Cross universal joint and manufacturing method thereof

A manufacturing method for manufacturing a cross universal joint;

• the cross universal joint comprising
• a pair of yokes;
• and
• a joint spider that connects the yokes so as to be able to pivotally displace freely;
• wherein
• each of the yokes is made of metal and comprises:
• a base section for connecting and fastening to an end section of a rotating shaft;
• a pair of linking arm sections that extend in an axial direction from two locations on an edge of one end in the axial direction of the base section on opposite sides in a radial direction of the rotating shaft;
• and
• a pair of circular holes that are formed in tip end sections of the linking arm sections so as to be concentric with each other;
• the joint spider being made of a ferrous alloy and comprises four shaft sections that are provided so that center axes of adjacent shaft sections are orthogonal to each other, and tip end sections of these shaft sections are supported by way of bearings inside the circular holes that are provided in the yokes so as to be able to rotate freely;
• reference lines, which are straight lines as seen from outside surface sides of the linking arm sections and that extends in an axial direction of the base sections, is displayed on the outside surfaces of the linking arm sections of at least one of the yokes, in at least the portions nearer to the base section than the circular holes;
• and
• the reference lines comprises protrusions that protrude outward or concave grooves that are depressed inward in the radial direction of the base section from the outside surfaces of the linking arm sections;
• the base section is connected to an end section of a cylindrical shaft section, and the yoke is integrally formed with the shaft section,
• the method comprising steps of:
• performing forward extrusion of a column shaped material made of metal, to obtain a first intermediate material wherein a column shaped section is provided on one end section in an axial direction, and a cylindrical shaft section is provided on the other end section from a middle section in the axial direction;
• obtaining a second intermediate material by pressing a punch in a center section of one end surface in the axial direction of the first intermediate material with the first intermediate material set inside a die having:
• cavities for forming a pair of arm sections that correspond to the linking arm sections in two locations on opposite side in a radial direction of an inner circumferential surface of a portion that surrounds the column shaped section;
• and concave grooves or protrusions that are long in an axial direction of the column shaped section that are formed in a center section in a circumferential direction of the cavities for forming these arm sections, and by forming the linking arm sections at the same time of forming protrusions or concave grooves which are the reference lines in a center section of an outside surface of these linking arm sections by crushing the center section in the radial direction of the column shaped section in the axial direction and pouring the metal material into the cavities for forming the arm sections;
• and
• performing a finishing process to the second intermediate material wherein the process includes formation of circular holes that are concentric with each other in the tip end sections of the linking arm sections to obtain a yoke that is integrally formed with the shaft section, then linking this yoke with another yoke by way of the joint spider.