Power Press: Definition, Design Princples ＆ Selection Guide

What Is A Power Press

A power press is a sophisticated metal stamping machine used for shaping, cutting, forming, and punching metal with speed and precision, making it an indispensable tool in mass production for manufacturing metal parts and components. These presses come in two main types: hydraulic and mechanical, each offering distinct advantages in the manufacturing process.

Operating on mechanical, hydraulic, or servo motor principles, power presses utilize different mechanisms to generate the force necessary for shaping metals. Mechanical power presses employ a system of clutches, flywheels, crankshafts, and fixed and moving plungers to convert circular motion into linear force. Hydraulic power presses, on the other hand, utilize the pressure of hydraulic fluid to compress and shape metals effectively. Servo power presses are driven by servo motors that control the movement of the press slider through eccentric gears, ensuring precise operation.

In all three types of presses, the final shape of the workpiece is determined by the meeting of the upper and lower halves of a die as they are pressed together under the force exerted by the press.

Before the advent of power press machinery, shaping metal sheets was a labor-intensive task requiring significant force and effort. However, the introduction of power presses revolutionized the process, introducing mechanical force and accuracy that greatly improved efficiency in metalworking industries.

Principles of Power Presses

Power press machines work on the principle of reshaping the metal sheets by applying the necessary force. The main parts used are a ram, bed, flywheel, clutch, and crankshaft. The ram and bed are furnished with a combination of dies that enable a metal sheet to be shaped into a particular form. The rotational motion of a flywheel is powered by an electric motor.

The rotating flywheel is joined to the crankshaft by a clutch. Upper and lower dies are joined to the ram, one workpiece on the bed is fed into the machine, and the process is initiated. As a result of the rotational motion of the flywheel, pressing and shaping jobs are done when the upper and lower dies apply a force together. Once the process is done, the formed workpiece is detached and replaced by a new workpiece, and the same process is repeated.

How to Calculate the Size of a Power Press

To properly calculate the size of a power press, the tonnage needed, the size of the worktable, and the press opening height must be defined.

• The tonnage is determined according to the type and thickness of the material to be processed and the shape and size of the press tool.
• To define the size of the worktable, it is enough to know the maximum size of the materials that need to be handled.
• To select the opening height for a press, the choice must be based on the stamping extent and the height needed to clear the workpiece.
• Working speed is an important aspect to consider, particularly for serial production.

Power Press Designs

Power presses vary in design based on how they generate force. In a mechanical power press, the key component is the flywheel, which accumulates rotational energy to drive the ram. Hydraulic power presses, on the other hand, rely on hydraulic fluid pressure to supply force, while servo motor power presses utilize a motor to generate rotational motion that is then converted into linear motion.

Choosing the appropriate type of power press depends on several factors. Mechanical power presses, being the oldest method, center around the concept of utilizing a flywheel. Hydraulic power presses, which are widely used, were developed as replacements for mechanical power presses. Meanwhile, servo motor power presses represent the newest advancement in power press technology.

Hydraulic Power Press Design

The hydraulic press was introduced over 200 years ago by a British engineer. During the first industrial revolution, it was used for forging as a way to replace steam hammers. Over the years, the tonnage of hydraulic presses has gradually increased into the thousands of tons with the capability of mass producing a wide variety of parts and components.

A hydraulic power press uses a pump, endplates, and a piston that creates pressure in a fluid to form and shape metal parts. The main component of a hydraulic press is its pump that pumps oil under pressure into a cylinder.

Cylinder

The cylinder contains a piston that moves up and down to create the compressive force. The piston of the cylinder acts like a pump in order to produce the force. It is the part of a hydraulic press that produces the power to apply force to the workpiece.

Reservoir

The reservoir contains the hydraulic fluid, collects contaminants from the fluid, removes air and moisture from the fluid, and sends heat into the system. The hydraulic fluid is sent from the reservoir to the cylinder through a tube.

Valve

The valve helps to relieve pressure and manages the flow of fluid from the pump to the cylinder. Additionally, the valve regulates the speed of the press and the amount of force it produces. It functions as a pressure limiter. A pressure gauge measures the pressure of the hydraulic fluid to ensure it is performing within its pressure range.

Hydraulic Pump

The hydraulic pump is the mechanical part of a hydraulic power press that moves hydraulic fluid to the reservoir and converts mechanical energy into hydraulic energy. It generates a powerful flow against the pressure at the outlet.

Plates

The press plates hold the workpiece in place and provide a platform for the press to bend, pierce, stamp, or puncture the workpiece. They are the part of the press that makes contact with the workpiece.

Hoses

The movement of the hydraulic fluid depends on a set of hoses that move the fluid from the pump to the cylinder and reservoir. The hoses are made of durable and sturdy material that is capable of withstanding the pressure and heat produced during the operation of the press. Common hose materials are thermoplastics, synthetic rubbers, and polytetrafluoroethylene (PTFE), which are materials capable of resisting corrosion and the effects of exposure to chemicals.

