In developing a custom hinge for a medical application and after reviewing several manufacturing approaches, hinge manufacturer Southco Inc in Honeoye Falls, NY, (southco.com), (585) 624-2545, opted to use Design for Manufacture and Assembly (DFMA) software from Boothroyd Dewhurst Inc, Wakefield, RI, (dfma.com), (401) 783-5840 to lower manufacturing and assembly costs.

The hinge was a load-bearing counterbalance of the type used to control the opening and closing of heavy lids, such as those on chest freezers. Southco evaluated the current hinge to propose a custom design and provide costing data.

The initial review put Southco’s manufacturing cost near the customer’s selling price for the hinge – not much of a profit margin there. Another option was to build the hinge overseas in Southco’s China or India facility, but this goes against a company policy. When possible, it manufactures and assembles products in the same area as their customer, in this case, North American.”

To make the hinge inexpensive enough to justify turning it into a new catalog product, the company focused on using DFA software to lower assembly costs. For the counterbalance hinge, this was important because manual assembly work was the bulk of product cost.

For simplicity, the team broke the DFA analyses into subassemblies:

The center housing subassembly sits at one end of the counterbalance spring. The column in the software identifying the minimum number of parts suggested there were good opportunities for reducing assembly times, particularly for screw driving and bracket assembly.

Out of this information came possible redesigns. One used a single steel bracket on a cast housing, reducing part count from seven to three. The other, a one-piece casting, eliminated the bracket as a separate part. “We prototyped and tested both options,” says Southco Manager of Manufacturing Technology and New Product Development Rick Langkamp. “Since they both passed, we opted for the one-piece casting.”

This analysis decreased component cost by 60%. It also eliminated the need for three stamping dies, one tapping operation, one screw-driving operation, and the tooling cost to build the fixtures.

The hinge point subassembly is at the other end of the spring and includes the die-cast housing that holds the hinge shaft and the torque elements in place. In spite of it being more complex (29 parts) than the center housing, the hinge point offered fewer redesign opportunities because a number of the parts were off-limits. “The sixteen torque elements are essential to all of Southco’s position control hinges,” says Langkamp. “They help make our positioning control smooth, and consistent.”

Many of the remaining parts made up the center shaft that attached the counterbalance to the stamped housing. By redesigning the shaft, engineers eliminated four fastening screws, cut out several other parts, and simplified the two die-cast housings at either end of the shaft.

Performing FEA on this and a second redesign revealed an opportunity to create a stronger hinge by splitting the stamped housing into two left-and-right pieces and adding extruded housing holes for the shaft in each piece. “Splitting the housing added a part to the assembly,” says Langkamp, “But the final subassembly still significantly reduced the overall number of parts. In fact, it trimmed part count from 29 to 23 and component costs by 28%.”

The counterbalance subassembly provides the force that makes the enclosure lid easy to lift, in spite of its weight. During DFA analysis, engineers quickly noted how much assembly time the two nuts and the rivets took. “Nonfunctional parts were taking well over half of the total assembly time,” says Langkamp. “Eliminating them was an easy decision.”

Engineers replaced the custom-turned shaft with a standard one that had threads on one end. At the top end they eliminated the nut (and its long assembly time) by adopting a floating-pin strategy similar to those used on many automotive valve train assemblies. They also removed a rivet by creating a tapered holding pin and substituting slots for the round holes of the spring guide.

“Sometimes it’s necessary to balance the improvement of a design against the drive toward reducing as many parts as possible,” says Langkamp. For example, we discovered during cycle testing of the prototype counterbalance that premature wear on the shaft caused squeaking. Adding a plastic bushing solved that problem. In addition, the customer asked for an increase in the force specifications on the counterbalance, which required an additional spring (mounted inside the original spring) to support higher loads, as well as a cup washer to provide centering.

Even with the additional parts, the final design reduced component costs 28% and eliminated one rivet peening (flattening) and one nut driving operation.

A lesson the company learned from its DFA analysis was how fasteners like nuts and bolts add significant assembly time — for example, orienting the nut on the bolt, picking-up the driver, driving the nut, and putting down the driver.

The redesigned counterbalance hinge is now in full production (10,000-15,000 a year). The hinge has more than met the cost goals. Total component count dropped from 45 to 30 pieces. Total assembly operations fell from 14 to 5. The overall cost of the hinge was slashed by 53%. The result: Profits are great enough to market the hinge at an attractive price.