Hydraulic And Mechanical Presses
Presses are different from hammers in that instead of delivering energy to a work piece through a collision, presses deliver energy through a force that acts over a distance or stroke. Presses are machines that may by used for many different applications. One important application is in forging manufacture. The energy of the press is used to close the mold forming the part within. The work is forged over a single long stroke instead of a series of blows as with a forging hammer.
Figure:176
|
Presses are very important in the forging industry, but presses have many more uses in modern manufacturing. Presses are used extensively in the forming of sheet metal, sometimes called pressworking. Metal extrusion is another large area in manufacturing in which press machines are used. Presses may also be used in the manufacture of plastic parts. Machining operations such as broaching may require presses. Press machines vary in size and in the amount of force they can output. The energy from a press is often used to do work requiring a tremendous amount of force, such as a large amount of plastic deformation of a sizable piece of metal. The method and nature by which a press machine will deliver its energy will vary, dependent on its type. Machine presses shown here are vertical, meaning that the force vector is up and down relative to gravity. In many cases these same machine types are utilized horizontally, meaning the force is delivered in a path perpendicular to the direction of the force of gravity. If the force be exerted vertically, horizontally, or at some intermediate angle, the working principles of each type of machine are the same.
Hydraulic Presses
Hydraulic presses derive the energy they deliver through hydraulic pressure. Fluid pressure, in a particular chamber, can be increased or decreased by the use of pumps, and valves. Sometimes devices and systems may be used to increase the capacity of the pumps in more powerful presses. These presses can operate over a long distance and at a constant speed. Hydraulic presses are generally slower relative to other press machine types. This involves longer contact with the work, therefore the cooling of the work can be an issue when hot forming a part with hydraulic force. Hydraulic presses are capable of being the most powerful class of presses. Some may be as large as buildings, and can deliver awesome pressure. The largest hydraulic presses are capable of applying 75,000 tons, (150,000,000 lbs), of force. The hydraulic press shown is being used to manufacture a forging. Extrusion is also a very common use for such a press, although extrusion is often performed horizontally.
Figure:177
|
The basic working principles of the hydraulic press are simple, and rely on differences in fluid pressure. Fluid is pumped into the cylinder below the piston, this causes the fluid pressure under the piston to increase. Simultaneously fluid is pumped out of the top channel, causing the fluid pressure above the piston to decrease. A higher pressure of the fluid below the piston than the fluid above it causes the piston to rise. In the next step, fluid is pumped out from below the piston, causing the pressure under the piston to decrease. Simultaneously fluid is pumped into the cylinder from the top, this increases the fluid pressure above the piston. A higher pressure of the fluid above the piston, than the fluid below it, moves the piston downward.
Mechanical Presses
Mechanical presses transform the rotational force of a motor into a translational force vector. Therefore the energy in a mechanical press comes from the motor. Mechanical presses are generally faster than hydraulic or screw presses. Unlike some presses, in a mechanical press, the application of force varies in both speed and magnitude throughout the distance of the stroke. When performing a manufacturing operation using a mechanical press, the correct range of the stroke is essential. Presses are chosen based on the characteristics of the manufacturing process. These types of presses are commonly used in forging manufacture, and sheet metal working. The desired application of force will dictate the type of machine required. Extrusion will often necessitate a more consistent force over a longer distance. However, a mechanical press may often be a good choice for impact extrusion, since a fast, quickly repeatable application of force over a limited distance is what is needed for that type of manufacturing operation. The most powerful mechanical presses in modern manufacturing industry will range at about 12,000 tons, (24,000,000 lbs).
Crank Press
The crank press uses a crank link attached to a drive shaft. The crank link rotates with the drive shaft and is attached to a connecting rod by a rotational joint. The connecting rod rocks back and forth during the motion of the crank. The connecting rod is in turn attached to a ram by a rotational joint. The ram operates in a slider joint and travels a one dimensional path in both directions. It is through this path that the crank press delivers it energy. The crank press does allow for a stoke of a relatively long distance.
Figure:178
|
Knuckle Joint Press
The knuckle joint press translates the energy of a motor through a powerful linkage design, and is capable of delivering a tremendous amount of force. The drive shaft crank rotates completely. The links are well grounded to support such pressure.
Figure:179
|
Eccentric Press
The eccentric press uses a motor to drive an eccentric shaft, rotating in a connecting rod. The connecting rod moves a ram in a slider joint one dimensionally. The eccentric shaft itself is round, therefore it may completely rotate within the connecting rod. The center of the drive is not the center of the overall shaft. As the motor rotates, the center of the drive remains stable but the overall center of the shaft changes. This causes the shaft to change position, providing motion. The actual principle of an eccentric press is very similar to a crank press.
Figure:180
|
Rack And Pinion Press
The rack and pinion press delivers the motors energy from a gear directly connected to the drive shaft. The rack is actually a round gear of infinite radius. A rotating gear (pinion), provides force through the rack. This gives the one dimensional translational motion desired of press machines.
Figure:181
|
Screw Presses
Forging screw presses use the rotational energy of a motor to turn a large screw. Typically a friction disk is used to translate the force from the drive shaft to the screw's head. The screw pushes a ram with great mechanical advantage. Screw presses are similar to hydraulic presses in that they are relatively slow and require a longer contact with the work. Screw presses are also similar to hydraulic presses in that they can produce a constant amount of force over a long stroke. Some screw press machines in modern industry can produce 31,000 tons, (62,000,000 lbs), of force.
Figure:182
|