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.
Today’s economic climate is forcing machine and vehicle builders to improve the productivity, efficiency, and longevity of the equipment they make and sell. As a result, operations across many industries are now demanding hydraulic cylinders that move faster, generate higher forces, and last longer than ever before.
In addition, societal demands are increasing requirements for cylinders that operate with high efficiency and little or no leakage – to diminish the environmental impact.
These conditions mandate high-performance sealing systems that control leakage, lower friction, and minimize costs over the expected life of the cylinder. This requires careful design and collaboration among cylinder and sealing-system engineers.
This article examines seal design considerations related to higher operating speeds. In a future installment, we’ll look at how the push for higher pressures and longer life affect seal design.
Fast cylinder movement can have a variety of meanings, including:
• High extension or retraction rates.
• More cycles in a given time.
• Less dwell time between extension and retraction.
• Higher acceleration and deceleration rates.
All these conditions affect the seals. They can generate more friction and wear, cause pressure buildup, and lead to unforeseen dynamic events that can play havoc with the sealing system. Here’s a closer look at each.
As hydraulic systems move faster, they tend to generate more heat that can adversely affect seals and cylinders. Heat sources include:
• Dynamic effects between the seals and bearings and the rod and housing, which typically worsens with higher velocities.
• Valves and other control units that shift faster or convey more fluid in a shorter time.
• The fluid and surrounding hardware, which has less time to cool.
• Fluid breakdown or viscosity changes.
• Hose and tubing routing.
• Braking devices that give quick, accurate stopping.
High temperatures can alter seal friction, leakage, and durability. For cylinders, heat can affect position control, energy consumption, life, and overall costs. For instance, seal performance can change with temperature, causing the cylinder’s feedback system to generate overshoot or undershoot and affect positioning. Changes in fluid viscosity and seal characteristics mean the pumps must work harder, reducing efficiency. Cylinder life is directly linked to seal life, which tends to degrade as temperatures rise. And downtime, lower performance, and higher energy consumption all affect costs.
To minimize the effects of heat, cylinder manufacturers should ensure proper alignment to uniformly load the sealing system, minimize external heat sources, cool the fluid, and ensure adequate fluid exchange near the seal.
Seal suppliers should investigate bearings and seals. For bearings, ensure the design, material, and location relative to the seals will handle the heat, limit additional heating from bearing friction, support the seals, and potentially aid in forming a fluid film under the seal. For example, some polymer bearings use a “teardrop” pattern to help form such a film.
For seals, primarily investigate high-temperature materials and designs. A secondary effort would be to decrease friction to lower temperature near the seal. Ways of doing this are to reduce seal contact stress, change the seal footprint, adjust fluid film under the seal, or change the material.
One example that incorporates all four changes is a new U-cup from Trelleborg. The design increases temperature capability and lowers friction compared to standard U-cups. It also contains a micro-structure combined with back-pumping channels on the dynamic seal slide segment that ensure a constant lubrication film underneath the seal sliding surface and back-pumping across its entire pressure range. This reduces friction and temperature near the seal. Tests have shown that at higher speeds, the Ucup produces less friction compared to other designs.
Although seal suppliers and cylinder manufacturers can work independently to minimize effects of heat generated from fast-running systems, a team approach works best. The sealing system is more than just seals and bearings. It also encompasses component layout, mating hardware, and operating conditions. Here are some examples of steps that both seal supplier and cylinder manufacturer can take together to handle the heat.
• Ensure proper surface finish. Inappropriate surface finish does not allow an adequate fluid film under the seal and thus generates heat. Too smooth a surface does not permit sufficient fluid film under seals. Too rough causes the seal to initially ride on the surfacefinish peaks, and that quickly causes surface and seal wear. This can also increase friction and leakage, and reduce life.
• Ensure proper rod hardness and surface coating. Hardness and coatings have to be adjusted with surface finish. Too soft a rod or insufficient coating thickness will potentially let the seal alter the surface finish. Too hard of a surface may damage the seal.
• Maximize bearing load distribution. Unit loading that’s too high adds to system heat and limits life. Consider the overall packaging envelope to determine proper bearing width and spacing.
• Change the lubrication pattern underneath the bearings. Adding grooves, cavities, patterns, or splitting the bearing will alter lubrication paths. This can reduce frictional heat from the bearings and help ensure the seal rides on an adequate fluid film. However, bearing costs may increase.
• Understand bearing spacing relative to the seals. Unsuitable bearing locations will not provide adequate support for proper alignment. This increases local loading and temperature of the seals.
• Understand seal component layout and its affect on fluid-film thickness. To reduce heat, seals must ride on a thick fluid film which can be pumped back into the system. Too few or inappropriate components can increase leakage, friction, or pressure buildup, while too many components will increase costs and frictional heat.
• Review fluid conditions. Changes in viscosity and potential fluid breakdown affect the fluid film on which seals ride. Too thin a film results in higher friction and heat. Too thick a fluid film may result in excess leakage.
• Understand potential hardware dynamics. Thermal expansion can cause hardware to balloon and high temperatures can make components bend. Designers must account for how these factors change the seal contact stresses, which affects the fluid film thickness. Other heat-related hardware dynamics include softening of the bearing supports or the bearings themselves. This limits bearing load capability and can mean uneven unit loading.