Reconstruction of strip loopers at a perforation unit

Abstract

Experience in operation of the perforation unit at the Cherepovets Metallurgical Combine has shown that the existing loopers, which store the strip below the level of the press die in pits (Fig. i), has a significant effect on the life of the dies and, thus, on the productivity of the unit and the accuracy of spacing of the perforated holes in the strip. The free hanging of the strip in the pits, combined with the original parameters of the pits and the servo and the fact that the strip is fed into the die periodically, leads to vibration of the die and produces dynamic loads on the press and die feed mechanism. The latter developments in turn shorten the life of the die. Taking into account the positive experience gained in operating similar automatic units at the Kama River Truck Plant (KamAZ) and the Volkhov Aluminum Plant (VAZ) in which the reserve strip is stored above the press, we developed and introduced at the Cherepovets combine an experimental design of looper with the same arrangement, i.e., location of the strip above the level of the die and the retention of a minimum reserve of strip in the looper. The design of loopers located ahead of and after the press represents a floor-type roller table with empty undriven rolls and two pairs of undriven rolls which form a reserve of strip between them in the form of a loop. The size of the loop is controlled by a servo. The distance between the loop-forming rolls is regulated. Taking into consideration the step-by-step feed of the strip into the die and the different linear speeds of the strip in the press feed mechanism and on the sections ahead of and behind the press, to ensure reliable operation of the perforation unit in the automatic regime it is necessary to know the minimum and maximum values of the raising of the strip above the die H (Fig. 2) and the distance between the rolls A. It is necessary to know the minimum strip-lifting height Hmi n to prepare the automatic control system for operation so that it will ensure feeding of the strip and die without slip in the press feed mechanism and its pulling out from the press and roller-straightening machine during operation of the coiler. Knowledge of the maximum value Hma x guarantees a maximum reserve of metal before the press and coiler, so that no plastic strains will be made in the strip. With a substantial reserve of metal and a fixed distance between rolls A, the strip may be sent so that it will be plastically strained and will require additional straightening. In connection with the fact that the force needed to keep a bent thin strip in the elastic state exceeds the force exerted by the weight of the strip, to simplify the determination of H (Fig. 2) we assume that the strip is bent symmetrically along the radii RI and R2 relative to the halfway point between the rolls, i.e., we will not consider the weight and the effect of inertial forces on the shape of the bent strip. These curvature radii are determined on the basis of the condition that no plastic strains will occur in the bent strip when they are minimal. For low-carbon steels, the relative curvature radii at which plastic strains will appear lie within the range