There are choices in presses, which is a good thing. The debate is not which is best, but which one is right for the job at hand. There are tradeoffs in cost, function and quality between the main types of presses one could choose.
It’s a friendly debate, and the cards are on the table for all to see.
According to Stephanie Price, senior application engineer at Promess Inc, Brighton, Mich., many people in industry don’t fully appreciate the advantages of servo press technology. Conversely, Mike Josefiak, mechanical engineer at Greenerd Press & Machine, Nashua, N.H., makes a good case that a hydraulic press is the best solution for some applications. And Jim Landowski, vice president at Chicago-based Komatsu America Industries LLC, would tell you there are still situations in which a traditional mechanical press is fine.
A mechanical press converts the rotational motion of a flywheel into the linear motion of the ram pressing into the workpiece. As Landowski described it, you can “imagine the circle, with zero at the top and 180 at the bottom. A mechanical press goes from zero through 180 and back to zero, or 360, in one continuous motion.” The stroke has no force at the top and maximum force at the bottom, so “depending on the die, you might start pushing the material at 160 degrees or so. But when it gets to 180, the part is finished, because your slide is going back up.”
As Bob Southwell, executive vice president of AIDA-America, Corp. Dayton, Ohio, explained, most servo presses are versions of the same arrangement, “except that you’re powering a mechanical drive train with a servo motor, versus a fly wheel with a clutch brake mechanism.” A mechanical press has a fixed stroke and a constant speed. But “add a servo motor to it, and now you’re able to program the motion profile. You can slow down, pause, perform rapid restriking, and do various things that were never possible with a standard mechanical press.” There is also a direct drive version (servo motor to ball screw), with better torque characteristics than the servo-mechanical hybrid.
A hydraulic press combines a set of pumps, valves, and hoses to engage the ram with pressurized fluid. While this approach has advantages, they don’t include the kind of motion control covered above. So a servo press offers additional capabilities and solves a number of problems that occur with pure mechanical or hydraulic presses.
Landowski observed that the move toward advanced alloys brought on by automotive light-weighting and other factors is driving demand for servo presses. As he put it, “think of steel as a liquid, it has to flow…You don’t work the material, you work with the material.”
Harder materials require fine adjustments to the ram velocity in order “to let the material flow correctly, or else it becomes like taffy and starts to come apart.” For example, he said, forming a cup in a hard alloy might require slowing from 30 to 15 IPM over the course of a 3" (76.2-mm) stroke, at a precise—and perhaps varying—rate of change.
Only a servo control could manage this, given that the adjustments occur in milliseconds.
The main advantage of a servo, Landowski stated, is the flexibility to work with different metals by dialing in the material flow. “That’s why we have people come in to try various options. I can make a good part or a bad part just by changing the velocity of the slide.”
Southwell concurred and reported that these material challenges have resulted in a servo press market share of roughly 80 percent in North American automotive manufacturing. “The high strength and ultra-high strength steels and aluminums are much more challenging to form than the materials of ten to fifteen years ago. And a servo’s ability to adjust the forming profile has proven extremely beneficial to the customer base.”
Josefiak from Greenerd agreed that servo control has an advantage in response time versus hydraulics, in which the response is dampened, but said he hadn’t “seen many applications where that level of control in the motion profile materially impacts whether or not you make a good product.” But he acknowledged that “restriking is a good example of a servo-only function. Going to the bottom and then re-striking within a fraction of a second is not something you can do with hydraulics.”
If you don’t need to control velocity, Landowski argued, you may not need a servo.
“If you’re making washers, for example, or small rivets or something like that, you’re not going to slow the press down, you’re not going to control velocity. You want to make as many parts as you can, as fast as you can.” That’s where a mechanical press shines, he said. It’s also where hydraulic presses are least appropriate.
Southwell added that the ability of a servo press to be easily reprogrammed for different parts is another factor in their success, even in the high-volume automotive world.
“Most press systems are designed to run multiple types of parts. They will run one tool for an hour, change it out and roll in another tool. Virtually no one sets up one press and just runs it…There’s no way they could stay competitive. We sell many systems to the OEMs through the Tier Ones and Tier Twos for large families of different parts or die sets through a single press.”
Servo press versatility goes well beyond easy programming and extends into delicate assembly operations, said Price from Promess.
