In a per se known manner and therefore not described in detail, the gripping elements  19 , through non-illustrated actuators operatively arranged between each of the gripping elements  19  and the annular support structure  18 , are movable along radial directions and with respect to the annular support structure  18  between a first configuration and a second configuration. In the first configuration, the gripping elements  19  are radially more spaced from the longitudinal axis “Z-Z” and circumferentially more spaced from each other than in the second configuration, in which they lie closer to said longitudinal axis “Z-Z” and are closer to each other. 
     The gripping elements  19  are also provided with needles on the gripping surfaces  20 , and by means of the aforesaid needles the gripping elements  19  are capable of retaining a tyre being processed, such as for example the crown structure  15 , against the gripping surfaces  20 . 
     The assembly station  1  comprises ( FIG.  1   ) a movement device  21  defined by an anthropomorphic robot with at least six axes and an auxiliary movement device  22 , also defined by an anthropomorphic robot with at least six axes, for example arranged on opposite sides of the transfer device  16 . 
     The movement device  21  comprises a terminal end  23  provided with gripping devices configured for coupling or releasing a forming drum  12  carrying the carcass structure  14 . 
     The movement device  21  is therefore capable of supporting and moving the forming drum  12 . Due to the movement device  21 , each forming drum  12  carried by said movement device  21  has six degrees of freedom and can be spatially oriented as desired. The forming drum  12  with the carcass structure  14  formed in the carcass building line is picked up from the movement device  21  in order to be operatively coupled to the transfer device  16 , as will be illustrated hereinbelow. 
     The auxiliary movement device  22  comprises a terminal end  24  provided with gripping devices configured for coupling or releasing an auxiliary drum  13  carrying the crown structure  15 . 
     The auxiliary movement device  22  is therefore capable of supporting and moving the auxiliary drum  13 . Due to the auxiliary movement device  22 , each auxiliary drum  13  carried by said auxiliary movement device  22  has six degrees of freedom and can be spatially oriented as desired. The auxiliary drum  13  with the crown structure  15  formed in the crown building line is picked up from the auxiliary movement device  22  so that it too is operatively coupled with the transfer device  16 , as will be illustrated hereinbelow. 
     The forming drum  12  comprises a shaft  25  coaxial with a rotation axis  26  thereof and a pair of semi-drums  27  mounted coaxially on the shaft  25  and movable, due to non-illustrated devices, along a direction parallel to the rotation axis  26 , mutually close to or away from each other. The terminal end  23  of the movement device  21  is configured for being coupled with a terminal end of the shaft  25  of the forming drum  12  so as to projectingly support said forming drum  12 . 
     The auxiliary drum  13  comprises a plurality of sectors  28  consecutively arranged around a rotation axis  29  thereof and defining a radially external deposition surface for the crown structure  15 . Mechanisms, not illustrated, allow radially moving the sectors  28  in order to move them between a radially expanded configuration and a radially contracted configuration. 
     The terminal end  24  of the auxiliary movement device  22  is configured for being coupled with a central portion of the auxiliary drum  13  placed at an axial end of the auxiliary drum  13  so as to projectingly support said auxiliary drum  13 . 
     In accordance with the method according to the present invention, in the assembly station  1 , while the gripping elements  19  are in the respective first configuration (radially more spaced from the longitudinal axis “Z-Z”) and the auxiliary drum  13  is in the radially expanded configuration thereof, the auxiliary movement device  22  carries the auxiliary drum  13  with the crown structure  15  within the transfer device  16 , i.e. in a radially internal position with respect to the gripping surfaces  20  of the gripping elements  19 . In such position, the gripping elements  19  are arranged around the auxiliary drum  13  and the crown structure  15  and the gripping surfaces  20  are directed radially towards a radially external portion of the crown structure  15 . 
     The gripping elements  19  are partially spaced towards the second configuration until the gripping surfaces  20  are brought into contact with the radially external portion of the crown structure  15 . The presence of the aforesaid needles allows making the crown structure  15  integral with the gripping elements  19 . At this point, the sectors  28  of the auxiliary drum  13  are moved into the radially contracted configuration and they are decoupled from the crown structure  15 , freeing said crown structure  15  which remains supported only by the gripping elements  19  of the transfer device  16 . The auxiliary device  22  therefore provides for extracting the auxiliary drum  13  from the transfer device  16  while the crown structure  15  remains on the transfer device  16 . 
