Triple play

By Lynn Stanley

Finding the press that suits your application may not be as hard as you think

December 2016 - Servo presses have migrated from the fringe to mainstream production with their ability to program stroke, slide position, motion and speed. A growing appetite among automakers for metals with high strength-to-weight ratios is also helping to cement servo’s star power. But before metalformers become distracted by “shiny new things” syndrome, one machinery builder suggests hitting pause and returning to basics. 

“People are buying, or seriously considering buying, new presses these days, probably more than at any other time during my career and they are especially interested in the latest technology like servo,” says Stamtec National Sales Manager Lee Ellard. “But many manufacturers still do not really understand the fundamentals of press operation and which type of machine—servo, mechanical or hydraulic—would work best for their application.” 

Stamtec is the North American subsidiary of one of the largest press builders in the world, with more than 60,000 presses sold worldwide, and more than 2,000 installations in North America. 

The global metal stamping market should exceed $180 billion by 2022, according to a 2016 report by Grand View Research. The consultancy expects that the aerospace, automotive, electrical and electronic equipment markets will drive the growth.   

“With stamping operations the basic question has always been: How can I successfully make parts faster, but use a slower slide velocity for better quality parts, have fewer equipment and die problems yet meet my budget requirements?” asserts Ellard.

FFJ 1216 stamp image1

A press, feedline and transfer system combination is a versatile production system capable of high performance, efficient processing for myriad materials and parts.

Door No. 1, 2 or 3?

A number of variables and criteria influence press choice but there are some guidelines companies can use. A traditional mechanical press offers high production speeds, a hydraulic press is versatile, while a servo press is both flexible and able to reach higher production speeds.

Part type also dictates press selection. Flat parts with simple, shallow specifications processed from coil stock through a progressive or transfer die are most often run on a mechanical press. Although a hydraulic press doesn’t usually achieve the high cycling speeds of a mechanical press, it is more versatile, featuring a variable stroke length, die space and pressure. Deep, complex components such as tanks, cylinders and bowl shapes—which need a lot of material flow but don’t rely on production speed—often work best on a hydraulic press. A servo press combines much of the versatility of the hydraulic press with production speeds comparable to a mechanical press. Servo programmability means it can process a large variety of part types and dies at a wide range of production speeds. 

A better understanding of the principles of tonnage and energy will also help stampers narrow their choices. “Press tonnage and press working energy are not the same,” explains Ellard. “Mechanical press tonnage rating is the maximum load that can be achieved and exerted without damaging the press or its components.” Because a servo press is still a mechanical press, it is limited to the same tonnage capacity as a comparable conventional mechanical press. The tonnage rating point is determined at ram [slide] position just above bottom dead center (BDC). As the distance from BDC increases, available tonnage capacity decreases. 

While tonnage rating is the largest load a press can withstand, a machine’s working energy is defined by applied press loads and the distance through which loads are applied. In a mechanical press, the motor is connected to a crankshaft that cycles the ram. Its electrical motor system must generate horsepower sufficient to power the flywheel. During the non-working portion of the stroke, the motor restores energy to the flywheel.

“The flywheel stores and delivers working energy while the electrical motor restores energy by maintaining flywheel speed and averting a slowdown,” explains Ellard. “If the flywheel slows down too much, a stamper can experience motor damage, or worse, the press will be unable to complete the job, an event that can result in the dreaded position of ‘stuck on bottom.”’ The servo press has full working energy at any press speed because all the energy is supplied directly from the servo motors.

A hydraulic press runs on pressurized hydraulic fluid, with full tonnage and working energy available in the full range of the stroke—making hydraulic presses well-suited to slower, super deep draw operations.  Deep drawing operations have a large appetite for energy due to their “long working distances, sometimes beginning several inches from BDC,” notes Ellard.

“Mechanical press builders have worked to build presses to fit specific applications, with high productivity the end goal, but the limitations of the working energy system of a mechanical press make the job of properly sizing it, at best, difficult and imprecise,” says Ellard. 

FFJ 1216 stamp image2

This medium depth, multi-formed part requires a slow draw operation that can be performed by a traditional mechanical press, but a servo press can offer improved throughput and part quality.

The end game

Servo motors replace the standard motor, flywheel and clutch/brake found in a crankshaft or eccentric-geared mechanical press. “This simplifies the issue of working energy, and at the same time greatly expands the capabilities of the press to run complex stroke profiles,” explains Ellard.

Understanding working energy and how it is achieved and configured is especially important when stampers weigh the differences between a mechanical press and a servo press. “You also want to look at how working energy between the two presses will impact electrical energy consumption and infrastructure investment,” Ellard notes.

“At Stamtec we can build a servo system that can achieve the highest level of productivity possible because we base it on the time-optimized configuration of a motion profile for the press, using parameters like maximum motor speed; forming operating area; maximum slide speed and optimal forming speed of the slide,” he continues.

Optimized productivity, though, comes at the expense of very high maximum torque and peak current demands on the servo system. Stamtec can minimize these costs, however, “by providing the right drive with a properly fitted gearbox for the mechanics and by partially or fully managing energy to prevent peaks from pulling directly from the power grid,” he says. “We do this a couple of ways. [First] we can semi-manage energy supply by using some capacitors or kinetic buffering motors that reduce the amount of energy drawn directly from the grid. [Second] we provide full management by adding capacitors or kinetic buffering motors to draw all working energy from the storage system.”

Two types of servo press drive technologies are available. A link drive is a lower initial-cost solution because it uses smaller, “off-the-shelf” A.C. servo motors, while applying the mechanical advantage provided by the link mechanism. A direct drive system uses larger, more expensive servo motors. One disadvantage of the link drive is, “It doesn’t support a pendulum motion,” that is, shorter stroke length at higher speeds, Ellard says. 

Direct-drive systems provide a higher rating point further off the bottom of the stroke, and are capable of pendulum motion. But the initial cost is higher and working energy capacity is de-rated when running higher speeds in pendulum motion due to heat generation in the drives.

In addition to educating customers, Stamtec employs an “open architecture” approach. “We offer an all-Siemens platform which uses off-the-shelf components,” says Ellard. “It means our customers are not captive to us for support and service of the Siemens product. Nor do they have to rely on us for proprietary servo motors, controls and other components. Our control platform can effectively run myriad servo press jobs. In addition to the pre-programmed, adjustable stroke profiles we can customize profiles or help manufacturers create their own.

“We’re not a cookie cutter organization,” Ellard says of Stamtec. “We want to put technology to work for our customers by giving them the tools they need to get the job done.” FFJ

Photos: Genesee A & B



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