Laparoscopic Surgical Device

A medical device manufacturer was using a Swiss style CNC-machined Knurled Solid Pin in a surgical device. This specific device is used as a “grasper” to allow the surgeon to operate through a laparoscopic port versus having to fully open-up the patient to perform the surgical procedure. The pin is used to drive the jaw movement at the distal end of the device. During pin installation, the customer was experiencing high insertion force, bending of the pins, damage to the sub-assembly, and generation of metallic debris. Since this device is meant to go inside the human body, the possibility of introducing metal chips or shavings is completely unacceptable.
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Original assembly using Knurled Solid Pin.
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Metallic debris and damage to the sub-assembly with Knurled Solid Pin vs. clean installation with Coiled Spring Pin.
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Improved assembly design using Light Duty Coiled Spring PIn.
One issue identified immediately was that pins made of 303 stainless steel with no subsequent heat treatment were being installed into a hardened 416 stainless steel shaft; a situation conducive to debris generation from the softer 303 stainless steel knurls. SPIROL performed a number of installation tests to validate this theory, and it was confirmed that the knurls of the softer Knurled Solid Pin were in fact shaving off and creating the observed metallic debris. The combination of the incompatible hardnesses of pin and host materials and the variation in the size (outer diameter) of the pin were determined to be the root causes of the debris generation, as well as the high insertion force, pin bending, and associated sub-assembly damage.

To address all of these issues, SPIROL recommended replacing the Solid Pin with a heat treated, 420 stainless steel Light Duty Coiled Spring Pin. The flexibility of Coiled Pin facilitates a low insertion force while providing adequate retention without protruding knurls damaging the hole and generating debris. A Light Duty pin provides the appropriate balance of flexibility and strength for this application. The fact that the pin is made from hardened 420 stainless steel provides the necessary corrosion resistance without risk of being damaged by the 416 stainless host during installation. Subsequent testing of the Coiled Spring Pins revealed a clean installation that required significantly less force - with no evidence of burrs or shavings and no damage to the sub-assembly.

By replacing the Solid Knurled Pin with a Light Duty Coiled Spring pin, the manufacturer permanently resolved all of the issues that they previously experienced with the Solid Pin in their medical device. Along with this crucial performance aspect, the manufacturer also benefited from the cost reduction associated with switching from a CNC-machined pin to a roll-formed Coiled Pin.

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