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Fluid power systems transfer and utilize mechanical power through a working fluid. Energy is transmitted, stored and used through the transfer and pressure of fluids. There are two main types of fluid power system:
A hydraulic cylinder is a linear actuator used to push or pull a load, or to selectively resist motion under load, by means of fluid pressure. Double-acting cylinders, the most common type, are able to push and pull (--> fig. 1). High pressure fluid pumped into the extend cham¬ber (port “A”) acts on the piston to push the piston rod out, thereby extending the length of the cylinder. Inversely, to retract the piston rod and reduce the length of the assembly, high pressure fluid is pumped into the retract chamber (port “B”) and acts on the opposite side of the piston.Other cylinder types are shown in figure 2.
A cylinder which can only push but not pull is referred to as a single-acting cylinder. High pressure fluid is pumped into the extend chamber (port “A”) and acts on the piston to push the piston rod out. An external force is required to return the cylinder to the retracted position. A typical application for a single-acting cylinder is a lift truck, where the load of the fork pushes the cylinder back. Multi-Stage cylin¬ders (also referred to as telescoping cylinders) have two or more piston rods in a coaxial arrangement to achieve greater extended length compared to the retracted length.
The type of cylinder and the application in which it is used are two of the main criteria when selecting the appropriate seals and guides. Applications are referred to as light duty, medium duty or heavy duty applications. These classifications are somewhat subjective but the duty levels are typically characterized by the following criteria.
Light duty cylinders
Light duty cylinders, e.g. cylinders used for stationary equipment indoor in a factory envir-onment, may be characterized by:
Medium duty cylinders
Medium duty cylinders, e.g. cylinders in agri-culture off-highway equipment, may be char-acterized by:
Heavy duty cylinders
Heavy duty cylinders, e.g. cylinders in off- highway earthmoving or forestry equipment, may be characterized by:
Hydraulic cylinder seals are used to seal the opening between various components in the hydraulic cylinder. Figure 3 shows an example of the seal and guide components of a heavy duty cylinder. There are two main types of hydraulic seals in the system:
They seal between components in relative motion. เท a hydraulic cylinder the rod seal-ing system seals dynamic reciprocating motion between the piston rod and head, while the piston sealing system seals dynamic reciprocating motion between the piston and cylinder bore.
They seal between components fixed together without relative motion. Hydraulic cylinders use static seals in various loca¬tions depending on the design and construc¬tion. The most common are static seals between the piston and piston rod and between the head and cylinder bore tube.
Each dynamic seal in a hydraulic cylinder has a special function to contribute to the perform-ance of the system:
Piston seal functions
Rod seal functions
Wiper seal functions
Guide rings (rod and piston) functions
The subsequent chapters contain additional information regarding the function of each seal or guide in the system and the selection of them.
Designing hydraulic rod and piston sealing systems requires careful attention to the dynamic seal interaction and the principles of tribology to ensure long service life, proper seal function, minimal wear, low friction and smooth operation. Tribology is the study of the design, friction, wear and lubrication of inter¬acting surfaces in relative motion. The most important considerations for tribological prop¬erties of a dynamic sealing system are:
For information about seal profile design, see the relevant product chapter.
The surface properties of the cylinder bore and the piston rod have a great influence on the function and service life of the seal. Parameters for specifying a surface finish are defined by ISO 4287. The most common sur¬face roughness parameter specified is Ra i.e. the arithmetic mean deviation of the surface profile. This value does not, however, completely describe how the surface can be expected to affect the seal. The reason for this is that two surfaces with the same values of Ra but with different sur¬face profile characteristics can lead to differ¬ent lubrication film thickness, resulting in varying seal performance and level of wear.
The material ratio curve (Abbott-Firestone curve) provides more information about the surface profile characteristics. It describes the ratio of the material-filled length to the evalu-ation length at a given cutting depth expressed in percent (--> fig. 1). The slope at the begin-ning of the curve represents the peaks in the profile, which are causing initial wear on the seal. The slope at the end of the curve repre-sents the valleys in the profile, which serve as lubricant reservoirs.
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 tempera¬ture range, contact with various hydraulic fluids and the outside environment as well as high pressures and contact forces. The appro¬priate 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:
Many different material properties should be considered to support and maintain the seal¬ing function over the expected seal service life, for example:
In addition to these considerations, the struc-ture and morphology of polymeric materials make selection and specification of seal mater-ials much more complicated than the standard materials used in mechanical engineering (e.g. aluminium or 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 con-ditions of the system (e.g. friction and wear) has a strong influence on the seal material behaviour and vice versa. Therefore, state-of- the-art sealing systems can only be developed by close cooperation between material experts and product designers, supported by advanced design tools like non-linear FEA and extensive seal testing capabilities.
Rubbers are widely used in the seals industry for rotary shaft seals, static sealing elements such as 0-rings and energizers, 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 withstand a wide variety of hydraulic fluids. NBR elastomers in a hardness range from 70 to 90 Shore A (shA) are the most commonly used rubbers in the fluid power industry. For higher temperatures and more aggressive hydraulic fluids, We recommends HNBR or FKM elastomers.
PTFE is a polymer with very unique proper¬ties. Due to its chemical composition, it is the plastic material with the highest chemical resistance and the lowest coefficient of friction, however, it has some restrictions in terms of mechanical properties and wear. Therefore, PTFE is very often modified by adding various organic and/or inorganic fillers to improve specific properties such as wear or extrusion resistance.
