Rubber seal products like O-Rings and certain gaskets are used in a multitude of different applications worldwide. A large proportion of these applications are static seal applications where the O-Ring or seal is held firmly in place in relation to the gland and would not require consideration of friction or abrasion related damage to the seal.
In a dynamic application where the gland surfaces move in relation to the sealing surface, there are a significant number of special considerations that OEM manufacturers should take into account when specifying an elastomer.
Key Dynamic Seal Considerations for New OEM Applications
As an introduction, dynamic applications have two unique factors that can influence rubber seal performance. Break-out friction is the force required to initiate movement of the seal. Running friction is the force required to keep the seal in motion. To compare the two, break-out friction can be up to 3X greater than running friction and therefore requires significantly more force to overcome.
Choosing the Rubber Compound
The first consideration for dynamic seal requirements would be verifying that the material chosen will be suitable for use in a dynamic application and to evaluate the materials native break out and running friction in your application.
The most common elastomers, Nitrile and EPDM are generally suitable for dynamic applications but will typically have higher than normal break-out or running friction without the use of internal or external lubrication.
Silicone and Fluorosilicone that are typically utilized in fuel or high temperature applications have very poor tensile strength and are not recommended for any high rate dynamic applications. These type of elastomers could only be used in very low movement dynamic applications with very smooth gland surfaces like single use syringes.
Fluorocarbon (Viton®) elastomers offer improved breakout and running friction over the other elastomers and some of the best high temperature and chemical performance but this elastomer can cost significantly more and cannot be used with steam.
Rubber Seal Lubricants
Once an elastomer has been chosen, the OEM Engineering team must then determine if the existing break-out and running friction is acceptable or if improvement would be beneficial in terms of operational efficiency or seal longevity. At this stage design goals can be determined and the following variety of internal and external lubricants can be evaluated. An experienced Seal specialist should be able to provide guidance to your group on the best lubricant to meet your goals.
The following are the most common options used by OEM manufacturers to provide reduced break-out and running friction in dynamic applications.
A. External lubricants:
External lubricants such as hydrocarbon lubricants, silicone base greases or barium base greases can be applied to rubber seals already in stock and can provide the best initial reduction in break-out or running friction.
Powder based lubricants can also be utilized like Molybdenum Disulfide (MoS2) or Graphite with similar effect.
Key issues to consider, before utilizing an external lubricant include chemical compatibility of the elastomer and whether the external lubricant will cause parts to stick together and have an impact on assembly operations.
External lubricants are available that are compatible with a variety of elastomers and can meet special high temperature or FDA requirements. External lubricants also provide additional benefit of protection of the elastomer from oxygen or ozone damage.
A drawback of external lubrication is that eventual loss of lubricant either through collection away from the seal or dilution of the lubricant in any fluid that may come into contact with the O-Ring or seal.
Experienced rubber seal manufacturers or suppliers can provide an OEM with guidance on external lubricants available and what might meet the performance and assembly requirements for your seal component.
B. Chlorination:
Chlorination is a permanent process that can be applied to existing stock O-Rings or seals to provide a much smoother seal surface with reduced running friction in dynamic applications. The improvement in break-out friction with a chlorinated O-Ring or seal is negligible but a chlorinated O-Ring can be used in combination with external lubricant to address both break-out and running friction.
C. Internal Lubricants:
Internally lubricated compounds have a friction reducing agent mixed into the elastomer that is designed to be chemically incompatible with the elastomer. This causes the elastomer to continually exude the lubricant, especially as the surface of the O-Ring or seal wears or is abraded in the application. Advantages of internal lubrication include greater consistency in performance and less impact on assembly.
Automatic vibratory feeders used in high volume assembly operations will see appreciable benefit with internally lubricated compounds which will cause less interference and greater assembly productivity.
Internal lubricants like PTFE (Teflon®), Graphite, Molybdenum Disulfide (MoS2) and specialty wax type products can be used with many elastomers to provide dynamic friction reduction and the most efficient automatic assembly.
O-Rings.com by Boyd Corporation proprietary material UltraSlip® provides some of the best friction reduction in the industry, little or no impact on assembly and can meet FDA requirements for food or beverage applications. UltraSlip® can be added to most commonly used elastomers.
Like external lubricants, it is critical to verify that the internal lubricant chosen will be compatible with the system fluids.
Performance Verification
The final step in implementing these types of friction reducing solutions would involve getting performance verified in your application and ensuring that assembly operations are not adversely impacted by the lubricant option chosen. An experienced rubber seal manufacturer or supplier should be able to provide guidance to your group on best options for your specific application.
Article Co-Authored by:
Ken Schaefer
Craig Webb – Director of Business Development, Boyd Corporation
O-Rings.com by Boyd Corporation
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