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Blank-and-Draw-Process Problems--Part Two

By: Peter Ulintz

Saturday, July 1, 2017
 

In my May 2017 column, I addressed a question submitted by a reader regarding press-tonnage calculations.

“On several occasions we had to move our dies to a larger press because the press we selected did not have enough power,” the reader commented. “In fact, the press slowed down noticeably during production. When we moved the die to a larger-tonnage press, the problem went away.”

The reader’s concern was that the engineers were using incorrect formulas for calculating the required press forces. As it turned out, the force calculations used did not include the force required to counteract the draw-pad forces, nor was the reduction in available working force (de-rated tonnage) considered as the distance increased above the bottom of the press stroke. Still, the calculations were close enough to get the job assigned to a press with adequate tonnage capacity. Why, then, did the reader experience problems (i.e., press slowing down) running this particular job? And more importantly, why did the problem disappear when the die was moved to a press with a greater tonnage capacity?

First, it is important to recognize the difference between press tonnage and press energy. The tonnage rating of a press is the maximum load that can be exerted in continuous operation without causing damage to the machine structure or its drive system. On the other hand, the energy rating of a press is the product of an applied press load and the distance through which the load must be applied. Since energy is expended with each stroke of the press—and this energy must come from somewhere—we must direct our attention to the main drive motor, the flywheel and geartrain.

The main motor is the only source of energy for the stamping press, and it must have sufficient horsepower to supply the demands of the entire stamping operation. The press flywheel, by virtue of its mass and rotational speed, serves as the energy-storage device. The energy stored in the flywheel often is expressed as in.-tons of torque. In combination, the flywheel stores and delivers the required work energy while the electrical motor restores depleted energy by maintaining flywheel speed and avoiding excessive slowdown.

Deep-drawing operations consume large amounts of press energy due to their long working distances, sometimes beginning several inches above the bottom of the press stroke. For example, pushing 50 tons through 1 in. of drawing requires approximately 50 in.-tons of energy, but pushing 50 tons through 3 in. of drawing requires approximately 150 in.-tons.

The accompanying table lists four mechanical presses, specifying both tonnage and energy capacity. Each machine listed has an adequate tonnage rating to carry out the deep-drawing process. However, only one of the machines has enough energy to draw a distance of 3 in. (approximately 150 in.-tons is required).

Unfortunately, most stampers don’t consider press-energy requirements in their machine-selection process, even though the data can be found in the press builder’s technical specifications. Omitting the energy-data column (in.-tons) from the table surely would lead to improper selection of a stamping press. Most stamping companies would not select a 300-ton press for a job requiring only 50 tons of force, especially when three other lower-cost (lower-tonnage) machines are available.

Drive systems represent the primary differences between the mechanical presses listed in the table. Nongeared presses do not use geartrains or gear reductions to transmit torque to the crankshaft. The electric motor has a pulley located on the end of its shaft and v-belts connect the pulley to the flywheel. A clutch is positioned between the crankshaft and the flywheel so that crankshaft rotation can be started or stopped as needed. The number of strokes/min. on nongeared presses generally are quite high in order to maintain flywheel energy.

Geartrains are used in mechanical presses intended for deep-drawing and forming applications. The geartrain may be a single- or twin-drive arrangement coupled with single or double gear reductions. By adding a single-reduction geartrain, an increase in mechanical advantage is achieved. Additional mechanical advantage is achieved with double-geared presses along with the possibility of longer strokes.

Another way to address energy-related problems with deep-drawing operations, especially when forming higher-strength materials, is to consider the benefits of a servo-driven mechanical press. Here, high-capacity servomotors and energy-storage devices replace the flywheel, motor and clutch/brake systems. A servo-drive press can provide full energy and constant torque even when speed is reduced to as slow as 1 stroke/min.

When comparing flywheel-drive and servo-drive press technologies, remember that although there are key differences between the press types, fundamental principles of tonnage de-rating apply to both. MF

 

Related Enterprise Zones: Tool & Die


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