by Larry Castleman, Technical Director of Product Development,
Martin Franz, Product Development Engineer,
John McLaughlin, Manager of Product Testing and
Mark Sitko, Product Development Engineer
Based on practical experiences, these 12 do’s and don’ts can help confirm a sealing system will perform as designed.
When designing and developing equipment that uses sealing systems, confirming the sealing system performance is an important part of the equipment validation process. Sealing system performance can be directly linked to overall equipment performance, such as seal leakage to hydraulic cylinder life, and seal friction to hydraulic motor position control.
Validation is not just a series of tests, it is a process. It starts off with flagging that ‘validation is required,’ and then moves on to identifying the items of concern, continues through the testing, and finishes with analysis and confirmation of results. Unfortunately, market forces and conditions do not permit all possible uses to be considered during validation. Decisions have to be made to balance the:
- Risk: Risk of failure mode or system functionality loss and associated liability
- Investment: Effort required to properly validate the product
- Reward: Value of knowing system performance
While the number of critical elements to consider for a sealing system may seem daunting, there is general practical experience with many of these factors that can provide some assistance to avoid common mistakes and replicate common successes. Some of these practical experiences, referred to here as DO’s and DON’Ts, are more applicable than others, so each individual application will determine which ones are appropriate.
Pressure produces the energy required for the seal function. Additionally, pressure can energize and deform the seal, and change the fluid film in which the seal rides.
• Consider pressure rise rate effects. Rapidly rising pressure can introduce the potential of blow-by effect.
• Consider that a change in pressure affects fluid film. Higher pressures thin down the fluid film.
• Account for the potential of pressure spikes from valving and plumbing. Bear in mind the role of pressure in relationship to the dynamic profile. Pressure conditions significantly differ during various motion directions.
• Underestimate hardware ballooning or internal deformations caused by pressure. Changes to the mating hardware can severely affect sealing performance.
• Misjudge the effect pressure can have on internal contamination. Higher pressures can give contamination more momentum or accelerate the generation of debris, increasing wear.
• Don’t concentrate testing on the pressure spike portion only or only on the high-end system pressure. System pressure energizes seals, but a low system pressure may not be enough to energize the sealing system.
• Assume that system pressure is exactly the same as the pressure conditions the seal is exposed to in application.
Load or force is what the sealing system must absorb to ensure proper guidance of the piston or rod. When possible, bearings or wear rings should be incorporated to absorb the majority of this load to limit rod or piston deflection.
• Understand that for isolated tests, there is a need to transform load to displacement. Isolated tests, because of the size difference between test rig and actual application, cannot simulate the particular load being applied to the wear ring.
• Be aware that the scarf cut on wear rings are better suited to be on the unloaded side of the bearing. This ensures maximum material coverage and subsequent distribution of loading by the wear ring.
• Neglect impact. Impact conditions can exert extremely high loads on the wears, which may be beyond the limits of some materials, especially as the speed of impact increases.
Speed of application plays a significant role in many situations. Items such as duty cycle, motion parameters and accelerations all can improve or degrade sealing system performance.
• Understand that changes in the operating cycle, such as in linear cycling, can introduce different failure modes. For example, a system with slow rod extension speed and a fast retracting speed will have a different performance compared to a system with an opposite cycle of fast rod extension and slow retract. The latter would be more susceptible for inter-seal pressure trap. In this case, a special seal may be required to reduce pressure build-up between seals.
• Be aware that speed plays a significant role in fluid film and thus affects leakage, friction and wear. Leakage can be increased if the hardware velocities are decreased.
• Underestimate the importance of speed in sealing issues. Fluid film thickness is highly dependent on speed, so properties such as friction, wear and sealing effectiveness will all be significantly impacted.
• Miss the significant impact of fluid property changes. Increased heating, viscosity breakdown, or chemical separation and degradation associated with accelerated speeds often introduce artificial failure modes.
System dynamics go beyond the general operating parameters of the process, encompassing other factors such as vibration, deformation and alignment, which can have a significant effect on sealing system performance.
