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Fluid power systems transfer and utilise mechanical power through a working fluid. Energy is transmitted, stored and used through the transfer and pressure of fluids. By utilising differential surface areas acted upon by the pressure of the working fluid, a mechanical advantage can be achieved. There are two main types of fluid power systems:
Hydraulic seals are generally relatively soft, non-metallic rings captured in groove or fixed in a combination of rings, forming a seal assembly, to block or separate fluid in reciprocating motion applications. Hydraulic seals are vital in machinery. Their use is critical in providing a way for fluid power to be converted to linear motion.
Hydraulic seals can be made from a variety of materials such as polyurethane, rubber or PTFE. The type of material is determined by the specific operating conditions or limits due to fluid type, pressure, fluid chemical compatibility or temperature.
A static hydraulic seal is located in a groove and sees no movement – only sealing within its confined space, acting like gasket.
A type of dynamic hydraulic seal called a rod seal is exposed to movement on its inner diameter along the shaft or rod of a hydraulic cylinder. A type of dynamic hydraulic seal called a piston seal is exposed to movement on its outer diameter along the tube or bore of a hydraulic cylinder.
Hydraulic Piston Seals seals ensure fluid or other media does not by-pass the piston as the system pressure pushes the piston down the cylinder during a pressure cycle.
Piston seals are dynamic seals, typically single-acting (pressure acting on one side only) and double-acting (pressure acting on both sides) seals. The choice of piston seal is determined by the way in which the cylinder operates.
Rod seals are considered the most critical element of a cylinder sealing system. They prevent the leakage of fluid from within a hydraulic cylinder to the outside as the rod cycles back and forth (dynamic pressure sealing). Rod seals work together with wiper seals to protect a hydraulic system against external contaminants such as dirt and outside weather conditions.
Rod seals are static seals installed in the glands or housings. They maintain a seal with the piston rod in dynamic motion.
Wiper seals are also known as Scraper seals. They tend to be used in conjunction with other sealing elements. Wiper seals create a tight fit whilst still allowing a reciprocating ram rod to pass through the inner bore of the seal. Despite it’simportant function, the wiper seal is possibly the most undervalued seal type in the hydraulic cylinder.
Wear rings are used to help keep the piston centered, which allows for even wear and pressure distribution on the seals. These seals are used in both piston and rod applications. Popular wear ring materials include glass filled Nylon, bronze filled PTFE and glass filled PTFE. Wear rings are available in butt cut, angle cut and step cut styles.
Materials play a major role in the performance and lifetime of seals. Generally, hydraulic seals are exposed to a variety of application and working conditions such as a wide temperature range, contact with various hydraulic fluids and the outside environment as well as high pressure and contact forces. The appropriate seal materials have to be selected to achieve a reasonable service life and service intervals. A wide variety of seal materials from four major polymeric material groups is available:
Polyurethanes combine the elastic properties of elastomers with melt processible thermoplastic materials. Seals made of polyurethanes provide excellent wear and anti-extrusion characteristics. Special sealing polyurethanes grades have a superior compression set and relaxation performance as well as temperature stability compared to commodity industrial grades. Due to their elasticity and flexibility, they are easy to install.
Rubbers are widely used in the sealing industry for rotary shaft seals, static sealing elements such as o’rings and energisers as well as dynamic seals in the fluid power industry. Depending on the chemical composition, rubbers can cover a wide temperature range up to 200°C (390°F) and more and can withstand a wide variety of hydraulic fluids. NBR elastomer in a hardness range of 70 to 90 Shore A Durometer are the most commonly used rubbers in the fluid industry. For higher temperatures and more aggressive hydraulic fluids, HNBR and FKM material is often used.
Often referred to its DuPont trade name Teflon, PTFE is a polymer with very unique properties. Due to its chemical composition and physiologically neutral properties, it is a plastic material with a very broad range of chemical resistance and very low coefficient of friction; however it has some restrictions in terms of mechanical properties and wear characteristics. Therefore PTFE is often modified by adding various fillers to improve specific properties such as wear or extrusion resistance.
One important characteristic of PTFE is the low friction coefficient giving outstanding stick-slip performance. Therefore PTFE is the preferred material in applications requiring accurate positioning of hydraulic cylinders. Due to increased modulus of elasticity compared to rubbers and polyurethanes, PTFE seals cannot usually be installed by simple snap-in measures therefore require special tools and procedures for installation.
Rigid Thermoplastics, thermosets and their composites are characterised by much higher hardness and stiffness as well as reduced elasticity compared to polyurethanes, rubbers and PTFE. Therefore they’re used for components where mechanical strength is more important than flexibility, such as guide rings, anti-extrusion rings and special seal arrangements for heavy duty applications. Rigid thermoplastics and composites.
