Using high-quality actuator components can protect industrial equipment designers—and the users of their products—from a lot of grief. Harsh environments that can damage equipment exteriors will eventually reach inside those devices, infecting its very musculature. Harsh can be defined by shock loads and contamination from fluids, dust, or debris, as well as extreme temperatures, presence of a vacuum, high humidity, or a corrosive atmosphere. If actuators are designed to fight off these effects, equipment will last a lot longer.
Appropriate grade stainless steel can protect rotational bearings from wet or acidic environments. Bearings and surfaces can also be protected with corrosion resistant plating such as a hard chrome material. For extremely corrosive environments, other bearing materials such as ceramics may also be considered. Such exotic materials, however, can prove expensive and also come with reduced load capabilities. In some cases engineered plastic bearings can also be used if speeds and loads permit.
Beyond material options for bearings, various methods of sealing and lubrication can be employed to further enhance the ability of a rotational bearing to protect from fluids. Also, aluminum and stainless steel can be selected for the body material of commercial stages used in wet environments.
Linear guidance system
Liquids and dusts can contaminate rolling element bearings used in linear guidance systems. Particles larger than the lubricant film can lead to spalling and bearing failure. Lack of lubricant and/or exposure to moisture can allow pitting or etching of metal bearing components due to corrosion that likewise can lead to eventual reduced life and failure.
Guided roller style bearings tends to be least affected by dirty environments, but the most costly aspect of any bearing technology is the assembly labor to preload it to reduce or eliminate play and to provide carriage stiffness. A cost effective approach to preloading is to allow for some level of compliant interference between rollers and track that can produce the optimum preload and system stiffness.
Common drive train mechanisms include timing belts, ball screws, lead screws, linear motors, friction drives, and rack-and-pinion drives. Belt drives can run in a dirty environment as long as contamination is not large enough to jam the drive train or abrasive enough to damage the belting over time. Steel cords offer less stretch than Kevlar or fiberglass cords; however, you should avoid their use in wet environments where cracks in the base belt material can allow water to penetrate to the cord and cause eventual corrosion and failure.
Ball screws are also sensitive to liquids from a corrosion standpoint and contamination that can lead to bearing failure. Seals can be added to each end of a ball screw nut; however, the complex seal form required to mate to a screw thread can be less than 100% effective in keeping out contamination. Thus, actuators that use ball screws need extra attention to minimize contamination in the area of the ball screw.
Linear motors can be designed to work well in both wet and dirty environments depending on the design. Contamination particle size must be managed so that it does not cause interference with the gap established between the forcer and the magnet track.
Which drive system is best for your application?
If a low-cost actuator is desired, and it must function in a harsh environment, a timing belt drive coupled with a roller wheel guidance system can prove to be a robust combination. Assemble these components into a housing that is corrosion resistant and design the body such that it limits ingress of fluids or contaminants and you have a motion system that can provide maintenance-free operation for many years.
This article contributed by Tom Baric, P.E., Engineering Manager, Parker Hannifin Corporation.
For more information, visit Parker Linear Motion Systems.
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