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How Dry Lubricants Can Help Overcome Stacked Tolerances

Many medical devices use gears similar to this 4-piece gear assembly, which is shown being spray treated with dry lubricant to eliminate binding of the spring against the cog
Photo courtesy of MicroCare Medical

One cost-effective way for design and manufacturing engineers to reduce the friction caused by stacked tolerances is to use a dry lubricant surface treatment

Jay Tourigny
MicroCare Medical

One familiar challenge that both design and manufacturing engineers face is 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 a process.

Addressing tolerance issues has implications all the way to the end-user, where he or she needs the device or product to perform its function precisely and smoothly. One option of dealing with stacked tolerances would be to design everything with tighter tolerances to ensure higher levels of precision. However, higher precision typically equals higher cost, because more frequent inspections and maintenance of the tooling and machines during manufacturing are necessary in order to obtain these high levels of precision. Therefore, in the interest of keeping costs low (especially in high volume production), this is not usually the most appropriate choice for the design engineer and manufacturer.

Stacked tolerances are a common issue for manufacturers of mechanical assemblies with multiple components and moving parts that work in unison. They present challenges in the assembly process because mating parts don't fit precisely, making assembly difficult, and also can present variables in the operation of the finished device. The design engineer can often specify component parts made of plastic or nonferrous materials with inherent lubrication properties to help minimize friction. They may also specify a surface treatment in the form of a silicone or hydrocarbon-based lubricant to further reduce friction from stacked tolerances. Both are common strategies used to overcome stacked tolerances, but end-use loading requirements may preclude the use of plastics and nonferrous metals. Silicone and hydrocarbon lubricants can present cleanliness problems because they remain wet and tend to migrate and attract surface contamination, which may be problematic in the end use.

The Dry Lubricant Solution

Given the challenges above, one cost-effective but often overlooked way of addressing the challenge of stacked tolerances is the use of dry lubricant as a surface treatment. Dry lubricant is compatible with most plastics and metals and readily conforms to virtually any surface geometry, including wire cables and complex meshing. These lubricants are easy to apply in-house and can easily be incorporated into the assembly process.

Coating a device's components can play a huge role in the ease of assembly and in the performance, quality, and consistency of the finished device; in fact, many precision devices manufactured today would not be commercially viable without this type of coating. Devices or mechanical assemblies treated with dry lubricants (a thin, smooth coating of PTFE particles--more commonly known by the brand name DuPont^TM Teflon^®) achieve an effective reduction in friction caused by stacked tolerances, thus ensuring device functionality and performance. In fact, dry lubricants can reduce the force needed to actuate a device or assembly by 25 percent to 30 percent.

Dry lubricants have inherent physical advantages of being clean and non-migrating, and can be easily applied in numerous ways (see The Easy Application of Dry Lubricants). There are certain advantages to each application method, depending on the function and performance of the device, manufacturing volumes, and surface geometries. Choosing the correct dry lubricant and coating process for your particular device can mean significant time and cost savings on the shop floor, so it is important to consider all the variables in play.

Calibration is Key

Particularly when applying the coating through a dip process, an important consideration for the engineer and manufacturer is maintaining a consistent coating on the treated surfaces. Dry lubricants are extremely effective, but they are most effective when the calibration of the fluid--the consistency of the ratio of PTFE particles to carrier fluid--is constant and correct. This is important to ensure a quality coating.

There are many issues at play in maintaining calibration. The first challenge is controlling the evaporation of carrier fluid. Many PTFE dry lubricants are mixed with a carrier fluid that evaporates very quickly. This is necessary, and good for the coating of the part, because it dries quickly enough to both maintain high production speeds and leave a very consistent surface coating on the device. However, this also means that on the manufacturing floor during the coating process, the fluid can evaporate quickly out of the vessel used for coating. In some cases, workers will add an unmeasured amount of carrier fluid to maintain approximate percentage saturation, but this is not precise and can affect the consistency of the coating.

To control evaporation and keep fluids calibrated for maximum consistency and quality, use of process-specific equipment for the cleaning and coating process is recommended. This may include hermetically sealed equipment, specialized carrier fluid recovery systems, controlled temperature baths, engineered parts feeding systems, such as hoists or conveyers, or engineered application systems, such as spray or brush applicators.

The next challenge to maintaining calibration is the "hang-time" of the PTFE particles themselves. The large PTFE particles found in many formulas can have a low "hang time" within the liquid carrier fluid, which means that as the fluid sits in the vessel and parts are dipped, more of the PTFE particles will sink in the fluid to the bottom of the vessel, thus affecting the calibration and consistency of the coating. Manufacturers can work to constantly agitate the fluid, but this may promote uncontrolled evaporation of the carrier fluid, and can lead to streaky coatings.

