Machinery's Handbook, 31st Edition
1466 Mechanical and Hydraulic Presses maintenance. So, for operations requiring very short strokes, such as punching and blanking, a hydraulic press can achieve speeds that compare favorably with those of mechanical presses, especially when the material is hand fed. Because drawing and forming operations must be run more slowly to allow time for the metal to flow, press speeds usually range from around 5 to 100 strokes per minute, depending on the part size and the severity of the operation being performed. Mechanical and Hydraulic Presses, Advantages and Disadvantages.— The mechani cal press has been the first choice of many press users for years. Training of tool and die makers and tool designers has been oriented toward applying mechanical presses to sheet metal pressworking. However, modern hydraulic presses also offer good performance and reliability. Factors that may favor the use of hydraulic presses over their mechanical counterparts may include the following: a) Full power during the stroke: Maximum power is maintained during the entire stroke of a hydraulic press. This allows for rapid movement to a position just above the part, when the stroke is slowed for the working stroke and an adjustable slowdown speed during forming. The result is more strokes per minute. b) Overload protection: Because the pressure is pre-adjusted, if the pressure exceeds a limit, such as might occur when a part is not properly ejected, the machine shuts down, eliminating catastrophic results to tooling or to the machine. c) Cost: Depending on the application, a hydraulic press may cost less than an equivalent mechanical press. d) Lower operating costs: Hydraulic presses have fewer operating parts; therefore, there are fewer things to break. Automatic lubrication of moving parts helps eliminate maintenance problems. e) Flexibility: Owing primarily to electronic controls and robotics, hydraulic presses fit well into such areas as flexible manufacturing systems (FMS) and factory automation. f) No design limitations: The principles of hydraulic force allow for creative engineer ing. Presses can be designed for traditional down-acting, up-acting, side-acting and multi-action operation. Power systems can be placed above, below, or away from the press and force actuators. Large bed presses can be designed for low tonnage applications and small bed presses can be designed for high tonnage requirements. g) Unlimited control options: The hydraulic press can be controlled in a variety of ways, ranging from basic relays to more sophisticated PLC or PC control systems. Operator interfaces can be added to press systems to facilitate ease of job set by storing individ ual job parameters for each die. Presses can be controlled for precise pressure and position, including pressure holding, speed control, and dynamic adjustments to real- time operating variances. Ram force and speed can be controlled in any direction with various levels of precision. h) Small footprint: Hydraulics allow for generation of high pressure over a small surface area. This reduces the overall structure required for support of the force actuators. When compared to mechanical presses, hydraulic presses consume almost 50 percent less space for the same tonnage capability. This size advantage results in lower manu facturing costs and a faster return on investment by requiring less long-term overhead expense. i) Hydroforming: Hydroforming is one of the areas in which hydraulic presses have no competition. The hydraulic press ram moves down to close the die and the hydroform ing high-pressure fluid fills and acts onto the inside tubular parts and expands it to fill the die. The mechanical presses are not capable of carrying out the hydroforming operation, as they do not have the speed control or the ability to stay on the bottom for performing high-pressure-forming operations. Although hydraulic presses have many advantages, they also suffer from some disadvantages when compared to mechanical presses. Some of these include:
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