In this third of four articles on forging equipment, hydraulic presses are covered. A general overview of the equipment is presented, and details about the physics associated with it are reviewed.

A 3,500-ton hydraulic press in action at a forging plant. (Courtesy Schuler SMG GmbH, Germany)


Figure 1. Overview of a hydraulic press with major components and features labeled

Unlike hammers and mechanical presses, in which mechanical energy is used to cause motion, hydraulic presses use fluid to apply pressure to a hydraulic cylinder. This pressure results in ram motion, the speed and load of which can be easily regulated by a proper control system. Hydraulic presses generally operate at relatively low speeds in comparison to hammers and mechanical presses. These low speeds cause workpiece deformation to occur at relatively low strain rates. Hydraulic presses are force-limited equipment, with the power and speed limits based on the hydraulic system employed.

Figure 1 shows a schematic diagram of a hydraulic press. The major features are labeled in this figure. In the illustration, the hydraulic pressure is supplied to the upper part of the press, causing the ram and the upper platen to move downward.

Figure 2. Typical hot-forged parts produced on a hydraulic press

Hydraulic-Press Applications

Hydraulic presses are well suited for forgings with large aspect ratios or for long extrusions. Also, they are often used to forge materials that are highly sensitive to strain rate, such as aluminum and titanium alloys. Since large loads can be generated by increasing the hydraulic cylinder size, the largest tonnage forging presses are hydraulic. Cold coining operations are frequently conducted on hydraulic presses since no flash is required as the press reaches its load limit. They are also suited to the forging of high-strength alloys, like nickel-based superalloys. Finally, hydraulic presses are used on most isothermal forging processes since die chill is a non-factor. Figure 2 shows some typical parts produced on a hydraulic press.

Figure 3. Operational features of a hydraulic press

Hydraulic-Press Physics

Figure 3 illustrates the basic physics of a hydraulic press. Initially, electrical power is used to run a pump (top left), which pressurizes the hydraulic fluid. The high-pressure fluid is stored in a reservoir or accumulator (top right). When press movement is required, a control valve is opened, and the fluid is allowed to flow through the piping to the press. These valves are frequently controlled by computer and can provide much precision as to the amount of high-pressure fluid allowed to flow to the press (bottom left). When the pressurized fluid reaches the press, it causes the upper ram to move downward. The upper die is attached to the ram, and during its downward motion it contacts the workpiece (bottom right). The energy stored in the pressurized fluid is dissipated as work in deforming the forging.

Because of its reliance on pressured fluid, a hydraulic press will operate within a power envelope. Figure 4 shows a schematic of such a power envelope. The curve is plotted on the axes of ram speed versus forging load. In order for the press to function, the parameters of speed and load must remain below this curve, or within the power envelope. In the extremes, the press can only operate at maximum speed when there is no load or at maximum load when there is no speed (i.e. a stalled press). In normal operations, the press will operate at a constant speed and increase load as the workpiece is deformed. If the load at this speed reaches the power limit curve, the load will continue to increase but at a decreasing rate. This situation is shown in Figure 4 (bottom).

Simulated Operation

Figure 5 shows the simulation of a hydraulic forging. Note that this is the same type of component simulated in the two previous articles on hammers and mechanical presses. The ram speed is highest during the movement through the daylight region of the press with no workpiece deformation. Once the ram makes contact with the workpiece, the ram speed begins to decelerate. As the forging load rises, the backpressure on the ram decreases the flow of hydraulic fluid and subsequently results in further deceleration of the ram. The maximum forging load occurs at the end of the stroke when the cavity is completely filled.

Figure 4. Schematic of a power envelope for a hydraulic press: (top) The curve represents the limits of operation in terms of speed and load; (bottom) The dashed curve shows the deformation of a large workpiece in which the load initially increases at a constant press rate. As the operation reaches the press power limit, the load continues to increase but at a decreasing rate.

Hydraulic-Press Features and Effects

Hydraulic-Press Controls and Drive Systems – The pressure in the hydraulic-press fluid is typically 3,000–6,000 psi, and it is normally set to a constant value in this range. The control valve can be opened or closed to modify the flow of hydraulic fluid, which is directly related to the speed of the press. When the valve is fully opened, the press will move at its fastest speed, subject to the power-curve constraint. The forging to a particular position or thickness in the workpiece can be done by closing the control valve to stop the fluid flow. Normally, the control of this important valve is done by computer, allowing the operator to set the final position and speed through a control panel.

