When to Use Servo stamping press？

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

When comparing a servo press vs. a hydraulic press, it's crucial to consider these machines' pivotal role in shaping, forming, and assembling various products.

They're essential for a broad array of industries, including aerospace, electronics, automotive, and consumer goods. By applying force to materials like metal, presses enable manufacturers to create components with precision and efficiency.

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Hydraulic presses and servo presses are two of the most prominent types. Each has unique mechanisms, advantages, and applications, making it suitable for different manufacturing needs.

The hydraulic press, known for its immense power and versatility, has been a staple in heavy-duty manufacturing processes for decades.

On the other hand, the servo press, with its advanced control systems and precision, is increasingly used for tasks that require high accuracy and repeatability.

What Is a Hydraulic Press?

 A hydraulic press generates a compressive force using hydraulic fluid. It operates according to Pascal's principle, which states that pressure applied to a confined fluid is transmitted equally in all directions. This principle allows hydraulic presses to exert significant force with relatively minimal input energy.

The hydraulic press was invented in the late 18th century. Over the years, it has undergone significant advancements, incorporating modern technology to enhance its performance, precision, and versatility.

How Does a Hydraulic Press Work?

A hydraulic press operates through interconnected components that generate and apply force. Understanding these crucial components and their functions is necessary to grasp how a hydraulic press works:

• Hydraulic Cylinder: The core component where the press's force is generated. It consists of a piston that moves within a cylindrical chamber.
• Hydraulic Fluid: Typically oil, this fluid transmits pressure throughout the system. Its incompressible nature allows for efficient force transfer.
• Hydraulic Pump: This pump moves the hydraulic fluid through the system, creating the necessary pressure.
• Control Valves: These valves regulate the flow and pressure of the hydraulic fluid, ensuring precise control over the press's operation.
• Reservoir: A storage container for the hydraulic fluid, ensuring a constant supply to the system.
• Ram: The component that applies force to the material being processed.

Step-by-Step Description of the Hydraulic Press Operation

There are six steps in the operation of a hydraulic press:

1. Activation of the Hydraulic Pump: The process begins with the activation of the hydraulic pump, drawing hydraulic fluid from the reservoir and conveying it into the hydraulic cylinder through a network of hoses and control valves.
   
   
2. Pressure Build-Up: As the hydraulic fluid enters the cylinder, it exerts pressure on the piston. Because the fluid is incompressible, this pressure causes the piston to move, generating a compressive force.
   
   
3. Force Application: The movement of the piston translates into the downward motion of the press's ram. Depending on the specific application, this ram can be equipped with various tools or molds.
   
   
4. Material Deformation: The downward force of the ram is applied to the material placed on the press's bed. This force can shape, mold, cut, or assemble the material, depending on the desired outcome.
   
   
5. Control and Adjustment: Throughout the process, control valves regulate the hydraulic fluid's pressure and flow, ensuring the press operates efficiently and safely.
   
   
6. Return Stroke: Once the pressing operation is complete, the hydraulic fluid is directed back to the reservoir, allowing the piston to retract and the ram to return to its original position. This prepares the press for the next cycle.

What is a Servo Press?

A servo press is a mechanical press that uses a servo motor to drive the ram, providing precise control over the press's movement and force. Unlike traditional mechanical presses that rely on flywheels and clutches, servo presses utilize advanced electronic control systems to deliver highly accurate and programmable motion profiles. This allows for exceptional flexibility and precision in manufacturing processes, making servo presses an increasingly popular choice in modern production environments.

The development of servo press technology can be traced back to the late 20th century, as advancements in electronics and motor control systems paved the way for more sophisticated machinery.

Servo Press Advantages

Servo presses offer several advantages over traditional hydraulic presses. Their ability to precisely control the ram's motion enables complex forming operations that would be challenging or impossible with other types of presses. Additionally, servo presses can operate at variable speeds, allowing manufacturers to optimize the press cycle for different materials and processes. The programmable nature of servo presses also facilitates quick changeovers and reduces downtime, further enhancing productivity.

As industries continue to demand higher precision and efficiency, servo presses are poised to play a crucial role in the future of manufacturing. Their advanced capabilities and versatility make them an ideal choice for applications that require meticulous control and adaptability, ensuring that manufacturers can meet ever-evolving market needs.

