Mon 5th Feb 2018 - Select the Best Sealing and Shielding for Linear Mechanics in Harsh Environments

Article Posted by Parker's Electromechanical Team on Thursday, January 25, 2018, on the Parker Motion & Control Technology Blog
 
Parker Many factors must be considered when determining which linear actuator to use in an application. Besides technical specifications such as speed, acceleration and payload, there’s another critical aspect that must be considered – the application environment. In fact, if you do not account for the environment, the rest of your application requirements can be out of specification when using the actuator.  
 
Application environment

Why are the details of an application’s environment so important?  Depending on the application, you may need to protect the actuator and its control from the environment. Electromechanical linear stages have critical, moving components that will only work properly within the correct environment. If you try to use the linear stage in the wrong environment, issues can range from the stage not working properly to causing damage to the unit that is beyond repair. If you have a "dirty” application (e.g., a cutting application that produces lots of scrap metal) the proper sealing and shielding are needed on the actuator to protect it from contaminants.
 
Parker However, it is not always the actuator that must be protected from the environment, but the opposite.  We know that linear stages will particulate over time due to normal wear and tear – no matter what application or environment. While we can minimize the amount of particulate from the actuator and increase its life cycle using proper seals and shielding, you must ensure the application’s environment is not compromised. Cleanroom or in-vacuum applications may require zero particulates, so it is critical that they use actuators with seals and shields to protect them. Remember, some linear mechanics are moving only microns at a time. Even the tiniest amount of contamination can compromise and ruin an application.   
 
Environmental factors
 
Consider these Environmental factors when designing for an application:
  • Temperature
  • Exposure to gases other than air
  • Moisture presence
  • Radiation
  • Pressure level (does it need to be able to perform in-vacuum?)
  • Cleanliness (does it need to be rated for cleanroom use?)
  • Surrounding objects impacting performance (example – does another unit cause vibrations that will affect the linear stage’s specifications?)
  • And more
You must consider these factors from both sides. Will the temperature be too hot and cause my actuator to burn out? If my actuator is exposed to moisture without the proper seals, will it be damaged? Conversely, can the actuator react poorly with a chemical in the environment causing contamination throughout? Issues will arise in your application if you do not make sure the actuator and environment are compatible. To find that compatible actuator, you will need to understand its seals and shielding capabilities.
 
Sealing and shielding for protection
 
Parker There are countless harsh environments that linear mechanics can be used for, but how are the units protected to work properly? The answer is using sealing and shielding technologies. A linear mechanism is comprised of various components. When the assembly is complete, there can be parts critical to the drive train and bearings that are initially exposed. If nothing is done, a harsh environment will destroy the linear stage in short time. Conversely, if the environment is a cleanroom, the actuator can particulate and cause contamination. By adding seals and shields where necessary, those critical components of the actuator are no longer exposed to the harsh environment, and the linear stage can run as it was designed to perform. For clean environments, the seals and shields on the actuator are protecting the application’s environment, not the actuator itself.
 
Seals and shields are both in place to keep contaminants out of a bearing. External seals must be able to prevent contaminants from entering the actuator. Integral bearing seals need to keep contaminants out and lubricant in the bearing cavity. Seals in contact with sliding surfaces are called dynamic seals and are used to seal passages between machine components that move relative to each other either linearly or in the circumferential direction. These dynamic seals must retain lubricant, exclude contaminants, separate different media and withstand differential pressures. The materials from which the seal is made should also withstand a wide range of operating temperatures, and have appropriate chemical resistance.
 
Ingress protection rating
 
How do you know if a linear stage has the proper protection from its environment? The stage’s Ingress Protection (IP) rating will tell you this. IP ratings are defined levels of sealing effectiveness of enclosures against intrusion from foreign bodies (dust, dirt, etc.) and moisture. An enclosure rating takes the form of "IP-" followed by two digits, each of which has a specific meaning. The first indicates the degree of personal protection from moving parts and also the level of protection of the equipment from foreign bodies. The second digit defines the protection level that the enclosure offers for exposure to moisture in the form of drips, sprays, submersion etc.
 
A complete reference standard for IP ratings may be found in IEC 60529. Following are some basic guidelines: 
 

 First Number
 Characterization of ingress limit
 0  No protection
 1  Objects larger than 50 mm
 2  Objects larger than 12.5 mm
 3  Objects larger than 2.5 mm
 4  Objects larger than 1 mm
 5  Protected from dust (limited ingress)
 6  Dust-tight
 

 Second Number
 Characterization of ingress limit
 0  No protection
 1  Dripping water - vertical
 2  Dripping water -up to 15 degree angle
 3  Water spray
 4  Water splash
 5  Waterjet
 6  Powerful waterjet
 7  Temporary immersion
 8  Continuous immersion
 
Checking the IP rating of an actuator initially can help rule out any that will not be suitable for the environment. It may also save you money. For example, the Parker 400XR Series has an IP30 rating. While it will not have any protection against moisture, it does have intrusion protection against fingers. The XE Series, an economical alternative to the XR Series, does not have an IP protection (i.e. 00). If there are concerns about injuring fingers, XR should be selected. However, if the intrusion protection is not necessary and the XE specs work for the application, the customer can save money by with the XE. Now if a customer needs protection against moisture as well, the XR Series is not the right choice. Instead, they should use the HMR Series, which has an IP54 rating.

ParkerCustom solutions

Parker offers a wide array of linear positioners suitable for applications in a variety of environments, even the harsh ones. Our IP rating differs from product to product and application to application, so we are confident the right fit can be found. Once determined, sealing and shielding guidelines are followed for all our linear mechanics to meet the required customer specifications. In addition to seals and shields, positive pressure ports can be included on linear stages as well. This allows customers to purge unwanted contaminants inside their unit, keeping the performance and life cycle at a maximum.

What if you are not certain which sealing and shielding technology is needed for their application? Parker will work directly with you to provide custom engineered solutions. We will discuss your requirements and can determine the best product to use based on the environment.  Forming this partnership throughout the process ensures you receive the best solution for your application  - which is Parker’s ultimate goal.


ParkerArticle contributed by Patrick Lehr, product manager for precision mechanics, Electromechanical and Drives Division North America, Parker Hannifin Corporation.