Managing Off-Center Press Loads—What Every Press Technician Should Know
May 13, 2024Comments
When the working forces in a die are not distributed evenly across the press ram, the ram will tip and move laterally in the direction of the greatest force. If this movement occurs while punches are engaged with other die components, excessive wear or damage may occur. Die wear increases die-maintenance costs and compromises dimensional consistency of the stampings.
For example, assume that the punching action in a die generates a 100-ton load on the left side and a 50-ton load on the right side, with both punches equally spaced from the press centerline. This situation generates a large left-to-right tipping of the press ram because the load on the left side measures twice that of the right.
Prior to the punches contacting the material, the press ram is parallel with the bolster. At contact, the press deflects, or stretches, to build the required force to punch out the material. As this force builds, the load distributes evenly to both punching stations. When the load reaches 50 tons, the right station begins shearing the material, while the left punch remains on top as the press crank continues to develop the force required to stamp out that area—100 tons. This condition causes the ram to tip in a clockwise direction about the press centerline.
Ram tipping can occur with multiple parts consolidating into a single stamping, or when down-gauging material thickness to reduce component weight increases part complexity. This complexity often requires dies with greater numbers of processing steps, which increases the total length of the die. As a result, distributing the die load evenly across the press ram becomes difficult.
Assess and address ram tipping when creating the die strip layout. Unfortunately, die designers often overlook or misunderstand this step, resulting in strip layouts that position the center of the progressive die with the center of the press ram. Many dies are designed, manufactured and installed in presses without consideration given to centering the load due to designers misunderstanding the negative effects of off-center loading on part quality and die life.
This is analogous to two children of different weights sitting on a seesaw. In Fig. 2, both children sit 3 ft. from the center fulcrum and the seesaw tips in the direction of the heavier child due to the larger tipping moment: 135 ft.-lb. vs 165 ft.-lb. Positioning the heavier child to 2.5 ft. from the fulcrum (Fig. 3), offsets the tipping moments: 138 ft.-lb. vs. 135 ft.-lb., thus balancing the seesaw.
It is not necessary to perfectly balance all the forces across the ram. The press structure and die guiding systems can handle some off-loading conditions. In our example, a press structure designed to handle more than 121.5 in.-tons of off-center loading requires no action.