Ram

The ram slides within the frame and applies pressure to the die. Depending on the design of the hydraulic power press, the ram can move horizontally or vertically with some hydraulic presses having multiple rams used for the forming process.

Bed

The bed is a flat supportive surface that supports the die as force is applied by the ram.

Servo Press Design

A servo press uses precision and a servo motor to control the movement of the ram. They are popular for their accurate positioning of the ram, which is ideal for the production of parts that require precision and optimal repeatability. The servo motor is connected to a form of linear actuator, such as a ball screw, that controls the upward and downward movement of the ram.

With a servo mechanical press, the main motor, flywheel, and clutch have been removed and replaced with a servo motor that makes the ram more controllable. The elimination of the parts of a traditional mechanical press results in a servo press having fewer driving parts and a simplified structure. In a typical mechanical or hydraulic power press, the ram moves down with great force and strikes the workpiece to create the desired shape, after which it returns to its original upward position. With a servo press, the ram can be controlled to the extent that it can strike the workpiece and remain in contact for an extended period.

Servo presses are used for applications that require exceptional precision and control, such as aerospace and electronics manufacturing. They are capable of producing the stamping, punching, and forming applications of mechanical and hydraulic power presses but with greater precision.

Servo Motor

The servo motor drives the ram of a servo press and provides power and force to the servo press system. Direct drive and servo motor drive with a reducer are the types of motors used in a servo press.

Direct Drive

A direct drive motor is connected directly to the actuator and is a low speed high torque motor with a simple structure, high efficiency, and low noise. It has limited torque, which limits its use to low tonnage servo presses.

Servo Motor with a Reducer

A servo motor with a reducer allows for rapid acceleration and deceleration. It has a speed reduction ratio that matches the inertia of the motor and gearbox with the inertia of the driven load, which makes the motor run more efficiently.

Servo motors with a reducer take three different transmissions, which are deceleration with a crank connecting rod, with a crank elbow rod, or a screw elbow rod. This type of construction makes it possible for a low torque, high speed servo motor to drive high tonnage presses.

Actuator

The actuator is the part of a servo motor press that changes rotary motion into linear motion. Ball screw actuators are the most commonly used, which consist of a screw and nut assembly with ball bearings to provide smooth, even, and efficient motion. The construction of a ball screw actuator consists of a nut mounted on a grooved shaft. As the screw turns, the nut moves up and down the shaft creating linear motion and precision control.

Controller

The controller receives input from sensors, which it uses to send output signals to the servo motor. Algorithms programmed into the controller regulate the motions of the press to ensure precise operation and accurate repeatability. With hydraulic presses and mechanical presses, it is difficult to control the stroke, the pressure of the stroke, and the motion of the slider. A servo press can be programmed to control the stroke, speed, and pressure with precision allowing the press to reach the desired tonnage at a low speed.

Sensors – For the controller to perform properly, it requires data in regard to the position, force, and speed of the ram. Internal and external sensors send feedback to the controller that converts the data into command signals for the press.

Human Machine Interface (HMI) – The HMI connects operators to the servo press and allows them to monitor, adjust, and change aspects of servo press operations, such as speed, force, and positioning. A necessary component of servo presses is a user friendly interface with graphics that are displayed in real time on the HMI, which can be programmed to the needs of the part being manufactured.

For complex systems of HMIs, a supervisory control and data acquisition (SCADA) system is used to interface HMIs in a factory or facility. Information and commands can be sent to a specific HMI or several HMIs using the SCADA system.

Mechanical Power Press Design

The major components for power transmission on a mechanical power press are the clutch, crankshaft, flywheel, moving ram, and stationary ram. The slide is joined to a crankshaft with connecting rods (“pitmans”).

The crankshaft is coupled with the flywheel, which is constantly rotating while the motor is running. A clutch connects the spinning flywheel with the crankshaft. The crankshaft converts the flywheel’s rotational motion to the upward and downward motions of the press slide.

Ram

The ram is the primary operating component of a mechanical power press, which operates directly during the reforming of a workpiece. The ram moves to and fro within its guides, which prescribe a stroke length and power. The transferred stroke length and power can be adjusted according to the requirements of the operation. The lower end of the ram carries the punch to process the workpiece.

Flywheel

A driven pulley or driven gear is made in the shape of a flywheel (which is used to store the energy reserve) in order to maintain a constant ram speed when the punch is pressed onto the workpiece. The flywheel is fixed at the driving shaft’s edge and is attached to it via a clutch.

The energy stores up in the flywheel when it is idle. If the machine has insufficient flywheel energy, it will come to a halt and won’t be able to finish the operation. Essentially, by employing a flywheel, the motor can work with less capacity. At the same time, maximum tonnage is supplied at the required need of the operation.

For a bigger working space (in case of a drawing process) and for quicker processing (in case of an automatic piercing or blanking process), more power and energy must be provided.

