Everything needs fasteners. From mop heads to telecommunications, from gun assemblies to complex medical equipment-all multi-pieced products need something to hold parts together.
Industry leaders like The Lee Co., Huck, and Penn Engineering produce fasteners. Yet, fasteners are misunderstood, says Robert Zavisza, manager of the Made-to-Print Division for Atlantic Fasteners (West Springfield, MA). "When an engineer chooses a fastener, not a lot of thought goes into the design or its characteristics," he says. But, fasteners are unique in what they do, in the clamp load they provide, and in the structural importance they provide in a clamping environment.
John Souza agrees. He works as the applications engineering manager at the fastener manufacturer Quality Screw & Nut Company (Leominster, MA), and has authored several papers on the cost of fastening. " 'Buy it cheap' or 'buy what's available at a local distributor,' is a common criteria for fastener selection," he says. It's a task typically assigned to the junior engineer or buyer. "All the attributes of a fastener should be considered and the highest level of performance should be incorporated at the design phase of every product," he says.
It is hard for some companies, however, to take a part that may cost anywhere from $0.15 to $0.75 seriously. But for those who do, the payoff is tremendous, says Zavisza.
These men should know. Zavisza and Souza help customers reengineer or redesign fastener needs everyday, always with an eye on the bottom line.
As the name implies, the Hidden Panel Latching System from Southco conceals all hardware behind the panel. The latch keeps out unwanted visitors, yet provides easy access for those who need it. Engineers designed the fastener for near-edge applications. Floating keep assists alignment.
Automatic success. Bayer wanted to automate the production of a blood sugar test device. The user's blood mixes with a reagent held in one of the internal pockets of the compact analyzer. The electronics, mounted on a flex print, are secured by four screws, which required a very precise preload. Bayer worked with an automatic screw manufacturer for a year and a half to totally automate production at a rate of 5,000 units/day. Once in operation, Bayer engineers found they were getting 1,100 screw-related failures a day.
The screwdriver manufacturer suggested that Bayer call Quality Screw & Nut Co. to test the fastener design. After just a day, Souza had a solution. First, he modified the recess. Then, he suggested changing the screw from an M2.5 PT, because it was the wrong thread for the application, to a Type WB with an #3 thread and metric head from Quality Screw. Finally, Souza swapped materials from the original stainless steel to steel zinc with a special JS 600 coating containing wax, which would not be affected by the chlorine solution used to clean the blood from the device.
Within the first week, failure rates dropped from 1,100 per day to 6 per week. In addition, Souza's design reduced screw cost by 40% and eliminated three rework stations.
Check the process. Zavisza suggests another way to save money is by analyzing the manufacturing processes. For example, a large flow valve company asked Zavisza to review the design of a vent screw. "The customer was using a 3/8-inch 316 stainless-steel bar stock and milling an undercut and point and rolling the thread in house," he says. "We suggested converting the screw to a headed part instead." By switching processes, the customer reduced the fastener cost from $0.53 each to $0.35-a 34% cost reduction. That translated into a $12,600 savings per year based on 70,000 pieces annually. "In terms of cost savings, this was a home run for the customer," says Zavisza.
A large medical equipment manufacturer asked Atlantic Fasteners to look at a Delrin 20% glass filled pump housing that had been redesigned from stainless steel. "The screws were loosening in the field, and failing on the assembly line," says Zavisza. The customer had been using a 302 stainless steel, 48-degree, twin lead, thread rolling screw.
After Atlantic Fasteners evaluated the application and design, they found a couple of problems. Zavisza says, "The customer had specified the fastener and purchased a year's supply, so we had to look at a two-step process." First Zavisza tested the screw currently in stock to find out why it was failing. He determined that the drive-to-strip ratio was an acceptable 5.8 to 1. But the average strip torque was 15.2 inch-lbs and the customer was using 15 inch-lbs to seat the fastener, which was slightly less than the torque required to strip the screw. He recommended reducing the seating torque to 6.8 inch-lbs, which provided the same clamp load. This decreased the stress on the fastener and provided a short-term solution.
Next, Zavisza looked to improve the performance. The current fastener would not work because the steep helix angle of the double lead screw would cause it to loosen or back out during assembly or in operation. Atlantic suggested a PT screw from Semblex Corp. (Elmhurst, IL), who helped evaluate the application. PT screw threads reduce and control stresses and to improve retention of clamp load. The 30-degree thread angle reduced radial stresses, provided increased thread depth, lowered installation torque, and improved material flow, says Zavisza. The thread root design, with a large flatten root, provided maximum resistance to relaxation, improved retention of clamp load, and prevented material jamming.
Do your homework. Chris Novak, project manager for Graetag Imaging Group (British Columbia, Canada) needed a latch for the top cover of the company's new "Sapphire" platesetter, a laser imaging devise that makes "direct to print" plates. Graetag also wanted a completely concealed fastener to ensure that only authorized service engineers would be able to access the machine. After doing some investigating, Novak chose the Hidden Panel Latch from Southco (Concordville, PA). The latch attaches to the lid of the machine and a mating keeper is attached to the frame. The latch has a button at one end. When the user pushes the button, the latch pushes the keeper away, releasing the lid. To close the lid, simply push it closed, and the latch and keeper snap together. Novak recessed the button down a small hole. The service engineer must push a tool into the hole to open the latch. According to Novak, the Southco latch offered the best price/performance ratio in the industry.
Choosing the right design saves on maintenance, part failures, fractures, increases reliability, decreases overall cost of production, and extends the lifetime warranty. Choosing the right process can save big bucks during assembly. Workstation efficiency, space, part counts, material handling and tooling are also affected. "This surprises most people," says Souza, "but minor changes to a fastener can bear big dividends."