Servo Press in Action

Explore the many slide movement profiles available with a servo press. 

How Does a Servo Press Work?

Servo presses are distinguished by their use of advanced servo motor technology, which provides precise control over the press's operations. Understanding the basic components and their functions is essential to appreciate the sophistication and versatility of servo presses:

• Servo Motor: The heart of the servo press, this motor converts electrical energy into mechanical motion. It offers precise control over speed, position, and torque, enabling highly accurate press operations.
• Control System: This system includes a computer or programmable logic controller (PLC) that manages the servo motor's movements. It allows for the programming of complex motion profiles and real-time adjustments based on feedback.
• Feedback Sensors: These sensors continuously monitor the ram's position, speed, and force. The data collected is sent to the control system for real-time adjustments, ensuring consistent and accurate performance.
• Ram: The servo motor directly drives and controls the ram's movement.

Step-by-Step Description of the Servo Press Operation

 Servo presses operate using six basic steps:

1. Programming the Operation: The process begins with the operator programming the desired motion profile into the control system. This includes parameters such as ram speed, position, force, and dwell time. The control system's flexibility allows for complex and precise operations tailored to specific manufacturing needs.
   
   
2. Initiating the Press Cycle: Once the program is set, the servo motor is activated, and the press cycle begins. The control system sends signals to the servo motor, dictating the exact movements required for the operation.
   
   
3. Ram Movement: The servo motor drives the ram with precise control over its speed and position. Unlike traditional presses, which may operate at a constant speed, the servo press can vary the ram's speed throughout the cycle, optimizing the process for different materials and tasks.
   
   
4. Feedback and Adjustment: As the ram moves, feedback sensors continuously monitor its position, speed, and the force being applied. This real-time data is sent back to the control system, which makes necessary adjustments to ensure the operation remains within the programmed parameters.
   
   
5. Force Application: The ram applies force to the material, performing tasks such as forming, punching, or assembling. The precise control of the servo motor ensures that the force is applied accurately and consistently, resulting in high-quality and repeatable outcomes.
   
   
6. Completion and Reset: Once the operation is complete, the control system commands the servo motor to retract the ram to its starting position. The press is then ready for the next cycle, with minimal downtime required for adjustments or changeovers.

Servo Press vs. Hydraulic Press: Which to Choose?

 When comparing servo and hydraulic presses, several critical differences in operation and technology become evident. Hydraulic presses rely on hydraulic fluid and cylinders to generate force, offering immense power and versatility. In contrast, servo presses use servo motors and advanced control systems to deliver precise and programmable motion, ensuring high accuracy and repeatability.

Hydraulic presses are best suited for heavy-duty applications such as metal forging, deep drawing, and tasks requiring substantial force.

On the other hand, servo presses are ideal for precision manufacturing, electronics assembly, and applications demanding high accuracy and repeatability, such as automotive fine blanking and medical device production.

While hydraulic presses are powerful and versatile, servo presses offer superior precision and efficiency, making them increasingly favored in modern manufacturing environments.

Frequently Asked Questions About Servo Presses vs. Hydraulic Presses

When choosing between servo presses and hydraulic presses, manufacturers often have questions to determine which type best suits their needs. Understanding the differences, advantages, and applications of each can help make an informed decision. Below are some common questions and answers to guide you through this comparison.

What Is the Main Difference Between a Servo Press and a Hydraulic Press?

The main difference lies in their operation and control systems.

Servo presses use servo motors and advanced electronic control systems for precise and programmable motion, while hydraulic presses rely on hydraulic fluid and cylinders to generate force.

This results in servo presses offering higher precision and control compared to the immense power and versatility of hydraulic presses.

Which Type of Press Is More Energy-Efficient?

A servo press is generally more energy-efficient than a hydraulic press.

It uses energy only when needed during the press cycle, whereas a hydraulic press consumes energy continuously to maintain hydraulic pressure.

This efficiency can contribute to significant cost savings in long-term operations.

What Are the Maintenance Requirements for Each Type of Press?

Hydraulic presses typically require more maintenance due to their reliance on hydraulic fluid, which can lead to leaks and the need for regular fluid replacement.

Servo presses, on the other hand, have fewer mechanical components and do not use hydraulic fluid, resulting in lower maintenance needs. However, the advanced technology in servo presses may require specialized knowledge for troubleshooting and repairs.

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