Hydraulic Press

1. INTRODUCTION

A hydraulic press is a device using a hydraulic cylinder to generate a compressive force. It uses the hydraulic equivalent of a mechanical lever, and was also known as a Bramah press after the inventor, Joseph Bramah, of England. He invented and was issued a patent on this press in 1795. As Bramah (who is also known for his development of the flush toilet) installed toilets, he studied the existing literature on the motion of fluids and put this knowledge into the development of the press. The hydraulic press depends on Pascal's principle: the pressure throughout a closed system is constant. One part of the system is a piston acting as a pump, with modest mechanical force acting on a small cross-sectional area; the other part is a piston with a larger area which generates a correspondingly large mechanical force. Only small-diameter tubing (which more easily resists pressure) is needed if the pump is separated from the press cylinder.

Pascal's law: Pressure on a confined fluid is transmitted undiminished and acts with equal force on equal areas and at 90 degrees to the container wall.

A fluid, such as oil, is displaced when either piston is pushed inward. Since the fluid is in compressible, the volume that the small piston displaces is equal to the volume displaced by the large piston. This causes a difference in the length of displacement, which is proportional to the ratio of areas of the heads of the pistons given that volume = area X length. Therefore, the small piston must be moved a large distance to get the large piston to move significantly. The distance the large piston will move is the distance that the small piston is moved divided by the ratio of the areas of the heads of the pistons. This is how energy, in the form of work in this case, is conserved and the Law of Conservation of Energy is satisfied. Work is force applied over a distance, and since the force is increased on the larger piston, the distance the force is applied over must be decreased.

A hydraulic press is the cause of death for the Terminator in the film of the same name, as well as Andre Delambre in The Fly. The room featured in Fermat's Room has a design similar to that of a hydraulic press. Boris Artzybasheff also created a drawing of a hydraulic press, in which the press was created out of the shape of a robot.

The hydraulic press is one of the oldest of the basic machine tools. In its modern form, is well adapted to press work ranging from coining jewelry to forging aircraft parts. Modern hydraulic presses are, in some cases, better suited to applications where the mechanical press has been traditionally more popular.

2. FUNDAMENTALS

Advantages of Hydraulic Presses

The mechanical press has been the first choice of many press users for years. The training of tool and die makers and manufacturing engineers in North America has been oriented toward applying mechanical presses to sheet-metal press working. Modern hydraulic presses offer good performance and reliability. Widespread application of other types of hydraulic power equipment in manufacturing requires maintenance technicians who know how to service hydraulic components. New fast acting valves, electrical components, and more efficient hydraulic circuits have enhanced the performance capability of hydraulic presses.

Factors that may favor the use of hydraulic presses over their mechanical counterparts may include the following:

1. Depending on the application, a hydraulic press may cost less than an equivalent mechanical press.

 2. In small lot production where hand feeding and single stroking occurs, production rates equal to mechanical presses are achieved.

3. Single stroking does not result in additional press wear.

4. Die shut heights variations do not change the force applied.

5. There is no tonnage curve derating factor.

6. Forming and drawing speeds can be accurately controlled throughout the stroke.

7. Hydraulic presses with double actions and or hydraulic die cushions are capable of forming and drawing operations that would not be possible in a mechanical press.

Example of a Gap-Frame Hydraulic Press:

Like the mechanical press, hydraulic presses deliver a controlled force to accomplish work. The style of the press frame and the hydraulic components vary depending on the intended use. Figure illustrates a gap-frame or C-frame hydraulic press. The bolster and ram provide a surface to mount tooling. The ram is actuated by a large hydraulic cylinder in the center of the upper part of the frame. Additional alignment is provided by two round guide rods. The motor drives a rotary pump, which draws oil out of the reservoir housed in the machine frame. The control system has electrically-actuated valves which respond to commands to advance and retract the slide or ram. A pressure regulator is either manually or automatically adjusted to apply the desired amount of force.

