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Added emphasis needs to be placed on understanding sealing system component layout and the effect on fluid film thickness. Thicker fluid film will reduce wear as seals ride on top of the fluid film. However, with thicker fluid films, there is potential for more leakage. Sealing system suppliers will want to add components to increase fluid film thickness under the primary seal to minimize the effect of wear while maintaining leakage control. Sealing system suppliers may also want to add components such as ingression rings to reduce the impact of contamination to the sealing elements. This is contrary to cylinder manufacturers’ desire to minimize the packaging space for sealing components and also cost. It is a balancing act of leakage versus wear versus cost, which needs to be understood between the sealing system supplier and cylinder manufacturer in order to meet the desired performance and cost targets.
PRESSURE BUILDUP – With systems running faster pressure buildup has an effect on the sealing system. Pressure buildup can come in these forms:
• Short duration buildup (i.e. pressure spikes) caused by
– Valving not responding fast enough
– Closing action of the valves
– Lack of appropriate accumulators
• Fluid film changing, allowing it to pass under the seal and not allowing it to enter back into the system, as the hydrodynamic pressure to feed back fluid is insignificant.
Whichever scenario pressure buildup is associated with, it affects life, leakage, and friction on the sealing system. This then has the potential effect on cylinders for
• Load limitations. Seals cannot handle the pressure to move the load.
• Position control. Friction is changing in the sealing system and this affects the feedback loop causing the potential for over and under-shoot.
• Life. Due to pressure buildup the seals can extrude and wear at a greater rate, limiting life of the sealing system.
• Energy Consumption. Increased energy is needed due to the higher friction.
• Cost. Cost increase due to increase energy consumption and downtime to due decreased sealing system life.
• Safety. There is a potential for having pressure between seals with the system not pressurized.
For cylinder manufacturers, minimizing the potential for pressure spikes is done by
• Investigating valving to ensure proper speed and smooth closing actions
• Ensuring appropriate pressure damping, such as appropriate size accumulators
• Ensuring appropriate pressure relief capability
• Moving the source of pressure pulses away from the sealing system
Sealing system suppliers counter pressure buildup via designs, but in different ways. For systems where rapid pressure buildup could occur, sealing systems are designed to knock down the pressure prior to the primary seal and to allow for fluid to be pumped back into the system. An example of this is a buffer seal (Fig. 5).
Pressure buildup between seals can be taken care of by either preventing the amount of leakage, which causes the pressure buildup by increasing the contact stress the seal has on the moving component, or by allowing the pressure to build up and then relieve it without damaging any components. Increasing the contact stress will eliminate hydroplaning of the seal, which allows a significant amount of fluid past the seal but will minimize fluid film thickness. This action can lead to thinner fluid films, increasing the changes for higher heat and wear. If the material and design can handle the higher heat and thinner fluid films over the expected life of the cylinder, then this is not a problem. The other alternative, to allow the pressure to build up and then relieve it without damaging any components, is done by different pressure-relieving product designs (Fig. 6). It shows a seal with an integrated check valve function, which automatically releases the inter-seal pressure when it is higher than the system pressure. In this case, as the pressure builds up between the seals, the O-Ring component in Fig. 6 moves, allowing pressure to be relieved through the pressure relief channel.
Another way sealing system suppliers deal with pressure buildup is to increase the strength of the design by changing design, changing material, and/or adding higher strength support materials, most commonly backup rings. Caution is used when proceeding with this route as assembly by the end user needs to be taken into consideration.
Although individual efforts done by the sealing system supplier and cylinder manufacturer can minimize the effect of pressure buildup caused by running faster, substantial additional progress can be made by working together. As mentioned previously, the sealing system is more than seals and bearings; it also is component layout, hardware conditions, and operating conditions. Combined efforts need to be done on
• Ensuring proper surface finish, rod hardness and/or surface coating. Inappropriate finish can allow too much fluid film to pass under the seal, leading to pressure buildup between seals. Conversely, inappropriate finish will not allow enough fluid film for the seal to ride on, causing excessive friction and wear.
• Understanding sealing system component layout. To reduce the effect of pressure buildup, adding or substituting components such as a buffer seal or the pressure-relieving seal in the sealing system may be the recommended course. However, packaging envelope limitations and cost could be the trade-off. Volume between seals could also be viewed as another alternative to reduce pressure buildup, but with more understanding of duty cycle to ensure that the pressure does not buildup over time and that the pressure is allowed to be relieved.
• Understanding fluid conditions. If the fluid viscosity is too high, the fluid may pass under the seal and not be allowed to reenter back into the system. Alternatively, lower viscosity fluid may not allow the pressure to build up between seals. However, this might not provide enough lubrication, causing higher heat and more wear.
• Understanding assembly. With pressure buildup, higher strength materials, which are less pliable, and/or added components, such as backup rings, may be the preferred direction. However, this adds complexity to the assembly process and needs to be understood and agreed on by both the sealing system supplier and cylinder manufacturer.
One additional item is the importance of understanding the event and communicating it. Without understanding the event and effectively communicating it, systems may be over- or under-designed, which will lead to increased cost, poor performance, and/or shorter life.
