Colin Macqueen is director of technology for custom seal manufacturer Busak+Shamban Americas. Mr. Macqueen has been with Busak+Shamban and its sister companies for the past 24 years, serving in various engineering, technical, and sales roles. He is a specialist in automotive fuel system, climate control, and emissions system sealing and leads a 21-person research and development group covering all of Busak+Shamban’s industrial, aerospace, and automotive market segments. Mr. Macqueen can be reached at [email protected] or 260 748-5744.
Joel T. Johnson is vice president of technology for Simrit, the industrial products division of Freudenberg-NOK. Mr. Johnson has been with Freudenberg-NOK since 1989. Currently, he oversees the transfer of technology from all global Freudenberg-NOK locations to Simrit, and he trains Simrit personnel on Freudenberg-NOK products and plant capabilities. He is also Simrit’s chief liaison on high-profile technical programs and is a member of the Society of Automotive Engineers. Mr. Johnson can be reached at [email protected] or 847 428-1261.
Q: What are some of the key issues users should be aware of when specifying a seal for a given application (e.g., chemical/material compatibility, pressure, temperature, etc.)?
CM: The most basic requirements to understand are pressure, temperature, dimensions, and fluid media to be sealed. But often it is the less obvious application requirements that can cause issues with seals. Surface finish, for example, significantly affects friction and seal life. Side-loading conditions can cause unexpected leakage and tolerances must always be considered, especially in high-volume applications where maximum and least material conditions are likely to occur sooner or later. The type of motion also can cause unexpected behavior — for example, in a very short-stroke dither or a very low-angle oscillatory application wear debris cannot escape and heat cannot dissipate as easily as in full reciprocating or rotary service, so different designs and materials may be required.
Temperature conditions can also hold surprises. Most seal users understand the basic service temperature of their equipment, but unusual conditions can cause temperature peaks that degrade materials. The running friction of the seal itself can cause local temperature spikes if it is not effectively dissipated by the fluid or the housing.
Specifying the ideal seal for an application is always a matter of balancing various characteristics. In dynamic applications, the most common consideration is friction versus leakage control. For this reason, Busak+Shamban application engineers work closely with customers to understand the broader view of the application and specify the optimum sealing system.
JJ: At Simrit we use the acronym STAMPS (speed, temperature, application, media, pressure, sizes) to help define basic sealing needs. The word seal is a very broad and generic term, and without functional data it is very difficult to choose the correct design. For example, a different seal design is used for static applications when compared to rotating applications, and reciprocating seal designs are completely different than rotating designs. One must also recognize that every sealing material has a temperature and chemical compatibility limit. Several questions to ask are:
- Where will the part be used?
- Will the seal contact a fluid?
- Is there pressure involved at the seal?
- Is there motion involved and is it defined?
- Are the component drawings available?
- Are the contacting surface finishes defined?
- What are the life targets (hours, cycle, etc.)?
- What is the temperature range of the fluid at the seal?
- Are material specifications necessary (UL, FDA, NSA, etc.)?
- Are the design parameters set or are they flexible?
- What are the dimensional quality requirements?
Q: How have the seals of today evolved from previous generations of sealing technology?
CM: New materials using nontraditional ingredients are allowing applications with higher pressures to be sealed with lower abrasion. Elastomers are now available with significantly higher and lower temperature capabilities and broader chemical resistance than ever before, and spring-energized seals are available to take over when the limits of elastomers are exceeded.
The use of nonlinear finite element analysis (NLFEA) allows designs to be mathematically modeled before producing prototypes. Laboratory techniques, such as compressive stress relaxation and dynamic mechanical analysis, can be used in conjunction with three-dimensional NLFEA to better predict the long-term behavior of seals. Advanced testing facilities are used to validate these models and allow the supplier to provide robust, proven solutions to their customers.
Seals today are also being asked to perform additional functions such as noise and vibration dampening, metering and monitoring tasks, and handling complex combinations of linear, rotary, and angular motion.
JJ: Some of the greatest advancements in seals today are due to material formulations. As the need for improved chemical resistance, higher temperature resistance, zero leak systems, and longer service intervals increasingly become key to improving the efficiency and value of a product, the sealing materials, and designs that compliment these materials, have advanced. The link between the design engineer and the materials lab has become a critical success factor not only in the functionality of a seal, but also in regard to its manufacturability.