• Understand that offset or misalignment has a significant effect on sealing performance. Offset puts higher localized stress on sealing components and limits life. Increased bearings allow for better alignment, which can help reduce the localized stress and thus increase seal life.
• Underestimate the effects of pressure hold on sealing components. Constant high pressures can cause creep of material, which can prematurely deform sealing components and limit effectiveness.
• Ignore the issues associated with impact, buckling, and other similar dynamic events. Events caused by instantaneous reactions or instability often are difficult to replicate, or become significant noise factors in a test.
Temperature is not only the outside environmental conditions or fluid movement through valves and hoses, but also the frictional heat caused by the sealing system interaction with mating components and operating conditions.
• Take into account the frictional heating due to the sealing system. This may significantly increase overall temperatures of operation near the seal. Keep in mind that higher temperatures can weaken seal and bearing materials and could increase deformation and thus change performance.
• Take into consideration that temperature cycling can affect temperature limits of materials. For example, cycling of temperature or extended durations at high temperatures may decrease the low temperature effectiveness of elastomers.
• Test a system at temperatures greater than sealing component limitations. Possible effects include severe premature aging of the seals and other failure modes.
• Underestimate the temperature rise due to equipment shutdown. Lack of fluid flow from shutdown can raise temperatures a number of degrees, which may put the temperature beyond the capability of the material.
• Exceed fluid temperature limitations. Excessive temperature breaks down the fluid film under the seal and creates premature wear and higher friction.
Seals and bearings are highly stressed components that have a life span. Thus, time has an effect on performance. Ideally the components will function the same after many hours of use compared to when they first start.
• Understand that seals and bearings have a time component associated with them. Elastomers tend to stress relax and plastics tend to creep or cold flow. Stress relaxation reduces force over time exerted by the seal to the sealing surface. Creep is an increase in deformation or strain over time under a constant load or stress.
• Understand the working fluid and the potential change of properties over time. Breakdown of fluid could change heat transfer rates, lower viscosity of the fluid, or change the chemical makeup of the fluid, making it more aggressive to the sealing components.
• Underestimate the effect of component fatigue. Fatigue of seals and bearings goes beyond the cyclic operation. As seals and bearings have an interaction with the mating components, fatigue may result in fluid break down, hardware changes due to shaft hardness or coating, or to component layout.
• Undervalue that the surfaces in contact with the seal can change over time. Time-dependent behavior like corrosion, fatigue, and other factors can significantly alter the sealing surfaces.
• Exceed fluid temperature limitations. Temperature breaks down the fluid film under the seal and creates premature wear and higher friction.
Proper assembly is needed from a functional and a practical standpoint, such as in assembly rates for volume product and for field assembly. Changes in this area will significantly affect end performance, as historical studies have proven many times that premature failure of sealing products is most often a result of poor installation.
• Take into consideration the recommended assembly dimensions and hardware for installation of seals and bearings. Use of proper chamfers, radius edges, and proper resizing and installation tools helps with avoiding sharp corners.
• Take care of installation over ports. Proper orientation, such as not allowing cut section of seals over ports, will lessen assembly damage. Also take into consideration seal cross-section to port size. The seal cross section needs to be larger than the port to limit potential damage.
• Take into account field assembly potentials, as these will differ from factory installation in many cases. Simple changes may be needed to ease field assembly, such as cut piston bearings having a tendency to be easier to keep in the groove upon installation as compared to rod bearings, which are more difficult to assemble and insert a shaft through successfully.
• Consider assembly accessibility. Some configurations may have seals or bearings installed deep within a cylinder. Available space for seal manipulation into the groove does not exist in real application, so resulting distortion or change caused by assembly can be significant.
• Take into consideration assembly lubricant. It needs to be compatible with the seals and system components, but not significantly different in makeup or application to induce excessive leakage or friction. For example, some lubricants used in installation can dry and cause the seal to stick to the shaft. This will lead to high friction or seal twisting.
• Keep in mind the media used for preheating hard seals for installation. Due to the stiffness of the seals or bearings, in some cases they need to be preheated prior to installation to soften the material enough to allow it to be distorted for installation.
