Conventional presses only provide unidirectional and constant-speed shaft rotation during production; stampers cannot fine-tune the slide speed for each forming operation. Conversely, servo presses allow for change in direction and speed. Hence, stampers can adjust a servo press to fine-tune the speed at impact or during forming while maintaining the highest possible production rate.
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Another advantage of a servo press: The shaft does not need to complete a 360-deg. rotation. Rather, stampers can program a servo press to change the direction of rotation at predefined angles (pendulum mode) to increase the production rate. For example, limiting the rotation per stroke between 90 and 270 deg. (180-deg. rotation) can increase the production rate compared to a 360-deg. rotation. Simply put, position and speed control in a servo press can improve the forming process and increase production rate.
Synchronization
Inputs and outputs (programmable limit switches, die protection, tonnage monitor windows, auxiliary outputs, blow offs, etc.) must be coordinated with the press motion. Doing so with a conventional press occurs rather simply, as the angular position of the shaft increases consecutively between 0 and 360 deg. As a result, each angular position of the shaft occurs a single time within the stroke. Conversely, with a servo press, due to the motion flexibility a given angular position of the shaft may repeat several times within a single stroke. This requires an additional step in the synchronization process.
To ensure proper function of all systems linked to a servo press, we use a real and a virtual angle. The real angle indicates the angular position of the shaft at a given moment, while the virtual angle is a constant-time-based representation of the complete motion profile. For example, a production rate of 60 strokes/min. will take 1 sec. to complete a full revolution, 0 to 360 deg., of the virtual angle, even though the real angle may accelerate, decelerate or change direction during the stroke. The press controller performs this conversion automatically.
The virtual angle indicates how far in time the motion profile is from completing a stroke. Consider the real angle oscillating between 90 and 270 deg. with the servo press operating in pendulum mode. In the forward stroke, the virtual angle will automatically be set in such way that 0 virtual deg. corresponds to the initial pendulum angle, 90 real deg., and 360 virtual deg. corresponds to the final pendulum angle, 270 real deg. (Fig. 2). If we add acceleration or deceleration to the motion profile, a virtual 180-deg. angle would not necessarily mean that the press is at BDC. Instead, it would indicate that the slide has moved half the time required to complete a stroke. The use of virtual angle opens an infinite number of possibilities for synchronization of ancillary equipment and other external actuators.
Torque Transmission
Mechanical presses, conventional and servo, apply vertical force as a result of the available torque within the system, with a direct relationship between the magnitude of the vertical force and the torque. Due to the nature of their systems, conventional and servo presses show opposite torque characteristics at low and high shaft speeds.
In conventional presses, torque available depends on the size of the flywheel and clutch, which in turn deliver torque to the shaft, either directly or through gears. The flywheel must rotate at or above a minimum speed in order to provide enough torque to satisfy the rated tonnage of the press. The clutch serves not only as a torque transmitter but also as safety mechanism. The clutch will slip should the press experience a reaction torque higher than the capacity of its components, interrupting torque transmission (assuming correct setting of clutch operating pressure).
In a servo press, which lacks a flywheel to deliver torque or a clutch to limit it, the servo motor must deliver the required torque to the shaft, either through gears or directly. With a motor direct-drive configuration, the motion controller is able to protect the system against excessive reaction torque. The absence of a flywheel and clutch in a servo press reduces not only the number of components and their maintenance, but also helps to reduce system inertia. (For example, the low-inertia design of a Nidec Minster servo press design allows for fast changes in motion conditions for the given torque capacity, and allows for high performance.)
Due to electric principles beyond the scope of this article, the torque available in a servo motor peaks and remains constant up to a rated speed, but decreases at a constant power rate above such speed (Fig. 3). This behavior has positive and negative connotations. On one hand, servo presses would outperform conventional presses for applications requiring full capacity at low speeds&#;die setup, for example. And, conventional presses will perform best for applications requiring full torque at maximum speeds.
Tonnage Available
As mentioned, the available tonnage is related to the torque available within the system. For constant torque, in a slider-crank press the available tonnage will increase with decreasing lever-arm length. In other words, for constant torque, available tonnage increases with decreasing distance off bottom (DoB), as described by the available tonnage curve (ATC, Fig. 4). Looking at the ATC, the minimum tonnage available occurs near the halfway point in the stroke, while the maximum tonnage available (or press tonnage capacity, PTC) occurs at the rated DoB. Theoretically, the available tonnage could reach infinity at TDC or BDC, where the lever arm is zero. However, that would compromise the press structure at tonnages beyond the PTC and it should be avoided.