In the blanking process, the work is finished in a very short portion of the stroke. So in this, energy is to be taken from the flywheel, which then instantly provides all the energy needed for operation. The same applies to the remaining cycle period. The drawing process takes a significant portion of the cycle. Since time is adequate, excess energy can be tapped from the motor and lacking energy provided by the flywheel.

Allowable Speed Reduction of Flywheel:

Its value for discontinuous operation = 20%

For continuous operation = 10%

• E = energy
• D = flywheel diameter
• W = flywheel weight.
• N = speed, R = gyration radius.

From operation E = P x K x L

• P = average force, L = stroke length.
• K is friction loss (constant).

If the energy of the flywheel is lower than P x K x L, the speed N must be increased.

Clutch

The mechanical clutch is used to connect and disconnect the driving shaft from the flywheel when it is essential to stop or start the movement of the ram. A clutch moves the torque generated by the flywheel and drives to the gear shaft. Two different kinds of clutches are used on power presses: full revolution and part-revolution clutches.

Full Revolution Clutch

As defined by OSHA, a full revolution clutch is a type of clutch that, when tripped, can’t be disengaged till the crankshaft has nearly done a complete revolution and the press slide a complete stroke. Presses with full revolution clutches are generally older and more dangerous because of their cycling operation.

Part-revolution Clutch

A part-revolution clutch, also defined by OSHA, is a type of clutch that can be disengaged at any time before the crankshaft has done a complete revolution and the press slide has done a complete stroke. The majority of part revolution power presses are air clutch and brake. When air is trapped and compressed in compartments, the clutch engages and the brake disengages. To stop the pressing, the reverse takes place.

Brakes

The brakes are utilized to stop the motion of the driving shaft promptly after it disconnects from the flywheel.

Brakes are very crucial in any mobile system. Commonly, two types of brakes are used. The first type is a normal brake that can stop the driven shaft quickly after disengaging from the flywheel. The other is an emergency brake which is offered as a foot brake to any power press machine. These brakes have a power-off switch with normal strong braking to bring all movements to rest quickly.

Base

The base is the supporting structure of the press and offers arrangements for clamping and tilting the frame in an inclined press. It supports the workpiece holding dies and various controlling tools of the press. The table size limits the size of the workpiece that can be processed on the power press.

Drive Mechanism

Different kinds of driving mechanisms are applied in various types of presses, such as piston and cylinder configuration in a hydraulic press, eccentric and crankshaft configuration in a mechanical press, etc. These mechanisms are utilized to drive the ram by moving power from the motor to the ram.

Control Mechanism

Controlling mechanisms are utilized to run a press under pre-programmed, controlled conditions. Normally, two parameters are configured by controlling mechanisms: the power of the stroke and the length of stroke of the ram. Transferring of power can be cut off with the help of a clutch offered with driving mechanisms as per requirement. In many power presses, controlling mechanisms are inherent to the driving mechanisms. Nowadays, computer-controlled presses are used where control is guided by a microprocessor. These power presses provide accurate and reliable control with automation.

Bolster Plate

This is a thick plate fixed onto the base or bed of the press. It is utilized to clamp the die assemblage rigidly to support the workpiece. The die used in press working might have more than one component, which is why the name “die assembly” is being used in place of the die.

Manually fed presses are cycled by either foot or by two hand controls or trips. With foot control, the press is triggered by pressing down on a foot pedal or switch.

It leaves the hands free while cycling the press. This free hand movement puts operators using foot control at a higher risk of getting an injury while operating. About twice as many press injuries come from foot-controlled presses. With two hand controls or trips, when a workpiece is positioned on the press, both hands should be removed from the operation point to depress the buttons.

Choose A Right Power Press Machine

Selecting the right power press is crucial for efficient operations and avoiding waste of equipment investment. Here are key considerations:

Understanding Processing and Operating Methods:

Different stamping and cutting methods should be understood to determine the appropriate punch type.

Production Volume:

For batches exceeding 3000-5000 pieces, automatic feeding is more beneficial. Continuous and transfer processing should be considered for large production amounts.

Material Shape and Size:

Understanding material characteristics, usage rates, and processing methods is essential.

Material Handling:

Efficient material handling, including supplying materials, taking out products, and waste disposal, is crucial for overall productivity.

Die Buffer Usage Frequency:

Die buffers should be considered for extension operations to facilitate difficult drawing processes, especially in single-action punches.

Processing Punching Capacity:

Calculate the required processing pressure and stroke curve, considering multi-engineering processing and eccentric loads.

Dimensional Accuracy:

Choose a power press based on required accuracy and tolerance, with servo presses offering superior precision control.

Understanding Punch Function:

Fully investigate punch specifications and select appropriate accessories to enhance productivity.

Reliability and Maintenance:

Choose a power press with safety features, considering the risk associated with pressing operations, and address noise and vibration concerns to comply with regulations.

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