Pullback power

When extruding cast billets, of which only the faces are machined, a shell forms in the container, the thickness of which is about 2 to 3 mm depending on the billet diameter. It containes oxides, dross and other casting impurities and eliminates the need for machining the billets. If this shell were to be pulled out of the container after the extrusion by means of the ram, a very high pullback power would be required. Therefore a loose dummy block, whose diameter is slightly larger than that... 

In most cases the pullback power chosen for the extrusion ram amounts to 10% of the extrusion power the counterpressure in the main cylinder should not exceed about 10 to 12 atmospheres if the plunger area was designed for a hydraulic pressure of 300 to 400 atmospheres. Under these circumstances idle speeds of about 200 to 300 mm/sec are attained in either direction. With a hydraulic pressure of 200 to 300 atmospheres the permissible counterpressure is 6 to 8 atmospheres. 

When extruding machined billets on the without shell method with the ram moving close-fitting in the container bore, it is recommended to increase the pullback power by 25%. If the press is equipped with advance- or piercer cylinders, the plungers of which move the main cross-head, as shown in Fig. 88, the maximum counterpressure occurring in the main cylinder is 2 to 3 atmospheres thus the pullback power required is decreased correspondingly. 

The piercing power is also rated at 10% of the extrusion power this calculation is based upon a pressure of 30 to 40 kg/mm on the face of the piercing mandrel. It is in special cases only that a higher piercing power is chosen when, for instance, piercing is carried out with a thick mandrel and the end of the container bore is closed by a blank die. In any case, pullback power and piercing j)ower should be carefully verified when the maximum mandrel diameter is known ). 

A two-valve control also answers the purpose when, for example, the pullback cylinders of an auxiliary device of the press are connected directly to the pressure conduit. In such case the main piston, while advancing, has to overcome the constantly acting pullback power and must... 

---