Machinery's Handbook, 31st Edition
Mechanical and Hydraulic Presses 1467 a) Tolerances: While some hydraulic presses do maintain high tolerances, most of them are limited to approximately 0.020 inch. When closer tolerances are required on these machines, they are usually accounted for in the tooling, which increases the cost of the die or other tooling. b) Speed: Although high speed is attainable in hydraulic presses, generally, a mechanical press will produce parts faster. This is especially true when short strokes are used. c) Automatic feeding: Because of fewer moving parts, there is less equipment on which to attach automatic feed mechanisms. Therefore, most automatic feed equipment must be integrated into the process via electronic components, which increases cost. Factors that favor the use of mechanical presses over their hydraulic counterparts may include these: • The mechanical press is faster than the hydraulic press. • The mechanical press is by far the most suitable for blanking and punching. • Mechanical presses do not require as large a motor as hydraulic presses because they can store energy in the flywheel and then dissipate the energy throughout the press stroke. • Mechanical presses can be easily adapted to use different rolls and transfer feeds for progressive dies. • Mechanical presses with short strokes are more economical than hydraulic presses. Lubrication System.— The lubrication systems of mechanical and hydraulic presses are very important. If the lubricating system should fail, not only will the press stop, but also many of the parts are likely to be damaged beyond repair. Therefore, when lubrication failure occurs, the press can seldom be run again without a major overhaul. The lubricating system delivers oil to the moving parts of the press to reduce friction and to assist in keeping the parts cool. Most newer and more modern presses are equipped with a pressure re-circulating lubricating system that delivers the oil under pressure to the bearings and bushings, and other lubricant points. After selection of the correct lubricant, the next most critical factors to long machine and lubricant life are keeping the lubricant clean and dry. Details such as machine criticality, operating environment, and component clearances as well as lubricant type, viscosity, flow rate, and economic issues must be carefully considered for optimum lubricant con tamination control. In any press application, the most important aspect for insuring maximum press and lubricant life is the selection of the correct lubricant. This process includes choosing the correct base oil, the correct oil viscosity, and the correct additives for the application. Next in importance is keeping the oil clean and dry. Particulate and water contamination can have devastating effects on machine and lubricant life. The primary source of particulate contamination in lubricants is ambient dust and dirt. While the composition can vary, in general, dust and dirt will contain materials such as silicon oxides and aluminum oxides. Elemental indicators of dirt ingression would be silicon (Si), aluminum (Al), and in some cases calcium (Ca) and magnesium (Mg). If a contaminant particle is larger than the clear ance between two slide surfaces, the particle will grind against them, removing metal from the slide surfaces in a process called abrasive wear. The resultant wear particles can cause a chain reaction by increasing the total number of particulates in the lubricant. Additionally, this newly generated abrasive wear material can get broken into smaller particles and become harder due to the process of work hardening. These more numerous and harder particles combine with the original solid contaminants to increase the amount of abrasive material in the lubricant. Under conditions of high velocity or high pressure or both, small particles can impinge on a press surface and result in erosive wear. In this case, particles can be much smaller than the machine clearances and still cause extensive damage due to the velocities and pressures involved. Particles generated during erosive wear also add to the overall contamination of the system and further increase machine
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