If the available torque changes, the ATC also will change, and so it&#;s important to understand the differences between the tonnages available in a conventional press compared to a servo press. For practical purposes, we can say that in a conventional press, the ATC does not change with shaft speed within the operational-speed range. Conversely, the ATC of a servo press is scaled down as we exceed the rated speed (Fig. 5). This results from the decreased available torque above rated speed in servo motors. Torque reduction in the servo motor also leads to a decreased rated DoB above the rated speed (Fig. 6). When looking at the ATC in a servo press, do not confuse shaft speed and production rate. Shaft speed refers to the actual rotational speed of the shaft, while the production rate refers to the number of parts being produced per unit of time.
In many cases, this scaling-down-of-ATC phenomenon does not represent a significant disadvantage for servo presses, as many jobs do not require maximum motor torque. For those that do, we can decrease the shaft speed during forming in order to have full tonnage available. And, increasing the shaft speed during the nonforming portion of the stroke would help to maintain the production rate as high as possible. Nonetheless, take care to understand the ATC versus shaft-speed variation with servo presses.
Available Energy
The power supply of the press dictates the energy available per unit of time. Conventional and servo presses employ different systems to store energy, and due to these differences, conventional and servo presses show opposite trends regarding energy available at low and high production rates. Again, do not confuse production rate and shaft speed.
In a conventional press, the energy available depends on the size and speed of the flywheel, and the size of the main drive motor. As the flywheel rotates faster, the amount of stored energy increases. This energy is consumed during the forming process and replenished by the drive motor during the nonworking part of the stroke. The faster the speed, the less time the motor has to restore the energy, thus the system is limited by the power of the drive motor and its ability to restore the energy in time. The available energy curve (AEC, Fig. 7)) captures this behavior, relating the energy available to the production rate.
In a servo press, the energy flows directly from the electrical supply line to the storage devices, and then to the servo motor. The capacity of these storage devices determines the maximum amount of energy available during forming. Starting at low production rates, the full amount of energy is available as there is enough time to replenish the storage system to its full capacity, an advantage over conventional presses. Once the replenish time becomes insufficient, the energy available starts to decrease above a threshold determined by the size of the electrical supply line and capacity of the energy storage. Due to the storage capacity of modern devices, servo presses typically have larger amounts of energy available. This makes servo presses more desirable than conventional presses when dealing with high-energy consumption applications.
Different energy-management systems exist for servo presses, depending on how the system delivers energy. Delivering the required energy directly through the servo motor does not require energy management. A semi energy-management system delivers energy by the motor and the storage system. And, a full-sized energy-management system provides all of the energy required in the storage system.
Applications implementing some level of energy management will experience a more uniform power demand from the grid, resulting in lower peaks (Fig. 8). Moreover, an energy-management system will reduce the power pulled from grid and the size of the transformer required, while increasing the efficiency factor of the energy used. On the other hand, these setups will increase the number of electrical components in the servo press, which can complicate maintenance procedures. In general, however, the benefits of an energy-management system overcome any maintenance concerns.
When to go Conventional, and When to Choose Servo
In general, servo presses can outperform conventional presses when applications require:
Full energy and tonnage at relatively low speed
Low forming speeds while maintaining high production rates
Reduction of vibration, reverse tonnage or impact load
A wide range of stroke lengths (one servo press can emulate the motion of various configurations of conventional presses)
Special motion profiles
Improved synchronization with transfer and feed systems.
In other cases, a conventional press can be a better option for:
Shops looking to make a lower initial investment
High-speed applications
Production of similar and relatively simple parts
Applications with low energy requirements (e.g., blanking or piercing)
Applications requiring maximum shaft speed and full tonnage.
MF
See also: Nidec Press & Automation
Technologies: Stamping Presses
Selecting the best press machine for your metal stamping operations is a very critical undertaking, which involves a great deal of understanding regarding the types of machines you choose to invest in. Some press types are better suited for certain production needs. In this article, we hope to give you a foundational understanding of the pros and cons of mechanical and servo presses, so you can be confident in choosing the proper press type for your next project or production expansion.
Mechanical Press Technology
The mechanical press machine has a long history in metal stamping seeing as how it was among the first kinematic mechanisms used in modern metal stamping. Mechanical presses are complex machines with multiple variations of the same machine type and have a wide range of applications and capabilities.