     At this point, the movement device  21 , which supports the carcass structure  14 , carries the forming drum  12  with the carcass structure  14  within the transfer device  16 , i.e. in a radially internal position with respect to the crown structure  15  supported by the gripping elements  19 . In such position, the crown structure  15  is arranged around the carcass structure  14 . 
     Through the mutual approaching of the two semi-drums  27  of the forming drum  12 , which contributes to determining a radial expansion of the carcass structure  14 , a radially internal surface of the crown structure  15  is coupled to a radially external surface of the carcass structure  14 . The crown structure  15  adheres to the carcass structure  14  and is released by the gripping elements  19 . 
     The movement device  21  can therefore bring the forming drum  12  with the tyre being processed provided with the carcass structure  14  and with the crown structure  15  towards further possible processing stations and/or towards the moulding and vulcanisation unit. 
     In order to ensure the correct positioning and assembly of the crown structure  15  with the carcass structure  14 , the positions of the auxiliary drum  13  with respect to the transfer device  16  during the pick-up of the crown structure  15  and the position of the forming drum  12  with respect to said transfer device  16  during the association of the carcass structure  14  with the crown structure  15  must be well-defined and precise. 
     In particular, each of the abovementioned forming drum  12  and auxiliary drum  13 , in the respective operating steps, must be longitudinally centred and coaxial with respect to the transfer device  16 . As already previously indicated, by “longitudinal centring” of a drum with respect to the transfer device  16 , it is intended the correspondence between the centre of the drum and the centre of the transfer device  16  and by “coaxiality” between a tyre being processed and the transfer device  16  it is intended that the longitudinal axis “Z-Z” of the transfer device  16  coincides with the rotation axis of the tyre being processed, i.e. said axes are not tilted with respect to each other and/or laterally offset with respect to the other. 
     In order to check and possibly adjust the centring of the auxiliary drum  13  and of the crown structure  15  with respect to the transfer device  16  when the auxiliary drum  13  and the crown structure  15  are placed within the transfer device  16 , the transfer device  16  comprises a measurement device  30  mounted on the annular support structure  18 . The same measurement device  30  also serves for checking and possibly adjusting the centring of the forming drum  12  and of the carcass structure  14  with respect to the transfer device  16  when the forming drum  12  and the carcass structure  14  are placed within the transfer device  16 . 
     Since the positioning of the crown structure  15  on the auxiliary drum  13  is precise, i.e. the rotation axis  29  of the auxiliary drum  13  coincides with a rotation axis of the crown structure  15  and a middle line plane of the auxiliary drum coincides with a middle line plane of the crown structure  15 , the centring of the crown structure  15  with respect to the transfer device  16  is operated between said transfer device  16  and the auxiliary drum  13 . 
     Analogously, since the positioning of the carcass structure  14  on the forming drum  12  is precise, i.e. the rotation axis  26  of the forming drum  12  coincides with a rotation axis of the carcass structure  14  and a middle line plane of the forming drum  12  coincides with a middle line plane of the carcass structure  14 , the centring of the carcass structure  14  with respect to the transfer device  16  is operated between said transfer device  16  and the forming drum  12 . 
     In the illustrated embodiment and in accordance with the method according to the present invention, the measurement device  30  is configured for detecting a longitudinal shift “ΔZ”, along a direction parallel to the longitudinal axis “Z-Z”, between a centre  31  of the transfer device  16  and a centre  32  of the crown structure  15  and of the auxiliary drum  13  and for detecting the coaxiality between the longitudinal axis “Z-Z” of the transfer device  16  and the rotation axis  29  of the crown structure  15  and of the auxiliary drum  13 . 
     Analogously, the measurement device  30  is configured for detecting a longitudinal shift “ΔZ”, along the abovementioned direction parallel to the longitudinal axis “Z-Z”, between the centre  31  of the transfer device  16  and a centre  33  of the carcass structure  14  and of the forming drum  12  and for detecting the coaxiality between the longitudinal axis “Z-Z” of the transfer device  16  and the rotation axis  26  of the carcass structure  14  and of the forming drum  12 . 
     The measurement device  30  comprises a sensor  34  mounted on a lateral portion of the annular support structure  18 . The sensor  34  is a laser sight sensor (e.g. the laser micrometer IG-028 KEYENCE™) which comprises an emitter  35  and a receiver  36  which are arranged in diametrically opposite positions of the annular support structure  18  and extend laterally with respect to the annular support structure  18 . The emitter  35  and the receiver  36  are mutually facing so as to generate a laminar laser beam  37  which is extended between the emitter  35  and the receiver  36  and lies in a radial plane of the transfer device  16 , i.e. in a plane where the longitudinal axis “Z-Z” ( FIGS.  2  and  3   ) also lies. The position of the emitter  35  and the receiver  36  is such that the laminar beam  37  emitted by the emitter  35  at least partly hits the radially external deposition surface of the auxiliary drum  13  (formed by the sectors  28 ) and hence at least in part does not reach the receiver  36 . Indeed, the longitudinal ends of the auxiliary drum  13  project from the opposite sides of the transfer device  16 . 
