Screw Presses Offer Versatility and Precision
This article examines the application of screw presses for forging operations. An overview of screw presses and typical applications are discussed first, followed by the physics of their operation. Simulations are used to illustrate these various aspects, which cannot be directly observed during production. Other features and characteristics, which are important for the proper use and operation of screw presses, are also described.
A screw press, like a forging hammer, is an energy-limited machine. It takes the rotational energy from a flywheel and translates it into the linear motion of a ram. Once the energy has been consumed in deforming the workpiece (forging), the ram movement stops. Screw presses are more widely used in Europe than the U.S., although there has been a wider adaptation of screw presses in the North American market over the last several years.
There are two basic drive motor types – electric or hydraulic – that can be used to impart energy into the flywheel. The way that the motion is translated from the motors to the vertical motion of the ram, however, can be divided into four types of screw presses. There are friction drives, direct electrical drives, indirect electrical drives and hydraulic drives. Generally (although there are exceptions), the flywheel is directly connected to the vertical screw of the press. When the drive is engaged and rotates the flywheel, the screw, which is connected to the ram via a nut, rotates and the ram is driven toward the workpiece. In a typical operation there is only one hit per forging operation. With some modern applications and drive systems, multiple blows are possible. Figure 1 shows a schematic diagram of a screw press. When the screw rotates, the ram moves downward, causing the two die blocks to compress the metal workpiece.
Screw-press sizes are often given as a nominal force capacity that generally relates to the diameter of the screw. Figure 2 shows a drive screw from a large screw press. Tonnage ranges can vary from small (160 ton) friction presses to 38,600-ton clutch-type screw presses. The small presses can operate at speeds up to 50 strokes a minute, while large sizes can run between seven and 12 strokes a minute. The ram speed, length of stroke and energy availability are variable in screw presses and programmable when modern equipment and control systems are used.
Screw presses can be used to forge a variety of metals and alloys, including steel, titanium, aluminum and brass, as well as many other exotic materials such as those used in the aerospace and medical industries. Compressor blades, “gear-like” parts, orthopedic implants and hand tools are among the many shapes that can be forged. Precision-forged and net-shaped parts can also be produced on screw presses. They are particularly suitable for thin-section parts as there is no fixed bottom dead center, as is the case with a mechanical press. Figure 3 shows a variety of typical parts forged on a screw press.
The Physics of a Screw Press
The schematic images accompanying this article illustrate the physics of a screw press. Electrical power is used to turn a flywheel. The spinning flywheel essentially acts as storage for this energy. Generally, the flywheel rotation can be generated via friction, gears or a belt drive. With the exception of a clutch-type press, the flywheel is directly connected to a vertical screw. The rotation of the flywheel turns the screw, which causes the ram (attached to the screw via a nut) to move downward. In effect, the downward movement continues until the energy in the flywheel is consumed by the deformation imparted to the forging. The rotational energy of the flywheel is converted to linear kinetic energy in the screw movement, which is converted to useful work in deforming the metallic workpiece. This energy can be approximated by the area under the load-stroke curve. The energy is then dissipated as adiabatic heating of the forging. Once the ram has reached the bottom of its stroke, the rotation of the screw is reversed and the ram returns to the top of its setting.
Figure 4 is a simulation of a screw press forging an axisymmetric gear blank. Three points are illustrated in the simulation. At the initial contact, the speed of the ram is at its maximum and the loads required for the deformation are at their least, as shown in Figure 4A. During the intermediate points (Figure 4B), the kinetic energy in the ram movement is being converted to deformation, causing the ram speed to slow. The load requirement goes up as the forging progresses. At the end of the stroke, the ram speed goes to zero and the load is at a maximum (Figure 4C). A feature of note at this last point in the forging is the area under the load-displacement curve. This area represents the energy needed for the deformation to occur. A screw press is an energy-limited piece of equipment. Once all of the energy from the flywheel has been dissipated in mechanical work, the blow has ended and no further deformation can occur.
There are several different types of screw presses. The differences are primarily in how the energy for operation is transmitted. Among these types are friction screw presses, clutch-operated screw presses and electrically driven presses. There are also a few hydraulically driven screw presses (utilizing hydraulic motors in place of electric motors), but they are not as common as the other types.
