- SPECIAL REPORTS
When David J. Zabrosky, North American sales manager for Schmidt Technology, gets a call from a customer asking for a servo-driven assembly press, the first
question he asks is, “Why?”
In many cases, the customer may not need a servo press, says Zabrosky. A less expensive pneumatic press may be able to handle the application just fine. “The customer almost has to talk me into supplying a servo press,” he says. “Precision control is really the only reason you would need a servo press.”
It’s not as if Zabrosky has an ax to grind with servo presses. After all, Schmidt offers a full line of manual, pneumatic, hydropnematic and servo presses. He just wants to ensure his customer gets the right tool for the job.
A case in point occurred a few years ago. A motor manufacturer asked Schmidt to design a workcell to press a magnet onto the shaft of an electric motor. The magnet had to be pushed close to the top of a circuit board that was soldered onto the motor housing. The press would have to position the magnet to a tolerance of ±0.12 millimeter. In addition, the shaft had to be supported on the opposite side to prevent damaging a bearing within the motor. No force greater than 9 newtons could be applied to any area of the circuit board.
Pressing the magnet would require a force of 670 newtons, and the expected cycle time was 6 seconds. The manufacturer also wanted some assurance that the interference fit between the magnet and the shaft was within specifications. Given that the entire assembly had a tolerance stack up of ±1 millimeter, the customer figured a servo press was the only way to go.
Zabrosky thought otherwise. Schmidt engineers designed a system equipped with a direct-acting pneumatic press, a controller, a speed control and a process monitoring package. They also designed a custom tool for the ram that incorporated a spring-loaded probe and sensor.
During operation, the operator loads the magnet into the bottom of the nest. The motor assembly is then placed into the nest, above the magnet. The operator initiates the cycle, and the ram extends at full speed, pressing the magnet onto the shaft. At a predetermined height, approximately 2 millimeters above the final pressing height, the speed control engages to slow the ram speed to a more controlled rate.
During this controlled stroke, the spring-loaded probe contacts an unpopulated section of the circuit board and is pushed into the tooling until it triggers the sensor. This signals the ram to retract.
Not only was the cell simpler and more cost-effective than a servo press, but it also exceeded the customer’s requirements. The pneumatic press positioned the magnet to a tolerance of ±0.05 millimeter and achieved a cycle time of 4 seconds.
“Pneumatic presses may be an older technology, but they’re still capable,” Zabrosky points out.
At its simplest, a pneumatic press consists of a cylinder, valves and pistons connected to the ram. Compressed air enters the cylinder and pushes down the piston. The force produced by the ram is equal to the diameter of the piston multiplied by the pressure inside the cylinder.
Pneumatic presses come in a wide range of sizes and capacities. For example, with BalTec Corp.’s line of standard, direct-acting pneumatic presses, maximum pressing force extends from 340 to 7,640 pounds. Stroke length ranges from 40 to 80 millimeters; throat depth ranges from 63 to 130 millimeters; and working height ranges from 40 to 340 millimeters.
In addition, the company recently introduced the MicroPress for low-force applications. Equipped with a square ram, this pneumatic press has a stroke length of 5 to 60 millimeters, a throat depth of 63 millimeters, and a working height of 43 to 208 millimeters. Eight models are available, with maximum pressing force ranging from 30 to 750 newtons.
“The MicroPress is for applications requiring force measured in newtons rather than kilonewtons,” says Charles A. Rupprecht, vice president and general manager of BalTec Corp. “It’s designed for medical devices, small electronic parts and precision mechanical assemblies.”
While servo presses provide ultimate flexibility, pneumatic presses are no slouch.
“Force can easily be changed by increasing or decreasing air pressure with the use of a filter-regulator,” says Vince Moll, sales and marketing manager at Janesville Tool & Manufacturing Inc. “The stroke can be altered by use of an adjustable down stop, and the speed of the ram can be controlled by flow controls located on the valve.
“Our two most common requests are for longer stroke lengths and increased press height. Our presses are designed to easily accommodate those requests with minimal lead time.”
Although the force generated by a pneumatic press can be adjusted by changing the air pressure, Rupprecht cautions engineers against pushing the envelope.
“Presses are optimized to run at their rated pressure,” he explains. “That’s why we offer a range of models. It doesn’t make sense to run a 12-kilonewton press at 1 kilonewton.”
If force isn’t widely adjustable with pneumatic presses, some models at least offer adjustable work height. The BP-500 press from Fancort Industries Inc. has a solid steel column that lets engineers position the cylinder along an 11-inch range. (The press itself has a 3-inch stroke.) Five models are available, with outputs ranging from 5 to 1,175 pounds.
“The adjustable-column press has been very successful because of the low force range we offer and the versatility of being able to adjust cylinder height to accommodate tooling of varying sizes,” says Ron Corey, president of Fancort Industries. “Many small presses do not have this range.”