     As a function of the blocked portion of the laminar beam  37 , the sensor  34  through a control unit, not illustrated and operatively connected to the measurement device  30 , of the assembly station  1  is able to supply the longitudinal shift “ΔZ” of the auxiliary drum  13  with respect to the transfer device  16 , and hence of the crown structure  15  arranged on the auxiliary drum  13  with respect to the transfer device  16  ( FIG.  3   ). In other words, the longitudinal shift “ΔZ” is detected by measuring a longitudinal distance, parallel to the longitudinal axis “Z-Z”, between the lateral end of the transfer device  16  and the longitudinal end of the auxiliary drum  13  and calculating the longitudinal shift “ΔZ” starting from said longitudinal distance. 
     In the same manner, the sensor  34  and the control unit are capable of supplying the longitudinal shift “ΔZ” of the forming drum  12  with respect to the transfer device  16  when the forming drum  12  is situated within the transfer device  16 . In such case, the laminar beam  37  emitted by the emitter  35  at least partially hits the longitudinal end of the forming drum  12  or the shaft  25 . 
     The illustrated measurement device  30  also comprises a first group of distance sensors  38 ,  39 ,  40 ,  41  mounted on a first of two longitudinally opposite lateral portions of the annular support structure  18  (on the right in  FIG.  1   ), and a second group of distance sensors  38 ′,  39 ′,  40 ′,  41 ′, mounted on a second of the two longitudinally opposite lateral portions of the annular support structure  18  (on the left in  FIG.  1   ). The function of such distance sensors is that of detecting the coaxiality between the longitudinal axis “Z-Z” of the transfer device  16  and the rotation axis  29  of the crown structure  15  and of the auxiliary drum  13 . The abovementioned distance sensors  38 ,  39 ,  40 ,  41 ,  38 ′,  39 ′,  40 ′,  41 ′ are fixed with respect to the annular support structure  18  when they are operating. Their position can be adjusted in order to calibrate the system. 
     For example, but not necessarily, each of the first and second group of distance sensors comprises four distance sensors substantially angularly equidistant from each other. The first and the second group of distance sensors  38 ,  39 ,  40 ,  41 ,  38 ′,  39 ′,  40 ′,  41 ′ are situated on a first plane P 1  and on a second plane P 2  which are symmetric with respect to a middle line plane  42  of the transfer device  16  ( FIGS.  1 ,  3  and  4   ). Each of the distance sensors is of laser type, e.g. of the series LK-G400 of KEYENCE™, and comprises an emitter and a receiver, not illustrated in detail, side-by-side each other. The emitter and the receiver are substantially pointed towards the longitudinal axis “Z-Z” of the transfer device  16 . 
     Each of the distance sensors  38 ,  39 ,  40 ,  41  of the first group measures a respective radial distance R 1 , R 2 , R 3 , R 4 , on the first plane P 1 , between said distance sensor and a radially external surface (defined by the sectors  28 ) of a longitudinal end (that on the right in  FIG.  4   ) of the auxiliary drum  13  (when said auxiliary drum  13  is situated within the transfer device  16 ). 
     Each of the distance sensors  38 ′,  39 ′,  40 ′,  41 ′ of the second group measures a respective radial distance R 1 ′, R 2 ′, R 3 ′, R 4 ′, on the second plane P 2 , between said distance sensor and a radially external surface (always defined by the sectors  28 ) of the other longitudinal end (that on the left in  FIG.  4   ) of the auxiliary drum  13 . 
     In  FIG.  4   , the first plane P 1  and the second plane P 2  are perpendicular to the plane of the drawing, parallel and symmetric to the middle line plane  42  of the transfer device  16 . 
     Since the opposite longitudinal ends of the auxiliary drum  13  project laterally from the transfer device  16 , these are used as a target for measuring the radial distances. The radial distances are detected at the radially external surfaces of the opposite longitudinal ends of the auxiliary drum  13  which project laterally (along a longitudinal direction) beyond end edges of the crown structure  15  wound on the auxiliary drum  13 . 
