Radial Seals are rotary oil seals to prevent lubricant leakage out of a system. While the primary purpose of such components is to prevent leakage out, they also ensure that debris and other contaminants do not enter the system. They mount the shaft with an interference fit and are used in both, static and dynamic sealing applications. In this application, the seals remain stationary as they slide on the shaft surface as the shaft rotates, creating a dynamic seal. Figure 1
below is an image of a radial lip seal, demonstrating the various parts it consists of. The seal has (1) a lip, which presses down on the surface of the rotating shaft, (2) an outer static seal, (3) a supporting metal reinforcement and (4) a garter spring with a pre-load, which provides a force for the lip to achieve sealing. This spring presses onto the seal-shaft interface to provide resistance to the fluid leaking out.
The sealing interface is a three-part tribological interaction between the seal lip, the sleeve surface and the thin film of lubricant in between them. This interface is critical in determining the resulting sealing system, successful or unsuccessful. The ‘success’ of a sealing system is also subjective; a perfect seal does not exist. The success of a sealing system is determined by how well the specific needs of the application as identified by the engineers have been met [2
]. Thus, there are several factors for the seal that effect the system i.e., seal material, seal radial loads, seal design and geometry, tolerances, etc. For the sleeve, factors such as coating, hardness, lead, texture, manufacturing method and surface finish all play a role in determining how the system will perform ([3
] referring to ISO 6194, DIN 3760/1), also described in [5
]. The motivation of this particular study was to understand the functional behavior of three common shaft coatings in the aerospace industry against seals of varying radial loads to learn about the factors that will affect the contact temperatures at the interface.
Various environmental factors also influence the wear, leakage, and overall performance of each sealing application. Temperatures, pressures, vibration, rotational speeds, oil selection, shaft coating and seal selection all play an important role and will eventually determine a successful or failed seal in each case [5
]. The temperature at the sealing interface i.e., the contact temperature is a critical aspect of determining the operational conditions of a sealing system. Several factors influence the contact temperature, including the radial load, circumferential velocity and lubricating capacity of the fluid.
The lubricant helps in cooling at the seal interface as well. However, it is important to note that in such applications (helicopter splash lubricated gearboxes), it is difficult to predict where the oil will end up and whether the seals are receiving oil, or are running dry. Seals that run dry experience a high degree of friction and experience pre-mature wear and failure. This is also a reason a polytetrafluoroethylene (PTFE)-lined seal is studied in this application as PTFE provides excellent lubricating properties at the interface [7
], in case lubricant is not reaching every seal. In reference to the seal, material is a critical factor. During selection of a seal material, the operating temperatures are essential limits to consider. Elastomers are the most commonly used seal materials, where various elastomers differ from one another based on the polymer and its’ cross-linking. Additionally, elastomers exhibit viscoelastic behavior, which further enforces the significance of working temperatures of the seals in service. The glass transition temperature, TG
, separates the defined states of the polymer at different temperatures. Most sealing materials will have a very low glass transition temperature so that they can seal effectively as a flexible elastomer material at ambient temperature [8
As seen in Figure 1
, the seal has a lip (marked as 1) which slides on the shaft surface. The seal lip temperature plays an essential role in determining and predicting overall seal performance and life. When exposed to extremely high temperatures, the rubber material of the seal lip will experience aging, coking and swelling, thus damaging the seal [9
]. Various rubbers have their own temperature limit, beyond which the rubber material will deteriorate. The respective aging of the rubber directly affects the useful life of the seal as well. Fluoroelastomer (FKM) rubber can usually withstand the most frictional heat and operates at a temperature range of −35 to 230 °C. However, it is important to note that the following rule of thumb applies: ‘for every 10° increase in temperature, aging increases by a factor between 2 and 4. The life of that component is then decreased by that same factor’.
In reference to the sleeve, the surface coating is critical in determining the functional behavior of the sealing system. As described in depth by ISO 6194-1 and DIN 3671 [3
], there are specific requirements for the sleeve that must be met for radial lip seals. These requirements include the hardness, the texture and lead as described in this particular manuscript. The surface coating determines the properties of the sleeve such as surface texture, wear and corrosion, hardness and thermal insulation [11
], all of which are factors in final sealing performance.
