Application Briefs
Proto Labs
Maple Plain, MN
www.protolabs.com

As products get smaller, their components need to follow suit, and springs are no exception. In a variety of industrial applications, flat-wire wave springs are replacing traditional coil springs. The reason is that a wave spring is half the height of a coil spring, yet can often perform the same job as the coil spring — deflect the same amount of weight under the same load — saving valuable space and, in most cases, weight as well.

Smalley's comparator, a sales demo device.

Smalley Steel Ring — a Lake Zurich, IL-based wave spring manufacturer that serves aerospace, automotive, oil and gas, medical, and other industries — offers more than 10,000 standard rings and springs, as well as custom designs in diameters ranging from 4 to 3,000 mm (0.165 to 120 inches). While the capabilities of wave springs are becoming more widely known, the company has long equipped its sales force with a small demonstration device — a handheld “comparator” — that clearly shows the relative size and performance of a wave spring compared with a larger, but functionally equivalent coil spring.

“Comparators have been a great way to demonstrate the advantages and benefits of wave springs,” said Lane Persky, Smalley marketing manager. “But they were complicated and costly to manufacture, so we only made a small number of them to distribute to our salespeople.”

Smalley would eventually turn to Proto Labs for help with this cost issue when, more recently, the company considered redesigning the comparator to reduce costs and, as Persky explained, “use them as ‘giveaways' to prospective customers. We were looking to go from about 20 of the original comparators, which each cost about $100 to produce, to an initial run of 1,000 redesigned comparators at a target cost of about $1 each.”

The project to design the new comparator was headed by Senior Research and Development Engineer Ben Moskalik, who noted that the original comparators each consisted of 23 parts, many of them individually machined or purchased. The most costly single part was a linear bearing that allowed smooth compression of the springs. Developers hoped that injection molding would allow multiple parts to be combined and a number of individual fasteners eliminated, simplifying production and significantly reducing costs.

Proto Labs' injection molding service reduced the number of components needed for Smalley's new comparator. The new design required just seven pieces; the original comparators each consisted of 23 parts.

The new comparator, as designed, would require just seven parts. Smalley engineers created initial prototypes on an in-house 3D printer, but while the resulting parts approximated the shape of the designed components, the printing process was slow, the resin was nothing like the material that would be used for molded parts, and it tended to absorb moisture, affecting its performance. In addition, the rough surface texture resulting from the layering process caused moving parts to bind instead of sliding smoothly, hampering functionality testing of the new device. Smalley sought out assistance from Proto Labs.

The same CAD models that had been used to print parts in-house were uploaded to ProtoQuote, Proto Labs' online quoting and design for manufacturability (DFM) analysis engine. ProtoQuote flagged potential mold-ability problems including a couple of features in which walls were too thick for effective molding, and would be subject to cosmetic defects like sink.

Based on Proto Labs' DFM analysis, a redesigned CAD model with thinner walls was resubmitted. The new CAD models raised no red flags with ProtoQuote, and met Smalley's requirements without the moldability problems. With a few additional iterations by Smalley engineers — some post heights were reduced — the CAD model was uploaded for production of injection molds.

One of the important decisions Smalley had to make was the choice of resin for the molded parts. Proto Labs sent resin sample “tiles” to help with choices of feel, finish, and color. With input from Proto Labs, the company chose Lustran ABS 433, a general-purpose grade of ABS that has a high impact resistance and glossy appearance. The hard material and glossy finish allowed elimination of the separate linear bearing in the original comparator, and reduced friction between moving parts.

The completed comparator requires just seven parts: three molded parts made by Proto Labs, a wave spring, two retaining rings made by Smalley, and a purchased coil spring that matches the performance specs of the wave spring, which was chosen for comparison purposes. Smalley ordered 1,000 injection-molded pieces from Proto Labs that the company assembled into the new comparators.

This article was written by Greg Kagan, contributing writer for Proto Labs, Maple Plain, MN. For more information, click here.

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