Universal joint with improved stiffness

A universal joint comprising:

• a first fork comprising a first sleeve with a first longitudinal axis, a first member for connecting to a first shaft, and a first pair of arms diametrically opposite to each other with respect to said first longitudinal axis and extending in a direction opposite the first sleeve;
• a second fork comprising a second sleeve with a second longitudinal axis, a second member for connecting to a second shaft, and a second pair of arms diametrically opposite to each other with respect to said second longitudinal axis and extending in a direction opposite the second sleeve;
• a spider comprising:
• a first pair of trunnions coaxial with each other and inserted into a first pair of aligned and opposite seats provided in the first pair of arms;
• and a second pair of trunnions coaxial with each other, orthogonal to the first pair of trunnions, and inserted into a second pair of aligned and opposite seats provided in the second pair of arms;
• wherein a respective needle bearing is provided between each trunnion and the respective seat;
• wherein:
• the first sleeve is connected to the first pair of arms through a first collar extending from the first sleeve and having a concave shape and an end edge, from which the first pair of arms extends;
• and the second sleeve is connected to the second pair of arms through a second collar extending from the second sleeve and having a concave shape and an end edge, from which the second pair of arms extends;
• and each collar has an inner concave surface of substantially spherical shape, with a center located on an axis of the respective aligned pair of seats of the respective pair of arms.