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Overcoming Stacked Tolerances with Dry Lubricants

Overcoming Stacked Tolerances with Dry Lubricants

Dry lubricants can give design engineers and manufacturers a solution to the familiar design and production challenge of stacked tolerances. Tolerances refer to the permissible limit or limits of variation in a physical dimension, and are specified by the design engineer to allow reasonable leeway for imperfections and inherent variability - without compromising performance of the finished assembly or process.

This challenge of stacked tolerances becomes an issue for design engineers when working with mechanical assemblies that have multiple components and moving parts that work in unison. One solution that could be used to address stacked tolerances is to design everything with tighter tolerances, thus ensuring higher levels of precision. This option is not usually the most appropriate for the design engineer and manufacturer because of the high cost associated with it. This is especially true if the finished assembly has a limited service life expectancy. Higher precision equals higher cost because more frequent inspections and maintenance of the tooling and machines during manufacturing are necessary to obtain high levels of precision.

Overcoming Stacked Tolerances with Dry Lubricants

A more cost-effective solution to address stacked tolerances is the use of a dry lubricant as a surface treatment. Dry lubricants using PTFE technology provide a thin, smooth coating over the finished device or mechanical assembly to reduce friction caused by stacked tolerances. This ensures device functionality and performance; in fact, dry lubricants can reduce the force needed to actuate a device or assembly by 25 to 30 percent.

A common example of a portable medical device found exclusively in the operating room that benefits from dry lubricant coating is the surgical stapler. A stapler is essentially a mechanical assembly that requires trigger activation to actuate the stapling action. The motion of springs and metal stamped parts sliding against each other is a process that requires the engineer to design with precision and allotted tolerances. In high-volume production, the use of dry lubricant can keep those precision costs down while ensuring device performance.

Manufacturing volumes and surface geometries influence what application method to choose for dry lubricant use. The various methods of application include: dipping, wiping or brushing, air spraying, air-less spraying, aerosol sprays, drying, and melt-coating for improved adhesion and durability. The most common method used is dip-coating because it offers the most consistency from part to part.

Dry Lubricant Selection Issues

Dry lubricant fluids are non-flammable and have a high degree of materials compatibility, permitting use on nearly all types of metals and most plastics including polycarbonates. Despite their wide applicability, when choosing a dry lubricant, design engineers must take many variables into consideration. Consistency and quality of the coating is an extremely important variable because it can affect how well the lubricant will perform. To address this, design engineers must pay attention to the calibration of the fluid. In other words, the ratio of PTFE particles to carrier fluid should remain at consistent levels throughout the coating process. This can also be impacted by the evaporation of the carrier fluid.

Overcoming Stacked Tolerances with Dry Lubricants

Another challenge to maintaining calibration is the "hang time" of the PTFE particles within the carrier fluid. It is common for the PTFE particles to sink in the carrier fluid to the bottom of the vessel, thus affecting calibration and consistency of the coating. When the part or device is dipped into the fluid, it may end up with a streaky and inconsistent coating. If manufacturers work to constantly agitate the fluid, this can help to maintain calibration. However, constant agitation requires a lot of extra work on the production floor and also can promote evaporation of the carrier fluid. By choosing a dry lubricant vendor that provides a ready-to-use, pre-mixed and calibrated formula that has a high "hang time" and maintains the ratio of carrier fluid to PTFE particles, design engineers can be assured that they will achieve a more consistent coating and smoother end-use actuation.

Another consideration for design engineers when choosing a dry lubricant are the environmental and safety qualities of the coating. Some vendors offer products that have better environmental properties, meeting strict Environmental Protection Agency (EPA) regulations. For example, the MicroCare DuraGlide has an EPA designated Volatile Organic Compound (VOC) exempt status and also has an Ozone Depletion Potential (ODP) of zero.

Jay Tourigny is vice president of operations at MicroCare Medical. Click here for more information.

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