Unique Features of Hydraulic Presses:

In most hydraulic presses, full force is available throughout the stroke. Figure illustrates why the rated force capacity of a mechanical press is available only near the bottom of the stroke. The full force of a hydraulic press can be delivered at any point in the stroke. This feature is a very important characteristic of most hydraulic presses. Deep drawing and forming applications often require large forces very high in the press stroke. Some mechanical presses do not develop enough force high enough in the downward stroke to permit severe drawing and forming applications such as inverted draw dies to be used without danger of press damage.

Another advantage is that the stroke may be adjusted by the user to match the requirements of the job. Only enough stroke length to provide part clearance is required. Limiting the actual stroke will permit faster cycling rates and also reduce energy consumption. The desired pre-set hydraulic pressure provides a fixed working force. When changing dies, different shut heights do not require fine shut height adjustment. Different tool heights or varying thicknesses of material have no effect on the proper application of force.

The availability of full machine force at any point in the stroke is very useful in deep drawing applications. High force and energy requirements usually are needed throughout the stroke. The ram speed can also be adjusted to a constant value that is best for the material requirements.

Built-in Overload Protection:

The force that a hydraulic press can exert is limited to the pressure applied to the total piston area. The applied pressure is generally limited by one or more relief valves. A mechanical press usually can exert several times the rated maximum force in the event of an accidental overload. This extreme overload often results in severe press and die damage. Mechanical presses can become stuck on bottom due to large overloads, such as part ejection failures or diesetting errors. Hydraulic presses may incorporate tooling safety features. The full force can be set to occur only at die closure. Should a foreign object be encountered high in the stroke, the ram can be programmed to retract quickly to avoid tooling damage.

Lubrication:

Hydraulic presses have very few moving parts. Those parts that do move, operate in a flood of pressurized oil, which serves as a built-in lubrication system. Should leakage occur, it is usually caused by the failure of an easily repairable part such as the ram packing, or a loose fitting. Hydraulic presses having guide rods or gibbing, may require a different lubricant than the hydraulic fluid. The same type of metered or recirculating lubrication systems used on mechanical presses are used in such cases.

Large Force Capacity:

Mechanical presses with high force capacities are physically much larger than their hydraulic counterparts. Few mechanical presses have been built with force capacities of 6.000 tons (53.376 mN) or more. Higher tonnages or more compact construction is practical in hydraulic presses. Hydraulic presses for cold forging are built up to 50,000 tons (445 MN) or greater force capacity. Some hydraulic fluid cell presses have force capacities over 150,000 tons (1,334 mN). Figure 3 illustrates how two pistons having different diameters both deliver 75 tons (667 kN) of force. The force developed by a hydraulic piston is the product of the area of the piston times the applied pressure.

Force Depends on Hydraulic Pressure and Piston Area:

75 tons (667 kN) of force can be achieved by applying 5,300 psi (36,538 kPa) to a 6-inch (152.4 mm) diameter piston. The same 75 tons (667 kN) of force is achieved by applying 1,910 (13,168 kPa) to a 10-inch (254 mm) diameter piston. There is no set rule on the best peak operating pressure for a press design. Obviously, higher pressures permit the use of more compact cylinders and smaller volumes of fluid. However, the pumps, valves, seals, and piping are more costly because they must be designed to operate at higher pressure.

 Advantages of Adjustable Force:

The force of a hydraulic press can be programmed in the same way that the movements of the press are preset. In simple presses, the relief valve system that functions to provide overload protection may also serve to set the pressure adjustment. This allows the press to be set to exert a maximum force of less than press capacity. Usually there is a practical lower limit, typically about 20% of press capacity. At extremely low percentages of force capacity, a stick-slip phenomena known as stiction in the cylinder rod and piston packing can cause jerky erratic action. Programmable controllers are a feature of many modern hydraulic presses. The correct pressure together with ram travel and other parameters is stored in memory by job number and automatically preset by the die setter. For deep drawing operations, the blank holder or hydraulic die cushion force can be varied through the press cycle for best results.

Press Construction Depends on the Type of Work Performed:

The bed size, stroke length, speed, and tonnage of a hydraulic press are not necessarily interdependent. Press construction depends upon the amount of total force required and the size of dies to be used.