There are two types of pneumatic presses: direct-acting and toggle. A direct-acting press produces a constant force over the entire length of the stroke. With a direct-acting press, the relationship between air pressure and force is linear. If a press generates 1,000 pounds of force with an air pressure of 90 psi, it will produce 500 pounds of force with a pressure of 45 psi.
In a toggle press, the air cylinder is connected to the ram via a toggle mechanism. The lever action of the toggle multiplies the force applied by the piston. With a toggle press, the relationship between air pressure and force is exponential. The press produces little force at the start of the stroke, but delivers maximum force at the end.
“If you’re inserting a bushing into a metal stamping, a straight-acting press is the way to do it,” says Rupprecht. “If you’re doing a punching application, you want a toggle press. You want all that force at the end of the stroke, so you can be sure that you’re taking out that material.”
Another difference between direct-acting and toggle presses lies in air consumption. Because it has a mechanical advantage, a toggle press can produce more force with less air than a direct-acting press. The mechanical linkage also ensures that the final position of the ram will be more consistent.
Another variation on the pneumatic press is the hydropneumatic, or air-over-oil, press. As the name implies, the press uses both pneumatic and hydraulic power to drive the ram. Upon activation, a hydropneumatic press is initially driven only by compressed air. The ram descends rapidly with low force. When resistance is encountered, the hydraulic power stroke is activated.
“A hydropneumatic press generates its maximum force over a smaller distance,” explains Zabrosky. “A hydropneumatic press might have 4 inches of total stroke, but it may only provide 0.25 inch of power stroke. With a pneumatic press, if it has 4 inches of total stroke, it can produce its maximum force over that entire length.”
Because air is more compressible than oil, a hydropneumatic press can generate more force than a comparably sized pneumatic press, albeit over a shorter
“Hydropneumatic presses…draw on the positives of both pneumatic and hydraulic presses without all the negatives."
distance. For high-force applications requiring short power strokes, a hydropneumatic press may be more cost-effective than a pneumatic press. For high-force applications requiring longer power strokes—2 inches or more—assemblers may be better off with a fully hydraulic press.
“Pneumatic presses are less expensive than hydropneumatic presses,” adds Zabrosky. “They’re also easier to maintain.
“But, once you get into higher-force applications—anything over 5 tons—it just makes more sense to go to a hydropneumatic press at that point.”
“Hydropneumatic presses…draw on the positives of both pneumatic and hydraulic presses without all the negatives,” adds Michael T. Brieschke, sales coordinator at Aries Engineering Co. Inc. “Simple pneumatic presses have great speed, but unless you have hard stops in the tooling, they offer limited control.
“Hydropneumatic presses also offer better force vs. size. For example, a 4-ton HyperCyl hydropneumatic press has a 3.25-inch bore, but puts out 8,700 pounds of force at 100 psi. A similar sized pneumatic cylinder will put out only 842 pounds at 100 psi.”
The basic pneumatic press hasn’t changed much over the years. What has changed are the optional equipment and accessories.
Topping the list are pressure switches, regulators and feedback devices, such as load cells, that give engineers greater ability to monitor and control the pressing process. Engineers can press to a force or a distance. They can generate a force-displacement curve for a known-good assembly and then set limits for acceptable variation.
“We can bridge the gap between a ‘dumb’ air press and a full servo press,” says Zabrosky. “Now, servos come in only when you need very precise control.”
A speed control is another technology that can improve repeatability when pressing to a force or distance. This device brakes ram shortly before it reaches its end position. A speed control can also improve cycle times by enabling the ram to rapidly traverse the distance to the workpiece, but slowing it down when precision is required.
“A good servo press offers closed-loop control over force and distance,” explains Zabrosky. “If you program the system to press to a certain force, the ram will automatically decelerate as it approaches that force.
“You can’t do that with a pneumatic press. What you can do, however, is signal the valves to shift when the target force is reached. That will retract the ram. How quickly the press can react to that depends on how fast the ram is moving forward. A speed control slows down the ram, which gives the system time to react.”
Die sets are another option that can help assemblers get the most out of their press. Die sets can be tooled to reduce setup times or enable one press to perform many assembly operations, says Moll.
Recently, Janesville’s engineers helped a customer develop a custom press system to progressively shear spring steel. Using mostly standard components, engineers designed a four-post press frame with three stock pneumatic cylinders. Each cylinder is connected to its own die set, so each set controls its own operation. The sets are designed to be interchangeable, so they can be swapped in and out of the presses quickly.
“It saved our customer hours of setup time between jobs and made the press universal for other projects it had,” says Moll.
Safety accessories have also improved. These include light curtains, two-hand touch controls, and shuttles for moving work into and out of the press.
Timers are another useful option for pneumatic presses, says Corey. These devices keep the ram in the down position for a set period of time. This could be as little as 1 second or as much as several hours. This feature is useful to ensure completion of a process or to clamp parts bonded with adhesive.