One of the most important characteristics of PTFE is the low coefficient of friction that pro-vides outstanding start-up behaviour as well as minimized stick-slip phenomenon. There fore, PTFE is the preferred material in applica¬tions that require accurate positioning of hydraulic cylinders. Due to the increased modulus of elasticity compared to rubbers and polyurethanes, PTFE seals can usually not be installed by simple snap in procedures and require special tools and procedures for installation.
Rigid thermoplastics and thermosets and their composites are characterized by much higher hardness and stiffness as well as reduced elasticity compared to polyurethanes, rubbers or PTFE. Therefore, they are used for components where mechanical strength is more important than flexibility, such as guide rings, anti-extrusion rings or special piston seal arrangements for heavy duty applications.
WARNING :At temperatures above 300 °c (570 °F) all fluoro elastomers and PTFE compounds give off dangerous fumes. If there is contact with your skin or if the vapours are inhaled, seek medical advice immediately. |
Fluids used in hydraulic systems serve multiple functions for the 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 a fluid or the resistance to flow. Seal per-formance is affected by the viscosity of the fluid and changes to the 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 (PAO) 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 the hydraulic system is used in dose proximity to ignition sources. The data, specifications and recom¬mendations in this catalogue are for common mineral oil fluids. For guidance on specifica¬tions of sealing systems for alternative fluids, contact Ecoseal.
The chemical composition of hydraulic fluids can impact the seal life and performance depending on compatibility with the seal material(s). Absorption and reaction of the seal material(s) with non-compatible fluids can cause, for example:
Generally, these changes are accelerated by higher temperature. To avoid these changes and the resulting damage to seal function and life, careful consideration should be taken to ensure compatibility between the fluid and all seal materials, as well as the temperature and mechanical loads on the seal material. Wehas a long history and extensive database of test results concerning compatibility of various seal materials and fluids, as well as unparal¬leled expertise in developing materials to meet customers’ needs for chemical resistance of seal materials.
The process by which seal material is forced into the clearances between components is called gap extrusion. The dimension of this clearance gap is referred to as the extrusion gap, or “e-gap” (-» fig. 4).
The resistance of a given seal component to gap extrusion is mainly determined by the material composition and quality. Materials of greater hardness and stiffness typically also have improved resistance to extrusion. There¬fore, fullface anti-extrusion or back-up rings of materials harder than the seal material may be used to prevent seal extrusion into the e-gap (—> fig. 5).
Pressure IS the main driver of extrusion, but the e-gap size and application temperature are also major factors. Diagram 2 shows the pressure resistance of different materials as a function of temperature. The values were measured on an ECoseal test rig. The tests were carried out with a rectangular sample, dimen¬sions 38,7 X 49 X 5 mm under static pressure and an extrusion gap of 0,3 mm. The pressure values were taken at an extrusion length of 0,5 mm (-» fig. 4). While these sample values illustrate the differences in extrusion resist¬ance for standard grades of typical seal mate¬rials, there are many variations of each basic composition that impact the extrusion resist-ance of seals. เท addition, the profile design and the seal friction affect extrusion. For maximum allowable pressure, temperature and e-gap of each seal profile, refer to the profile data for each profile in the relevant chapters.
The maximum e-gap in a hydraulic cylinder occurs when the cylinder components are at the maximum radial misalignment of compo¬nents. This misalignment is affected by:
Therefore, it is necessary to calculate the e-gap at the maximum misalignment at min¬imum material conditions of the cylinder and guide components.
For rod seals, the maximum e-gap should be calculated with the following conditions (-> fig. 6):
For piston seals, the maximum e-gap should be calculated with the following conditions (-> fig. 7):
The maximum allowable e-gap is provided in the profile data for each rod seal and piston seal profile in the relevant chapter. The e-gap can be kept within these limits by specifying and controlling the tolerances of dimensions described above and shown in figs. 6 and 7.
During storage, the properties of elastomer products can be damaged either by chemical reactions or by physical processes. Chemical reactions are basically caused by the influence of heat, light, oxygen, ozone or contamination by chemicals. The physical processes, which are called physical ageing, are either due to the influence of external stresses leading to cracks and permanent deformation, or due to the migration of plasticizers, which makes the material more brittle and can lead to deforma-tion of the parts.
Therefore, elastomer products only main¬tain their characteristics for several years without major changes, if they are properly stored. The ageing behaviour of elastomer products and their reaction on storage condi¬tions depend considerably on their chemical structure. Unsaturated elastomers (e.g. nitrile rubber) age more quickly under improper storage conditions than saturated elastomers (e.g. fluorocarbon rubber).
Storage conditions
Elastomer products should be stored in accordance with the following recommendations,which are in line with the recommendations provided in ISO 2230 or DIN 7716.
Shelf life
When stored under the conditions mentioned above, elastomer products retain their typical properties for several years (--> table 9). The typical shelf life may be prolonged based on the actual product conditions at the end of the typical shelf life. rained and experienced experts can approve a prolonged storage period based on a visual inspection of representative samples. The samples should not reveal any permanent distortion, mechanical damage or surface cracking. The material should not show any signs of hardening or softening nor any kind of tackiness.
Seal housing grooves
The type of seal housing determines the method of installation, required equipment and the degree of difficulty. There are four main types of seal housings.