     The laser beam emitted by the emitter of each distance sensor  38 ,  39 ,  40 ,  41 ,  38 ′,  39 ′,  40 ′,  41 ′ hits the auxiliary drum  13 , is reflected and then captured by the respective receiver of the same distance sensor  38 ,  39 ,  40 ,  41 ,  38 ′,  39 ′,  40 ′,  41 ′. 
     Starting from said measured radial distances, R 1 , R 2 , R 3 , R 4 , R 1 ′, R 2 ′, R 3 ′, R 4 ′, the control unit calculates the position of the rotation axis  29  of the auxiliary drum  13  with respect to a reference system integral with the transfer device  16  and hence with respect to the longitudinal axis “Z-Z” of said transfer device  16 . The abovementioned two axes  29 , “Z-Z” can be: coinciding, parallel and spaced from each other, tilted with respect to each other and intersecting, tilted and spaced (oblique). Merely by way of example, a calculation method example is reported hereinbelow. 
     For such purpose, the employed reference system is the clockwise triad “x”, “y”, “z” which has origin in the centre  31  of the transfer device  16 . The axis “z” coincides with the longitudinal axis “Z-Z”, the axis “x” is vertical and consequently the axis “y” in  FIG.  4    exits from the plane of the drawing. 
     With reference to  FIG.  4   , since the distance “Rz” in “z” is known between the sensors  38 ,  38 ′ and the sensors  40 ,  40 ′, the distance sensors  38 ,  40 ,  38 ′,  40 ′ detect the radial distances R 1 , R 3 , R 1 ′, R 3 ′ parallel to the axis “x” which allow calculating, in the plane “x, z”, the tilt “ax” and the shift “dx” along “x” between the rotation axis  29  of the auxiliary drum  13  and the longitudinal axis “Z-Z” of the transfer device  16 . 
     Analogously, since the same distance “Rz” in “z” is known between the sensors  39 ,  39 ′ and the sensors  41 ,  41 ′, the distance sensors  39 ,  41 ,  39 ′,  41 ′ detect the radial distances R 2 , R 4 , R 2 ′, R 4 ′ parallel to the axis “y”, which allow calculating, in the plane “y, z”, the tilt “ay” and the shift “dy” along “y” between the rotation axis  29  of the auxiliary drum  13  and the longitudinal axis “Z-Z” of the transfer device  16 . 
     Such tilts “ax”, “ay” and distances “dx”, “dy” are used for evaluating the non-coaxiality between the rotation axis  29  of the auxiliary drum  13  and the longitudinal axis “Z-Z” of the transfer device  16 . 
     According to a calculation variant, the distances R 1 , R 2 , R 3 , R 4 , R 1 ′, R 2 ′, R 3 ′, R 4 ′ allow calculating, in each of said first plane P 1  and second plane P 2  in which such distances are measured, a respective first centre “C 1 ” lying on the first plane P 1  and a respective second centre “C 2 ” lying on the second plane P 2 , in which the rotation axis  29  of the auxiliary drum  13  passes through said first and second centres “C 1 ”, “C 2 ” ( FIGS.  4  and  5   ). The distances between each of said two centres “C 1 ”, “C 2 ” and the longitudinal axis “Z-Z” are used for evaluating the non-coaxiality. 
     In the same manner, the distance sensors  38 ,  39 ,  40 ,  41 ,  38 ′,  39 ′,  40 ′,  41 ′ with the control unit are capable of detecting the coaxiality between the longitudinal axis “Z-Z” of the transfer device  16  and the rotation axis  26  of the carcass structure  14  and of the forming drum  12 . In such case, the radial distances are detected at the radially external surfaces of the carcass structure  14  carried by the forming drum  12  or of the shaft  25 . 
     In a preferred embodiment, the assembly station  1  is capable of generating warning and alarm signals if the centring (intended as coaxiality and/or longitudinal centring) between the auxiliary drum  13  and the transfer device  16  and/or between the forming drum  12  and the transfer device  16  does not fall within predefined thresholds. Such thresholds can be the same for the forming drum  12  and the auxiliary drum  13  or they can even be different. 
     The control unit can be programmed for generating a first warning signal if the longitudinal shift “ΔZ” exceeds a first threshold of longitudinal shift, e.g. +/−2 mm, and for generating a second alarm signal if the longitudinal shift “ΔZ” exceeds a second threshold of longitudinal shift, greater than the first, e.g. +/−3 mm. 
