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Combating dieseling and explosive decompression for sealing system suppliers is done via material selection. Dieseling requires higher strength materials that will withstand the combustion of the air. However, higher strength materials are less compliant. Friction, leakage control, and assembly are additional issues that are associated with higher strength materials. Explosive decompression is also dealt with in materials, but in a different fashion. Limiting the materials’ ability to absorb gases, maximizing the material to release gases, or improving the strength to resist damage are the characteristics desired.
For viscosity changes and fluid breakdown, sealing system suppliers investigate both bearings and seals.
Bearings- For bearings, the main target is to ensure that the design, material, and its location relative to the seals will be able to handle the pressure, provide the necessary support and aid in forming a fluid film for the seals, all while staying within the system cost targets.
Seals- For seals, the main emphasis is to investigate the use of materials and designs that will handle the increased pressure. A secondary effort would be to lower the friction, causing a decrease in the temperature near the seal. Ways of doing this are to reduce seal contact stress, change the design to allow for a different footprint, adjust fluid film under the seal, or change the material.
Between the cylinder manufacturer and the sealing system supplier, there are additional actions which can be taken to ensure fluid film integrity due to cylinders running stronger. They are similar to areas previously covered:
• Ensuring proper surface finish to allow for adequate fluid film under the seal
• Maximizing the load distribution on the bearing. Too high of a local loading will add to the system heat and limit life.
• Changing lubrication pattern underneath the bearings
• Understanding spacing and location of the bearing relative to the seals
• Understanding sealing system component layout and the effect on fluid film thickness to reduce friction
• Understanding fluid conditions
• Understanding potential hardware dynamics
• Understanding assembly
Hardware Dynamics – As hardware is pushed to operate in stronger conditions, there is increased potential for various hardware dynamic issues to affect the sealing system and thus the cylinder performance:
• Hardware ballooning /internal hardware deformation. Deformation of the hardware due to pressure
• Side loading. Moving more load over the total stroke imparts side loading.
• Fatigue. Cycling modes of high stress over an extended period of time has a potential impact on both sealing components and various cylinder components.
• Vibration/noise. With higher pressures, vibration of the cylinder may not allow for the seal to track the mating components effectively.
• Pressure buildup. Pressure buildup is due to the valving/plumbing allowing for pressure spikes.
• Contamination. With higher pressures, contamination has greater momentum and may destroy the sealing components.
The listed hardware dynamics are a result of various factors such as pressure, system plumbing (valving and accumulators), filters, component connections, and operating conditions, to name a few. In general, these dynamic issues affect the sealing system leakage, friction, and life, which affect the cylinders’ position control, loading capabilities, energy consumption, life, and overall cost.
Cylinder manufacturers try to minimize any of these effects while sealing suppliers do their best to try to both manage and minimize them. There are also joint efforts between the sealing system supplier and cylinder manufacturer that can be done to better deal with the various hardware dynamics. Both the individual actions and joint activities as listed in the previous sections “Faster-Pressure Buildup” and “Faster-Hardware Dynamics” are applicable here. Regardless, any of the potential dynamic effects listed needs to be considered individually, and any changes must take into account the effect on performance and cost. This approach ensures all possible failure modes are considered and that the components are not over-engineered, which leads to increased cost.
Higher Heat and Wear – With stronger operating conditions, higher heat and wear are effects that change the performance of the sealing system. Both higher heat and wear are related. With increasing heat, more wear can be expected. Likewise, an increase in wear changes the friction of the sealing system and adds heat to the system.
Similarly, for both higher heat and wear, the same basic causes and effects on the sealing system and cylinder as referenced in the sections “Faster-Higher Heat” and “Faster-Wear” apply here. However, higher heat and wear, in this instance, are caused by higher pressures instead of higher velocities. For example, the dynamic effect of the seal in contact with a running surface worsens with higher pressures. As many of the sealing components are pressure-energized, this causes the seal to impart greater contact stresses, leading to potentially higher friction and wear.
As with causes and effects, the actions done individually by the cylinder manufacturer, the sealing system supplier and jointly between them can also be applied for stronger operating conditions. The need to maximize cooling, ensure alignment, and ensure an adequate fluid film is crucial to dealing with the higher heat and the wear from stronger operating conditions. Again, refer to the “Faster-Higher Heat” and “Faster-Wear” sections for more details.
One additional item which also needs to be jointly addressed is assembly. Although the above actions are generally the same for “Faster” conditions, assembly is one area with significant changes. With stronger operating conditions, typically higher strength seal designs are needed. These higher strength designs, whether by materials change, added components, or complete design changes, mean less compliant seal materials, increased number of seal components and/or greater spacing between the seals. All these factors lead to less assembly-friendly conditions. Limitations in ability to deform the seal for assembly or the inability to assemble into a closed groove configuration need to be understood by the cylinder manufacturer and sealing system supplier for success.
DISCUSSION
There are a lot of commonalities on both the effects on the sealing system and the actions needed by cylinder design engineers, sealing system engineers, and both disciplines together to deal with hydraulic cylinders moving faster, stronger, and longer. In general, the effects on sealing systems are leakage control, life, and friction. This in turn affects cylinders’ position control, load capability, energy consumption, life, and cost.
