Machinery's Handbook, 31st Edition
Presses for Sheet Metal Working 1465 advantage gained in the open-end press is great accessibility. All of the large gears and flywheel are at the back and out of the way. Only smaller tonnage presses are made in this style, however. Straight Side Presses.— Straight side presses are so named because of the vertical col umns on either side of the machine. This design eliminates the problem of angular deflec tion. Also, die life and part accuracy are enhanced. Straight side presses have frames consisting of a crown member, two upright side members, a bed or fundament of the press, and the bolster, which mounts on the press bed and accommodates the die while strength ening the bed. These components are often secured in a preloaded position by four tie rods. They may also be bolted and keyed together or welded into one piece. As a result, straight side presses are stiffer vertically than gap frame units, and any deflection under load tends to be symmetrical. Straight side presses are suitable for progressive die and transfer die applications and cover an enormous range of types, sizes, and speeds. Hydraulic Presses.— Hydraulic presses are manufacturing devices designed and built to operate dies using hydraulic drive systems to deliver a controlled force. Drive System: These systems are relatively less complex, and are comprised of a motor, a pump, a valving system, and a hydraulic cylinder with the piston connected to the ram. The cylinder is usually double-acting so that hydraulic fluid (oil or water) is pumped under pressure into the top, above the piston, to make the ram move down; to make it come up, fluid is pumped into the bottom, under the piston. The pressure of the fluid under the piston is generally held between 10 to 15 percent of the pressure of the fluid above the piston. This is done to keep the ram from dropping by force of gravity, and also to help control the ram throughout the stroke. Valving System: The valving system ensures the desired control and direction of flow either into or out of the cylinder. Fig. 75 schematically illustrates a hydraulic press drive system with its basic components.
Fig. 75. Schematic Illustration of Hydraulic Press Drive System Frame: Hydraulic presses have various frame types, including C-frames, straight-sided frames, H-frames, four-column frames, and other shapes depending on the application. Tonnage: The tonnage of a hydraulic press is easily calculated: the piston area times the hydraulic pressure in the cylinder. By changing the pressure, the tonnage may be varied. The tonnage of a hydraulic press ranges from about 50 tons to 50,000 tons. Stroke: The stroke is variable on the hydraulic press and can vary from 0.4 in. to 48 in. Strokes per Minute: Hydraulic presses are faster and more reliable than ever. The tech nology has undergone constant change. Improvements in seals, more efficient pumps, and stronger hoses and couplings have virtually eliminated leaks and minimized
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