To support sealing function and keep the contact force for long seal life, many different properties should be considered to provide effective seal performance, examples include:
In addition to these considerations, the structure and morphology (the study of form and shape) of polymeric materials make selection and specification of seal materials much more complicated than the standard materials used in mechanical engineering (such as aluminium and steel). Mechanical properties of polymeric materials are strongly influenced by time, temperature, load and rate of motion. Highly complex intermolecular processes affect the stress relaxation and retardation phenomena. Furthermore, the tribology of the system (e.g. friction and wear) has a strong influence on the seal material properties and vice versa.
Fluids in hydraulic systems serve multiple functions for system performance:
The fluids used in hydraulic systems come in various chemical compositions and viscosity grades as suited to specific applications.
Viscosity is a measurement of the thickness of the fluid or resistance to flow. Seal performance is affected by the viscosity of the fluid and changes to viscosity during use. Most typical hydraulic fluids exhibit decreased viscosity with increasing temperature and increased viscosity with increasing pressure.
The most commonly used media in hydraulic systems are mineral oil based fluids with various additives. However, a variety of alternative fluids may be encountered in special applications. For example, biodegradable fluids such as synthetic (HEES) or natural esters (HETG) and polyalphaolefines (POA) may be used to reduce environmental impact in the event of accidental spills. Flame retardant fluids based on water or synthetic esters may be safely used in confined spaces or where hydraulic systems are used in close proximity to ignition sources.
The chemical composition on hydraulic fluids can impact the seal life and system performance depending on fluid compatibility with the seal material. Absorption and reaction of seal materials with non-compatible fluids can cause for example:
Generally these changes are accelerated by higher temperatures. To avoid these changes and the resulting damage to seal function and life, careful consideration should be taken to ensure compatibility between fluid and all seal materials, as well as the temperature and mechanical loads on the seal material.
The following table is a guide for seal materials and their compatibility with common grades of hydraulic fluids.
|Material||Polyurethane||Polyurethane||Hydrolysis Resistant Polyurethane||Hydrolysis Resistant Polyurethane||Ether-Based TPU||Ether-Based TPU||Polyester||Polyester||NBR/HNBR||NBR/HNBR||FKM||FKM||EPDM||PA/PF||PM||PEEK||PTFE|
|Hydraulic Fluids||Up to 60°C||Up to 100°C||Up to 60°C||Up to 100°C||Up to 60°C||Up to 100°C||Up to 60°C||Up to 100°C||Up to 60°C||Up to 100°C||Up to 60°C||Up to 100°C|
|Mineral Oils HL, HLP, HVLP||A||B||A||A||A||B/C||A||A/B||A||A||A||A||D||A||A||A||A|
|ATF: Automatic transmission fluid||A||B||A||A||A||B/C||A||B||A||A||A||A||D||A||A||A||A|
|HETG (triglycerides, rape seed oil)||A||B/C||A||A||A||C||A||B/C||A/B||A/B||A||A||D||A||A||A||A|
|HEES (synthetic esters)||A||B/C||A||A||A||C||A||B/C||A/B||A/B||A||A||D||A||A||A||A|
|HEPG (polyalkylene glycols)||B||D||A||C||B/C||D||C||D||A||A/B||A/B||C/D||A||A||A||A||A|
|Fire Resistant Fluids, Water Based||Up to 40°C||Up to 60°C||Up to 40°C||Up to 60°C||Up to 40°C||Up to 60°C||Up to 40°C||Up to 60°C||Up to 40°C||Up to 60°C||Up to 40°C||Up to 60°C|
|HFA Fluids (oil in water)||B||D||A||A||B||B/C||A||B||A||A||A||B||D||C||A||A||A|
|HFB Fluids (water in oil)||B||D||A||A||B||D||A||B||A||A||A||A||D||C||A||A||A|
|HFC Fluids (water – glycol)||C||D||A||B/C||B||B/C||C||D||A||A||A/B||B/C||A||C||A||A||A|
|Fire Resistant Fluids, Water Free||Up to 60°C||Up to 100°C||Up to 60°C||Up to 100°C||Up to 60°C||Up to 100°C||Up to 60°C||Up to 100°C||Up to 60°C||Up to 100°C||Up to 60°C||Up to 100°C|
|HFD-U Fluids (polyol amd carboxylic esters)||B||D||A||A||B||D||A||B/C||C||C||A||A||D||A||A||A||A|