Closed housing grooves
Closed housing grooves are the most common seal housings († fig. 8).
They require the most planning and effort to ensure that the seal is installed properly without damage. Not all seal cross section sizes and material combinations can be installed into this type of seal housing.
Open housing grooves
Open housing grooves allow the seal to be pressed in with minimal deformation and are therefore a good choice when the seal design, material or size prevent installation into a closed or stepped housing. Some seals, such as press-in wiper seals, have a metal sleeve that retains the seal in an open groove by press forces (--> fig. 9)
whereas other seals may require a snap ring (--> fig. 10).
Plastic snap rings, such as RI for rods or RR for piston, are available from ECoseal on request. Open housing grooves require specified edge radiior chamfers to prevent seal damage when the seal enters the housing groove or passes the snap ring groove.
Split two-piece closed grooves
These grooves incorporate two separable machine components to provide an open groove when the seal is installed and a closed groove when the machine is fully assembled (--> fig. 11).
Stepped grooves
Stepped grooves allow seals to be installed with less deformation (--> fig. 12). Snap-in wiper seals are a common example in hydraulic cylinder applications.
Corner radii
The corner radii inside the groove should be sized to avoid inadvertent contact with with the adjacent portion of the seal. Static side corner radius recommendations are provided in the product tables of the relevant chapter.
Groove edge radii
All outside groove edges and any other edges that may come into contact with the seal during installation or use should be broken with a small radius. Otherwise, the sharp edge may damage the seal. Unless otherwise specified,all outside groove edges should have a radius of approximately 0,2 mm (0.008 in.).
Installation chamfers
Installation chamfers should be designed into the cylinder bore and onto the assembly end of the piston rod to ensure that the seal can easily transition from its free state diameter into its installed diameter. The installation chamfer should also be blended into the cylinder bore or piston rod diameter with a generous radius.The chamfer angle and minimum length recommendations are provided in the product tables of the relevant chapter.
Installing rod seals
The method of installation and the possible groove types for rod seals depend on the materials, seal design and ratio between the diameter and cross-sectional height. Table 10 provides general recommendations for profiles made of rubber or TPU with a hardness ≤ 95 shA. PTFE or other harder materials may require a smaller radial depth S or even open grooves. The recommendations in table 10 are not a substitute for careful installation tests in the particular application.
Installing rod seals in closed grooves
Rod seals can often be installed into closed grooves through carefully bending the profile similar to a kidney shape and then inserting it into the groove. It is very important to avoid sharp bending. Thin and flexible profiles can be installed by hand (--> figs. 13 a and b).
Installation tools for TPU rod seals help to install profiles of greater section thickness (--> figs. 14 a to f).
After installation, the seal may need to be reshaped to a round form using a cone-shaped tool. For PTFE seals, small diameter seals or for serial assembly, special assembly tools (--> fig. 15)
may be required to save time or avoid seal damage. For additional information about such special installation tools
Installing piston seals
Piston seals installed in closed grooves must be expanded or stretched into position. Seals with step cut slide rings such as CUT or SCP (--> Piston seals with rigid split slide rings) are relatively easy to expand into position. Non-split profiles should not be expanded to a material deformation of more than 20% for TPU or 30% for rubbers. Otherwise, the permanent deformation would be too large. Heating the seal, e.g. in an oil bath, decreases the required expansion force, but cannot increase the maximum material deformation.Piston seals with a TPU slide ring can usually be installed by hand or with simple tools (--> fig. 16).
PTFE seals or those with thicker radial sections may require special assembly tools to save time or avoid seal damage (--> fig. 17).
For additional information about such special installation tools. The recommendations cannot substitute for careful installation tests in the particular application.
Installing wiper seals
Snap-in wiper seals, which are installed in stepped grooves (--> fig. 12), are typically of a small radial section per diameter and close to the end of the cylinder head component. Therefore, installation by hand is usually possible. Press in wiper seals require special equipment and careful planning for ease of installation without damaging the wiper seal or housing.Assembly tools adapted for each press-in wiper seal size should be used in conjunction with appropriate steady force in a hand perated press. Installation by impact (e.g. striking the assembly tool with a hammer) is not advised. The press assembly tool should be configured to stop when the wiper seal has been pressed flush with the groove edge (--> fig. 18). Pressing beyond flush can damage the wiper seal.
Piston seals (--> fig. 1) maintain sealing contact between the piston and the cylinder bore. Differential pressures acting on the piston to extend or retract the piston rod can be in excess of 400 bar (5 800 psi). The pressure acting on the piston seal increases contact forces between the piston seal and cylinder surface. Therefore, the surface properties of the sealing surfaces are critical to proper seal performance (--> Counter-surface finish properties). Piston seals are typically classified into single-acting (pressure acting on one side only) and double-acting (pressure acting on both sides) seals.
Depending on the profile and the required characteristics of its components, a piston seal can consist of one or several materials. Common materials used for piston seals are thermoplastic polyurethane (TPU), polytetrafluoroethylene (PTFE), polyamide (PA), and nitrile rubber (NBR). The standard materials used for a specific profile are provided in the Profile overview and in the relevant profile sections below. For additional information, refer to Materials.
External forces acting on the piston rod,reacted by the fluid inside the cylinder, can result in abrupt pressure peaks. These peaks can be far in excess of the system operating pressure and may press a piston seal into the gap between the piston and bore, thereby causing damage to the seal and adversely affecting seal performance and cylinder operation. Seal materials must be carefully chosen to avoid gap extrusion. This risk of gap extrusion can also be minimized by using seals with anti-extrusion rings (--> Piston seals incorporating anti-extrusion rings).
