Overview of Materials Used for the Basic Elements of Hydraulic Actuators and Sealing Systems and Their Surfaces Modification Methods

04 Nov.,2022


high pressure hydraulic seals

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/ ).

The article is an overview of various materials used in power hydraulics for basic hydraulic actuators components such as cylinders, cylinder caps, pistons, piston rods, glands, and sealing systems. The aim of this review is to systematize the state of the art in the field of materials and surface modification methods used in the production of actuators. The paper discusses the requirements for the elements of actuators and analyzes the existing literature in terms of appearing failures and damages. The most frequently applied materials used in power hydraulics are described, and various surface modifications of the discussed elements, which are aimed at improving the operating parameters of actuators, are presented. The most frequently used materials for actuators elements are iron alloys. However, due to rising ecological requirements, there is a tendency to looking for modern replacements to obtain the same or even better mechanical or tribological parameters. Sealing systems are manufactured mainly from thermoplastic or elastomeric polymers, which are characterized by low friction and ensure the best possible interaction of seals with the cooperating element. In the field of surface modification, among others, the issue of chromium plating of piston rods has been discussed, which, due, to the toxicity of hexavalent chromium, should be replaced by other methods of improving surface properties.

In this review article, a division of materials and methods has been applied according to their purpose (for cylinders, pistons, etc.) Such systematics can be helpful when there is a need to know the latest manufacturing technologies applied to a given hydraulic actuators’ components. For each actuator part, materials were selected into subgroups such as ferrous materials, light alloys, etc. During the review, items from recent years were selected to discuss the latest publications presenting modern materials and methods of surface modification. The database of available materials is constantly being expanded, so it is important that the knowledge presented is as up-to-date as possible.

In the next part, the analysis of materials used for selected parts of hydraulic cylinders was performed. Material groups such as iron alloys, light metal alloys, composites, and polymers are discussed. For each of the discussed materials, the most important mechanical properties are presented, such as tensile strength, yield strength, hardness, impact energy, or elongation at break.

To systematize the materials and surface modification methods used for hydraulic actuators elements, an extensive review of the existing literature has been made, which allowed for the preparation of a work presenting the current state of art in the field of hydraulic actuators manufacturing. This paper focuses on several of the most important elements of hydraulic actuators: pistons, piston rods, cylinders, glands, bottoms, and seals. The most common causes of failure of these parts are described, with particular emphasis on operational factors such as abrasive wear and material fatigue. Errors during the component manufacturing stage were also noted.

Hydraulic actuators are used in many branches of industry, often requiring large working forces. They are used in construction and transportation equipment (excavators, forklifts, telescopic handlers, basket elevators, booms) [ 2 , 12 , 13 ]. Industries, where hydraulic cylinders are used, include mining, robotics, and aviation [ 6 , 14 ]. Therefore, manufacturers of actuators are outdoing each other in new material solutions and methods of improving the operational parameters of actuators.

The piston is usually connected to the piston rod using a thread connection. In this case, the sealing element must also be located here. Guide elements mounted on the piston transmit side forces and ensure that the piston is centred in the cylinder [ 5 , 10 ]. In addition to the guides, there are also sealing rings that prevent the working fluid from moving between the chambers. The piston seal acts as a pressure barrier which, for example, keeps the piston rod in a specific position [ 11 ]. Piston seals can be single- or double-acting. The thickness of the acceptable lubricant film depends on the design of the actuator. Single-acting actuators require a small oil film; in double-acting actuators, the oil film can be slightly thicker [ 10 ].

There are also guide elements of the piston rod in the gland. They must be sufficiently lubricated with the working fluid of the hydraulic system. These elements absorb radial loads and hold the piston rod axially [ 10 ].

The piston rod is the component that transfers the load to the receiving element. The gland is usually attached to the cylinder pipe using a threaded connection, which is why sealing between the two components is also necessary here. The gland contains the entire sealing system for the piston rod. Most often there are one or two seals to prevent the working fluid from flowing outside and a wiper ring to prevent contamination from being transferred from outside to inside the cylinder. Such a sealing system contributes to increase the life of the cylinder [ 5 , 7 , 8 , 9 ].

The cylinder cap can be assembled with a pipe using a welded connection—this is the most common way. Another method is to use a threaded cap. In the case of the latter solution, it is necessary to use a static seal to ensure adequate tightness here [ 4 , 5 ].

The casing of the actuator units is a cylinder tube. Inside the tube, there is a piston rod with a piston fixed at the end. On one side, the cylinder is closed by the end cap, and on the other side, there is a so-called actuator gland [ 5 , 6 ].

The construction of a hydraulic actuator is described on the example of the simplest piston actuator ( ).

In a plunger actuator, there is the so-called plunger instead of the piston assembly. A telescopic actuator consists of a set of several pipes, extending one from the other—such an actuator allows obtaining large strokes.