A key solution to the short "hang time" of PTFE particles in a coating application is to find a provider that supplies a pre-calibrated fluid with micro-particles. For example, MicroCare Medical and its Duraglide^TM dry film lubricants provide a ready-to-use, pre-mixed and calibrated formula that maintains the ratio of carrier fluid to PTFE particles. MicroCare Medical uses proprietary "microdispersion" PTFE technology to deposit a thin, smooth film over the treated surface. These micro dispersions actually suspend the PTFE in unique carrier fluids engineered to create a better "hang time," resulting in a more consistent coating and smoother end-use actuation.

Environmental Benefits

Another consideration for the engineer and manufacturer is the environmental impact of the dry lubricant and other cleaners or coatings that they use during the design and manufacturing process. Some vendors, such as MicroCare Medical, offer products that meet strict Environmental Protection Agency (EPA) regulations and are free of volatile organic compounds (VOCs). There are also dry lubricants that are free of any hazardous air pollutants (HAP) and have an ozone depletion potential (ODP) of zero.

Outside of actual product selection, a more important decision is to find a coatings provider partner who understands your needs in terms of production floor safety, Environmental Protection Agency (EPA) regulations, and selecting the best chemistry for your process. The coatings partner should be able to provide a dimension of expertise on the EPA regulations and chemistry parameters to make the design and manufacture of the device as consistent, efficient, and sustainable as possible. They should also be able to evaluate and optimize your process, i.e., the footprint of your application system, the electricity and maintenance required, etc., and identify all types of potential waste.

For design engineers and manufacturers, the bottom line is that the coating and the coating process you use for a certain device can play a huge role in the performance and consistency of the finished device. Although there are plenty of challenges that can impact the process, the most important thing to remember is that a partnership with a vendor whom you trust can go a long way to improving the quality and consistency of your coating process. A partner who can address questions and concerns while providing new ideas will ultimately help you achieve maximum profitability.  

Jay Tourigny is vice president of operations at MicroCare Medical, New Britain, Connecticut, and can be reached at JayTourigny@microcare.com.  

MicroCare Medical (www.microcaremedical.com) is a supplier of advanced cleaners, carrier additives, coatings, and lubricants to medical device designers and manufacturers throughout North America and Western Europe.  
  

The Easy Application of Dry Lubricants  Many people know the benefits of a dry lubricant with respect to the performance of a device, but they don't realize how easy it can be to apply the lubricant. Following  is an overview of the different application methods: Dipping Most common when coating small parts, coils of wire, and items of varied shapes. Full finished devices are also sometimes dipped. The pieces are dipped in a PTFE-carrier fluid dispersion. The coating level is determined by the concentration of solids, rate of withdrawal, and number of applications. A single dip is adequate for most uses. Wiping or Brushing Useful for coating continuous surfaces such as rods, tubing, or sheets. This application method is especially useful for braided wire cable that has a relatively complex surface area and can benefit significantly by the presence of a dry lubricant when subjected to friction from guide channels and drive wheels. In addition, wiping and brushing are appropriate for coating small, selected areas of a larger part. One variation of this method is flood coating, followed by wiping. Air Spraying Spray equipment can be used to apply dilute dispersions. A solvent with low evaporation is recommended in order to ensure a consistent coating. Air-less Spraying Applied with a hand-held spray gun or with automatic spray heads operated either intermittently or continuously. Typically, the technique is to apply a succession of thin coats, allowing the surface to dry between applications. Results from this technique are typically better than the application of a single, thick coating, which can take longer to dry and can cause "mud cracks," or uneven coverage, and poor adhesion. Aerosol Sprays Allow convenient surface application and quick coverage. Many aerosol sprays will contain other additives to impart special properties. Drying All coatings dry best in an area that is relatively free of dust. Adequate ventilation improves drying, as does allowing adequate spacing of covered pieces to avoid crowding. Drying times of coatings are dependent on dispersion type and thickness of application, but increasing the temperature in the drying room can reduce all drying times. Melt-Coating for Improved Adhesion Sometimes, the dry lubricant coating can be improved through melting to "fuse" the coating. This is particularly helpful if the coating needs to be more permanent, or if it needs to be invisible on the device or part. (Heating turns many dry lubricants completely clear, so it appears that nothing is on the surface of the device.) In all cases, you should consult your coatings provider for the recommendation on which process is best for your particular application.