Chuck for a tyre changing machine

1. A tyre-changing machine comprising a chuck ( 5 ) having a self-centring group ( 51 ) for blocking a rim ( 100 ) of a wheel, which self-centring group ( 51 ) comprises a plurality of blocking jaws ( 7 ) of the rim ( 100 ) arranged angularly equidistanced along a circumference centred on a rotation axis (A) of the chuck ( 5 ), and means for moving ( 52 , 57 ) for radially distancing and nearing the blocking jaws ( 7 ) with respect to the rotation axis (A) of the chuck ( 5 ) such as to vary a diameter of the circumference on which the blocking jaws ( 7 ) are arranged while maintaining an inclination of each blocking jaw ( 7 ) constant with respect to the rotation axis (A) of the chuck ( 5 ), wherein each blocking jaw ( 7 ) comprises a first portion ( 70 ) which is connected to the means for moving ( 52 , 57 ), and a second portion ( 71 ) which is movably coupled to the first portion ( 70 ) such as to be able to change an overall development of the blocking jaw ( 7 ) in a radial direction with respect to the rotation axis (A) of the chuck ( 5 ), and wherein the second portion ( 71 ) is coupled to the first portion ( 70 ) by means of a joint ( 72 ) for defining a reciprocal rotation axis (B).

2. The tire-changing machine of claim 1 , wherein the reciprocal rotation axis (B) is perpendicular and skewed with respect to the rotation axis (A) of the chuck ( 5 ).

3. The tyre-changing machine of claim 1 , wherein each blocking jaw ( 7 ) comprises means for fixing ( 73 ) for fixing the second portion ( 71 ) in at least two distinct operating positions with respect to the first portion ( 70 ).

4. The tyre-changing machine of claim 3 , wherein when the second portion ( 71 ) is in a first operating position, the blocking jaw ( 7 ) develops in a prevalently parallel direction to the rotation axis (A) of the chuck ( 5 ), while when the second portion ( 71 ) is in the second operating position, the blocking jaw ( 7 ) exhibits a prevalently radial development with respect to the rotation axis (A) of the chuck ( 5 ).

5. The tyre-changing machine of claim 4 , wherein the first portion ( 70 ) and the second portion ( 71 ) are conformed such that when the second portion ( 71 ) is in the first operating position, the blocking jaw ( 7 ) exhibits a shape having diminishing steps in a parallel direction to the rotation axis (A) of the chuck ( 5 ), which diminishing steps make available a plurality of surfaces ( 76 ) for contact with the rim ( 100 ).

6. A tyre-changing machine comprising a chuck ( 5 ) having a self-centring group ( 51 ) for blocking a rim ( 100 ) of a wheel, which self-centring group ( 51 ) comprises a plurality of blocking jaws ( 7 ) of the rim ( 100 ) arranged angularly equidistanced along a circumference centred on a rotation axis (A) of the chuck ( 5 ), and means for moving ( 52 , 57 ) for radially distancing and nearing the blocking jaws ( 7 ) with respect to the rotation axis (A) of the chuck ( 5 ) such as to vary a diameter of the circumference on which the blocking jaws ( 7 ) are arranged while maintaining an inclination of each blocking jaw ( 7 ) constant with respect to the rotation axis (A) of the chuck ( 5 ), wherein each blocking jaw ( 7 ) comprises a first portion ( 70 ) which is connected to the means for moving ( 52 , 57 ), and a second portion ( 71 ) which is movably coupled to the first portion ( 70 ) such as to be able to change an overall development of the blocking jaw ( 7 ) in a radial direction with respect to the rotation axis (A) of the chuck ( 5 ), wherein the second portion ( 71 ) is distinct from the first portion ( 70 ) and is movably coupled thereto for allowing a relative movement of the second portion ( 71 ) with respect to the first portion ( 70 ), and wherein each blocking jaw ( 7 ) comprises fastening means ( 73 ) for blocking the second portion ( 71 ) in at least two distinct operating positions with respect to the first portion ( 70 ).