DYNAMIC EVENTS – With cylinders moving faster, there are a variety of dynamic events, other than pressure buildup, which need to be addressed:
• Starting/stopping
• Dithering
• Ballooning
• Vibration/Noise
The above listed effects can be a result of: system plumbing (valving and accumulators), filters, component connections, or momentum of the system running faster. For a cylinder manufacturer, this equates mainly to life, position control, energy consumption, and cost, and can be minimized by focusing efforts on cylinder design, valving, accumulators, and filters to minimize the starting/stopping and dither. For vibration/noise the concentration is on
• Increasing vibration/noise absorption
• Minimizing alignment issues for more uniform loading of the sealing system
• Keeping tighter tolerances, which minimizes variations for which the sealing systems need to compensate
As cylinder manufacturers try to minimize any dynamic effects, sealing system suppliers try to both cope and also minimize dynamic effects, which effect sealing system life, friction and leakage control, by looking at these areas:
• Ensuring that the sealing component can handle a wider variety of tolerance conditions. This includes ballooning.
• Ensuring adequate fluid film is possible under the seal in all conditions. This is done by design, contact stresses, and sealing component layout. Having an adequate fluid film thickness lowers friction, heat, and wear of the sealing system and allows it to survive a variety of conditions.
• Ensuring that the designs and materials used are strong enough to handle higher impact or dithering conditions. An example of this is specially formulated materials that are designed to operate in a dithering environment without affecting the mating hardware (Fig. 7). It shows the wear factor of different materials in various environments, including dithering.
• Ensuring that the designs can track the dynamic components throughout the cylinder duty cycle even with high vibration conditions
In order to optimize the solution, there must be a joint effort between the cylinder manufacturer and sealing system supplier to deal with the dynamic effects of cylinders moving faster. Some areas to consider jointly are
• Ensuring proper surface finish, rod hardness, and/or surface coating. Inappropriate surface finish and rod hardness will not allow an adequate fluid film under the seal and not allow the seal to handle all operating conditions.
• Maximizing the load distribution on the bearing. Too high of loading may add to system noise and vibration rather than dampening them.
• Understanding spacing and location of the bearing relative to the seals. Unsuitable location will not allow adequate support for alignment and will result in higher unit loading of the sealing components, the potential for vibration and noise issues, and a possible inability for the seals to track the dynamic surfaces in all conditions.
• Understanding sealing system component layout and the effect on fluid film thickness. With striving to have an adequate fluid film under the seal in all conditions allows the sealing system to better cope with the dynamic effects of starting/stopping, dithering, and vibration/noise.
• Understanding fluid conditions. Changes in viscosity and potential fluid breakdown affects the fluid film thickness which the seals ride on, too thin will result in poor sealing system performance, while too thick may result in leakage.
Similar to the “Pressure Build-Up” section, it is important to understand the operating conditions and duty cycle in which the dynamic event could incur and effectively communicate it. Communicating history on similar applications, both by the cylinder manufacturer and sealing system supplier, are important, as lessons learned from those instances can help avoid over or under designing. Regardless, any of the potential dynamic events listed here each needs to be considered individually and any changes must take into account its effect on performance and cost.
LONGER LASTING
Cylinders are being pushed for increased useful life. Sealing systems, which are crucial for functionality of the cylinder, are highly stressed components having a defined useful lifetime. As they are asked to last longer, fatigue, aging, wear, and contamination are the primary effects to address to ensure effectiveness for extended periods of time.
Fatigue – Fatigue is caused by repeated cycling of high stress over an extended period of time. In sealing systems, fatigue is most evident by fractures and/or missing material in the sealing system components. This has the effect of limiting life and changing friction and leakage performance of the sealing system. For cylinders, fatigue of the sealing system can cause
• Position control issues. Friction is changing in the sealing system and this affects the feedback loop, causing the potential for over and under-shoot.
• Load limitations. Seals cannot handle the pressure and thus limit load capacity.
• Increased energy consumption. Increased energy is needed to move the load due to the higher friction caused by changing seal performance.
• Shorter Life. Sealing system life is a factor that equates to cylinder life. As seals fatigue and eventually fail, life of the cylinder is affected.
• Increased Cost. A cost increase due to increase energy consumption, downtime due to decreased seal system life and potentially hardware damage due to debris generated from fatigue.
As fatigue is caused by repeated cycling of stress over an extended period of time, cylinder manufacturers need to look at reducing the amplitude or frequency of the cycling. One cause of this is by feedback related to the valving of the system, other design changes may be necessary.
Sealing system suppliers dealing with fatigue issues mainly investigate ways to limit the amount of stress on the components of the sealing system. This is done by
• Changing design. Lower the internal stresses in the sealing components. One way is to use a spring energized seal with a special spring, which has very low internal stresses (Fig. 8).
• Adding components, such as backup rings, to limit deformation, which will limit stresses.
• Changing materials to ones that have higher capacities of loading.
• Lowering the friction to lower internal stresses on components. This can be done by reducing seal contact stress, changing the design to allow for a different footprint, adjusting fluid film under the seal, or changing the material.