All-electric and hydraulic-electric press brakes do more while consuming less
January 27, 2009
Electric and hybrid electric-hydraulic press brakes give fabricators an energy-saving bending alternative.
"Going electric" seems to be the latest step in the evolution of fabricating equipment. From turret punches to stamping presses, all seem to follow a natural progression from mechanically to hydraulically to, finally, electrically driven power. If electricity can drive a motor to provide sufficient force, the machine can be just as, if not more, accurate as its hydraulic and mechanical predecessors. With no hydraulic fluid or mechanical clutches, the machine requires less energy to run, and it produces less waste. In other words, it runs green.
And with hybrid hydraulic-electric and fully electric systems on the market, press brakes are following the same evolutionary path.
The first electric systems entered the market in the early 1990s. Over time two basic designs have dominated the market: one with a ball screw-driven ram, and another driven by a pulley system.
In the ball screw-driven design, an AC servomotor generates the energy. A Kevlar® timing belt goes around the motor's pinion gear and another, larger-diameter gear positioned dead-center over the screw. Depending on the size of the machine, either the screw rotates within a stationary nut attached to the ram, or the nut can spin around a stationary screw. The result is that the ram moves up and down with clockwise and counterclockwise motor rotation (see Figure 1).
Larger ball-screw machines increase available torque through gear reductions: If a drive gear connects to another gear three times smaller, the amount of torque the smaller gear can deliver is three times more. This transfers more tonnage from the screw to the ram and, ultimately, to the workpiece, without additional energy from the servomotor.
Another kind of all-electric system has an AC servomotor connected to a series of pulleys acting as work multipliers (see Figure 2). If the servomotor produces 100 units of force, an eight-pulley system can produce 800 units.
The motor-driven ram pulls against large springs, hidden under the sheet metal covers on the left- and right-hand side of the machine, as it descends toward bottom dead center. When the ram reaches the target position, the motor stops rotating, and the springs take over and bring the ram back up to its original position.
Electrics have three principal advantages: efficiency, accuracy, and ease of maintenance. They have accuracy from 0.0004 in. down to 1 micron (0.00004 in.) for some ball screw-driven models (see Figure 3). Also, like some higher-end hydraulic systems, some electrics feature material thickness detection on-the-fly. On electric systems, thickness detection occurs by measuring the torque feedback on the ball screw and motor.
The power put into electrics nearly equals the power put out by them. Also, electric systems resemble hydraulic press brakes in that full tonnage is available throughout the cycle. A 36-ton electric brake will produce, for all practical purposes, 36 tons throughout the stroke.
Hydraulic systems must manage hydraulic fluid whether or not the ram is moving, and this takes energy. Electric systems do not consume any significant amount of energy when the ram isn't moving (and in a job shop, the ram doesn't move for a significant part of the day). When idle, in fact, some electrics consume only about 1 amp, mainly for the control's LCD screen.
The ball screws themselves are, depending on the brake model, about 4 in. in diameter with about a 0.393-in. thread pitch and 0.393-in.-diameter ball bearings. The ball screws should be lubricated about once a year with heavy-duty grease. But otherwise, there's little maintenance.
The ball screw technology's disadvantage is tonnage. Most electric brakes are for low-tonnage applications, though higher tonnage is possible. Although ball screw-driven machines up to 300 tons are on the drawing board, for economic reasons the largest ball-screw systems being used in the field have about 88 tons. For higher tonnages, many have chosen a different route: the hybrid hydraulic-electric press brake.
Hybrid hydraulic-electric systems were developed in the late 1990s and began emerging in the marketplace earlier this decade. Although "hydraulic" is in their name, these machines bear little resemblance to fully hydraulic press brakes.
In conventional hydraulic systems, the ram fast-approach descent happens via free fall; the ram's weight sucks the oil to the top of the cylinder head. So the fast-approach speeds on hydraulic machines are set by what hose diameter feeds the intake of the hydraulics. Various tweaks of the hydraulics—larger valves, larger hoses, and so on—allow the ram to suck more oil while it falls and, hence, increase its fast-approach velocity. To switch to bending speed, the oil is diverted with a spool valve that closes up, allows less fluid to pass from the pump, and slows the ram. When the valve shuts off, the ram stops. How accurate a hydraulic system is depends on how quickly and accurately these valves can react when the ram descends.
Hybrid hydraulic-electric machines take a fundamentally different approach. The stroke initiates and stops with the rotation of the servomotor, and fluid flows only when the operator depresses the foot pedal.
Here's how it works. The AC servomotor's sealed shaft extends into the hydraulic tank under the oil bath. Attached to the shaft inside the oil tank is a common, fixed-angle piston pump, which looks like a Gatling gun (the cylinder rotates and shoots oil instead of bullets). This hydraulic pressure, in turn, drives the press brake ram (see Figure 4).
The ram positioning accuracy comes to within 5 microns, made possible from the direct control the servomotor has over the piston pump. The control's so close, in fact, that industry has come to call them "servo pumps." Any change in servomotor speed and direction is immediately followed by a change in velocity or direction in the servo pump.
An operator pushes the pedal, which actuates the AC servomotors that spin at a certain velocity, per the program in the controller, which can change the ram velocity at any point in the stroke. Ram position is monitored by linear scales on the left and right side of the machine, so the control always knows exactly where the ram is.
When the ram's punch gets to the programmed bending speed position above the workpiece, the linear scales send a signal to the control, which tells the motor to change RPMs and initiate bending. As the metal is bent to the desired angle, the servomotor's creep rotation guides the ram to hit, within a few microns, bottom dead center. The hydraulic pressure doesn't rise until the ram meets resistance when contacting metal, and only at this point is more current drawn into the servomotor.
During the bend, hybrid and all-electric systems draw the same amount of power as their fully hydraulic counterparts. Metal takes the same amount of force to deform, no matter what machine does the deforming. The difference is that electric and hybrid systems draw virtually no power when idle. Also, like electrics and hydraulics, hybrids offer thickness detection (using pressure sensors on the hydraulic lines), as well as crowning compensation.
And like all-electrics, hybrids don't require frequent maintenance. The fluid doesn't warm, because there are no gear pumps that send fluid in constant motion.
Still, hybrids do have a limited number of valves. For instance, when the ram reaches the bottom of the stroke, makes the bend, and reaches the target position, the motor stops rotating and a directional valve opens to allow oil back into the hydraulic tank. But it is still the servomotor, not the valves, that ultimately determines the ram position.
An all-electric press brake is, of course, the most environmentally friendly, with no hydraulic oil to worry about and less overall energy consumption. Hybrid brakes also save energy and use about a third to half the oil of conventional hydraulic brakes. The hybrid's oil also has to be replaced less often, and oil disposal isn't as easy as it used to be.
It's certainly true that hydraulic press brakes have made great strides in accuracy. And with years of reliable service behind them, they probably will remain the industry workhorse for years to come. But as green technologies like hybrid cars and wind towers have stepped to the footlights in recent years, so have the environmental benefits of electric and hybrid press brakes.