Fluid power systems in which the working fluid is liquid (instead of air) rely on hydraulic cylinders to push and/or pull a load or selectively resist motion under load using fluid pressure. Hydraulic cylinders, in turn, depend on seals to close gaps between various cylinder components (such as pistons and cylinder bores and rods and head glands) for optimized system performance. These seals are expected to hold hydraulic pressure, withstand wide temperature ranges, and be compatible with hydraulic fluids.
With hundreds of different hydraulic seal designs and material combinations — and existing innovations and those in the pipeline — an understanding of the basics can help point the way toward the appropriate sealing technology for an application.
Seal types and functions
Hydraulic seals fall into one of two categories: dynamic or static.
Dynamic types seal between components in relative motion. For example, in a hydraulic cylinder, a rod sealing system seals dynamic reciprocating motion between the piston rod and head, while a piston sealing system seals dynamic reciprocating motion between the piston and cylinder bore.
Each dynamic seal in a hydraulic cylinder performs practical functions:
Piston seals act as pressure barriers and prevent fluid from passing the piston (important for controlling the
cylinder motion, maintaining position and holding a load).
Rod seals act as pressure barriers and keep the operating fluid inside the cylinder, regulate the fluid film that extends with the surface of the piston rod (important to inhibit rod corrosion and to lubricate a wiper seal and the rod itself) and accept the lubrication film back into the cylinder when the rod retracts.
Buffer seals protect the rod seal from fluid pressure peaks in excess of system pressure, attenuate the fluctuations in system pressure (thereby improving rod seals performance by allowing the rod seal to accommodate more constant or gradually changing pressure) and act as internal excluders to prevent system contaminants (such as metal particles) from damaging the rod seal.
Wiper seals exclude external contaminants from entering a cylinder assembly and the hydraulic system and accept the lubrication film back into the cylinder when the rod retracts.
By comparison, static types seal between components fixed together without relative motion. Hydraulic cylinders use static seals in numerous locations, depending on the cylinder’s design and construction. The most common are static seals between the piston and piston rod and between the head and cylinder bore tube.
Criteria for seal selection
Selecting the appropriate hydraulic seal material and profile is governed largely by application parameters. Among the application considerations are:
- Fluid pressure range, including frequency and pressure peak severity
- Temperature range of the fluid and cylinder assembly when operating and at rest
- The stroking speed of the reciprocating piston rod
- Fluid media, including type and viscosity
- Hardware dimensions such as rod and bore diameters, seal groove dimensions and gaps, overall cylinder length and stroke length and surface finish specifications
- Cylinder application, including the type of equipment for the cylinder and how the cylinder will operate in the equipment, as well as installation, duty cycles and environmental factors (such as external temperature and contaminants)
A material difference
Materials play vital roles in enabling optimized performance and service life of hydraulic seals. Evaluating the different properties exhibited by different materials and/or material combinations will help guide in proper seal selection.
Included in the overall "wish list" of material properties for consideration are:
- Good elasticity over a wide temperature range, especially at low temperatures.
- Excellent compression set and stress relaxation behavior to maintain the sealing force.
- Adequate hardness and flexibility to avoid leakage and allow easy installation.
- Superior gap extrusion resistance to cover the increased pressures of fluid power equipment.
- Adequate working temperature range.
- Good chemical compatibility to cover a wide assortment of hydraulic fluids, such as mineral and synthetic oils, biodegradable and water-based fluids or fire-resistant fluids.
- Excellent tribological properties (low friction values and high wear resistance) to achieve a high efficiency and avoid early failures, especially when sealing against rough counter-surfaces. (Tribology is the study of the design, friction, wear and lubrication of interacting surfaces in relative motion.)
Example of a hydraulic cylinder integrating seals between various components. All graphics courtesy of SKF USA Inc.
In general, the world of seal materials is populated by the major polymeric material groups (with modifications and special grades always possible to meet the requirements of an application). The most common materials considered for hydraulic seals include:
- Thermoplastic elastomers, such as polyurethanes, provide excellent resistance to wear and pressure and exhibit elasticity and flexibility.
- Rubbers, such as nitrile rubber (NBR), are widely used as dynamic seals in the fluid power industry. Depending on the chemical composition, rubbers can cover a wide temperature range and withstand many hydraulic fluids.
- Polytetrafluoroethylene (PTFE) is a polymer with a unique composition and properties and, among plastics, offers the highest chemical resistance and the lowest coefficient of friction (providing superior startup behavior, minimized stick-slip phenomenon and accurate positioning of hydraulic cylinders). This polymer is often modified by adding organic and/or inorganic fillers to improve specific properties, such as wear or extrusion resistance.
- Rigid thermoplastics and thermosets (and their composites) are characterized by much higher hardness and stiffness and reduced elasticity compared with polyurethanes, rubbers and PTFE. As a result, they will suit components for which mechanical strength is more important than flexibility, such as special piston seal arrangements for heavy-duty applications.
In addition, innovative material variations have expanded the portfolio of sealing technologies. As examples, bonding urethane to PTFE and other rigid plastics has demonstrated improved performance at high pressures, temperatures and speeds, while bonding thermoset materials to PTFE and other plastics can enhance sealing capabilities and extend a seal’s service life.
Fluid compatibility a ‘must’
The fluids used in various hydraulic systems consist of formulated chemical compositions and viscosity grades consistent with an application. While mineral oil-based fluids with various additives represent the most commonly used media in hydraulic systems, alternative fluids may be encountered.
A notable example is an increasing use of biodegradable fluids. However, these fluids are more susceptible to entrained water and most seal-grade urethanes will fail from hydrolysis when exposed to high temperature, water or humidity. As a solution and performance upgrade, hydrolysis-resistant urethanes have been introduced.
Because fluids essentially vary by manufacturer, types of additives and contaminant levels, careful consideration should always be taken to ensure compatibility between the fluid and all seal materials (as well as the temperature and mechanical loads on the seal material). A best practice is testing the compatibility of a seal in the actual fluid and under actual operating conditions.
Sealing technologies for fluid power applications will continue to evolve in response to advances in technology and the demands imposed by applications. These demands include increased operating pressures, wider temperature ranges (higher and lower), more aggressive system fluids and longer expected service life, among others. Innovations in hydraulic seal solutions will follow.
Scott Barth is business development manager for Hydraulic Seals at SKF USA Inc. He can be reached at [email protected] or 801-973-9171.