Guide rings avoid sliding metal-to-metal contact between the piston and cylinder bore and accommodate the radial loads of forces acting on the cylinder assembly. Although piston seals are designed to accommodate slight radial motion between the piston and bore, effective guide ring function to accurately centre the piston within the bore is important for piston seal performance. For additional information about piston guidance, refer to Guiderings and guide strips
When the piston rod is at rest and held in position by fluid, any amount of flow passing the piston can result in an unintended movement of the piston rod and cause drift. Although piston seal leakage is a possible source of drift, internal valve leakage, external system leakage and flow between the piston and rod static connections should also be carefully checked. In some applications, a minimal amount of flow passing the piston seal within specified limits is permitted. This accepted flow allows the use of piston seals of low friction designs and materials or split slide rings for easy installation.
The piston can be welded to the rod (--> fig. 2)
if the disassembly of the cylinder can be done by removing the rod end attachment. The piston can also be fixed to the rod end by a lock nut (--> fig. 3)
which enables removing the piston from the rod during complete disassembly of the cylinder. When using a lock nut, static sealing (--> O-rings and back-up rings) is required between the piston and the rod end.
Double-acting piston seals
Double-acting cylinders are the most widely used cylinder types. They operate with pressure on both sides and, therefore, require double-acting seal arrangements (-->fig. 1).
Double-acting piston seals have a symmetrical profile (cross section) and identical sealing functions in both directions. Typically, double-acting piston seals consist of a slide ring and an energizer. The deformation of the elastomeric energizer when installed provides adequate force to keep the slide ring in dynamic sealing contact with the cylinder bore, while sealing statically against the seal housing groove.
A double-acting cylinder typically has the same fluid on both sides of the piston. Therefore, a relatively thick lubrication film can be permitted between the piston seal and the cylinder bore to minimize friction and wear. The transportation of fluid occurring during dynamic operation is, however, small and insignificant in most applications.
In some older cylinder designs, O-rings were used as piston seals. To allow easy replacement of O-rings with the equivalent piston seals, the housing dimensions for some double-acting piston seals are the same as those for dashnumber O-rings. Therefore, these housings are also called O-ring dash-number housings.
Ecoseal supplies double-acting piston seals in many different profiles and in a wide range of series and sizes, which make them appropriate for a wide variety of operating conditions and applications.
Piston seals with polyurethane slide rings
Piston seals with thermoplastic polyurethane (TPU) slide rings have a nitrile rubber (NBR) energizer. The wear-resistant slide ring has a profiled dynamic sealing surface. Its geometry is optimized to reduce friction and resist gap extrusion. Notches in the slide ring edges ensure rapid pressurization of the seal to react to abrupt changes in pressure. These profiles can usually be installed without special equipment and are resistant to damage during installation and cylinder assembly.
Piston seals with PTFE slide rings
PTFE slide rings may be preferred in applications with demands for low breakaway friction and when it comes to chemical and thermal resistance. Notches in the slide ring edges ensure rapid pressurization of the seal to react to abrupt changes in pressure. PTFE is hard and non-elastic when compared with polyurethane and rubber materials. For additional information about piston seal materials, refer to Materials (-->Piston Seals : Materials).
Piston seals incorporating anti-extrusion rings
These Ecoseal piston seals incorporate patented locking anti-extrusion rings made of polyamide (PA). They are split for easy installation. Their snap-in design makes it easy to identify the correct installation direction (--> fig. 10)
holds them in place when installed and prevents damage during assembly. Piston seals incorporating anti-extrusion rings have an improved high pressure performance and minimize the risk of gap extrusion at abrupt pressure peaks (--> Gap extrusion).
Piston seals with rigid split slide rings
These Ecoseal piston seals have a rigid split slide ring made of glass fibre reinforced polyamide and a nitrile rubber (NBR) energizer. The rigid slide ring has high resistance to wear and gap extrusion. The slide ring also provides low
friction, even at high pressures. The split slide ring design facilitates the installation process into a closed housing.
Piston seals with integrated guide rings
These seals are designed as compact sets that incorporate the piston seal and guide rings into one assembly. Typically, they are applied as an all-in-one piston seal solution.
A single-acting piston seal is designed for cylinders where pressure is applied from one side only. The piston in single-acting cylinders may have oil on the pressure side only, while the opposite side is open to atmosphere. Therefore, the piston seal must leave a minimum of oil film when passing along the cylinder bore since the transportation of oil otherwise would result in a leakage to the exterior. In single-acting cylinders, the open end may push air out and draw air in as the piston reciprocates. This air may carry moisture and contaminants into the cylinder, which can lead to seal damage. Vent filters may be fitted to the open side of the cylinder to reduce contaminants entering the inside of the cylinder. The cylinder bore may be hard chromium plated to prevent corrosion. In addition, to prevent damage to the cylinder bore or piston seals, ECoseal can supply special piston wiper seals on request. For additional information, contact Ecoseal.
Single-acting piston seals in double-acting cylinders
Two single-acting U-cup profile seals, facing in opposite directions, can be used in a doubleacting cylinder. It is important to select seal designs which can relieve reverse pressure for such arrangements to prevent build-up of pressure between the two seals.
