By: Tom Klemens, FF Journal
FFJournal: What is reverse tonnage and why is it a problem?
Sutherland: Reverse tonnage, also called snap-through tonnage, is the shock that occurs in a press when the energy is released instantaneously as the metal piercing or blanking is completed but before the bottom of the stroke. It is one of the most damaging things to metalforming presses, but most customers and end users don’t know what it is or how to address it.
Through its drivetrain—the gears, driveshaft, bushings, connecting rod and ball seats—a press has a stack-up clearance of a variety of connection points. Those stack-up clearances are taken up with an air counterbalance. But when all the built-up tonnage needed to actually pierce or blank a part is released suddenly, all of those connection points reverberate up and down. As one sheet after another is blanked, the connection points in the press get an egg-shaped motion going.
The cause of reverse tonnage is normally flat-type blanking or shearing work. The effect, if not addressed, is extensive, severe damage to a press exposed to that type of work on a regular basis.
As a rule of thumb, a yellow light should go on if you have more than 10 percent reverse tonnage compared to the total press tonnage. If you’re at 15 percent, the red light should go on. Over 15 percent you are doing severe damage to your press. Put a quarter on the bolster of the press and die area and if it is bouncing, you’re in the red light warning zone.
Q: Do all presses require these damping systems?
It depends how the press will be used. Presses are made for forward tonnage, not reverse tonnage. However, if you know there is going to be reverse tonnage exposure, installing a hydraulic punching damper system, for say $40,000, can reduce the reverse tonnage. But if it’s not taken care of, it can mean periodically rebuilding the press to the tune of $200,000.
The tensile strength of the material has a great effect on reverse tonnage. Today’s automotive steels, for example, have a lot more resistance compared to mild steel, so the percentage of reverse tonnage goes way up.
Older people in the industry know about the effects of reverse tonnage. But people in administration—and sometimes even those in engineering who aren’t on the plant floor—may not be aware of it. And commonly the operators are just concerned with making parts.
Any time we hear the term “flat blanking” it raises reverse tonnage concerns. But if the die has an angle, there will be shear on the die and the reverse tonnage will go way down. So when someone calls our company for a press proposal for blanking, our first question is whether there is shear in the tooling or whether it will be done flat and all at once.
Q: Wouldn’t just opting for shear on the die solve the problem?
There’s a tradeoff. When you have shear on the die, sometimes that can deform the part. With flat blanking, you normally get a clean, good edge. The tradeoff is that with flat-type blanking, you may be exposing the press to something you may not be considering, so a damping system might be advisable.
Of course, dampers take up space. They are short-stroke, 10 in. to 12 in. diameter hydraulic cylinders, so when the forward tonnage is complete, they absorb probably 90 percent or more of the reverse tonnage. You lose valuable die space by putting them on the four corners of the press, so we devised a system that puts them in the frame of the press, above the side windows that are still open and available for coil feeding or part or conveyor exits. We also build an adjustable striker plate to accommodate and work within whatever range of tool and die height the press will handle.
We installed our first dampers as a test more than 20 years. A customer was having trouble with one of five identical 200-ton presses, and it turned out to be the one dedicated to louver work. After a month of running with the dampers, the 96 tons went down to 5 tons, and that press is still running today with no rebuild. It sold me on the concept, and it sold them, too. FFJ