What are the components of a servo press?

By: Stephanie Price, Applications Engineer, Promess, Inc

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Industrial automation can seem like magic, especially when used in assembly operations. Learn what you need to about the basics to help in your next automation project with our servo press guide.

Two 500-kN presses used with a Platen Press Work Station to give users the required force.

Servo mechanisms are at the heart of 21st century automation. The basic elements are a servomotor; a device to convert rotary motion into linear motion; a suite of sensors to provide the required feedback; a controller to convert feedback data into command signals; and enabling software. This is the &#;magic&#; of high-speed, precision assembly operations. With 21st century advances in their base components, servo presses are a game-changing technology.

They are not just fast and precise&#;they have gone big as well. In the metal forming and stamping industries, servo presses are available with capacities up to 5,000 tons (4,536 metric tons) and larger units are under development. These presses are changing the way users approach process design by giving engineers precise control of force, speed and position in real-time during an entire stroke of a process.

Thirty years ago, industry pioneers combined a precision ballscrew with a servomotor, rotary encoder and load cell to produce the first Electro-Mechanical Assembly Press (EMAP) designed specifically for assembly operations.

Successful Early Adopters

Early adopters found the ability to measure process parameters during assembly allowed them to literally &#;clone&#; products by comparing the force/position &#;signature&#; of each operation to that of a known good cycle and adjusting press parameters in real-time to duplicate it. That basic concept has been used in hundreds of different assembly applications, from simple riveting to installation of high-value electronics. Here are a few examples.

Universal Joint Assembly: A Cardan-type universal joint has a center cross or &#;spider&#; that is attached to a pair of U-shaped arms by pressing a bearing cup through a hole in the arm to capture a machined journal on the &#;spider.&#; Once assembled, the bearing cups are staked to the arm to keep them in place.

The challenge is to keep the spider centered in the arm while the bearing cups are pressed in and staked. This is accomplished with a pair of EMAPs that are synchronized to apply the same force to each bearing cup while it&#;s being inserted. Once the cups are in place, the spider is precisely centered and the EMAPs simultaneously perform the staking operation. Since equal force is applied to opposing legs of the spider, it remains centered and the result is a good assembly each time.

This application works because the EMAPs are monitored during the operation and adjusted for force and position in real-time by the control. Data from the whole process can be captured and stored for quality assurance, providing 100-percent traceability for each assembly. That data is also useful for identifying and correcting anomalies in the parts being assembled, which improves the quality of the entire supply chain as well as the assembly process.

Medical Catheter Assembly: The critical operation is a crimping process that attaches a small-diameter metal tube to a larger tube that&#;s attached to the flexible portion of the catheter. If the crimp isn&#;t perfect, it will either come apart when it&#;s pulled on or it will close off the tube completely, rendering the catheter useless. Consistently crimping a tiny metal tube to one that&#;s only slightly larger proved to be a monumental challenge.

The key to maintaining consistent quality in the catheter crimping operation was monitoring both the amount of force being applied and the exact position of the crimping tool simultaneously. Once both parameters of a known good operation were captured, the force/position &#;signature&#; was used as a benchmark to measure subsequent operations.

An EMAP-based crimping station, complete with external position transducers, is used to perform the crimping operation on the catheters. The EMAP provides repeatable crimping force and the transducers monitor the tooling to make sure the crimp is neither too shallow nor too deep. The result is a 100-percent effort test certification for every catheter produced and the virtual elimination of crimp failures in the field.

Riveted Ball-Joint Assembly: Automotive ball-joint assemblies are safety-critical components that typically are attached to upper and lower control arms with rivets. There are three possible failure modes: 1) a rivet may be too long or too short, 2) a rivet may be too hard or too soft, and 3) a rivet may be missing entirely. Because the assembly is safety critical, a 100-percent post riveting inspection has traditionally been performed.

Using an EMAP-based system instead of a traditional hydraulic press eliminates the 100-percent inspection requirement by monitoring the process while it is being performed and comparing the &#;signature&#; to a known good operation. Three individual load cells are mounted into the tooling to independently measure the force applied to each rivet, while a single-position transducer measures the ram&#;s travel distance. Rivets that are too hard or soft, or too short or long, will produce a distinct change in the signature, as will out-of-tolerance details such as hole diameters.

The system provides: long, short, hard, soft, and/or missing rivet detection; 100-percent certification of every assembly; built-in data acquisition; and a record of force and position data for every part produced&#;all in real time during the process cycle. The results are consistent, accurate, and traceable, which means post-process inspections on every part are no longer necessary to assure quality.

