Machinery's Handbook, 31st Edition
2792
INTENSIFIERS AND HYDRAULIC PRESSES
V C = 0.1 V oil p r 1000 ------ is the compressibility allowance, where p r is the pressure in the high pressure circuit, and V oil is the total volume of oil in the high pressure circuit l = any pretravel of the work cylinder required A r = the area of the ram S 2 = the stroke of a dual pressure intensifier V o = the oil volume required to move the work cylinder through the high pressure portion of the stroke X = the stroke required to close the high pressure seal on the intensifier (often assumed to be 2 inches) Intensifiers can be either pneumatically driven or fully hydraulic. Fully hydraulic inten sifiers are more efficient than air driven ones. They operate on the same principle, with liquid used on both sides of the actuator. Commercial hydraulic intensifiers are generally powered by electric motors or diesel engines. Hydraulic intensifiers can be either single or dual pressure. A single pressure intensifier is used when the work cylinder requires a high pressure for the entire stroke. A dual pressure intensifier is recommended if the high pressure is required only during the last portion of the work cylinder stroke. For best economy, select the smallest driving cylinder bore size for a given intensifier ratio. Minimize pressure losses by keeping lines short and by sizing lines and fittings as large as practical. Size the lines so that fluid velocity does not exceed 7 ft/sec. Self-bleeding boosters be used wherever possible to ensure that air does not build up in the system and cause “spongy” operation. Hydraulic presses, like intensifiers, use two pistons of different diameters. They operate on a shared fluid volume. When a force is applied to the small cylinder to create pressure, that pressure is reflected in the large cylinder. When the large cylinder is exposed to high pressure, it can exert a lot more force than was applied to the small cylinder. The mechani cal advantage of the press is the ratio of output to input force. Hydraulic presses are subject to the following equations: p 1 p 2 F 1 s 1 F 2 s 2 = R where F is force, p is pressure, and A is area, s is stroke, and R is the gear ratio of the press. Note that the stroke of the smaller diameter cylinder will be greater than that of the larger diameter cylinder. Rotary Actuators.— These actuators use fluid pressure to induce rotary motion, and tend to be double acting. Rotary actuators can have either internal or external stops. External stops can generally withstand greater impacts than internal stops. Stops on rotary actua- tors are often adjustable. Rotary actuators can be hydraulic, pneumatic, or a combination of both. Types: Rotary actuators come in two main types: vane and rack and pinion. Vane type actuators offer the smallest package and zero backlash, but are usually capable of only up to 270° of rotation. They employ a rotary vane mechanism to turn the actuator under fluid pressure. Rack and pinion actuators use one or more linear actuators to move racks back and forth, thereby turning a pinion shaft. They can be outfitted to have multiple stopping positions, and are capable of 360° rotation. Many packaging options are available for both vane and rack and pinion types, with many mounting and output shaft options. Sizing Rotary Actuators: When sizing rotary actuators, one must know the accuracy and actuation time required, the required angle of rotation, any resisting torque, and the moment of inertia of any mass loading about the center of rotation. Torque values for both directions of rotation as a function of input pressure are normally provided by F 1 A 1 --- F 2 A 2 --- = = = F 1 F 2 --- s 1 s 2 -- = = = A 1 A 2 ---
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