There are two major types of fluid drive systems: direct-drive and accumulator-drive systems. In a direct-drive system, the fluid flows directly from the pump to the press. With direct drive, the pump capacity will define the power that the press can produce. For example, inadequate pump capacity can result in lower system pressure for long extrusions and the inability to complete the stroke. In an accumulator-drive system, the pressurized fluid is stored in a tank, or accumulator, with compressed nitrogen gas above the fluid in the accumulator. Large accumulator systems are popular when the power required exceeds the pump capacity. In other words, accumulators are used to store energy – a concept similar to a hammer or screw press. When the control valve is opened, the amount of pressured fluid available to the press is larger, and it does not rely upon the pump to replenish the fluid during the press stroke.

Recent Advancements in Hydraulic Presses – Advanced control systems provide the operator with a great deal of flexibility in the use of a hydraulic press. Systems are available that accurately control the speed, position, forging load or average strain rate during workpiece deformation. Recent work in fast die-changing technology also increases the usability and flexibility of hydraulic presses. The development of high-speed hydraulic presses allows them to be used in a wide range of critical service applications.

Press Structures – The main types of press structures are rod and housing designs. The rod design uses large round bars (two or four) for ram guidance and load containment. These limit the off-center loading capabilities of the press. Most housing designs use fabrications or castings for ram guidance but still use rods for load containment. The major features of housing-type presses with tie rods include: a multi-pieced frame; keyed alignment of press components; components clamped together with pre-stressed tie rods; and the maximum guiding area is designed to counter off-center loading. Single-piece housing frames do exist but have size constraints associated with them.

Potential Issues

Slower hydraulic presses will increase die contact time with the workpiece, which can result in higher die wear and shorter times before the die cavity loses tolerances. Surface cracking is possible in the forging of metals that are crack sensitive and when the process times are long. Die chill can influence cavity fill when the dies are not heated properly.

Figure 5. A simulation is used to illustrate the power limit of the hydraulic press. In the early part of the forging stroke when contact is sparse, the load is relatively low (blue). The press operates near maximum speed. Simulation of a hydraulic press forging with the ram speed and forging load is indicated. Late in the stroke when the load is high, the ram speed is limited by the available power due to the hydraulic system.

Summary

Hydraulic presses are a popular type of forging equipment. They are very flexible, especially when used with quick die-change techniques. Some hydraulic presses employ additional cylinders such as side rams or multiple ejectors. They can be designed to produce high tonnages and, coupled with modern computers, can be controlled with a great deal of precision. They are slower relative to other types of forging equipment, which can be an advantage when forging some alloys. However, this also increases the contact time between the die and workpiece, potentially leading to more die wear. Because their energy source comes from pressurized fluid, they operate within a range of speeds and loads (i.e. a power envelope).

Acknowledgements

We thank Dale McCartney for supplying information used in this article. The support given these articles from Ajax Manufacturing Company, the Forging Industry Association, the Forging Defense Manufacturing Consortium, Scientific Forming Technologies Corporation and the PRO-FAST Program is appreciated. The PRO-FAST Program is enabled by the dedicated team of professionals representing both 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 (FIA).

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 cvantyne@mines.edu. Co-author John Walters is vice president of Scientific Forming Technologies Corporation, Columbus, Ohio. He may be reached at (614) 451-8330 or jwalters@deform.com.

SIDEBAR: PROS AND CONS OF HYDRAULIC PRESSES

Pros
  • They can have very long strokes and considerable “daylight” is available, which allows them to be commonly used in extrusion applications.
  • Quick die-change technology makes hydraulic presses ideal for short-run jobs.
  • Full (rated) tonnage is available throughout the forging stroke.
  • Sophisticated speed controls are used on hydraulic presses for a wide range of critical aerospace forging applications.
  • Full (or partial) tonnage dwell is possible for an extended time.
Cons
  • Hydraulic presses are rarely used for platters of steel forgings with flash due to the slow speed and extended die contact time.
  • They are not used when extremely high production rates are required (such as high-volume automotive production lines).
  • The ram speed of hydraulic presses is slower than other forging equipment. This makes the contact time between the workpiece and the die longer, causing increased chilling of the workpiece.