What is in this article?:
- Upping the ante on seal performance
- Wear resistance
Todays hydraulic cylinders move larger loads, faster, and over longer distances. Heres how it affects the seals.
Higher wear related to faster cylinder movement can be caused by:
• The time-related dynamic effect of seals and bearings in contact with the rod and housing.
• Poor lubrication.
• Increased impact of contamination in the system.
• Heat-related changes to material properties.
Similar to the above discussion, wear adversely affects a cylinder’s position control, energy consumption, life, and overall cost; and sealing system friction, leakage, and life. And wear severe enough to damage seals and mating hardware will drive up costs.
As mentioned previously, ensuring proper cylinder alignment and cooling helps reduce wear. Also, minimal system contamination reduces the abrasive impact particles have on seals; tighter tolerances narrow the variations which the sealing system must handle; and limiting conditions such as hold-underpressure and dithering, where the seal operates under poor fluid-film conditions will all have a positive effect.
The same basics with regard to heat also apply to reducing seal wear. However, when reviewing designs and materials, wear resistance instead of temperature capability is the desired characteristic. Designing for wear is significantly more complicated, as wear is a function of surface finish, running surface hardness, fluid, material, design, velocity, temperature, and loading. Engineers must consider all these factors when testing to create meaningful data on wear characteristics of materials and products.
For instance, the accompanying graphic shows some typical test results, and how the choice of material may change based on operating conditions. Review test data with regards to product design, system component layout, operating conditions, and mating hardware – as the seal material is just one part of an effective sealing system.
The same recommendations to combat heat also apply to reducing wear. But designers must place added emphasis on understanding sealingcomponent layout and its effect on fluid-film thickness. A thicker film reduces wear as seals ride on top of the fluid, but it increases the potential for leakage. Seal suppliers often add components to increase fluid film thickness and control leaks, as well as ingression rings to reduce the impact of contamination on the seals.
Cylinder manufacturers, on the other hand, want to minimize the space and cost for sealing components. Thus, seal suppliers and cylinder manufactures must balance leakage versus wear versus cost to meet desired performance and price targets.
Pressure buildup can affect the seals in fast-running systems. Causes include short-duration buildup (pressure spikes) due to slow valve response, closing action of the valves, and lack of appropriate accumulators, or from letting the fluid film pass under the seal but not re-enter the system, as hydrodynamic pressure to feed back fluid is insufficient. Resulting effects on cylinders include:
• Load limitations. Seals cannot handle the pressure required to move the load.
• Position control. Frictoin is changing in the seal ing system, and this affects the feedback loop and causes the potential for overand undershoot.
• Life. Pressure buildup can extrude seals and increase wear rate, causing premature failure.
• Energy consumption. Higher friction demands more energy to move the cylinder.
• Cost. Higher energy consumption and downtime increases costs.
• Safety. There is a potential for residual pressure between seals when the machine shuts off.
Cylinder manufacturers can minimize pressure spikes by ensuring valves operate at the proper speeds and have smooth closing actions. Also maintain appropriate pressure damping with accumulators and pressurerelief valves. Moving the source of pressure pulses away from the sealing system also helps.
Sealing-system designers can counter pressure buildup in various ways. Systems with rapid pressure buildup can minimize the impact by reducing pressure before it reaches the primary seal, such as using buffer seals.
Several methods mitigate pressure buildup between seals.
One is to increase contact stress between the seal and moving component and prevent leakage that causes pressure buildup. Higher contact stress eliminates hydroplaning which lets fluid get past the seal, but results in thinner fluid films that may increase heat and wear. If the material and design can handle higher temperatures and thinner fluid films over the expected life of the cylinder, then this is not a problem.
Another alternative uses pressurerelieving seal designs. As the name implies, it vents built-up pressure without component damage. The accompanying graphic shows a seal with an integrated check valve that automatically releases inter-seal pressure that exceeds system pressure.
A third option increases strength by changing the design, material, or adding support materials such as backup rings. Use caution with this route as it can complicate assembly and have other adverse effects, such as increased frictional heat and wear of mating hardware.
Other ways to minimize the effects of pressure buildup include:
• Ensure proper surface finish, rod hardness, and surface coating. An inappropriate finish can let too much fluid pass under the seal, leading to pressure buildup between seals. Conversely, it may mean the seal rides on an insufficient fluid film causing excessive friction and wear.
• Understand sealing system component layout. Adding or substituting components such as a buffer seal or pressure-relieving seal may reduce pressure buildup. However, packaging envelope limitations and cost could be the trade-off. Adding fluid volume between seals could also reduce pressure buildup. But understand the duty cycle to ensure that pressure can be relieved and does not build up over time.
• Check fluid conditions. If fluid viscosity is too high, fluid may pass under the seal and not reenter the system. Alternatively, low-viscosity fluid may prevent pressure buildup between seals but might not provide enough lubrication, increasing heat and wear.
• Review assembly. High-strength materials, which are less pliable, and extra components such as backup rings may be the preferred method to combat pressure buildup. However, this complicates assembly.
Fast-moving cylinders can produce abrupt starts and stops, dithering, ballooning, vibration, and noise. Sources include system plumbing (valves and accumulators), filters, connectors, and the momentum of fast-running systems. These impact cylinder life, position control, energy consumption, and cost. Good system layout and the right valves, accumulators, and filters can minimize start/ stop and dither effects.<
For vibration and noise, concentrate on vibration damping and noise absorption; minimizing misalignment for uniform seal loading; and tighter tolerances, which minimize variations for which the seals must compensate.
Seal suppliers can also alleviate dynamic effects in these areas:
• Ensure that sealing components handle a wide range of tolerance conditions, including ballooning.
• Ensure adequate fluid film under the seal in all conditions through design, optimum contact stresses, and seal layout.
• Ensure designs and materials are strong enough to handle high impact or dithering conditions. Specially formulated materials are designed to operate in dithering environment s without affecting the mating hardware.
And design factors such as proper surface finish and rod hardness ensure an adequate fluid film that lets the seal handle all operating conditions.
Good bearing load distribution helps damp noise and vibration, while excessive loads may add to the problem. Unsuitable bearing locations means inadequate support and alignment issues, resulting in higher unit loading of sealing components, the potential for vibration and noise, and inability of the seals to track the dynamic surfaces in all conditions.
Finally, understand sealing system component layout and the effect on fluid film thickness. Striving to have an adequate fluid film under the seal in all conditions lets the sealing system better cope with the dynamic effects of starting/stopping, dithering, and vibration/noise.
For more information, contact Trelleborg Sealing Solutions, Fort Wayne, Ind. at www.tss.trelleborg.com/us.