Sticking with an automotive example, Price pointed to assembling a door hinge. She explained that a servo press offers both high precision and an inherent feedback loop capable of closely monitoring position and force. So, in pressing the hinge together, Promess is also able to gauge the resulting resistance in the joint, such that they can ensure that the door neither swings open too easily nor is too stiff to be uncomfortable for the car’s owner.
This ability to activate a moving part and measure forces in real time also yields the opportunity to loosen part tolerances, thereby lowering component costs. As Price explained, without feedback during assembly, engineers are often forced to design and manufacture to very tight tolerances in order to ensure that parts fit together properly.
“They use the fact that the press went to a certain depth, and based on their tight tolerances, assume that the part was assembled correctly. They have no signature analysis to verify that.”
With a servo press, they could instead loosen the tolerances and watch the data during the assembly process to determine that what they’ve pressed together is actually properly seated. Price said the built-in sensing capabilities of their servo presses has yielded scrap rate reductions of up to 50 percent in some cases.
Price also pointed out that if an application required additional sensing (beyond the feedback from the servo motor), it’s easy to integrate with their systems.
“We have customers using nine to ten different pressure transducers, or position transducers, or external load cells. We can take in all that information to understand what’s going on within the process,” he said. “And we can react to that during the process. And because everything is electrical, it’s very simple to set up. Just plug in a transducer into a digital signal conditioner. The controller can then take in that signal and use it to make a determination.”
Hydraulic presses are not blind in this area. Josefiak said there are motion controllers dedicated to hydraulic systems with “extremely fast scan times that look at the pressure on either side of a hydraulic actuator. And then using fast-acting pressure transducers, we can show the actual force being applied to the work.” One such system updates the force measurement in under a millisecond. In his opinion, applications requiring faster force measurement are “few and far between.”
According to Southwell, servo presses are much better than hydraulic presses in making complex parts that require a series of dies. Years ago this would have been done by transferring parts by hand from press to press, he explained. But now “the only way to compete” is to mechanically transfer parts from stage to stage within a single press. But “when you use multiple stations to make a part, you have off-center loading, which is very detrimental to a hydraulic drive train.”
Josefiak countered that “off-center loading is detrimental to both mechanical and hydraulic systems. Both handle these off-center loads with appropriate construction and guiding of the steel framework. We have systems using multiple hydraulic cylinders to allow off-center loading much larger than an off-the-shelf servo-mechanical press.”
There is also some controversy about applications requiring the use of food-grade oil as a lubricant. Landowski reported that “several customers have switched from hydraulic to servo mechanical presses solely due to cylinders weeping and the slide gibs dripping onto the material. All parts need to be cleaned after they are formed to remove any and all possible contamination. Customers have also told us that cleaning food-grade lubricants is less costly than non-food grade due to FDA or EPA regs.”
Josefiak said they have satisfied both medical and food safety standards on a number of projects “by modifying the sealing in their presses to use food-grade oil in place of standard industrial oils.” Whereas Landowski stated that their standard off-the shelf servo press don’t need any modifications, “just the food-grade oil for the press drive and the slide lube.” One customer “makes rubber stoppers for test tubes. Every stroke of the press delivers 65 to 75 rubber stoppers, and non-food-grade lubricants would invalidate this particular process.”
According to Southwell, “the advantage of a hydraulic press is that you have full tonnage or force capability through the entire stroke. So, if it’s a 200-ton press and you have a twelve inch stroke, you can apply 200 tons of pressure all the way through that stroke. With a servo press that has the same mechanical eccentric drive train as the original mechanical press, you have gearing, the crank shaft or a centric shaft, and a center gear drive. There is a tonnage or torque curve, and the force you can apply varies depending on the motor shaft’s angle off the bottom.” This is not the case for direct drive servo presses like those made by Promess, but these systems become very expensive as tonnage increases. Promess tops out at 1 MN (~100 tons) in a single cylinder, for example.
The ability to apply full force through the whole stroke make hydraulic presses perfect for deep-draw applications, and Josefiak went so far as to say it’s “the only option that really makes sense.”
One recent example he cited is a project to produce ”relatively large pressure tanks. We installed an automated system that loads large, flat blanks into a deep draw press that has a working stroke of five feet.” The system has multiple operations, he explained. The first uses a 170 Ton press to draw two halves of the tank. This is followed by an automated punch press and trimming and welding downstream. The key here, said Josefiak, is that such a working stroke “isn’t something that’s easily replicated with a servo press. So deep draw is an area where hydraulics still dominate. And that’s across quite a few industries. It’s more the process than the industry.”
