B Ball Bearing ▶ Bearing Bar Extrusion

Introduction Bar extrusion is a process in which a block of metal (billet) is forced to flow by compression through a tool (die) opening of a smaller crosssectional area than that of the original billet. There are two basic types of bar extrusion: direct and indirect. The most important and common method used is the direct extrusion (Lange 1988). Figure 1 shows the principle of direct bar extrusion where the billet is placed in the container and pushed through the die by moving a ram towards the die (Saha 2000). In indirect bar extrusion, the die at the front end of the hollow ram moves relative to the container, but there is no relative displacement between the billet and the container as shown in Fig. 2. Therefore, indirect extrusion is characterized by the absence of friction between the billet surface and the container. The absence of friction leads to reduced axial extrusion forces compared to direct extrusion. Therefore, indirect extrusion is used for the forming of heavy extrudable alloys. But the major disadvantage of the method is the strong limited die design due to the required hollow ram geometry. A third type of bar extrusion, which is rarely applied, is hydrostatic bar extrusion. This process utilizes a hydrostatic medium and there is no direct contact between ram and billet (Fig. 3). The pressure of the ram is transferred though the hydrostatic medium to the billet. In hydrostatic extrusion, friction as well as shear forces