Coating methods refer to the deposition process of the coating onto the metal. There are several methods of applying these coatings that include but are not limited to thermal and plasma spraying, chemical vapor deposition (CVD) and physical vapor deposition (PVD). These processes have varying outcomes on the final specimen in the form of microstructures, surface texture and durability. The desired coating method is selected based on the required functionality of the specimen [12
The surface roughness of the sleeve is key in achieving proper lubrication at the interface, preventing the seal from running dry. Lubricant-free seal contact with the asperities also increases shear loads, friction and raises local seal surface temperature leading to thermal degradation [13
]. Owing to the fact that the surface roughness is determined by the coating and coating method, the coating becomes an invaluable parameter in controlling increased temperatures at the sealing interface.
Further, due to high rotational speeds, frictional heat at the seal shaft interface and overall high sump temperatures can enhance the damage caused by thermal degradation. The heat generated by friction at the contact interface raises the seal lip temperature. An Finite Element Method (FEM) study conducted by Stakenborg [14
] finds that while the rubbers are chosen such that their stiffness is not effected by the lip temperatures, for any temperatures above 100 °C, the rubber stiffness will rapidly change due to physical aging, especially under a dynamic load. Datasheets for the operational limit of such seals [6
] suggests that for a shaft diameter of 50 mm, at 6000 rpm the excess temperature should be around 40 °C.
] conducts a study to find how the aging of oil impacts the seal performance of a radial lip design. Aged rubber seals and samples are obtained to investigate the changes in material properties with respect to aging time. His work shoes that the micro-surface topography of the seal lip is a critical factor in achieving successful sealing and this changes with aging time. More specifically, he found that the root mean square (RMS) roughness, known to be a good marker of the surface properties, decreases with aging time.
Additionally, similar thermal studies on rotary components conducted to study the relationship between contact temperatures and other factors of the rotary application are enlightening. Tzanakis [16
] finds that a rise in the temperatures at the contact interface can directly affect the surface geometry. This can lead to severe local wear at the surfaces and thus, lubrication breakdown. The prediction and measurement of the surface temperatures of the interacting bodies is an integral step in preventing the failure of tribological systems, including radial lip seals. Additionally, the surface roughness of the components will likely affect the distribution of temperature increase during sliding contact. Given that increasing contact temperatures can enhance wear and thus, the surface roughness of the specimen, this is undesirable. Thus, there is a co-relation present between the contact temperatures and surface roughness with time, which we investigate in this manuscript as well.
Other conditions, such as misalignment of the seal and high oil viscosity can also contribute to heat generation at the lip. All these factors can result in reduced seal life and increase the maintenance intervals for seals. This is additionally confirmed as for every 14 °C drop in the sump temperature, the seal life can double [17
]. Thus, it is clear why two alike seals can display stark differences in sealing efficiency, wear and life simply based on their location in the aero-engine [14
In this manuscript, we investigate the contact temperatures at the sealing interface of radial lip seals sliding on rotating shafts. The contact temperatures as a function of the sliding velocity, shaft surfaces (roughness and coatings) and the seal radial loads are studied. The aim of this work is to identify the parameters that result in higher contact temperatures than expected. This study will shed light on the factors that influence temperatures at the interface for systems where controlled temperatures are essential to successful sealing.
Sleeves are economically and logistically more feasible to surface treat rather than entire shafts and are thus used for testing. Further, if there is considerable and excessive wear, only the sleeve will need replacing instead of the shaft. The sleeve rotates along with the shaft when the rig is operational as they are connected using a hydraulic coupling. Several factors determine the overall performance of the sealing system concerning the seal-mating surface i.e., the sleeve. The specific coating, hardness of the sleeve, surface roughness and the manufacturing method all play vital roles in a successful or unsuccessful sealing system.
