Fasteners are typically the last thing engineers consider when designing medical devices. As a result, they often specify expensive, tightly toleranced parts, when less costly fasteners would suffice.

For example, a medical device manufacturer was using seven different solid pins as free-fit axles in a surgical stapler. Made from 303 stainless steel, the pins were slip-fit and held in position by a plastic shroud that went around the outside of the device. The OD of the pins was held to a tolerance of ±0.001 inch, while the length was held to ±0.003 inch. Because 303 stainless is only available in bar stock, the parts had to be machined rather than cold-headed or roll-formed—much less expensive production methods.

medical device assembly
A medical device manufacturer was using a knurled solid pin
(left and center top) in this surgical tool. Inserting the pin
required so much force that it deformed the fastener, damaged the assembly, and generated debris. A coiled spring pin installed cleanly. Photo courtesy Spirol International Corp.

We advised the manufacturer to widen the length tolerance to ±0.01 inch (the difference of two to three human hairs) and change from 303 stainless to 305 stainless, so commercially available wire could be used. These changes enabled the pins to be cold-headed, instead of machined, with no change in performance. And, the manufacturer saved more than $2.3 million annually.

There are two types of spring pins: coiled spring pins and slotted spring pins. Both are flexible and can accommodate wider hole tolerances than nonflexible fasteners, such as solid pins.

However, certain aspects of slotted pins limit their applicability in medical applications. A slotted pin is less flexible than the coiled pin, and it only flexes 180 degrees from the gap. This limited flexibility can result in plowing and debris generation during installation. Under load, stress is concentrated 180 degrees opposite the gap. This can cause premature failure of the assembly. Slotted pins are also difficult to automatically feed and install, since they have uneven ends and the slots can interlock. The best applications for slotted pins are noncritical industrial products that are manually assembled and made out of mild or hardened steel.

Simple changes saved the manufacturer more than $2.3 million annually

Coiled pins were invented to compensate for the deficiencies of solid pins, slotted pins and conventional fasteners, such as rivets and bolts. Easily recognized by their unique 2¼ coil cross-section, coiled pins are self-retaining fasteners that compress when installed. They are the only pins with uniform strength and flexibility after insertion. Truly an “engineered fastener,” coiled pins are available in three “duties” to enable designers to choose the optimum combination of strength, flexibility and diameter to suit different materials and applications. Their shock-absorbing design dampens forces and vibration to prevent hole damage and prolong assembly life. With square, burr-free ends, coiled pins require less insertion force than other pins, making them ideal for automated assembly.

A medical device manufacturer had been using a machined, knurled solid pin in a “grasper,” a tool used in laparoscopic surgery. The pin is used to drive the jaw movement at the distal end of the device. Inserting the pin required so much force that it deformed the fastener, damaged the assembly, and generated debris. Understandably, metal shavings were unacceptable in the device.

Made from 303 stainless steel without heat treatment, the pin was installed in a hardened 416 stainless shaft. A combination of incompatible hardness ratings and variation in size between the pin and host material were the root causes of the problems.

To address these issues, the manufacturer replaced the solid pin with a light-duty coiled spring pin made from heat-treated 420 stainless. This pin provided the right balance of strength and flexibility so it could be inserted with less force but still provide adequate retention without damaging the hole and generating debris. Better yet, the coiled pin was also less expensive.

For more information on fasteners for medical devices, call Spirol at 860-774-8571 or visit