Piston actuators are the most common type of actuators. They are characterized by the fact that at the end of the piston rod on which the piston is mounted.

The most general division of the actuators includes single-acting, double-acting, and rotary actuators [ 4 ]. A characteristic feature of single-acting actuators is the presence of one working chamber and the possibility of executive (active) movement only in one direction. The return movement can be carried out by an external force or a spring force. Double-acting actuators, in turn, are characterized by a working stroke in both directions.

Hydraulic actuators are elements converting the energy of the working fluid into mechanical energy related to the reciprocating motion. The pressure of the working fluid acts on the piston and creates a force causing the piston assembly to move. As a result, the piston rod can perform useful work [ 1 , 2 ]. Hydraulic actuators are an executive element in power hydraulic systems. These structures have several advantages, which include the possibility of obtaining large working forces and low operating speeds [ 3 ].

The material properties can be modified by appropriate heat or heat-chemical treatment. Another way is to apply coatings required to meet several requirements, such as reduced friction, vibration damping, appropriate hardness, abrasion resistance, and good adhesion to the substrate [ 23 ].

In , there is another graphical representation of materials. The chart shows polymers in terms of density and glass temperature. Elastomers are marked in red and thermoplastic polymers are marked in blue.

There are many tools for selecting materials. The optimum material is one that has the right values for the relevant parameters for the specific application. An interesting tool is the CES EduPack software, which allows analyzing materials and drawing material diagrams taking into account the values of various temperature, mechanical, and economic parameters. An example of such a material chart can be found in . In this case, it is a graphical representation of different iron alloys in terms of selected parameters: tensile strength (horizontal axis) and price (vertical axis). In this case, stainless steel is marked in green and cast iron in red. Blue is responsible for low alloy steels and yellow for carbon steels. Each dot or oval represents a different material.

Additionally, a tendency to reduce the weight of machine elements is observed. Less weight is usually associated with energy savings, increased efficiency, and reduced financial outlays [ 19 , 20 , 21 , 22 ]. Reducing the weight of an element by changing the material can be carried out, for example, by changing steel to light metals or plastics. However, the new material must meet the needed requirements for the designed part—it must not be too weak or, for example, not resistant to working conditions.

Nowadays, when selecting the material, great emphasis is also placed on ecology. Progressive pollution of the environment entails the necessity of actions limiting the so-called carbon footprint, defined most simply as the impact of a product or process on the climate [ 17 ]. Industrial production contributes to the emission of a huge amount of greenhouse gases into the atmosphere [ 18 ], so there is a need to look for the most environmentally friendly methods of manufacturing products.

The working conditions of the part, the loads acting on it, and possible exposure to harmful substances should be considered. Financial outlays are an important issue. It is worth looking for cheaper solutions and improving the technology in such a way as to ensure minimization of costs while obtaining similar or even better mechanical and functional properties.

Generally speaking, the selected material and shape should be complementary with the method of producing the given object during the design [ 16 ]. Even the selection of the best material with a badly applied manufacturing method may not bring the expected final effect.

The selection of appropriate material should be carried out taking into account the shape of the designed part, both in terms of external dimensions and cell structure. The issue of choosing the appropriate material for pistons, piston rods, and cylinders has been briefly discussed in [ 15 ].

Many articles have been written about research on failures of hydraulic cylinders. Thanks to a detailed analysis, conclusions can be drawn that will help to avoid failures in future projects and to ensure that the cylinders will work as long as possible without failures. Prevention of failures usually involves changes in the actuator design, both structural and material.

Many factors have to be taken into account during designing a hydraulic actuator. The aim is not only that each element performs its functions as well as possible, but also that all elements work together in the best possible way. Both mechanical (e.g., strength) and operational (utility) factors must be considered.

3. Failures of Hydraulic Actuators

Failures of hydraulic actuators may result from various reasons. These reasons are either related to the actuator manufacturing process or operation. The literature extensively describes research on these breakdowns, and solutions were often proposed that could prevent similar defects in the future.

The most common causes of damage and failure of hydraulic actuators are three factors: abrasive wear, material fatigue, and friction [24].

Abrasive wear can lead to a risk of internal leakage and a decrease in the volume efficiency of the actuator [25]. Material fatigue (associated with the occurrence of variable loads) contributes to so-called fatigue cracking, hence the need to design the actuators in such a way as to ensure the highest possible resistance to this type of cracking [26].

Failures caused by fatigue are also investigated in [2]. During the fatigue failure analysis, it is necessary to locate the places where the stress concentration in the actuator is high and may negatively affect the strength. Usually, such concentration occurs in places of geometric discontinuities [27].