Friction Screw Presses
These are relatively simple machines with few operator variables. Friction presses are very suitable for the forging of thin parts, but they require regular replacement of the friction belts and are unsuitable for large tonnages. These presses also have longer cycle times relative to other types of screw presses. The operator has little control over the amount of energy in the blow. The blow characteristics can also be influenced by atmospheric conditions such as high humidity, etc. The side wheels are permanently rotated via an electric motor and a drive belt. These side wheels are moved from left to right to impart rotation to the horizontal flywheel in either direction to either lower or raise the ram.
Clutch-Operated Screw Presses
The flywheel on a clutch-operated screw press is continuously running in the same direction. There is a clutch built into the flywheel that connects to the screw of the press. When the clutch is engaged, the rotation is transmitted to the screw. When the bottom of stroke is reached, the clutch disengages and a hydraulic motor reverses the direction of the screw and raises the ram back to the top stroke position. Because of the clutch, the setting of force and stroke can be widely varied. These types of presses have very high amounts of forming energy available. It should be noted that the flywheel speed has no influence on the forging force. These presses obtain full force and energy capability after about 30% of the stroke. They are also capable of running at full ram speed after only 10% of the stroke. These machines are much more expensive than friction screw presses. They are also more expensive to run and require a higher level of knowledge by the maintenance staff.
Electrically Driven Screw Presses
There are two basic types of electrically driven presses. The first is where the electric drive motor or motors are arranged around the flywheel, which is directly attached to the screw (Figure 5). The motors directly impart rotational energy into the flywheel. When the bottom of stroke is reached, the motors reverse rotation of the flywheel and raise the ram to its top of stroke position.
The second type is where the screw on the press is directly driven by the electrical motor, and the motor effectively acts as the flywheel (Figure 6). Again, the motor first drives the screw in one direction and then at bottom of stroke it stops and reverses the direction of rotation to raise the ram. The direct-drive systems are designed for precision forgings requiring low to medium forging energy. These types of presses can be built with a hydraulic overload protection, giving better energy utilization over a wide range of forging forces and very precise energy repeatability.
Screw presses are versatile pieces of forging equipment that are well suited to precision and net-shape forgings. They are energy-constrained like hammers and operate at speeds greater than hydraulic presses. The sizes range from very small to tonnages that are normally associated with the largest hydraulic presses. There are a number of different drive mechanisms that can be used to rotate the flywheel and the screw, converting the rotational energy in the system into the linear motion needed to move the dies together and achieve workpiece deformation.
The support for these papers from SMS Meer (Eumuco Hasenclever Division), the Forging Industry Association (FIA), the Forging Defense Manufacturing Consortium (FDMC), Scientific Forming Technologies Corporation and the PRO-FAST Program is appreciated. The PRO-FAST Program is enabled by the dedicated team of professionals representing the Department of Defense and industry. These teammates are determined to ensure that the nation’s forging industry is positioned to meet the challenges of the 21st century. Key team members include: R&D Enterprise Team (DLA J339), Logistics Research and Development Branch (DLS-DSCP) and the Forging Industry Association.
Co-author Roger Rees is with SMS Meer (Eumuco Hasenclever Division), Cranberry Township, Pa. He may be reached at (412) 320-4580 or Roger.Rees@sms-meer.us. Co-author John Walters is vice president of Scientific Forming Technologies Corporation, Columbus, Ohio. He may be reached at (614) 451-8330 or firstname.lastname@example.org. Co-author Dr. Chet Van Tyne is FIERF Professor, Department of Metallurgical Engineering, Colorado School of Mines, Golden, Colo. He may be reached at (303) 273-3793 or email@example.com.
Advantages and Limitations of Screw Presses
- High-precision forming
- Variable bottom dead center can be beneficial in a number of situations
- Tool height setting is not required
- Net-shaped closed-die forgings are possible
- Thin-section parts can be forged
- There is no need to readjust the ram to compensate for heat
- Workpiece contact time is shorter, leading to longer tool life relative to hydraulic presses
- Can use multiple blows on a single workpiece
- Jamming of the ram under load does not occur
- Instant and efficient energy conversion
- Continued high-energy, off-center loads are difficult
- They are slower than eccentric or crankshaft mechanical presses
- More noise is generated from a blow relative to an eccentric-shaft mechanical press
- Higher vibration levels are transmitted through to the ground and foundation
- They are less suited to automation
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