Moving Beyond The Basics

It didn&#;t take long for the early adopters to realize that the detailed process data generated by an EMAP-based system had uses far beyond simply comparing &#;signatures&#; and &#;cloning&#; assemblies. EMAP suppliers were also busy enhancing both hardware and software capabilities to support more advanced applications.

One of the first applications to take advantage of these advances was the assembly of automotive control arms, a product that requires geometric precision to achieve proper function, yet is made of components that cannot be produced economically with very close tolerances. The control arm is made up of heavy-duty stamping or casting with rubber-encased bushings pressed in place&#;clearly not a candidate for extreme dimensional precision.

What automotive engineers typically do is define the geometry required in the assembly and leave the &#;how-to-achieve-it&#; part up to the supplier. Suppliers call this a &#;phantom&#; dimension, and it&#;s quite common in a variety of industries.

The Conventional Approach

The conventional approach to meeting the specification for a &#;phantom&#; dimension is to build precise tools and fixtures and then continuously adjust them to deal with constant, unpredictable variations in the parts. Other suppliers choose to &#;press and hope,&#; then &#;measure and sort&#; and accept scrap and re-work costs. To meet this challenge instead with an EMAP-based system requires advanced software to handle additional sensors outside of the load cells integrated in the press. The assembly is done with two EMAPs and two digital probes. These probes are needed because the resilient bushings flex during installation, making it difficult to know their precise location. The probes also compensate for machine and load cell deflection.

To assemble the control arm, the bushings are pressed to an initial position, the force is removed, and the location is measured by the digital probes. One bushing is pressed to a dimension that is relative to the ball joint. The probe measures the position and feeds the information back to the controller, which tells the press how much further to press. This sequence repeats until the bushing is in place. The other bushing uses the same installation sequence but is pressed to a dimension relative to the first bushing. This is the &#;phantom&#; dimension and the system can efficiently and repeatably achieve it regardless of variations in the control arm and/or bushings.

The system just described significantly improves the functional quality of the control arm it assembles with no change in the dimensional specifications of the component parts. In fact, it&#;s quite possible that the tolerances on those component parts could be loosened to reduce manufacturing costs with no impact on the finished product&#;s functional quality. Function is the consumer&#;s measure of quality, and with the kind of intelligent assembly pioneered by the control arm system it can be the manufacturer&#;s as well.

A Mature Technology

As intelligent assembly applications have proliferated, so have the hardware and software systems required to enable them. Today, EMAPs are available with force outputs ranging from 0.2 kN to 1,000 kN and can be equipped with a broad range of integral and external sensors. They are available as individual components to system builders, H-frame presses and flexible stand-alone workstations for end users.

Innovative engineers have made EMAPs light enough to be used as robot end effectors and even human hand-held models are available. Both of these products are intended for applications in which the press is brought to the part, which means that the reaction force of the pressing operation cannot be transmitted to the robot or human operator.

Linear EMAP/Rotary Actuator Combo

Another trend is the combination of a linear EMAP with a rotary actuator to facilitate functional testing during assembly. For example, an automotive hood latch is assembled with a rivet using an EMAP. While the rivet is being peened, the latch is actuated and the required force continuously measured until it reaches a specified level, at which point the process is stopped. The process produces latches with uniform actuating force regardless of variations in rivets or stampings. Similar systems are used to assemble automotive seat latches, pliers, and even to check gear backlash under load in automotive differentials.

All of these applications depend on sophisticated controllers and software to handle and integrate multiple data inputs in real time and generate the necessary servo commands. Controls are now available with the ability to synchronize multiple EMAPs driving the corners of a platen press so that each one generates the force necessary to keep the platens parallel, even though the load is not uniform.

Control software has evolved along with hardware, and today&#;s systems are much easier to program than their predecessors of even a few years ago. Suppliers have invested significant time and talent in systems that simplify the process to the point where professional control engineers often are not required.

As EMAP technology has matured, the concepts of intelligent assembly and assembly to a functional specification rather than a dimensional one have become increasingly practical across a broad range of industries. Maturity, however, does not imply stagnation. The technology continues to grow by enabling truly innovative solutions to challenges that have plagued manufacturers for decades, and in some cases centuries.

