Trigger Pin in Surgical Stapler

A Solid Dowel Pin was used to fasten a stainless steel trigger plate to a plastic yoke subassembly within a surgical stapler. This functioned as a hinge to actuate the stapler as the surgeon pulled the trigger. The Solid Pin was retained in one side of the yoke by a friction fit between the plastic host and the pin. The trigger plate and the other side of the yoke were a free fit, allowing the pin to pass freely through them during installation.

The Solid Dowel Pin held the assembly in place by displacing the plastic host on the friction fit side of the yoke. This was accomplished by pressing the Solid Pin into a hole with a smaller diameter than the pin. Due to the stress exerted onto the plastic by the inflexible pin, the Solid Pin caused cracking of the plastic yoke during installation. The manufacturer attempted to modify the plastic hole geometry in order to reduce the surface area of plastic contacting the Solid Pin, but this did not fix the cracking issue and this also resulted in reduced retention of the Solid Pin within the assembly. The medical device manufacturer approached SPIROL for assistance in designing a self-retaining trigger pin that eliminated cracking of the plastic yoke within the surgical stapler.

SPIROL Engineering recommended using an oil free Coiled Spring Pin as the trigger pin. Coiled Pins are flexible, easy to install, and allow for wider hole tolerances than any other pin. They are ideal for plastics because they compress during installation and exert low stress on the host component after installation. Additionally, the Coiled Pin remains flexible and will adapt to the hole size even if the hole in the yoke experiences plastic relaxation over time.

SPIROL Engineering also recommended the two (2) holes in the plastic yoke be the same diameter to allow for better retention of the subassembly. In fact, SPIROL recommended modifications for all three (3) holes in order to optimize the pin’s performance and retention. The modified design resulted in friction fits in both plastic yoke holes and a free fit (larger hole) in the trigger plate, as shown in the image below.

By incorporating SPIROL’s recommended solution, the cracking of the plastic yoke was eliminated, the function of the trigger pin was enhanced, and the overall quality of the surgical stapler was improved. 

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