Rod seals used as single-acting piston seals
Some rod seal profiles are designed with similar inside and outside sealing geometry and, therefore, can also be used as single-acting piston seals in single- or double-acting cylinders (--> fig. 17). PTB, STD and DZ rod seal profiles (--> Rod and buffer seals) can be used for those applications. Rod seals with loaded-lip U-cup profiles may not relieve reverse pressure, but it is possible to remove their energizer (X-ring) from one of the seals to allow reverse pressure relief (--> fig. 17).
The piston seals listed in this catalogue represent the preferred profiles in common sizes. Ecoseal supplies many additional sizes and profiles. The following profiles are also manufactured in series production. Ecoseal can provide customized sealing solutions also for the toughest application conditions. For additional information about these profiles or if the application requires a solution outside of what is provided in this catalogue, contact Ecoseal.
More PTFE slide ring piston seals
Piston seals with PTFE slide rings are available in a wide variety of profiles and materials (--> fig. 18). For additional information about material options, contact Ecoseal.
SPECTRASEAL
SPECTRASEAL is a PTFE seal that can be used as a single-acting piston seal (--> fig. 19). The metal spring energizer adds radial load to the seal lip contact areas. SPECTRASEAL is intended for extreme condition applications including high temperature or aggressive media. For additional information, contact Ecoseal.
Customized machined seal profiles
Ecoseal can manufacture a broad variety of piston seal profiles with different materials and sizes with its industry. For additional information about customized machined profiles, refer to publication Customized machined seals – Product range or contact Ecoseal.
Rod and buffer sealsRod and buffer seals maintain sealing contact in sliding motion between the cylinder head and the piston rod. Depending on the application, a rod sealing system can consist of a rod seal and a buffer seal (--> fig. 1)
or a rod seal only (--> fig. 2). Rod sealing systems for heavy duty applications typically consist of a combination of both seal types, whereas the buffer seal is arranged between the rod seal and the piston in the cylinder head. Rod seals determine the tolerance for the rod diameter d.
In addition to the sealing function, rod seals also provide a thin lubrication film on the piston rod that lubricates themselves and the wiper seals. The lubricant also inhibits corrosion of the piston rod surface. However, the lubrication film must be thin enough so that it returns to the cylinder during the return stroke.
Selecting profiles and materials for a rod sealing system is a complex task, considering all possible cylinder designs and application criteria. Ecoseal supplies rod and buffer seals in many different profiles and in a wide range of materials, series and sizes, which make them appropriate for a wide variety of operating conditions and applications.
Materials
Depending on the profile and the required characteristics of its components, rod and buffer seals can consist of one or several materials. Common materials used for the sealing and energizing elements of rod and buffer seals are thermoplastic polyurethane (TPU), polytetrafluoroethylene (PTFE) or nitrile rubber (NBR). Common materials used for rod seal anti-extrusion rings are polyamide (PA), polyacetal (POM) or PTFE. The materials used for a specific profile are provided in the Profile overview.
Anti-extrusion rings
External forces acting on the rod can cause pressure peaks. They can be far in excess of the system operating pressure and may press a rod seal into the gap between the piston rod and the cylinder head. This risk of gap extrusion
can be avoided for rod and buffer seals by using anti-extrusion rings. These hard and temperature-resistant rings can be integrated in the seal or a separate full-face anti-extrusion ring can be used. This ring can be added to a rod seal by simply extending the housing length (--> fig. 3). Integrated anti-extrusion rings fit into a notch in the rod or buffer seal and do not need an extended housing length.
Rod guidance
Although rod sealing systems are designed to accommodate minor radial motion between the piston rod and cylinder head, effective rod guidance is important to ensure best rod seal performance. Guide rings accurately center the rod within the head, which reduces radial deflection and motion acting on the seals. Guide rings also accommodate the radial loads acting on the cylinder assembly and avoid direct metal-to-metal contact between the piston rod and cylinder head. For additional information about rod guidance, refer to Guide rings and guide strips
Buffer seals protect the rod seals by reducing the magnitude of pressure peaks. Abrupt pressure peaks can occur by external forces acting on the piston rod, initiated by the fluid inside the cylinder. These pressure peaks can be far in excess of the system operating pressure. Buffer seals in combination with rod seals provide an effective rod sealing system for cylinders in heavy duty applications at high temperature and pressure.
Hydraulic cylinders operate in a variety of applications and environmental conditions, including exposure to dust, debris or outside weather conditions. To prevent these contaminants from entering the cylinder assembly and hydraulic system, wiper seals (also known as scrapers, excluders or dust seals) are fitted on the external side of the cylinder head (--> fig. 1).
Wiper seals maintain sealing contact to the piston rod when the equipment is stationary (static, no reciprocating motion of rod) and in use (dynamic, reciprocating rod), whereas the tolerance for the rod diameter d is determined by the rod seal. Without a wiper seal, the retracting piston rod could transport contaminants into the cylinder. The outside static sealing of the wiper seal within the housing is also important to avoid moisture or particles from entering around the outside of the wiper seal.
