Don't Shut Off the Air
July 1, 2007
Although cordless tools and DC electric tools have become increasingly popular on assembly lines today, there’s still a place for pneumatic screwdrivers and nutrunners.
Given the proliferation of cordless tools and DC electric tools on today’s assembly lines, one might conclude that air tools have, well, run out of gas. In truth, however, pneumatic screwdrivers and nutrunners are still the best tools for many fastening applications.
“There’s always going to be a place for air tools,” insists Scott White, global product manager for assembly tools at Ingersoll Rand Productivity Solutions (Annandale, NJ). “They are tried and true, and they have benefits that set them apart from electric tools in certain applications.”
For starters, air tools are lightweight and ergonomic. They have high power-to-weight ratios. They run cooler than DC electric tools. And, of course, they pose no risk of electrical shock.
Air tools are ideal for mass production, says Lori Logan, marketing manager for Deprag Inc. (Lewisville, TX). “Even with batteries getting smaller, cordless screwdrivers are big, heavy and clumsy,” she says. “Cordless screwdrivers are fine if you’re just running 50 screws a day, but if you’re running 5,000, weight makes a big difference.”
Air tools are also smaller than comparable electric tools. “There aren’t many electric tools that can accommodate the very small fasteners in cell phones and laptops,” says Logan. “You don’t want to run a #2 screw with a driver that weighs 5 pounds.”
Air tools are fast and powerful. Many cordless tools simply cannot generate as much torque as a comparably sized air tool. “Few cordless tools can give you 40 ft-lb reliably, even with lithium-ion batteries,” says Alan Hirschmugl, training manager for AIMCO (Portland, OR).
Air tools are durable and require little maintenance. When they do need to be fixed, the repairs are simpler and less costly than repairs to electric tools. “An air motor can’t burn out when stalled,” notes William Staiger, marketing manager for Bosch Production Tools (Mount Prospect, IL). “Air tools can take a lot of abuse without the need for protection circuits, load monitoring or overload control devices. Any electric or cordless tool, no matter how well-designed, runs a slightly higher risk of being damaged from overload.”
Not insignificantly, air tools are much less expensive than DC electric tools. “It’s not always practical to invest in a DC electric tool to do the work of an air tool if you simply do not need the performance or capability of a smart tool,” says Staiger. “Sometimes, a good, durable, dumb shut-off tool is just right for the job.”
Logan agrees. “Through the controller, you can program an electric tool to drive a fastener to 5 in-lb, reverse for two revolutions, and retighten to 15 in-lb,” says Logan. “But, do you really need that level of control just to put a car radio together? I don’t think so.”
Besides a lack of programmability, air tools have other disadvantages compared with DC electric tools. For one, compressed air is an expensive power source. Most pneumatic tools require clean, dry, lubricated air at a dynamic pressure of 90 psi. Another disadvantage is torque range. DC electric tools typically have a much wider torque range than comparable pneumatic tools.
“Each power source-electric, battery and pneumatic-has its advantages, and those advantages will always exist,” says Hirschmugl. “It’s up to the manufacturers and their suppliers to determine which power source is best in a given assembly situation. Manufacturers often have [all three types of tools] running on their assembly lines, many times right next to one another.”
How Air Tools WorkAll power tools-pneumatic, electric and cordless-can be classified into two groups: continuous drive and discontinuous drive. A continuous drive tool applies torque to the fastener in a constant, uninterrupted process until it is fully tightened, says Hirschmugl. Stall tools and shut-off tools are continuous drive tools. A discontinuous drive tool applies torque to the fastener in a series of separate “torque events.” Impact tools and pulse tools are discontinuous drive tools.
Regardless of drive style, pneumatic tools are powered by compressed air running through an air motor. “Think of the air motor as a windmill,” says White. “Compressed air blows against the windmill and turns a driveshaft, and that rotary motion is eventually transferred to the fastener.”
On a continuous drive tool, the driveshaft is connected to a gear train that turns some of the speed of the air motor into torque. The gear train, in turn, is connected to the clutch, which either disengages the gear train or shuts off the tool when the desired torque has been reached.
A common clutch design works on spring pressure. It consists of two round steel plates. One plate has a machined profile, in which sit steel balls. The other plate sits on top of the balls, holding them in place under pressure from a spring. As the driveshaft rotates, the plates, moving together, rotate with it. As the fastener tightens and the motor encounters resistance to the force it’s applying, the plates slip past each other. The balls ride up on the profiles, pushing up on the spring. That, in turn, triggers a mechanism that shuts off air to the tool.
By adjusting the tension on the spring, engineers can change the torque output of the tool. “The more pressure you apply to the plates, the more torque you can overcome,” says White.
Discontinuous drive tools apply energy from the air motor indirectly. With an impact wrench, the air motor spins a cage containing one or two hammers. As the cage rotates, the hammers swing to strike an anvil at the end of the output shaft. Each time the hammers strike the anvil, the shaft transfers torque to the fastener.
With a pulse tool, the air motor turns a hydraulic mechanism that drives the output shaft with bursts of power, says Hans Mandahl, marketing manager for assembly tools at Atlas Copco Tools and Assembly Systems LLC (Auburn Hills, MI). When the fastener is running free, the tool isn’t pulsing. Once the fastener is snug, hydraulic pressure builds within the pulse mechanism. When the pressure reaches a certain level, the mechanism pulses out power to the output shaft.
The main advantage of pulse tools is that they generate little or no torque reaction, even at high torque levels. “You couldn’t apply 200 newton-meters with a DC electric nutrunner and still hold it in your hand. It would have to be fixtured,” says White. “With a pulse tool, you could.”
Like DC electric tools, air tools can be fixtured together to tighten multiple fasteners simultaneously. Such an arrangement boosts productivity and saves the cost of setting up multiple air lines.
Quality AssuranceWith all the attention on DC electric tools lately, engineers may not realize just how accurate some air tools can be. For example, Deprag’s Micromat pneumatic screwdriver is accurate to within ±3 percent of the set torque value.
In addition, a number of accessories are available to get extra capability out of air tools. For example, Ingersoll Rand offers an electronic controller that can be attached to the air line. By monitoring the air flow to the tool, the system can determine how many fasteners have been tightened and whether they have been tightened correctly. The device also contains a pressure regulator to ensure that the air going to the tool is at a constant pressure.
Air tools can also be equipped with transducers that feed torque data to electronic monitors for error-proofing and statistical process control, says Mandahl. These monitors can also count the number of fasteners installed during a sequence or they can time the fastening sequence. Sequences that take more or less than a set time could indicate an assembly error.
Tools can also be equipped with sensors to measure how deep the screw goes into the part. “There are applications where it’s more important for the screw to be seated than it is for the driver to hit a specific torque,” Logan explains. “If you’re assembling wooden furniture and you hit a knot in the wood, the driver resistance will go way up. The tool will think that it has reached the required torque and shut off, but the screw won’t be in all the way. The sensor will tell you when the screw is in all the way.”
Besides adding accessories, engineers can optimize the performance of air tools in less costly ways. First, engineers should avoid using air tools at the high end of their torque range. The desired torque should be in the middle of the tool’s torque rating. Preventive maintenance and regular calibration will ensure that pneumatic tools perform optimally.