It is also worth mentioning the issue of uniformity of working movements in the actuator. It is important because it affects the accuracy of piston rod positioning, the uniformity of the contact force, and the safety of the user of the hydraulic actuator. The uniformity of piston movement is related to the type of sealing and piston rod guiding elements, piston rod load, and wear of the actuator elements. Non-coaxial load influences the change in the uniformity of piston movement [14,28].

The work [29] discusses a case of damage to a hydraulic actuator caused by design errors of the hydraulic system and operator’s errors during manual control of the machine operation.

Another cause of problems with the actuator are vibrations and oscillatory movements. The article [30] proposes a solution in the form of vibration reduction using rotary viscous dampers.

Corrosion of cylinders, pistons, and piston rods can be a significant problem. As a result, it is necessary to apply corrosion-resistant materials on the actuator elements and/or to apply an appropriate anti-corrosion coating [31].

In the article [23], it has been noted that corrosion and tribological extortion entails the risk of unfavourable structural changes in the material used for the actuator, which leads to a deterioration in its functional properties and, consequently, to system failure.

3.1. Failures of Cylinders

Failures of hydraulic cylinders can result from many factors. Most often it is material stress caused by high pressure, loss of stability, corrosion, and fatigue cracking [3,12,26].

The fact of high pressures in a hydraulic system should be taken into account during the cylinder design process. Cylinders under the influence of pressure may suffer from deformation manifested by e.g., change of diameter [32]. A pipe exposed to high pressures may swell. The solution is to select appropriately thick walls, which will prevent deformation and thus reduce the risk of seal failure and loss of tightness. Welding joint failures also often occur in high-pressure cylinders. The material of pipes working in such conditions is required to have a high yield strength, good weldability, and right impact capabilities [33].

Cylinders can be subject to axial and radial forces and fretting vibrations. That results in abrasive wear on the pipe surface [34]. It can significantly accelerate corrosive wear.

Fatigue cracking can occur in the cylinders due to stress concentration at welding joints, e.g., at oil connections [26,35]. This leads to oil leakage near the working fluid ports. The article [35] proposes the introduction of a washer made of heat-resistant material or glue to fill the gap between the cylinder surface and the oil inlet. By protecting the gap against oil ingress, the propagation of fatigue cracks in the joint can be prevented.

3.2. Failures of Piston Rods and Pistons

Piston rods are those elements of hydraulic actuators which most often fail [36]. This is because these components are often exposed to an aggressive environment and compressive cyclic loads [37].

Because the piston rod slides out of the cylinder, it is highly exposed to external factors. Temperature changes, precipitation, dirt, and dust often cause damage to its surface, which leads to failures that can be dangerous for people working nearby [13].

The forces in the piston rod should be applied axially, but sometimes lateral forces may also occur, which is not beneficial for the piston rod operation. This can occur either intentionally or accidentally. The piston rod is subjected to quite high tensile or compressive stress. In the case of single-acting actuators, the piston rod is only subjected to compressive stress, so only buckling is considered in the strength calculation. This is different for double-acting cylinders. Here, there are both compressive and tensile stresses, so fatigue strength must also be taken into account in the strength calculations [1,38].

In an element as important as the piston rod, the correct choice of surface modification is crucial. The article [39] analyzes the piston rod failure related to its fracture. The results showed that the crack could have been the result of improperly selected heat treatment, which led, among others, to susceptibility to stress corrosion cracking (SCC) and low toughness of the piston rod material.

In [40], the piston rod failure related to the crack that appeared in the centre of the welding joint and then propagated further was described. The results showed that the carbides formed during the nitriding process contributed to the brittleness of the joint. This phenomenon resulted from the coexistence of two factors: the use of unsuitable welding material and subsequent nitriding of the piston rod.

The issue of cracking was also examined in works [37,41], where it was noticed that the segregation of copper and nickel in the material of the piston rod can cause cracking resulting from improperly performed heat treatment.

Another piston rod failure was described in the article [42]. Here, the destruction resulted from fatigue related to the stress concentration at the rounding point on the piston rod (so-called “fillet”). Fatigue life reduction was also featured in the article [43]. In this case, the stress concentration caused the change of the thread used to connect the piston rod with the piston to a smaller pitch thread. During operation, the piston rod is also exposed to abrasive wear [44].

The piston and piston rod together form a unit, so the materials used to make the pistons and piston rods should have similar hardness and high impact strength [33]. The piston is much less likely to fail than the piston rod, which is a result of different operating conditions. Scratches or pits appear most often on the piston surface and they disturb the degree of geometric accuracy and may contribute to the loss of tightness. In [45], it was noted that the critical point may be the piston edges.

3.3. Failures of End Caps and Glands

The end cap and the gland are not among the most loaded elements of the actuator, but there are a few things to consider during designing them.

