SPIROL’s Christie Jones shares an innovative case study on how overhauling the design of free-fit hinge pins for medical device handles saved costs and retained performance
A leading surgical device manufacturer was using seven different machined solid pins as free-fit axles in a surgical stapler assembly. The pins were slip-fit into place and retained by a plastic shroud that completely enclosed the outside of the medical device.
The machined solid pins were specified with an outer diameter (OD) tolerance of ±0.0127mm (.0005”) and a length tolerance of ±.003” (0.076mm). The pin material was specified as X8CrNiS18-9 (1.4305) / AISI 303 (UNS S30300) austenitic stainless steel (SST). Since this grade of stainless steel is most readily available in bar stock, the material specification, combined with the tight dimensional tolerances, essentially dictated that the pins be machined rather than manufactured using a significantly less expensive production method such as cold heading.
THE COST-SAVING SOLUTION
SPIROL’s application engineering team conducted a thorough review of the performance requirements and determined that the tolerances specified for both length and diameter were significantly tighter than necessary, given that the pins served solely as free-fit hinge pins in the surgical device. In other words, the original specifications were adding significant manufacturing cost without providing any functional benefit to the assembly.
SPIROL posed a simple question to the medical device manufacturer: “If you could save millions of pounds on the cost of the pins used in this surgical device, with no change in assembly performance, by allowing wider dimensional tolerances on the diameter and length, would you be interested?” Of course, they were.
SPIROL Engineering recommended widening the diameter tolerance from ±0.0127mm (.0005”) to ±0.025mm (.001”) and the length tolerance from ±0.076mm (.003”) to ±0.25mm (.010”). To put this into perspective, even though the allowable length tolerance was more than tripled, the difference is only about the width of two to three human hairs.
SPIROL also recommended changing the material from X8CrNiS18-9 (1.4305) / AISI 303 (UNS S30300) stainless steel, to X4CrNi18-12 (1.4303) / AISI 305 (UNS S30500) stainless steel. While both grades are austenitic stainless steel, the change to 1.4303 / 305 stainless steel, combined with the wider diameter and length tolerances, enabled the pins to be cold headed from commercially available wire rather than machined from bar stock – without compromising the fit, function, or performance of the assembly.
By replacing the seven machined solid pins with cold headed solid pins, the medical device manufacturer realised annual savings of more than £2.2 million.
Christie Jones is at SPIROL
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