Check out this article to learn more about intelligent press systems

In stamping, when you get right down to it, it's not about tonnage. It's about maximizing energy, or the machine's ability to deliver tonnage, where it's needed most: between the die and workpiece. And until recently the only way to increase tonnage in a mechanical press was through bigger presses with bigger motors and flywheels.

But what if a press delivered tonnage differently?

That question spurred a new wave of mechanical press designs. Press-makers removed the main motor, flywheel, and clutch, substituting it all with a servomotor that focused energy only where needed and, in effect, made the ram a controllable axis.

The flywheel-clutch mechanical press likely will remain the industry's workhorse for some time. Still, its servo-driven cousin probably won't stay a niche player forever. Toyota, for instance, has switched several lines over to high-tonnage Komatsu servo presses, producing panels for the Tundra® in San Antonio, Texas, and the RAV4® in Woodstock, Ont. According to Executive Vice President Jim Landowski of Wood Dale, Ill.-based Komatsu, Toyota plans to adopt more servo-driven mechanical presses during the next several years, with the intent to make its pressrooms more flexible.

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Flexibility sums up where the servo-driven mechanical press stands in its evolution. Early adopters are seeing that flexibility and asking, "What if?" What if I could control ram motion throughout the stroke and dwell for a certain period at bottom dead center (BDC)? According to sources, those "what ifs" have led to new ways of thinking about forming metal.

"In a servo press, you always know, within a few microns, what the slide position is," said Dennis Boerger, product manager for Dayton, Ohio-based AIDA-America Corp. "That opens up a lot of possibilities."

Capabilities

As Boerger explained, the ram motion of a press can be boiled down to a physics equation: "Energy comes from the mass times velocity, or mass times rotating speed." The faster that source&#;be it a flywheel or servomotor&#;spins, the more energy it has. But a flywheel-driven press has inherent inefficiencies. Energy must be delivered from the flywheel through a clutch, down the connecting rods, which drive the ram that provides the maximum tonnage at some point above BDC. The main drive motor then has to get the flywheel back up to speed before the punch hits the material again. For this reason mechanical presses can't run too slowly because the minimized rotating speed of the flywheel won't be able to provide enough energy to produce the needed force to cut through and form metal.

"But if I replace the flywheel and clutch with a servomotor, I can deliver maximum torque at any speed," Boerger said, from next to zero to the maximum rating.

With a servomotor, "you can match the velocity and dwell and stroke [length] based on the application," Landowski said. Consider a part that requires forming through a 3-inch stroke, and say the slide on the press stroke is 7 in. "You can set the stroke length so you travel only 3 inches, allowing for a certain height to clear a flange after it's formed up So, you can come down at a fast velocity, then slow down that last quarter inch to make the form, then speed back up to a 4-inch dimension height in order to clear the flange," maintaining fast cycle times.

"As you shorten the stroke length, you can significantly increase speeds," Boerger added. A hydraulic press also can use shorter stroke lengths, but the nature of hydraulic power gives those presses some speed limitations, he said.

Also, because the servo press's slide can slow and the ram can dwell at or just above BDC, more in-die operations such as tapping can occur inside the press. The ram's die, dwelling at the bottom, actually holds the part stable, like a fixture, securing the part as the in-die operation takes place.

A servo press can perform progressive forming under one die. Landowski referred to a titanium eyeglass frame application. Titanium springback can be a bear to deal with, so the application traditionally has called for a progressive-die setup, with each hit forming it 1 in., 1.25 in., 1.5 in., and so on, perhaps through five or 10 steps. The servo press can be programmed to perform all these steps in one stroke, with the ram stopping above BDC and then slowly progressing down to form the part, moving back up, then going back down a bit farther, and so on, until the part is formed.

Because the ram speed can be controlled precisely, the amount of shear can be controlled as well. During prototype work or testing, stampers can see exactly when a fracture will start to occur in the metal, then design the process to suit. Stampers also can combine this with sensors, such as linear glass scales and other closed-loop setups, to monitor off-center loading and account for thickness variation and hard spots caused by variances of carbon in the sheet, depending on the press model and application, sources said.

A shop buys a standard mechanical press with specifications designed for what it needs to do. Not so with a servo press, said Boerger. "The servo drive doesn't care. It can give stroke lengths from 1 to 12 inches long or more. It can give full tonnage at 1 stroke per minute to maximum speed, and you can program the stroke length and the profile." Blanking work can be done one day, deep draws the next.

