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Tuesday, February 9, 2016

Stress Levels Are Increasing In The Press Shop

Pete Ulintz
PMA Technical Director

Stamping high-tensile-strength materials can affect the size, strength, power and overall configuration of every major piece of press-line equipment, including, presses, feed systems and coil-straightening systems.
Consider the following:
Cutting, blanking and punching stresses produce unloading forces in stamping presses called “snap-through” or “reverse-tonnage” loads. Because high-strength materials require greater stress to blank and cut as compared to mild steel, they generate proportionally greater snap-through loads. These forces can easily exceed the limits for which the machine was originally designed, resulting in premature wear, damage and sometimes catastrophic failures.
Energy is expended with each stroke of the press and this energy must be replaced. Critical attention must focus on the size of the main drive motor (horsepower), flywheel mass and the rotational speed of the flywheel when stamping higher-strength materials. The main motor, with its electrical connection, is the only source of energy for the press and it must have sufficient horsepower to supply the demands of the stamping operation.
But stamping presses are not the only equipment affected by higher-strength materials.
High-strength materials have a greater tendency to retain their coil set, which makes straightening them to an acceptable level of flatness very challenging. Straightening in a traditional five- or seven- roll flattener may require larger-diameter straightening rolls and wider roll spacing in order to work the stronger material more effectively. But increasing roll diameter and center distances will limit the range of other materials that can be straightened. Additionally, roll defection - due to material strength and the deeper roll penetration required - can present significant problems. Close-center precision straighteners, having nine to 21 straightening rolls that are backed up with support rolls, often are required instead. 
Feed systems may require additional servo-motor power and/or torque capability to pull the stronger material through the straightener. Additional back tension between the feeder and straightener also is required as the higher yield strength material in the loop tends to push back against the straightener or the feed system.
Don’t assume that stamping higher-strength materials will be a “business as usual” condition for an existing press line – that can be a very costly mistake. These materials can easily push press line performance requirements well beyond the capabilities and limitations for which they were originally designed.

Interested in learning more about press line technology? Join us in Nashville, TN,on February 25, 2016 for PMA’s Press Technology Seminar.

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