3. GUIDE LINES FOR PRESS SELECTION

Bed size does not directly relate to press force capacity. Both if these illustrations show presses that use the tie rods for ram guiding which is suitable for jobs that do not produce lateral or side loads. Hydraulic presses are available in many types of construction which is also true of mechanical presses. There are many factors to consider when deciding between a hydraulic and mechanical press. These include stroke length, actual force requirements, and the required production rate.

Long Stroke Lengths Can be an Advantage:

Since the stroke length can be fully adjustable, long stroke lengths provide for ease of setup and flexibility of application. The full stroke may be used to open the press up for the installation of dies. In production, the stroke length can be set as short as possible to provide for stock feeding and part ejection while maximizing stroking rates.

Hydraulic Press Speeds:

Most press users are accustomed to describing press speeds in terms of strokes per minute. Speed is easily determined with a mechanical press. It is always part of the machine specifications. The number of strokes per minute made by a hydraulic press is determined by calculating a separate time for each phase of the ram stroke. First, the rapid advance time is calculated. Next the pressing time or work stroke is determined. If a dwell is used that time is also added. Finally the return stroke time is added to determine the total cycle time. The hydraulic valve reaction delay time is also a factor that should be included for an accurate total time calculation. These factors are calculated in order to determine theoretical production rates when evaluating a new process. In the case of jobs that are in operation, measuring the cycle rate with a stopwatch is sufficient. Most hydraulic presses are not considered high speed machines. In the automatic mode, however, hydraulic presses operate in the 20 to 100 stroke per minute range or higher. These speeds normally are sufficient for hand fed work. The resulting production rate speeds are comparable to that of mechanical OBI and OBS presses used single stroking applications. Here, there is no additional clutch and brake wear to consider in the case of the hydraulic machine.

Force Requirements:

When choosing between a mechanical or hydraulic press for an application a number of items should be considered. The force required to do the same job is equal for each type of press. The same engineering formulas are used.

There is always a possibility that an existing job operated in a mechanical press requires 20 to 30 % more force than the rated machine capacity. The overloading problem may go unnoticed, although excessive machine wear will result. If the job is placed in a hydraulic press of the same rated capacity, there will not be enough force to do the job. Always make an accurate determination of true operating forces to avoid this problem.

Machine Speed:

The forming speed and impact at bottom of stroke may produce different results in mechanical presses than their hydraulic counterparts. Each material and operation to form it has a optimal forming rate. For example, drop hammers and some mechanical presses seem to do a better job on soft jewelry pieces and jobs where coining is required. In some cases, a sharper coined impression may be obtained at a rapid forming rate. In deep drawing, controllable hydraulic press velocity and full force throughout the stroke may produce different results. Often parts that cannot be formed on a mechanical press with existing tooling can be formed in a hydraulic press that has controllable force throughout the press stroke and variable blank holder pressure as a function of the ram position in the press stroke.

The Type of Press Frame:

Like the mechanical press, open gap-frame machines provide easy access from three sides. Four-column presses such as the machine shown in Figure 6 insure even pressure distribution provided that there is little or no off-center loading. Some two piston presses similar to the design shown in Figure 6 feature a system of linear position transducers and servo valves to vary the force to each piston in order to maintain the ram level with the bed. How well such a system works depends on the accuracy of the position sensors and reaction speed of the servo valving. If snap through energy release is involved, the servo system may not be able to react quickly enough to prevent ram tipping that may be harmful to the press, tooling and process. Straight side presses such as that illustrated in Figure 7 are much better able to withstand off-center loading and snap through energy release than the type shown in Figure 6. Quality features to look for in a press designed for severe work when ram tipping is to be minimized are a single piston design together with a large ram or slide with long guiding and eight point gibing. However, loading should be carefully balanced and cutting dies timed to minimize snap through shock to the best extent possible in any press working operation.

4. HYDRAULIC PRESS LIMITATIONS

The fastest hydraulic press is slower than a mechanical press designed for high-speed operation. For example, the high speeds together with short stroke and feed progressions used for electrical terminal production favor the use of mechanical presses.