Josefiak said hydraulic also “does very well in situations with a very long cycle time, where we can manage very low power draw in a consistent pressure across the bed area and relatively inexpensively from a capital cost perspective.” Compression molding offers a major example. “Usually, compression molding is going to be a combination of time, temperature and pressure forming a material into a shape,” explained Josefiak. The press would hold a relatively thin material against a positive or negative die form under pressure. “The duration could be as short as five seconds, or as long as two hours. And very often…we’re trying to maintain a constant platten temperature across the working area of anywhere from around 300 to 700 degrees, and trying to control a very consistent pressure across the working area.” That ensures the material being formed is uniform throughout. The technique is used for things like automotive bed liners (including the new composite bed liners) and automotive headliners made with a carpet-like material. Another example he listed is “powder compaction for making aluminum oxide grinding wheels.”
Broadly speaking, the capital investment for a servo press exceeds that of either traditional mechanical or hydraulic presses. But there are operating costs and related factors to consider that make this comparison nearly useless. What’s more, not all presses of a given type are equal, even for the same tonnage/torque ratings.
Let’s start with energy consumption. A hydraulic press must maintain pressure in the lines to move the ram on demand, and that means running the pumps through the cycles. That compares unfavorably with a servo press, which uses electricity only when the ram is moving. According to Landowski, that yields a roughly “50 percent power savings with a servo press, depending on the size of the machine.” Price referred to a study by the University of Kassel, which found the servo press to be 90-percent efficient in energy conversion, versus 57 percent for the comparable hydraulic system. Southwell said Honda studied their own systems and published the finding that servo presses delivered a 30-percent savings in actual power consumption.
Southwell also indicated that some AIDA presses use a “100-percent capacitor-based energy management system.” This stores the required working energy in capacitors, which are recharged during the non-working part of the stroke. This “vastly reduces the peak load,” he explained, versus a mechanical or hydraulic press, which have a “huge spike when they first engage.” AIDA current draw is “fairly flat. So your actual peak flow might be only 20 to 30 percent of the peak load of mechanical or hydraulic systems. That’s critical, because power companies have to size the electricity they deliver to the customer by the peak load.”
Josefiak countered that in a high-production environment there’s little or no idle time, so “it really doesn’t matter too much” that the hydraulic pumps are running continuously. And “in systems where we have long idle times of 10 minutes or more, we can install a ‘soft start’ motor control that shuts down the motor to conserve energy.” Interestingly, although this option adds only 2 percent to 3 percent of the system cost, Josefiak reported that there has never been a strong demand for it. He added that switching from a fixed displacement pump to a variable displacement pump can also “dramatically lower our idle power consumption.” But that again is an option that has yet to become the norm in the U.S.
With all its pumps, valves, pipes and hoses, hydraulic technology is often dinged for being more complex and more maintenance-intensive than servo-based systems. Price said their servo presses require nothing more than twice-a-year greasing of the ball screws—and even that is being very cautious. Conversely, keep hydraulic lines under high pressure for months, through cycle after cycle, and sooner or later something is bound to leak or a sub-component is bound to fail. The counterargument, said Josefiak, is that “nobody is using NPT fittings anymore. There is a range of metal-to-metal and O-ring style seals, built with better materials, that have done a much better job controlling leakage.” Plus, he said, the individual components are relatively inexpensive and easy to repair, while “doing repairs to a servo system is dramatically more expensive.”
This last point brings us to the topic of correctly sizing the components for the job. It’s true that if you burn out a servo motor in a few years you’re in for a big repair bill. But Price said their systems routinely run for 20 years without any such failures, because they are designed with a safety factor of 2.5×. The drives are sized to run in the continuous current of the servo motor, instead of the peak, so the press can hold the part indefinitely without overheating and failing.
Likewise, the ballscrews will have a dynamic load capacity of 2.5× the force rating of the press. For example, the ballscrew in a Promess 40-kN press has a dynamic load capacity of 134 kN and a static load capacity of 320 kN. Price said such a system can be expected to perform without a failure for 22-plus years when running a job with an average force of 30 kN, with 16 cycles/min over 14 hours/day. Compare that to only 32 weeks for a ballscrew rated at 40 kN dynamic load; even at a rating of 80 kN, the system would last under five years.
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