     The coaxiality, or non-coaxiality, is for example evaluated as a function of the position of the first centre “C 1 ” with respect to the point of intersection of the longitudinal axis “Z-Z” of the transfer device  16  with the first plane P 1  and as a function of the position of the second centre “C 2 ” with respect to a point of intersection of the longitudinal axis “Z-Z” of the transfer device  16  with the second plane P 2 . 
     Considering a first radius of a circle with centre in the point of intersection of the longitudinal axis “Z-Z” with the first plane P 1  and passing through the first centre “C 1 ” and a second radius of a circle with centre in the point of intersection of the longitudinal axis “Z-Z” with the second plane P 2  and passing through the second centre “C 2 ”, the control unit can be programmed for generating a second warning signal if the first radius and/or the second radius exceeds/exceed a first threshold of non-coaxiality, e.g. +/−1 mm, and for generating a second alarm signal if the first radius and/or the second radius exceeds/exceed a second threshold of non-coaxiality, greater than the first, e.g. +/−2 mm. 
     The alarm signals can be followed by the blocking of the building apparatus or of the assembly station  1 . 
     The control unit is also configured for recording the errors of centring (in terms of coaxiality and/or of longitudinal centring) relative to subsequent assemblies of tyres, so as to collect historical data that will serve for the evaluation of the progressive course of the errors and for their correlation with the data relative to the uniformity of the produced tyres. 
     In a preferred embodiment, the assembly station  1  is capable of feedback correcting the position of the auxiliary drum  13  and/or of the forming drum  12  with respect to the transfer device  16  when said drums  12 ,  13  are situated within the transfer device  16 , so as to always obtain an optimal centring. Such control is carried out by for example resetting the reference coordinates of the movement device  21  and of the auxiliary movement device  22  as a function of the detected errors of mutual positioning (longitudinal centring and/or coaxiality). 
     For such purpose, the control unit is operatively connected to the measurement device  30  as well as to the auxiliary movement device  22 , and is programmed for: receiving, from the measurement device  30 , an auxiliary signal relative to a longitudinal shift “ΔZ” of the auxiliary drum  13 , calculating, from said auxiliary signal, a first error of longitudinal centring of the auxiliary drum  13  with respect to the transfer device  16 , feedback controlling the position of the auxiliary movement device  22 , so as to longitudinally centre the auxiliary drum  13  with respect to the transfer device  16 . 
     The control unit is also operatively connected to the movement device  21  and is programmed for: receiving, from the measurement device  30 , a signal relative to a longitudinal shift “ΔZ” of the forming drum  12 , calculating, from said signal, a second error of longitudinal centring of the forming drum  12  with respect to the transfer device  16 , feedback controlling the position of the movement device  21 , so as to longitudinally centre the forming drum  12  with respect to the transfer device  16 . 
     The longitudinal centring is intended as achieved if the first and the second longitudinal shift “ΔZ” are cancelled or better yet brought below a reference longitudinal shift equal for example to 1 mm. 
     The control unit is also programmed for: receiving, from the measurement device  30 , distance signals relative to radial distances R 1 , R 2 , R 3 , R 4 , R 1 ′, R 2 ′, R 3 ′, R 4 ′ from the auxiliary drum  13 , calculating from said distance signals the position of the rotation axis  29  of the auxiliary drum  13 , feedback controlling the position of the auxiliary movement device  22 , so as to render the auxiliary drum  13  and the transfer device  16  coaxial, i.e. to make the longitudinal axis “Z-Z” of the transfer device  16  substantially coincide with the rotation axis  29  of the auxiliary drum  13  and of the crown structure  15 . 
     The control unit is also programmed for: receiving, from the measurement device  30 , distance signals relative to radial distances R 1 , R 2 , R 3 , R 4 , R 1 ′, R 2 ′, R 3 ′, R 4 ′ from the forming drum  12 , calculating from said distance signals the position of the rotation axis  26  of the forming drum  12 , feedback controlling the position of the movement device  21 , so as to render the forming drum  12  and the transfer device  16  coaxial, i.e. in order to make the longitudinal axis “Z-Z” of the transfer device  16  substantially coincide with the rotation axis  26  of the forming drum  12  and of the carcass structure  14 . 
     The coaxiality is intended as achieved if the first radius and the second radius are cancelled or better yet if they are brought below a reference radius, e.g. of 0.5 mm. 
     The types of drums described in the present description (forming drum and auxiliary drum) are non-limiting. In other embodiments, not described in detail, the transfer device can operate with other types of drums which carry a tyre being processed.