Q: What are the most common causes of seal failure?
CM: Early seal failures are usually attributed to installation issues — damage, misassembly, or incorrect seal selection. In-service failures over time are related to degradation caused by any of the service conditions I detailed earlier. Poor surface finish and fluids contaminated with debris lead to accelerated abrasion and wear; unexpected temperature peaks can rapidly degrade elastomers if above the anticipated service range; and unusual loading conditions can reduce the loading significantly. Many failures are hard to diagnose, as the seal appears to be undamaged and mating components are often unavailable. In investigating failures, remember that the seal is part of a system in which all elements need careful review before defining a true root cause.
JJ: The most common cause of seal failure is seal installation. Since most seals are made from a material that can be cut, nicked, bent, or stretched, proper installation is critical. Behind each design is a theory on how the cross section should look, how the load should be applied, and how the system should function. If this system is compromised upon installation, failure occurs. Consulting your seal supplier on how to install a part is key to success. If the seal is installed correctly, the most common failure mode is usually heat, chemical, or wear-related. When seals are exposed to an environment outside of what they were designed for, there is an increasing chance of failure. Many times a countermeasure can be identified if the end-user returns the seal in good working condition for analysis. Unfortunately, removing a seal without damaging it is often difficult or overlooked. At Simrit, we provide local application engineers for on-site expertise in these issues.
Q: How can users extend the lifespan of seals? Are there certain best practices users can implement to ensure the longevity of seals and prevent failure?
CM: By working with an integrated sealing system supplier, seals and bearings can be specified as a package and their interactions clearly understood. If multi-stage sealing systems are required, it is vital that these seals are specified together. For example, an over-aggressive, inner-rod seal can starve the outer seal of lubrication, causing rapid degradation and loss of ability to perform when it is most needed. A piston seal with inadequate support can deflect underside load causing leakage and damage to the cylinder bore. Many users now are asking Busak+Shamban to supply seals and bearings preinstalled into their housing, leading to simpler assembly, fewer suppliers, reduced inventory, and the security of knowing that one reputable supplier is responsible for the whole seal system.
The most important best practice is to involve the seal supplier early in the design process. Too often there are unnecessary constraints, which could have been eliminated if the sealing system were specified earlier.
JJ: In general, you can say that by minimizing the effect of the temperature, pressure, speed, and aggressiveness of the media involved you can help to extend seal life. The condition of the mating components is also extremely critical. A design is usually based around a nominal condition and any time you deviate from the nominal you affect the end result. This includes things like machining tolerances, exposure to contamination, variation of installation process, and many other extenuating variables.
Q: What are some of the key warning signs of seal wear? How do users know when they need to replace seals?
CM: Most seals are completely contained in their housings and therefore inaccessible. The first sign of impending failure is often low-level leakage of system fluid. Many users therefore choose to replace seals as a routine part of the preventative maintenance programs. When replacing seals, check for distortion, wear, or permanent set of the old seal as an indicator of aging. Damage to the mating dynamic surfaces may also indicate a fundamental issue with the sealing system specified. Do not install new seals onto a previously damaged surface; wear will only accelerate as a result.
JJ: At Simrit we recommend determining what the seal life limitations are through testing and then setting up a regular maintenance schedule to change the seals before failure. Unfortunately since most seals are hidden, often there are no warning signs until the part starts to weep fluid. This could obviously cause damage to the system, so changing the seals before failure is recommended. It is important to always use certified parts in critical applications.
Q: How can users best determine the appropriate seal dimensions for a given application?
CM: Many suppliers produce seals for use in standard gland dimensions and literature is widely available with recommendations. For nonstandard applications however, it is always best to work closely with the supplier’s application engineers to determine an optimum sealing system.
JJ: For most standard type seals, there are standard bore, housing, surface finish, etc. specifications that can be found in many reference materials. The RMA, DIN, JIS, MIL, and other standards exist for these types of products. If you have a custom application, it is recommended to contact your seal supplier for this information. Often the seal is overlooked until the end of the design, but in actuality this should be reversed. Good sealing solutions require a certain amount of physical space, and by starting at the seal you will ensure that whatever you are making does not leak when your design is completed. It is also a very good way to minimize the complexity and cost of the system. Simrit has design engineers that specialize by product family to guarantee the best match between the seal and the operating conditions.
For More Information: www.busakshamban.us; www.simrit.com
