Page 31 - MetalForming May 2010

150 in.-tons of energy. The table describes four mechanical presses, specifying tonnage and energy capacity. Referring back to the previous example, every machine listed in the table has an adequate tonnage rating to carry out the 3-in. deep draw (50 tons is required). However, only one of the presses has enough energy (150-in. tons is required). This company literally has an energy shortage. Only one of the four machines
300 in-tons
200 in-tons
100 in-tons
actually has enough energy to run the job.
Unfortunately, most stamping com- panies don’t consider energy require- ments in their press-selection process, even though the data is readily available from the manufacturer’s technical spec- ifications. Omitting the energy data col- umn (in.-tons) from the table would surely lead to the improper selection of a stamping press. Most stamping com- panies would not select a 300-ton press to run a stamping process requiring only 50 tons of force when three other lower cost (lower-tonnage) machines also are available.
What happens when the customer changes the part material to a higher- strength steel grade? Let’s assume that the material change will require 80 tons of force working through the same 3-in. distance. This would require a machine with 240 in.-tons of energy. Again, all four machines are within the rated force capacity, but only one has enough energy.
Or does it?
Stamping high-strength steels requires higher blankholding forces and punch forces that result in increased frictional forces and interface tempera- tures. When interface temperatures reach a critical point, lubrication break- down and other forming- and die-relat-
20 40 60 80 Strokes-per-minute (SPM)
ed problems can arise. During a pro- duction run, it would not be uncom- mon to reduce the press speed in order to lower the process operating temper- atures and eliminate thermal-related forming problems.
Notice that the energy data for each of the machines in the table relates to specific operating speed, reported as strokes/min. (SPM). When the press speed is reduced below a critical point, available energy can diminish rapidly.
The illustration above represents the energy curve for the 300-ton press in the table. This machine has full-rated ener- gy at 60 SPM. If press speed were reduced to 40 SPM to reduce the amount of heat being generated or to mitigate other forming-related prob- lems, the available energy drops to less than 200 in.-tons.
This company now is in the midst of an energy crisis. They have plenty of machines with the required tonnage rating but none have the necessary ener- gy capacity. This supplier soon will dis- cover that they cannot produce these parts for their customer.
If the words “energy crisis” aren’t enough to turn your attention to the energy capacities of the machines in your press shop, especially if you plan to stamp any advanced high strength steels, maybe “breech of contract” will. MF
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METALFORMING / MAY 2010 29
Press Energy Capacity