Overview - Mechanical Stamping Presses
A mechanical press machine is a type of press that can exert extreme amounts of force driven by mechanical means which involves a flywheel, crankshaft, and clutch-brake mechanism. Mechanical presses are fundamental machines in various metal forming operations due to their reliability, efficiency, and capability to produce a high volume of parts with consistent quality. One of the more common variations of a mechanical press is within the frame which we compare in another article here: C frame vs straight side presses.
Working Principle & Applications - Mechanical Presses
The fundamental working principle of a mechanical press involves the conversion of rotational energy into linear motion. This is accomplished by the flywheel storing rotational energy and releasing a controlled portion of said energy to drive the ram of the press. The ram then uses the force applied to shape the material based on the composition of the tool and die. This entire process is done with a great deal of speed and precision which is one of the many reasons why mechanical press machines are essential in high volume production environments.
Mechanical Presses are versatile machines and are suited for several common metal stamping applications such as:
Advantages - Mechanical Presses
Speed & Efficiency
Mechanical Stamping Presses excel in operating at higher speeds than servo press machines. This can be particularly advantageous for large-scale operations where every second of production matters. The continuous & rapid cycles of a mechanical press ensure a high output making them an ideal choice for repetitive high-volume production tasks.
Reliability & Durability
Mechanical Presses offer unmatched robustness & dependability. They are constructed with technology that has been proven and refined over decades. Their sturdy construction helps reduce unexpected breakdowns and maintenance leading to continual operation and minimized downtime.
Power
These press models are known for the power that they can provide to a given operation. Mechanical presses can produce extreme amounts of force which makes them suitable for heavy-duty tasks and applications that use thicker more resistant materials, or processes with significant material deformation like deep drawing.
Cost Effectiveness
When looking at costs mechanical presses often have an advantage over servo press machines. Mechanical options typically require less upfront investment, which can benefit businesses operating with budget constraints. Another area where these presses have an edge is when considering maintenance costs. Costs associated with maintenance are often more straightforward and inexpensive leading to a lower total cost of ownership.
Limitations - Mechanical Presses
Flexibility
One of the primary limitations of mechanical presses is the reduced flexibility of the stroke length and profile. Unlike servo presses, which can adjust these parameters mechanical presses often have fixed stroke lengths and speeds.
Energy Consumption
Mechanical presses can be more energy-intensive. The flywheel, which is a key component in these machines, needs to continuously operate throughout the press cycle. This constant motion leads to a steady draw of power, which can result in higher energy consumption, especially in comparison to servo presses that use energy only when in motion.
Noise Levels
Noise is another factor to consider with mechanical presses. They tend to operate at higher noise levels due to the nature of their mechanical operations. This could require additional noise reduction measures in the workplace, such as sound enclosures or protective gear to ensure a safe work environment for operators.
One way to counteract the limitations of a mechanical press is by utilizing the adjustable stroke technology that we incorporate into our presses. When an adjustable stroke is paired with a variable frequency drive you can expect servo-like flexibility with the speed, consistency, and simplicity of a mechanical press.
Servo Press Technology
What makes a servo press stand out in terms of press technology is its incorporation of servo motors, which distinguishes it from traditional mechanical presses. These machines leverage the control of servo motors to drive the press ram mechanism, offering a high degree of accuracy and flexibility. Unique for their programmability, servo presses allow users to control the slide position, speed, and pressure with precision. Their adaptability makes them suitable for complex and varied stamping tasks.
Working Principle & Applications - Servo Presses
At the heart of a servo press is the servo motor, which directly controls the press&#;s ram movement. Unlike mechanical presses that operate on a fixed cycle determined by the flywheel, a servo press can adjust its stroke and speed in real time based on the task at hand. This is made possible by software that controls the servo motor, allowing adjustments to be made on the fly This results in a highly adaptable press, which can be tailored to specific production requirements.
Most of the applications that apply to a mechanical press can also be performed with a servo press.
Advantages - Servo Presses
Precision & Control
Some of the standout aspects of servo press technology lie in the amount of precision and control that it provides users. Unlike a mechanical press which operates on a fixed cycle, a servo press has a highly programmable stroke profile. This allows the user to adjust the motion, speed, and position of the slide at any point in the stroke. Servo presses can pause or slow down during certain points in the stroke to ensure proper time for feeds or transfer systems to complete their function. This makes very intricate or complex forming processes much more viable in applications where gradual or variable force is needed to guarantee sufficient part quality.
Energy Efficiency
Servo press machines contribute to energy-efficient manufacturing thanks to the press only consuming power during the stamping process whereas with a mechanical press energy consumption is constant when the machine is powered on. Another way servo presses contribute to energy efficiency is by optimizing the stroke profile for specific jobs.
