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Until recently, the most common production press used in industry was the mechanical press, a force-delivering machine where power is derived from a rotated crankshaft. But in 1991 shipments of hydraulic presses surpassed mechanical ones, and they have become a major factor in the U.S. market ever since.
During the past decade hydraulic presses have gone through continual changes in technology, including seals, hydraulic pumps and stronger hoses and couplings designed to minimize maintenance, along with programmable logic controllers and other electronics offering speed and flexibility.
With markets changing rapidly many manufacturers are reluctant to make large investments in feeding equipment. Just-in-time manufacturing has further increased the incidence of short runs, so hand feeding remains the primary method—something well-suited for the hydraulic press.
Of course, hydraulic presses do not fit every circumstance. But for many situations, they offer significant advantages. The following presents a brief synopsis of hydraulic press characteristics to consider when evaluating the equipment for an application.
Tooling Flexibility. Hydraulic presses have height adjustments, so the ram can come down and stay in a set position. Adjustments can be made to reduce or increase the hydraulic press' shut height. It also allows for tooling flexibility because the hydraulic press is fully adjustable within its daylight area. Tooling does not have to be built around a specific press.
Tonnage Control. A hydraulic press delivers full tonnage at any point in the stroke. Therefore, there are no allowances for reduced tonnage at the top of the stroke. For example, in drawing operations a press' full power is available at the top of the stroke, so there is no need to use a 200-ton press to get 100 tons throughout the stroke. A 100-ton hydraulic press will exert only 100 tons of pressure (or less, if it has been set for less) no matter what mistakes are made during setup. This eliminates the worry of overloading or damaging the press or smashing a die. When a hydraulic press reaches its set pressure limit, it won't go beyond it, because the relief valve opens at that limit, and there is no overload danger.
Stroke lengths of 12, 18, and 24 inches are common, and extra stroke length can be provided. Open gap (daylight) can also be added without much additional cost. In addition, press tonnage doesn't dictate the bed size, so larger table areas and small presses with big bed areas can be produced.
Tonnage versus Space. A typical 20-ton hydraulic press is 8 feet high, 6 feet deep and 2 feet wide. A 200-ton press is 10 feet high, 9 feet deep and a little more than 3 feet wide. At 10 times the capacity, the 200-ton press takes up 50 percent more floor space.
Maintenance. Hydraulic presses are relatively simple. Moving parts are few, and these are fully lubricated in a flow of pressurized oil. Replacements of packing, solenoid coils and occasionally a valve are typical maintenance items. These parts are inexpensive and usually off-the-shelf standard components. They also can be replaced without tearing the machine apart. (Editor's Note: For more on press maintenance, see Hydraulic Press PM: Getting in Front of the Eight Ball, available at www.fandmmag.com.)
Noise Control. With fewer moving parts and the elimination of a flywheel, overall noise levels from a hydraulic press are reduced. Properly sized and mounted pumping units meet and exceed current federal standards for noise, even with the pump under full pressure. Because each phase of the ram movement can be controlled, these noise levels can also be controlled. A hydraulic ram can be controlled to pass through the work slowly and quietly.
Ram Control. The ram force, direction, speed, force release and pressure duration of dwell can all be adjusted to fit an application. Jobs with light dies can be done with the pressure turned down. The ram can be made to approach the work rapidly, then shifted to a slower speed before contacting the work; this prolongs tool life. Timers, feeders, heaters, coolers and a variety of auxiliary functions can be brought into the operation to suit the job.
Hydraulic presses can be run in either run or jog mode. The jog mode, with the ram moving slowly, eases the stress on a part. The ram can also be held in the closed position for additional forming time if needed—a particular advantage for deep-draw work—then released quickly on its up-stroke.
Hydraulic presses have built-in overload protection that works for the tools, too. If tools are built to withstand a certain load, there is no danger of damaging them because of overloading. Tools can be sized to withstand the load of a particular job, not a particular press. The pressure of the press can be set down to suit the job. The lack of impact, shock and vibration promotes longer tool life.
Editor's Note: Tom Lavoie is applications engineering manager at Greenerd Press & Machine Co., Nashua, N.H., www.greenerd.com. Photo courtesy of Greenerd.
author: By Tom Lavoie