Installation
Housings for wiper seals are typically designed either as open or stepped housings (--> fig. 2). Wipers for open housings are bonded to a steel retaining ring and pressed into the housing. Therefore, they are called press-in wiper seals. Wiper seals for stepped housings do not have metal components and can usually be deformed by hand for installation. They “snap” into the groove and, therefore, are called snap-in wiper seal
Wiper Seal : Materials
Polyurethane (TPU) is the most common material for wiper seals in modern hydraulic seal applications. SKF wiper seals made of TPU are developed for wear resistance and flexibility as well as with the right physical properties to effectively remove contaminants.Ecoseal can supply wiper seals in a wide variety of materials, including rubber elastomers and PTFE
Press-in wiper seals
As their name implies, press-in wiper seals are pressed into the housing in the cylinder head. These seals are designed with a steel retaining ring that provides a robust static sealing on the outside surface. The polyurethane (TPU) wiper lip is bonded to the drawn sheet steel retaining ring and maintains a sealing force by tension as well as cantilever action (flexing) to the seal lips. This provides a robust preload of the wiper lips even under minor radial misalignments between the rod and head. Pressin wiper seals are available with a single-lip or double-lip design.
Snap-in wiper seals
Snap-in wiper seals are designed without any metal component and are easy to install without any special equipment. A common problem with snap-in wiper seals is that they become loose in the housing resulting in poor static sealing on the outside surface and reduced preload on the dynamic wiper seal lip (--> fig. 8). Furthermore, if the inside edge of the wiper seal could contact the rod, it may become an unintended sealing lip (--> fig. 8) and could trap pressure between the rod and wiper seal. Therefore, Ecoseal snap-in wiper seals have special sealing and venting features to help ensure proper operation.
In hydraulic cylinders the most commonly used guides are guide rings and guide strips. They accommodate radial loads of forces acting on the cylinder assembly and guide the rod in the cylinder head as well as the piston in the cylinder bore (--> fig. 1). Guides are made of polymer materials and prevent metal-to-metal contact between moving parts in a working hydraulic cylinder. Compared to metal guides, polymer guides provide the following advantages in hydraulic cylinders:
low speeds
larger contact area (--> fig. 2 and
fig. 4) due to higher degree of
elastic deformation distributes the load andreduces stress to counter-surface
certain self-lubricating properties
Ecoseal supplies different precision machined guide rings with different polymer materials and sizes, which make them appropriate for a wide variety of operating conditions and applications. Guides of PTFE are also available for applications where start-up friction must be minimized. PTFE guides have limited load carrying capability and should only be used in applications with light loads.
Guide lubrication
The guide must receive ample lubrication at all times. Rod guides are typically placed inward of both the rod and buffer seal and should be lubricated on assembly with the same medium as used in the system.Ecoseal recommends to place guides not outside of the rod seal, means between the wiper and rod seal. However, in certain conditions, PTFE guides may be used outside the rod seal due to their certain selflubricating properties.
Materials
The demands on reliability are continuously increasing. At the same time, the service conditions are getting tougher to match the development towards higher effectiveness of the hydraulic systems. Therefore, it is very important to be familiar with the operating conditions and parameters, such as operating temperature and pressure, load, speed, and fluid when choosing the most appropriate guide material. The most common materials for guides listed in this catalogue are:
Polyamide
Glass fibre reinforced polyamide guides are suitable for medium and heavy duty applications and are characterized by the following properties:
P-2551 is the standard polyamide material for guide rings. Technical specifications are provided in table 1
Fabric reinforced composites
Fabric reinforced composites consist of cotton fabric bound with thermoset phenolic resin. Its structure and the ability of the fabric fibres to absorb a certain amount of oil make these phenolic guides almost self-lubricating. However, cotton reinforced phenolic guide rings should not be used at high stroke speeds over 0,5 m/s (1.6 ft/s). They are suitable for medium and heavy duty applications and are characterized by the following properties:
wear resistant
reduce vibrations
protect seals from particles
protect components from diesel effect
low thermal expansion
easy to install
tight tolerances
withstand heavy side loads
PTFE
PTFE is typically used in guides where low friction and resistance to chemicals, heat or wear are essential. However, PTFE should only be used in applications with low surface pressure. To obtain optimal wear resistance, PTFE materials are available with different fillers, such as bronze or carbon powder. PTFE guides are characterized by the following properties:
wide temperature range
low friction
anti-adhesive, low breakaway friction
good wear resistance
reduce vibrations
protect seals from particles
protect components from diesel effect
tight tolerance machined guide rings available
Concentric alignment of cylinder components
Hydraulic cylinders and all their components are designed to minimize radial movements at load or pressure changes. It is also important that the piston and rod remain in a concentric position during the entire stroke to maintain seal effectiveness, especially at low temperatures, and to minimize the buckling loads on the piston rod. This in turn depends on the combined tolerances of the cylinder bore, the rod, the radial thickness of the guide rings or strips, and the housing diameters.
Guide distance
The bending moment on the cylinder components and the load on guides at any point in the cylinder stroke are a function of the radial loads and the distance between the rod and piston guides. Therefore, the distance between guides should be considered when designing the cylinder and calculating the guide loads.
Load distribution model
With metal guides, the close machining tolerances would cause narrow contact area and high surface pressure (--> fig. 3).
That could cause damage or wear to the contact surfaces. The higher degree of elastic deformation of polymer materials provides larger contact surfaces and a better utilization of the guide width (--> fig. 4).
While the guide ring load is realistically not an even distribution, the guide ring load and width requirements are estimated with the projected area of the full dynamic surface (inside diameter for rod guides or outside diameter for piston guides) assuming the load is carried evenly across the surface (--> fig. 5).