Versatility
Thanks to the programmability of the stroke profile the servo press can offer a great deal of versatility for metal stamping operations. They can adapt to different materials, thicknesses, and forming requirements without the need for physical or manual adjustments to the press. The versatility of the servo press makes it a quality choice for operations that tend to see a high mix of parts that vary in requirements.
Reduced Set-Up Times
With the right controls that offer tool/program storage capabilities, these presses can recall and adjust to the different programs that have been stored with ease. This helps drastically reduce setup and changeover times for operators which can be a huge factor for high-mix, low-volume stamping operations.
Limitations - Servo Presses
Initial Investment
Due to the advanced controls, programmable features, and other components a servo-driven press comes at a much higher initial cost compared to traditional mechanical presses. This type of upfront capital requirement may serve as a barrier to entry for smaller operations, or those with limited budgets.
Maintenance
Since servo presses rely on sophisticated software and controls, this makes the maintenance of these machines more complex than that of their mechanical counterparts. The requirement of specialized knowledge for both the mechanical and software aspects necessitates a higher level of technical expertise. Operations may encounter increased maintenance costs from training their existing operators and personnel. These factors and the need for specialized diagnostics, updates, and system checks may lead to a higher total cost of ownership.
Operator Training
The complexity and programmability of these presses demand a higher level of operator training and understanding. Maintaining skilled personnel is essential to fully leveraging the capabilities, and maintaining efficiency. This can be challenging in times and areas where skilled labor is scarce or for companies that place less value on developing and educating their personnel.
Reduced Operating Speeds
Although servo presses offer unmatched control and programmability they operate at reduced speeds compared to mechanical presses. For operations that require high-speed stamping, a servo press may not serve as the best available option.
Comparing Mechanical and Servo Presse
s
Performance
Mechanical presses lead the pack in terms of raw speed and simplicity, making them more suitable for high-volume job requirements. Servo presses, however, offer much more programmability and flexibility making them a great option for applications that require control and adaptability.
Efficiency
Servo presses are often more energy-efficient, as they consume power on demand as opposed to running continuously. This efficiency can lead to cost savings in the long term.
Precision
Servo-driven presses when utilized properly can provide exceptional precision in operation thanks to the control and programmability. Mechanical presses serve as a consistent option and precision can be determined by the quality and condition of the press.
Flexibility
Typically servo presses offer more flexibility, but for the tradeoff of less operational speeds. This makes them an ideal candidate for high-mix, low-to-mid-volume operations. Traditional mechanical presses tend to lack the flexibility to run multiple jobs optimally. This is one of the reasons the adjustable stroke feature is standard on a Sangiacomo press, so you can tune the stroke of the press to the application at hand which ensures efficient production. Also as mentioned earlier when you pair an adjustable stroke press with a variable speed option you can obtain similar flexibility to that of a servo press without giving up operational speed.
Operational Implications
Whether you are a seasoned metal stamper or a beginner looking to bring part production in-house it is important to know what will be required of your business to run a stamping operation successfully. This includes having skilled operators and maintenance personnel that can run the press and keep it running optimally, also you must invest time and money into ensuring that operators who run the press have the knowledge and skill to do so properly. This can be quite a challenge for those who are new to stamping, and servo presses present unique challenges in terms of programming the profile of the stroke. With mechanical presses the operation can be significantly simpler, so keep that in mind when considering your options for purchasing a press. If you are not going to be committed to investing in the proper support and training required to run the press you can run into significant issues down the road.
The choice between mechanical and servo presses should be informed by the specific needs, operational requirements, and future objectives of your metal stamping operation. Mechanical presses, with their speed, reliability, and cost-effectiveness, are ideally suited for high-volume production tasks where the simplicity and robustness of the machine can translate into operational efficiency and reduced downtime. On the other hand, servo presses offer precision, control, and versatility, making them perfect for complex stamping applications.
The decision should also consider the long-term operational implications, including the availability of skilled labor, maintenance requirements, and the total cost of ownership. Mechanical presses are generally simpler to operate and maintain, while servo presses require a higher level of skill and technical knowledge due to their sophisticated control systems. Therefore, investing in proper training and support is crucial regardless of the technology chosen to ensure optimal performance and avoid potential operational issues.
Ultimately, the selection between a mechanical and servo press should align with your operational goals, technical capabilities, and the nature of the stamping tasks at hand. By carefully assessing these factors, manufacturers can choose the press technology that not only meets their current production needs but also positions them for future growth and adaptability in the evolving landscape of metal stamping.
Need Help Finding the Right Press For Your Operation?
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