He added that because the ram represents a controllable axis, the speed may be controlled and reverse-tonnage effects minimized after material fracture. This means blanking operations can use a greater percentage of a press's overall tonnage rating. Using a 250-ton press, traditionally stampers would have blanked at a 125-ton maximum (half the tonnage rating). With the servomotor, a 250-ton-rated press could blank up to, say, 220 tons, depending on the application, Boerger said.

Note, however, that servo presses still cannot deliver full tonnage throughout the stroke, as hydraulic presses can. "The servo press has a tonnage rating curve like a [flywheel-clutch-driven] mechanical press," Boerger said. "A 150-ton standard mechanical press might be rated 6.5 mm above the bottom of the stroke; the higher up the stroke, the less tonnage there is available." The same rules apply for the servo press. The difference? A servo press can stop anywhere in the stroke, then descend to BDC and provide maximum tonnage. How? A servomotor, unlike a flywheel, can provide maximum torque almost immediately.

Servos: Custom or Off-the-Shelf?

During the late s came a fork in servo press development. Some press suppliers decided to develop their own servomotors offering much more torque than anything commercially available. Others used off-the-shelf servos together with leverage components that, thanks to Newtonian laws, multiply torque and, hence, tonnages&#;up to 5,000 tons to date.

The direct-drive approach has for the most part been limited to lower tonnages, but tonnage ratings have been steadily gaining ground. Direct-drive presses are approaching capacities of 2,000 tons as a result of recent developments, including the higher torque capacities provided by 400-volt motors (double that of previous generations) and the ability to use multiple servomotors to directly drive a single ram, Boerger said. "Instead of having one high-torque servomotor, you can have a gear on the driveshaft and build a housing that holds [multiple] servomotors, all with pinions that drive off the same gear," he explained.

AIDA took the direct-drive route. The company's first motors developed in-house "had five times as much torque as the largest commercially available motor did," Boerger explained. "At the time, [one manufacturer] had one that had about 3,000 foot-pounds of torque. The one we came up with had about 15,000 foot-pounds."

Amada also took the direct-drive approach, using a high-torque, low-RPM motor specifically designed for the company's press. According to David Stone, product manager, the direct drive maximizes the energy; the ram has more energy available along a greater portion of the stroke. "The direct-drive [servo] press can provide more strokes per minute and high energy for the ability to apply force high up off the bottom of the stroke," which, he said, is advantageous for deep draws and similar work.

So what makes these servos different from their off-the-shelf counterparts? As Stone explained, "The fundamental difference is the number of poles in the motor. A standard servomotor may have six to eight [magnetic] poles" that drive the motor rotation, while the motors used in Amada direct-drive servo presses have 24 poles. The more poles, the more torque a motor has at low speeds. This enables the press to develop full torque and energy at fewer strokes per minute.

Even so, due to the physics involved when using a crankshaft to drive the slide, full tonnage isn't available through the full stroke, as it is with a hydraulic press, although the high energy still allows many applications to be run (blanking or forming) at very slow speeds, at 1 SPM or less&#;something impossible with a flywheel-driven press. Nevertheless, due to stroke-length limitations of a mechanical press, a hydraulic press still may be the best option for extremely deep draws.

For its servo presses, Komatsu took the torque-multiplier route. "We use a standard, off-the-shelf motor and torque multiplier" consisting of a shaft and knuckle arrangement, Landowski said. The latest servo presses using this technology go up to 5,000 tons, he said. It's about leverage; the greater the lever effect (produced in this case through knuckles and rods), the more torque is produced. Also, servos aren't designed to take the harsh ram forces directly, so they're set apart, coupled to the ram assembly with timing belts or other coupling methods based on the press's capacity.

Taking the Tough Jobs

"I haven't had one scenario where the servo press hasn't done it better," said Tom Ward, vice president of Ward Manufacturing Co., Evanston, Ill. "Our standard way of looking at a job became very rigid. I had to run a certain job that couldn't exceed a certain tonnage. We always asked, 'How do I build the tool to withstand the shock of running a certain speed so I can make money?' With the servo press, we threw all that out the window."

In Ward replaced some 30-year-old equipment with four 250-ton AIDA gap-frame servo presses. The impetus for the purchase came from an upcoming job, but the job itself didn't necessarily require a servo press. Ward said company management looked beyond that one job. "We could have saved money and bought a standard mechanical press, but we asked ourselves, where does that leave us? Does that give us any technical advantage?"