When calculating the requisite guide housing width L, the above assumption should be taken into consideration by using a safety factor.Ecoseal recommends using a safety factor f of at least 2 for operating temperatures up to 80 °C (175 °F). For operating temperatures above 80 °C (175 °F), the safety factor should be increased. However, at temperatures above 120 °C (250 °F), the selection of guide materials is significantly restricted. The reduced effective load carrying width B of the guide († fig. 5) also need to be considered. It is approx. 2 mm (0.08 in.) smaller than the housing groove width due to the manufacturing and installation tolerances and the reduction by the chamfers and radii. Furthermore, dynamic forces, accelerating forces, vibrations and angular forces should be considered when calculating the transverse forces from the rod ends of the cylinders. For additional information, contact Ecoseal.
Calculating the guide width
The requisite guide width can be calculated
for:
where
L = requisite guide housing width [mm]
D = cylinder bore diameter [mm]
d = rod diameter [mm]
F = radial load [N]
f = safety factor († Calculation considerations)
p = maximum recommended bearing load pressure [N/mm2]
Calculation example
What is the required guide housing width L for a PGR piston guide ring made of phenolic resin with cotton fabric laminate (PF), a cylinder bore diameter of D = 100 mm, considering a radial load of 20 000 N and an operating temperature of 80 °C (normal conditions)? The maximum recommended bearing load pressure p = 30 N/mm2. The safety factor is chosen with 2. The requisite guide housing width L is
The requisite guide housing width is 15,3 mm. However, choose a 20 mm housing width, which is the nearest larger housing and guide width L, such as guide ring PGR 100x94x20-PF
Ecoseal guide strips are made of PTFE as standard and should only be used in light duty applications or when fluid, temperature, friction, or speed do not allow any other material. They are typically used with PTFE sealing systems (--> fig. 6).
At system operating pressures over 200 bar (2 900 psi), contact Ecoseal. PTFE guide strips are available with different designs (--> fig. 7)
and can be cut with different configurations (--> fig. 8).
Guide strips cut to length Based on the hardware dimensions, Ecoseal can supply guide strips with specified lengths. They are designated according to a system that states the type and design, dynamic diameter, housing groove diameter,housing groove width, type of cut and material
Basics
O-rings are one of the most common sealing solutions. SKF supplies O-rings in a wide range of sizes and different materials, which make them appropriate for a wide variety of operating conditions and applications. They are easy
to install and they enable a simple and costeffective seal housing design.
O-rings maintain sealing contact force by radial or axial deformation in the seal housing between two machine components. The most important criteria that influence the maximum operating pressure at which O-rings in static radial sealing can be used are the following:
Under specific conditions, there is a risk for gap extrusion († Extrusion gaps and back-up rings, page 298). Back-up rings prevent O-rings from gap extrusion in static radial sealing. O-rings are used in a wide variety of applications sealing various media. This catalogue focuses on sealing systems for hydraulic cylinders. Therefore, this chapter and provided recommendations apply to static sealing of common mineral-based hydraulic fluids.
Materials
The O-rings listed in this catalogue are made of nitrile rubber (NBR) with 70 shA hardness as standard. This is the most common O-ring material and hardness used in hydraulic cylinder applications. On request, SKF can supply alternative hardnesses such as 80 shA or 90 shA. However, SKF generally recommends hoosing O-rings with 70 shA and combining them with one or two back-up rings. At operating temperatures above 100 °C (210 °F), fluorocarbon rubber (FKM) or hydrogenated nitrile rubber (HNBR) can be an appropriate materials, depending on the fluid. Ecoseal back-up rings are made of polyurethane (TPU). On request, Ecoseal can supply alternative materials and various hardness grades. Common back-up ring materials are listed in the designation system.
Standards and sizes
Dimension standards
Ecoseal can supply O-rings in a wide range of sizes in accordance with various O-ring standards. Table 3 provides the most common national and international O-ring standards and their relevant sizes.
Inside diameter/cross section proportion
O-rings used in more demanding applications, such as those with higher operating pressures or larger misalignments, may require larger cross sections. Ecoseal recommends inside diameter ID and cross section proportions as provided in diagram 1.
Tolerance standard
Ecoseal supplies all O-rings with dimensional tolerances in accordance with ISO 3601-1 Class B (--> table 4). They are suitable for any elastomer material provided that appropriate tooling is used, whereas the tooling most commonly used is based upon the shrinkage of nitrile rubber (NBR) with 70 shA hardness.
Surface standard
Ecoseal supplies O-rings that all have surfaces in accordance with ISO 3601-3 (--> table 5). This standard provides maximum acceptable imperfections and quality criteria for O-ring surfaces.
Housing design and dimensions
Housing dimensions for static radial sealing
O-rings for static (non-moving) sealing can be used in a widevariety of applications and arrangements. The most common arrangement in hydraulic cylinder applications is static radial sealing between coaxial cylindrical parts. The O-ring is installed in a housing that is machined either as an outside or inside groove (--> fig. 1) in one of the two cylindrical parts. The housing dimensions for static radial sealing O-rings are listed in the product tables.
Housing groove edges
All housing groove edges should be smoothed and rounded off (--> fig. 1) to r = 0,1 to 0,2 mm (0.004 to 0,008 in.).
Lead-in chamfers
All edges and openings through which the O-ring has to pass during the assembly should have appropriate lead-in chamfers and should be well rounded off (--> table 6).
