Assembly lines go cordless

Towson, MD--Historically, there has been limited acceptance of cordless tools in the world's large assembly plants. Instead, appliance manufacturers, the auto industry, and others have relied almost exclusively on pneumatic assembly tools--primarily for their durability and accurate torque control.

Air tools, however, present problems: Hoses restrict mobility; compressors, oilers, and regulators require constant maintenance; vibration and noise lower productivity. That's why worldwide assembly manufacturers such as Ford and General Motors--in the U.S. as well as Europe--are taking a close look at, and using, a radically new cordless design.

Developed from a blank sheet of paper, the Black & Decker 2920 heavy-duty screwdriver offers the benefits, but not the drawbacks, of traditional pneumatic tools. Specifically, it is the first cordless assembly tool to offer precise torque control. "We had three objectives in designing the tool," explains Project Engineer Craig Schell. "Torque repeatability; no torque drift over time; and insensitivity to joint rate." A rolling-element clutch, he says, proved the key to fulfilling these requirements. Standard clutches call for an input and output cam disc with three interfaces for stability; spring force keeps the discs engaged. Each 120 degrees of rotation brings the clutch from the point of decoupling to the next recoupling point.

Air-tool drawbacks. While the above design works well for an air tool, it can lead to ratcheting, and therefore poor torque accuracy and shorter life for tools driven by electric motors. Simply shutting off the air supply will stop an air motor within 120 degrees of output spindle rotation. An electric motor, however, requires a longer period for speed decay, allowing additional time for secondary torque spikes.

"We're talking about a tool that runs 700,000 to a million cycles per year," Schell points out. "If a tool ratchets seven times before stopping, that's seven million cycles."

A rolling-element clutch extends the time allowed to brake the motor. That's because rolling elements move at onehalf the rotational velocity of the input cam disc. Here's how the clutch works:

Each cam disc features three female pockets. When the clutch is engaged, the rolling elements sit in the pockets of both discs. When the clutch disengages, the balls roll up and out of the pockets, forcing the two discs apart. At 240 degrees of rotation, the system recouples.

Since the spindle has enough time to slow down before recoupling, there are no secondary torque spikes higher than the first one. As a result, Schell claims, "we have a single decoupling clutch, rather than a ratcheting clutch." Less torque drift, less wear. Schell also points to another advantage: The rolling elements reduce wear for less torque drift over time and remain insensitive to joint rate. "The beauty of the rolling-element design," he says, "is that one mechanism satisfies all clutch criteria."

The new tool's ability to hold torque, coupled with the advantage of cordless operation, has had an enthusiastic reception by men and women who work on assembly lines.

"The cordless tool performs very well," reports Abba Vice, a trim operator who has worked with GM for 22 years. "It always fastens the screws smoothly on the first try."

Adds Steve Bemiller, a worker on the transportation assembly line at Harley Davidson, "The 2920 is a real timesaver. Because it doesn't vibrate in my hand like an air tool, I can get more pieces finished per hour with less fatigue."

Prototype, prove, improve

Computer modeling and rapid prototyping, says B&D's Group Product Manager Allen Brelsford, proved critical to quick development of the precision torque screwdriver. Starting with the tool's clutch, the company relied on ADAMs software (Advanced Dynamic Analysis of Mechanisms, from Mechanical Dynamics) to program in inertias, set up spring rates, and specify cam profiles.

Proprietary in-house software followed, generating 3-D animation to drive a simulated fastener and evaluate dynamic response of the clutch itself. The program also looks at the transfer of forces from one component to the next, to aid in optimizing design with respect to stress.

Moving back to the gearing, B&D uses TK Solver from Universal Technical Systems, as well as in-house gear simulation packages. "By simulating the gearing," Brelsford notes, "we can evaluate losses within the system, separation forces, and even net shapes for tooth profiles."

As for the motor, "we focus on the efficient use of steel." This, claims Brelsford, lets B&D determine flux density to model the magnetic pathway inside the laminations. Working closely with Johnson Motors, Black & Decker also models commutation losses, EMI, and braking characteristics.

Allegro software from Cadence helps B&D model the assembly tool's electronics package. Working through the system, B&D engineers lay out the circuit diagram with all components in place, input the motor model, and verify response of the circuit itself. Another Cadence package designs the actual pc board and performs thermal modeling.

DYNA 3D--a program originally developed for automotive crash test simulation by Livermore Software Technology Corp.--allows B&D to look at the dynamic inter-relationships of all the subassemblies and how they affect the housing.

"With our old FEA package," explains Ted Mitrou, director of product reliability, "we simply assigned the tool's housing a lump center of gravity for drop simulation. The ability to look at particular bosses at the interface of separate components allows us to generate a more accurate strain plot. "

Once thoroughly modeled, Black & Decker prototypes the new assembly tool using a stereolithography-built housing. SLA, says Project Engineer Schell, lets the company assess manufacturability and place the screwdriver in the field for testing and feedback before final tooling.

"In retrospect," Schell recalls, "the only thing we might have done differently is prototype the molds rather than the housing. Selective laser sintering allows us to make a steel mold useable in production. We can pull out the binder, infiltrate with copper, and be ready to produce parts."

That may be so, but it is hard to imagine improving upon development time for the 2920 screwdriver: Twenty-six months from initial concept to commercial product.