Stroke Depth Control:

 While hydraulic presses are available with an reasonably accurate built-in method of stopping the down stroke, generally stop or bottoming blocks must be provided in the tooling. Under production conditions, stroke depth typically can be controlled to within 0.020-inch (0.51 mm), even though readout devices with higher resolution may be provided on the machine. Hydraulic presses are often provided with controls to reverse the machine at a pre-set pressure. This feature used in conjunction with stop or bottoming blocks in the die, can result in excellent part uniformity.

Shock after Breakthrough in Blanking:

Problems with snap-through energy release are common to both mechanical and hydraulic presses. Damage to the hydraulic press structure may result. Severe snapthrough shock can damage lines, fittings, valves, and the press electrical controls. Presses that are robustly constructed have lower deflection and are preferred for heavy blanking applications. Snap through energy release is directly proportional to the amount of machine deflection at the moment of breakthrough when blanking.

Arresting Snap-Through Energy:

Some hydraulic press manufacturers build snap-through arresting devices into hydraulic presses used for heavy blanking applications. Hydraulic damping cylinders on each corner of the machine arrests snap through energy. Hydraulic snap-through arrestors are also available as add-on devices for retrofitting to existing mechanical and hydraulic presses. Hydraulic dampers are effective snap-through arresting devices on both mechanical and hydraulic presses. While the dampers are an effective solution, they add to equipment cost, may require time-consuming adjustment for different jobs and increase energy consumption. Snap-through energy control should be achieved through good die timing wherever possible.

5. MODERN DEVELOPMENT IN HYDRAULIC PRESSES

 As hydraulic presses continue to improve, there is no doubt that they will play an increasingly significant role in industrial production. Hydraulic presses are already in far more widespread use in Europe than in North America.

Response Time and Precise Control:

Hydraulic press speeds have increased over the last few years. Hydraulic component manufacturers have developed new valves with higher flow capacities, faster response time, and precise flow control capability. But unless a radically different hydraulic circuit design is developed, it is unrealistic to predict that hydraulic press speeds will overtake the mechanical press.

Feeders and Auxiliary Equipment:

 With the exception of high speed operations, mechanical press crankshaft-driven feeders are seldom specified for new installations. Today, hydraulic presses use the same roll feeders, and other auxiliary equipment designed for mechanical presses. Actuation is by one or more microprocessor-based programmable controllers. Important features of such systems are easy programming and multiple job memory capability.

Programming the Press:

Modern control systems permit the press sequence to be programmed for each job. Based on job memory parameters the correct pressure, stroke length, speed and dwell time, retraction force can be set-up quickly.

Safety, Human Engineering and Ergonomics:

Important improvements on all types of presses are continuing to increase the comfort and safety of the operator. Better illumination, quieter machines, comfortable work positions, semi unattended operation, and provisions for simplified machine adjustments, all add to operator comfort and increased productivity. Hydraulic presses are increasingly specified for production applications where mechanical presses were once used almost exclusively. The proper selection and use of the machine can be enhanced by a greater understanding of the characteristics of a hydraulic press. The manufacturing engineer should view the press as only one part of a total system which includes tooling, part feeding, personal protection, and part unloading equipment.

Programmable Hydraulic Blankholder Force Control:

Hydraulic die cushions are used on both mechanical and hydraulic presses. They have several advantages when compared to an air cushion.

These include:

1. Much larger forces can be obtained in the same press bed space.

2. Timed cushion lock-down or return delay: this feature is used to avoid deforming the part as the press opens.

3. The ability to control the instantaneous cushion pressure with a servo valve. This feature can be used to optimize the blankholder force as a deep drawing operation is in progress. By controlling the hydraulic die cushion pressure with a servo valve, optimization of blankholder force can be achieved. Typically the pressure of air-actuated die cushions increases 10% or more between initial contacts to the end of travel. A pressure increase of up to 40% is typical for self-contained nitrogen cylinders and some manifold systems. Metal movement on the blankholder may be severely retarded at the end of the forming cycle by this pressure increase. The result may be failure due to fractures. A programmable hydraulic die cushion can optimize blankholder forces through the forming sequence.