The company saw stamping work going overseas, so to keep profitable, Ward said the company had to focus on precision, low-volume, difficult, "China-unfriendly" work. For instance, Ward took on a job that involved aluminum and a perforated sheet layered on top, designed to provide heat shielding. The inner perforated material had limited formability, tearing easily under the forceful ram of a standard mechanical press. For this application, the servo press could move down quickly, stop just before the material, then form the material extremely slowly, balancing loads and ensuring smooth material flow. "This could all be done in one press stroke," he said.

Ward added that the technology has allowed him to automate material handling. "If I dwell at any point in the stroke, I can come in with a mechanical part extractor and remove the part during that programmed dwell. And to ensure quality, I can tell the ram, 'Give me two seconds while you're at the top so I can confirm, via sensors, that the part has come out of the tool with the extractor.'

"Some jobs that would have taken me four weeks now take me four days," Ward added.

The presses also have freed up enough capacity so that the company could perhaps get rid of some of its 30-year-old behemoths, opening up much-needed floor space. "In that space we could integrate new state-of-the-art equipment, further adding to our flexibility."

Designing for New Technology

For the past year PTL Manufacturing, Belleville, Ill., has used a Komatsu gap-frame servo press to perform deep-draw and forming work. "It can slow down when in the drawing area," said Daniel Stock, vice president of engineering. "When you're at bottom dead center, that's when you're doing all of your work. And for some material, we need to have that ram go at a certain [slower] speed to prevent cracking" in the forming portion of the cycle, while speeding through the rest of the stroke. The press has allowed the company to increase speed for one job from 40 SPM to 75, stamping materials like spring steel as well as higher-carbon steels like .

But integration at PTL hasn't been plug-and-play. The servo press is a different animal, and with that comes a learning curve. According to Stock, PTL Manufacturing has had to relearn the stamping process. Yes, the new press allows the company to be creative, to control the ram and material flow. But with that control comes a whole new way to operate a press. "We had dies designed with the old technology in mind. If you put a die designed in the traditional way in the servo press, you sometimes may not be harnessing all the [servo press's] capabilities," Stock said.

Specifically, with a traditional die design, all of the piercing and forming operations happen at BDC. "With the servo, you have more flexibility, with more tonnage available at different locations. It allows you to start engaging your stripper at a different time," along with the piercing pilot and forming operations.

This brings up a tradeoff, Stock said. If a die design takes advantage of the added capabilities of a servo press, the die may not run on standard mechanical presses.

Servo's Status

Servo-driven mechanical presses won't replace their standard counterparts any time soon, sources said. The flywheel-driven mechanical press still can do high-volume, relatively straightforward work faster and cheaper than the servo press. And the hydraulic press still remains the only technology with the maximum tonnage available throughout its stroke, ideal for extremely deep draws. (But sources said the stroke's positioning accuracy does not match that of the servo press, which can move a ram to a certain point within a few microns.)

Boerger said he sees the servo press undergoing the same "adoption curve" as other servo technologies used on the stamping house floor, such as servo-driven coil feeds and servo transfer mechanisms on a transfer line.

"I believe the servo press is going to change the landscape of stamping," Stone added, "but it's not going to happen overnight." If a traditional mechanical press is working well for a company now, it most likely will work in the foreseeable future. Areas where Stone sees the greatest impact include complex forming and exotic-material applications in which parts can't be formed any other way.

As Ward put it, the servo press will push shops toward high-value, creative work&#;in other words, China-unfriendly. "For us, that's a very good place to be."

The Good Neighbor

In Japan many stamping shops hit close to home&#;literally.

Amada's David Stone related a story of one shop in Japan that installed a servo press just to reduce the sound of those hits. "In Japan many shop owners live right next to their shop, and neighbors can be closely packed together." They don't appreciate the noise.

In stamping the noise comes not only when the die collides with the metal at high speed, but also from the breakthrough. This particular shop had to run some jobs in 410 stainless more than 1/2 inch thick. "With the breakthough noise measured at 115 decibels, nobody even wanted to be in the building when those jobs were running," Stone said. "And the neighborhood didn't appreciate it, either."

So the shop invested in a direct-drive servo press that could make those snap-throughs less noisy. Now the ram descends in a pulsing motion to a point just before the breakthrough, and then slowly pushes the punch through the remaining web of material. This resulted in a reduction of noise down to 74 dB. Since each 3-dB increase equates to a doubling of sound energy, a 115-dB noise has almost 14,000 times the sound energy of a 74-dB noise. Under the new conditions, the operators were not even required to wear hearing protection.

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