Jan 22

Servo Hydraulic versus Electronically Balanced

What’s the story? Hydraulic Press Brakes such as Pearson, Hydrabend and Kleen Press Brakes date back to the early ’70s in Australia. They quickly started replacing the old mechanical bangers such as the Cincinnati’s and Chalmers and Corners. As good as the old mechanical Press Brakes were, the hydraulic machines presented many advantages (particularly safety). The days of flywheel driven brakes were coming to an end. Hydraulics was an inexpensive way to generate the force required to bend even the heaviest plate.

The operation of a hydraulic press brake is reasonably straightforward. The press has a hydraulic cylinder on the C-frames at each end of the machine that moves the rams. Over the years, the method of synchronising the two cylinders has evolved. Hydrabend used a complex set of mechanical levers (toggle action), and others such as Pearson used chain-driven hydraulic balancing valves others used massive mechanical torsion bars. The current technology is electronic linear scales at each end of the machine monitoring beam position that feedback to the computer that in turn and controls electronically controlled hydraulic balancing valves. Whatever the method, this balancing left, and right cylinder to the required accuracy of +/-0.1mm has always been the challenge.

Whatever the method, the flow control feeds the hydraulic oil proportionally into the cylinders to move pistons, which in turn controls the movement of the ram.

In the old days, typically a limit switch was used for controlling the bottom dead centre. On the Hydrabends and Promecams, they even had innovative turret stops that automatically rotated to engage the limit switch, and this provided a sequence of six or eight bend angles. Quite revolutionary in the 80s. Now CNCs are used and along with highly precise hydraulic packages Press Brakes have emerged to offer remarkable user control of speed, high precision bend angles and versatility in the length of stroke.

Modern CNC Press Brakes have also evolved in reliability by eliminating or minimising much of the plumbing. Today’s hydraulic press brakes cannot be confused with those of twenty or even ten years ago.

So what’s all the fuss with Servo Hydraulic Press Brakes or sometimes as they are called Hybrid Press Brakes?

Hybrid Brakes are becoming more prevalent and more widely available. With a servo-hydraulic drive, when you turn the pump on each cylinder in one direction, you are pumping the ram down, when the computer sets the pump in the other direction, you are pumping the ram back up. By controlling the speed of the servo, you can also control the ram speed. In effect, this methodology almost simulates a screw and nut.

Benefits of this technology include substantial power saving as you don’t have a conventional AC motor running all or most the time. The servos only activate as required. You also get incredibly fast response time and minimal piping as it does away with a central tank and proportional valves. You can expect higher speeds for both high-speed approach and high-speed return, around 200mm/sec.

Unfortunately, there is no such thing as a free lunch, and hybrid technology is a little more than your electronically balanced proportional valve hydraulic systems.

So what benefit can you expect for this improved cycle time with such fast approach and return? Let’s say you have a 5 to 10-year-old Press Brake and you are knocking out around 500 -600 bends per hour if it is running at top speeds. A Servo Press brake can make up to 900 bends per hour. That’s a genuine improvement. But, it’s important to note that the machine is only bending a certain amount of time, of course, this depends on many factors, mainly your product, but it might only be 20 percent of the time during a shift.

Consider a batch size of 25 with two bends per part. You might have 300 seconds for setup, five seconds to pick up and place the part against the stop, five seconds to bend, five seconds to reposition to a second bend, five seconds to bend and then five seconds to stack the part. Total production time might be around 16 minutes, but the bending time is only four minutes. So 25 percent of the whole time.

So when is high speed an important advantage?

It’s usually with easily handled parts or parts and preferably with slightly longer runs, so the total time production time isn’t elongated by set up time. Conversely, as the parts get physically bigger and perhaps more time-consuming in their material handling, then the benefit of speed might be less.

In saying that, every shop is different, and owners all have their philosophy, methods and certainly different machine operators. But it is an interesting exercise to consider. Even the rising cost of power in Australia could sway the decision one way or the other.