May 2006 Mechanical Engineering Departments: Tech Focus, Pt. 2

This section was written by
Associate Editor Alan S. Brown.

Materials and Assembly

Worked Up About Work- benches

Workbenches usually merit no more than an afterthought in assembly line design. Yet when contract electronic manufacturer Cirtronics Corp. of Milford, N.H., decided to move to lean manufacturing, it spent up to eight months evaluating four different workbenches.

Why the fuss? Cirtronics is 20 percent owned by its employees. "We wanted our employees to have a comfortable place to work," said team leader Irene Lemay, who helped evaluate the ergonomic effectiveness of workbenches. "If they're comfortable, they'll have fewer health problems and higher productivity."

The workbench chosen by Cirtronics has motorized adjustment, foot pads, and shelves and bins for parts supply, along with a clear, well-lit surface.

This goal was complicated by Cirtronics' team-oriented assembly approach. While most teams work in assigned areas, members often move to other teams to smooth out spikes in workload. As a result, employees switch workbenches on a regular basis.

That made bench height an important consideration. One bench was a set height. "You had to adjust your chair or put a wood block underneath so you could put your foot up," Lemay said. That didn't prove as comfortable over a full shift as the benches with crank or motorized height adjustments.

The company's decision to move to lean manufacturing involved setting up kanban-style part bins that visibly signal the need for restocking. In the past, those bins either took up valuable workbench real estate or went on stacked trays on the side. That left workers either cramped for space or reaching for parts. Lemay's team wanted workbenches with shelves to keep parts within easy reach without expropriating valuable work space.

Cirtronics also wanted an aesthetically pleasing color that reflected its high-tech image.

Ultimately, Lemay's team opted for the Align workbench from Lista International Corp. in Holliston, Mass. Its motorized system adjusted work surface height between 25.5 and 41.5 inches, moving the kanban-ready shelves with the bench so they retained their relationship to the work surface. The unit also featured a switch that stored three height adjustments and leveling guides for uneven floors.

The units came with adjustable, glare-free lighting, adjustable footrests, and privacy panels that also served as bulletin boards. "They even created a special gray color for us, Cirtronics gray, so the benches would look the way we wanted," Lemay said.

Liquid Armor Stiffens When Threatened
by Harry Hutchinson

Personal armor may stop bullets and bombs, but its protection comes at a cost. Today's bulletproof vests are made of Kevlar aramid or other high-strength fabrics. Adequate protection takes multiple layers of fabric, which makes armor bulky and uncomfortable to wear.

One potential solution involves a little-known class of materials called colloidal shear-thickening fluids. During normal use, they flow as easily as conventional liquids. When subjected to sudden stresses that make them flow at higher shear rates, they instantly turn rigid and act like a solid material.

Impregnating conventional aramid fibers with shear-thickening fluid creates a more effective barrier, says armor developer Norman Wagner, a professor of chemical engineering at the University of Delaware's Center for Composite Materials. When he plunges an ice pick through four layers of Kevlar fabric wrapped around a foam block, it goes right through. Four layers of fluid-impregnated Kevlar fabric stop the ice pick cold.

Wagner's demonstration is especially interesting because conventional Kevlar vests usually fail to stop ice picks and knives, although they resist penetration by high-velocity bullets. Wagner's fluid-impregnated fabrics defeat bullets, too, and do so with fewer layers. This leads Wagner to believe that switching to fluid-impregnated fabrics will enable designers to create thinner, more flexible armor.

Wagner began unraveling the secrets of shear-thickening fluids more than 10 years ago. The syrupy colloids consist of submicrometer-size silica particles suspended in polyethylene glycol liquid. Wagner found that shear forces created "jamming clusters" of silica particles that freeze the fluid into a solid. The effect is similar to cement, which grows harder to mix the faster it is stirred.

Wagner originally intended to use his knowledge to improve the manufacture of coated and photographic papers. Instead, he began collaborating on armor with Eric Wetzel of the U.S. Army Research Laboratory's Weapons and Materials Research Directorate.

The University of Delaware recently licensed the technology to Armor Holdings Inc., a leading manufacturer of personal and vehicle armor systems. Wagner sees other non-armor applications as well, ranging from aircraft engines, car doors, tires, and sporting apparel to bomb blankets and paratrooper boots that stiffen on impact to protect a jumper's ankles.