• Underestimate the effects rapid stretch has on sealing components. Although the seal might have an ASTM callout of 100% elongations, do not assume this is possible within a severely reduced time. Higher speeds on installations increase the possibility of cracking, fracture or permanent deformation.
• Underestimate the effect scaling of seals (moving up or down in size of seals) has on assembly. Significant difference in assembly effort can be noticed when installing different diameter seals; for example a 1-in. diameter u-cup versus a 12-in. diameter.<
Media is both the fluid being retained and the contamination the seal is trying to keep out.
• Understand that fluid properties can lead to chemical attack on sealing components. This leads to additional wear and changing friction.
• Understand all potential sources of contamination. For example, water into the system may increase the potential for hydrolysis, which will affect certain seal materials greatly. Even excessive amounts of air can significantly alter performance.
• Underestimate the cleanliness of fluid. Contamination can cause greater wear, abrasion and impact damage to the seals and also affect the fluid film under the seal.
• Ignore the effect of energy density and size. As the energy density of some systems increase, there is more momentum of the contaminants in the system.
• Miscomprehend the replacement/replenish profile of fluid. This could affect fluid properties. Tests run using fluids of much greater or poorer conditions than those seen in the real application often result in erroneous conclusions.
9. Mating surfaces
The sealing bearing interface is made up of the mating materials along with the resulting surface finish. For dynamic seals, this is particularly important, as dynamic seals ride on a fluid film.
• Understand that various surfaces have different material properties that affect fluid retention under the seal and thus wear and friction.
• Understand that changes to the finishing processes can have an effect on the resulting surface finish and thus sealing and bearing performance.
• Underestimate the value in surface finish recommendations from sealing suppliers, as the optimization process for best sealing performance is dependent on achieving the right balance of friction, wear and leakage.
• Underestimate any change to the mating surfaces. Often validation requires a significant number of components that are expensive or hard to duplicate during scaling.
Process is the manufacturing, assembly, inspection and other steps for producing seal and bearing components.
• Realize that there are process and material limitations in producing some geometries due to size or complexity of the part.
• Understand that there are sometimes significant differences in performance depending on how sealing or bearing geometries are produced.
• Make significant process changes between prototypes and end production methods.
• Ignore the understanding of all process steps involved in production and handling of an acceptable production part. Any step in this process can significantly alter time, temperature, media, or other factors that critically affect sealing performance.
Design is the particular geometry of the seals and bearing.
• Understand the importance of working with sealing suppliers to determine the potential sealing system. The particular geometry, material and layout of a sealing system depend on both a variety of factors along with the particular weighing of importance of these factors.
• Understand the design limits of some geometries and how to measure them. For example, on composite bearings, depending on the method of manufacturing and the location being measured, this can alter the particular results of measurements.
• Assume that scaling (linear change in dimensions) of geometry will equate to the same factor change in the outputs of the sealing system (friction, life and leakage).
• Change layout (location, spacing, orientation, etc. of and between elements) without first considering possible effects of such changes. For example, changes in spacing can significantly affect the heat transfer and pressure buildup between system elements. Another example is significantly altering the position of the bearings in the system, as the resulting deflection required of the seal can be altered.
Materials are the composition making up the structure of the seals and bearings. Materials, process and design are linked together as part of the variables required to produce and use a product.
• Understand that with the same seal or bearing geometry, materials have a significant effect on component outputs. Substituting materials for seals, bearings, mating surfaces or fluids should require careful consideration of the risk and reward for this activity.
• Understand that specific materials may allow for additional performance benefits for seals or bearing components. For example, the teardrop shape in Turcite bearings offers benefits, as the material is designed to improve the formation of a lubricant film to assist with the sealing system life, friction and wear.
• Underestimate the value of sealing and bearing materials. There are special materials in which typical callouts, such as ASTM callouts, cannot fully describe the benefits.
• Underestimate the influence of materials on the seal/bearing/fluid/dynamic sealing surface. Many seals work using transfer films, exposure of certain fillers, reduction of peaks on the dynamic surfaces, and other mechanisms.
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