Even though they tend to be the smallest components in medical devices, fasteners are usually the most important elements of the assembly as they literally hold the entire unit together. When a fastener fails in a medical device, this usually means that the device will also fail. The correct fasteners ensure that the device goes together and stays together for the intended life of the assembly, and that the device performs as desired.

All too often, these critical elements end up being the very last thing that engineers consider in the design process. As a result, fasteners used in medical devices are usually specified as extremely tightly toleranced parts. In addition, due to the unfamiliarity of the various fastener industry standards and associated manufacturing processes, engineers frequently tie the hands of the fastener manufacturer into using high-cost manufacturing processes to meet the specifications.

Slotted pins have a “C” shape. Coiled pins have 2¼ coils of rolled material.	Surgical stapler.
Plexiglass sample shows the stresses exerted by light, standard and heavy dutycoiled pins and slotted pins.

What most people do not realize is that design engineers can play a significant role in their companies’ profitability through the fasteners they select. Fasteners can overcome challenges in assembly, solve quality problems, and significantly reduce the total cost of the device. Engineers can lower design and assembly costs by working directly with knowledgeable fastener manufactures early in the design stage in order to ensure the most cost-effective components are designed into the device without having to go through costly redesigns after the product has launched.

Here’s what you need to know.

Cold-headed vs. machined pins

In the process of working with one surgical device manufacturer, it was discovered that they were using seven different machined solid pins as freefit axles in their surgical stapler. The pins were slip fit into place and held in position by a plastic shroud that went entirely around the outside of the device. The pins were specified with an outer diameter (OD) tolerance of ±.001” and a length tolerance of ±.003”. The material of the pin was specified as 303 stainless steel (SST). Since this type of stainless is only readily available in bar stock, this essentially dictated that the parts had to be machined rather than cold headed or roll formed – two significantly less expensive production methods.

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