Individual efforts can be done by both the cylinder manufacturers and sealing system suppliers to improve sealing system and cylinder performance. For the sealing system supplier, the individual efforts can be seen as launches of new designs or materials and a better understanding on how to apply them. For the cylinder manufacturer, it is better plumbing/valving, alignment, and understanding the capability leading to general improvement in cylinder performance. However, there are actions where individual efforts are insufficient. Joint effort is needed, as there are significant interactions of the sealing system with the cylinder, and any action will have a direct effect on either the sealing system or cylinder performance. Although not listed for every area of concern, joint actions include
• Ensuring proper surface finish
• Ensuring proper rod hardness and/or surface coating
• Maximizing the load distribution on the bearing
• Changing lubrication pattern underneath the bearings
• Understanding spacing and location of the bearing relative to the seals
• Understanding sealing system component layout and the effect on fluid film thickness to reduce friction
• Understanding fluid conditions
• Understanding potential hardware dynamics
• Understanding assembly
• Effectively communicating
With each of the above, both the cylinder manufacturer and sealing system supplier need to view how these joint activities will affect issues like leakage control, friction, life for the sealing system supplier and position control, load capability, energy consumption, life, and cost for the cylinder. Also, the underlying effect on how the above will impact both packaging envelopes and cost needs to be taken into consideration early. With teamwork, the outcome of these efforts can be cylinders running faster, stronger, and longer than ever before.
As discussed several times, many of these effects are linked. For example, heat and wear are effects caused by cylinders moving stronger, lasting longer or moving faster. Heat and wear themselves are also linked; higher heat effects wear, and increased wear can lead to higher heat. Achieving the proper cost, life, leakage, and friction out of a sealing system is a balancing act. As an example, Fig. 15 shows a spider chart meant to demonstrate how the factors associated with sealing system performance can vary depending on the desired characteristics. The ideal solution is long life, low leakage, low friction, and low cost. In most cases, there is a baseline solution upon which improvement is desired. As shown in Fig. 15, if one factor, such as life, is increased, then the other factors like cost are negatively impacted. Conversely, lowering the cost usually means increasing the leakage, increasing the friction or decreasing the life.
Individual efforts by cylinder design engineers and sealing system engineers alone can allow movement in the different scenarios, but individual efforts combined with working together will allow both the sealing system supplier and cylinder manufacturer to move much closer toward both parties’ mutual goals.
CONCLUSION
The sealing system is more than just a compilation of individual seals and bearings. It also encompasses component layout, hardware conditions, and operating conditions. Therefore, an effective sealing system is jointly owned by both the cylinder manufacturer and sealing system supplier. Regardless of the characteristics wanted out of the cylinder (faster, longer, or stronger), getting there means effective communications between the cylinder designer and the sealing system engineer should occur upfront and as early as possible. Moving these discussions early in the design stages allows all to see which efforts can be done by the cylinder manufacturer and sealing system suppliers by themselves, and which actions need to be jointly done to have a cost-effective sealing system which produces the desired friction, leakage control, and cost targets for the desired life of the cylinder.
CONTACT
Trelleborg Sealing Solutions R&D, 2531 Bremer Road, Fort Wayne, IN 46803 USA, (260) 749-9631, www.tss.trelleborg.com/us, mark.sitko@trelleborg.com, larry.castleman@trelleborg.com.
Additional Sources
• Trelleborg Sealing Solutions Hydraulic Seals-Linear catalog, April 2007
• Trelleborg Sealing Solutions Zurcon® U-Cup RU9, May 2006
• Trelleborg Sealing Solutions Zurcon® Buffer Ring
• Trelleborg Sealing Solutions Turcon® Stepseal® V, April 2007
• Trelleborg Sealing Solutions Design Guide Variseal®/Varilip®, 2007
Definitions, Acronyms, Abbreviations
Backpumping: This is the action in which fluid film passes between the seal and the mating dynamic surface during an extend stroke and then is returned to the system during the retract stroke through hydrodynamic principles.
Hold under pressure: Operating condition in a cylinder where a load is held for an extended period of time without any rod movement. This puts the seal in a static condition under high pressure which increases the likelihood for permanent deformation and extrusion and eliminates or reduces the fluid film under the seal, leading to higher wear and friction.
Dithering: Refers to an operating condition in a cylinder where the dynamic surface moves a very short distance and at high frequency, which drives out fluid film under the seal to create higher friction or wear or creates little fluid exchange, leading to higher heat
Hydroplaning: This is a situation in which the hydrodynamic forces lift the sealing contact off the dynamic surface and allow fluid to pass under the seal.
Ballooning: Expansion of the cylinder housing caused by either thermal or pressure situations
Creep: The increase in seal deformation when under a constant pressure/load
Stress Relaxation: The decay of sealing stress over time when placed under a constant strain/deformation
Dieseling: This is the sudden pressure increase in an oil-air mixture in which the air bubbles ignite.
Explosive Decompression: An event in which high pressure can permeate the surface of the sealing material. When the system pressure is rapidly released, the sealing component can experience a sudden degradation or complete failure caused by the fluid trapped in the sealing material suddenly expanding and ‘exploding’ the material.