“Stacked tolerances” is a term that describes a production problem in the engineering of medical devices when the cumulative effects of allowable variation aggregate towards a single vector or dimension. Normally, variances are expected to “average out” towards a design ideal. However, it is common — especially with injection-molded parts — to see tolerances gradually eliminate any of the designed-in buffers. These phenomenon can be predicted statistically, but solving it on the factory floor can be problematic.
Hypothetically, if a device features four or five nested components, random variations affecting all of the components can fall within the worst-case limits of their design tolerances. This presents two challenges to manufacturers. First, during the assembly process, mated parts may fit too tightly, making assembly slow and difficult. Alternatively, friction from overly tight components can cause the device to function poorly because the accumulated tolerances restrict free movement. In surgical tools, uneven movement is an unacceptable condition. So stacked tolerances increase scrap rates, degrade product design and slow assembly throughput.
One great answer is to apply a thin, even coating of a dry lubricant to the parts. By solving stacked tolerance issues and improving the quality and performance of finished devices, dry lubricants such as the Duraglide products from MicroCare Medical are a powerful addition to an engineer’s tool box. They reduce the coefficient of friction on the surface of treated parts to 0.06, reducing actuation forces by 25 to 30% and smoothing device operation.