The chamfers facilitate assembly and protect the O-ring from damage during the installation process. The O-ring and all surrounding parts should be well lubricated before assembly, preferably with the same fluid as used in the hydraulic system, ensuring compatibility with seals and cylinder components.
Extrusion gaps and back-up rings
The size of the permissible extrusion gap depends mainly on the seal material, temperature and operating pressure. Harder materials provide a certain resistance to gap extrusion. When the permissible extrusion gap for the pressure and temperature in application is exceeded, back-up rings may be used to prevent the seal pressing into the gap and causing extrusion damage and possibly even premature failure. Figure 2 shows the O-ring behaviour at different operating pressures and conditions. In applications where the O-ring is exposed to pressure from one side only, the back-up ring is installed at the zero pressure side. For an O-ring exposed to pressure from both sides, a back-up ring is installed on both sides.
Diagrams 2 to 4 provide guideline values for the maximum extrusion gap in relation to the operating pressure for standard O-rings without back-up ring (--> diagram 2)
and different back-up ring materials and sizes (--> diagrams 3 and 4).
These guideline values are based on extensive tests conducted in laboratories at 90 °C (195 °F) and 100 000 pressure pulses. However, other factors such as temperature and fluid can influence these guideline values and should be considered.
Housing groove width
To accommodate the additional back-up rings, the O-ring groove width L needs to be increased to L1 for one back-up ring or L2 for two back-up rings (--> fig. 2). The groove width dimensions are listed in the product tables.
Dynamic radial sealing Under certain conditions, O-rings can be used for dynamic sealing with relative motion between the coaxial parts. These sealing arrangements are limited to slow reciprocating or oscillating motions. The radial depth S should be increased to S1 using
where
S1 = increased radial depth [mm]
CS = cross section [mm] († product tables)
S = radial depth [mm]
= (D - d1) / 2 for outside grooves
= (D1 - d) / 2 for inside grooves
Static axial sealing
O-rings can also be used for static axial sealing between two opposing parts. Although, static axial sealing arrangements are not common in hydraulic cylinder applications, some examples are shown in fig 3.
PTFE encapsulated O-rings
Ecoseal also supplies O-rings type ECOR that are encapsulated with PTFE materials (FEP or PFA). These O-rings have a core made of silicone or fluorocarbon rubber. The seamless and uniform PTFE encapsulation protects the core material against fluids and air.
ECOR O-rings are preferred for static sealing and not appropriate for continuously dynamic applications due to its thin and soft encapsulation. They are characterized by the following properties:
Back-up rings made of thermoplastic polyester elastomer
Ecoseal also supplies back-up rings made of thermoplastic polyester elastomer (TPC).
Back-up rings made of PTFE
Ecoseal back-up rings made of PTFE are suitable in applications with high temperatures or aggressive fluids. PTFE back-up rings are available unfilled or with an appropriate filler. Back-up rings made of unfilled PTFE can be machined from tube blanks with outside diameters ranging from 1 to 1 500 mm (0.039 to 59 in.). Therefore, they can easily be adapted to customer specific installations and delivered on short notice.
Hydraulic rock breaker hammers are a demanding application requiring hydraulic reciprocating seals working in short strokes and extreme high velocity. ฎแนหำฟส has many special solutions for rock hammers, including TEFLATHANE Seals with a special high-temperature polyurethane U-cup seal that is bonded to a low-friction PTFE anti-extrusion ring to extend service life for significant maintenance savings.
Each hydraulic press application is unique and should work reliably for many years without costly downtime or expensive repairs. Ecoseal has decades of experience developing and manufacturing customized press sealing solutions
for original equipment manufacturers, as well as retrofit assemblies for existing equipment. With moulding and machining capabilities and virtually unlimited diameter range, Ecoseal can customize and manufacture sealing solutions
to optimize system performance and decrease operating costs.
Hinge pin seals retain pin joint bearing lubricants and exclude contaminants on off highway equipment that operates in harsh outdoor environments across all climates and weather conditions. PAK-L press-in seals are optimized for demanding pin joint applications. The wear resistant polyurethane seal materials reduce relubrication intervals and extend both seal and hinge joint bearing life. In addition, Ecoseal has experience developing customized pin joint seal solutions.
Rotary unions
Rotary unions (also known as rotary manifolds or swivel joints) accommodate the flow of fluids between machine components with relative rotating or oscillating motion. They convey gases, lubricants, coolants and working fluids for fluid power.Ecoseal has sealing solutions and capabilities to accommodate virtually any rotary union application. The IM profile seal for hydraulic rotary manifolds is optimized for low friction and is available with a bonded energizer to improve energy efficiency and extend service life.
Hydraulic motors
Hydraulic motors rotate with high torque from high-pressure hydraulic fluid flow. Certain motor designs and applications require the shaft seal to handle high pressure. The demands for higher power, efficient operation and long life require other solutions than typical rubber shaft seals. Ecoseal designs and develops customized highpressure PTFE shaft seals for hydraulic motors for reduced friction and wear to support energy saving and extend service life.
Pneumatic fluid power applications
Special gas-sealing applications with high pressure, high speed, extreme high temperatures, cryogenic low temperatures or special gases place high demands on pneumatic seals. Ecoseal can provide customized solutions that meets your most demanding pneumatic sealing needs, designed specifically to enhance reliability and performance in your application.
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