If the cylinder cap is welded—a crack may occur at the joint. The situation of that failure is described in the article [2]. In this case, the welding joint is fatigued, mainly due to cyclical loads. On the other hand, in the case of the ends connected to the cylinder by means of a thread, the problem may be caused by leakage of working fluid. In such cases, it is necessary to use a static seal at this point.

The gland is usually fastened with a thread. In this case, also it is usually necessary to use a seal here to prevent working fluid from leaking from the actuator chamber. It is worth mentioning that in the case of higher pressures, screwing the gland is not recommended due to the risk of swelling of the cylinder pipe and the possibility of loosening of the threaded connection [33]. In such cases, e.g., structural modifications of the existing threaded connection are made.

The article [46] describes the failure of the hydraulic actuator gland in aviation related to leakage. It was shown that the pressure bolts broke during the machine start-up test. It was related to material fatigue. Measures are proposed here to increase the fatigue strength of these bolts, such as modifications to the way the bolts are manufactured.

3.4. Failures of Sealing Systems

Sealing systems are often a critical element of hydraulic cylinders [47]. The main mechanisms responsible for seal failures are swelling, thermal degradation, deformation, and wear associated with contact with the other surface [48]. Destruction of seals leads to loss of tightness and, as a result, to leakage of working fluid.

Seals are exposed to many negative external factors such as high temperature, radial loads, aggressive environment, and harmful substances. During designing the seals, it is necessary to take into account the pressure range of the fluid in the system and possible pressure peaks, the temperature range, the speed of the piston rod, the condition of the cooperating surface, and the type of working fluid. All these factors affect the durability of the sealing system and the performance of the entire hydraulic system [10,49,50].

The basic requirements for the dynamic sealing system in cylinders are of course low friction coefficients and leaks close to zero [51]. The issue of friction is related to the performance of seals and their durability [52]. Seals must be designed with materials of appropriate module and hardness [10]. The design of the seal should ensure appropriate resistance to friction and corrosion, easy assembly/disassembly and a possibility to work in a wide temperature range [6]. There should be a small layer of working fluid on the piston rod seal. The lubricating film reduces friction, which contributes to increasing the life of the seals. Another advantage is the prevention of corrosion on the piston rod surface [5,10,48,53].

In [54], it was noted that the friction force and sensitivity to load variation is related to the type and shape of the seal used. It was also found that friction may negatively impact the accuracy of position adjustment.

Apart from friction, the wear of piston rod seals often results from oil contamination, which is not without influence on the life of the entire system [55].

The piston seal separates two chambers of different pressure. If the seal expands too much under the influence of temperature and presses too hard against the inner surface of the pipe—the lubricant may be completely removed, causing the seal to wear. Therefore, the seals used for large diameter and high-pressure pistons should be reinforced with different materials. The use of fabric can reduce thermal expansion and compensate for the excessive pressure of the seal on the sliding surface of the hydraulic cylinder. Additionally, it is important that the piston seal should be symmetrical to the transverse axis of the piston, otherwise a loss of tightness could occur when the piston is loaded [32].

The article [50] examines the behaviour of seals on chromium plated piston rods. The tests showed that the seal material and the condition of the piston rod surface have a key influence on friction and wear of the seals. The issue of friction can only be neglected if the hydraulic power of the actuator is sufficiently high. For more demanding applications with the necessary high positioning and control accuracy, tribology cannot be neglected.

The article [56] confirms that the material used for the seals and the pressure conditions in the chambers influence the dynamic friction conditions.

The research described in [57,58] showed that tribological issues and tightness of the system are influenced by the roughness of cooperating surfaces.

The high roughness of the sliding surface causes a thick lubricating film. The edges of the seals wear out quickly as they come into contact with the roughness peaks. Too small roughness in turn makes the film thin, so the friction forces are much higher [5].

The article [59] discusses the influence of anisotropic surface roughness on friction at the contact between the two elements. Anisotropic roughness is created during the machining of a part. It was shown that the friction force decreases if the cylinder surface is grooved perpendicularly to the direction of motion.

The stick-slip phenomenon occurring at low sliding speeds is also important. It consists of causing vibrations by changing the friction force. In the article [60], it is noted that the phenomenon was related to the transfer of carbon monoxide from carbon steel to the sealing surfaces. This leads to noise and accelerates the wear of sealing elements.

Problems with seals such as extrusion and cracking often result from excessive pressure [33]. In the article [61], it is noted that high pressure in the actuator system used in aviation can cause the formation of thicker fluid layers and thus more effective lubrication.

It is noted that the design of the wiper ring has a major impact on the control of leaks from the actuator gland [9]. It seems obvious that double lip scraper rings can cause less leakage than single lip ones. Unfortunately, however, double lip wipers, due to their design, can cause the wiper ring or seal ring to be ejected from the groove.

Sealing failures can lead to dangerous failure of the entire hydraulic system, so it is important to regularly check the condition of the seal and replace worn seals [48].