According to DIN 3760/61 [4
] the sleeve must be hardened to at least 45 HRC (Rockwell hardness), and as the running speeds increase, the HRC of the shaft must too.
Hard chrome, chrome oxide and tungsten carbide coatings are some of the commonly used coatings in this particular application. Some of the common methods used to apply these coatings are Hard Chrome Plating (HCP), High Velocity Oxy Fuel (HVOF), Physical Vapour deposition (PVD) and Chemical Vapour Deposition (CVD). The various methods used to apply the coating will determine the properties and effectiveness of the coating. This will then impact how it will degrade or experience wear during its usable life.
Chrome oxide coating is applied using combustion powder thermal spraying, chrome plate (CP) coating is applied using hard chrome plating, and the tungsten carbide coating is applied using CVD.
HVOF is a type of thermal spraying technique where particles are sprayed onto the substrate material surface at a high velocity of 600 m/s or more with high pressure combustion energy of fuel and air (oxygen) serving as a heat source. The coating particles are in powder form and are injected into the hot gas stream resulting from the combustion process. High kinetic energy and high impact forces result in bonding of the coating and the substrate material. With chrome oxide, the HVOF process doesn’t generate enough heat in the flame to melt the ceramic, so a combustion powder gun is used. It uses the same powder feed set up and attaches to the robot, but utilizes an alternative fuel which burns hotter. This type of gun has a lower velocity in the flame as compared to HVOF, which is why the porosity is higher. The porosity is controlled by the particle velocity and heat in the flame mainly. A review on plasma sprayed coatings [18
] suggests some methods to reduce the porosity in chromium oxide coatings, as this may be detrimental to the application.
Hard chrome plating is an electroplating process to coat a specimen with a later of chromium. Thickness of hard chrome plating ranges from 2 to 250 µm. Hard chrome plating is a technique that has been in commercial production for over 50 years and which is a critical process associated with manufacturing and maintenance activities for commercial and military aircraft [19
]. However, in recent years, European Union directives have highlighted economic and ecological repercussions of HCP and several industries are finding appropriate alternatives for this coating [20
CVD, on the other hand is a completely different process. It is a more robust version of PVD as it provides a thicker layer of coating to protect from heat damage. While PVD uses primarily physical processes to apply source material to the substrate, CVD involves mixing the source material with chemicals that interact with the material to create the desired coating. This results in a chemical bonding between the two materials, leading to a more reliable and improved coating process for aerospace applications. It is an environmentally compliant and technically superior replacement for HCP and HVOF coatings, providing enhanced protection against corrosion and chemically aggressive media, wear, galling, fretting and fatigue. Hardide-A matches HCP in thickness and hardness and outperforms the material in several key properties including enhanced protection against corrosion, wear and chemically aggressive media [22
]. Additionally, the tungsten carbide coating (called Hardide A) is by Hardide. It is porosity free and is an excellent alternative to HCP. According to a recent study [13
], it outperforms HCP by 14 times, demonstrating excellent wear resistance. They also go as far as to refer to it as ‘seal friendly’, as it is qualified by Airbus as ‘an environmentally friendly replacement for HCP’.
The four various sleeves are described below, three of which are coated and one remains uncoated.
Chrome Oxide (CO): Chrome Oxide is a common shaft coating used in many industries, especially aerospace. The specified standard Ra and Rmr values: Ra = 0.2/0.4 and Rmr = (0.8) > 70% with a 5% reference line surface finish parameters. The thickness of the coating is 0.1 mm and hardness to be > 55 HRC. The surface finish process is through diamond grinding. A university verified local manufacturer manufactured this sleeve. The manufacturer for this coating was a UK based company (Thermal Spray Coating, B&B precision engineering, Huddersfield, UK).
Chrome Plate (CP): Hard chrome plating is a common shaft coating for aerospace applications. However, due to its susceptibility to ‘polishing in’ with high fatigue and wear, industry is moving away from this coating [23
]. The specified standard Ra and Rmr values: Ra = 0.2/0.4 and Rmr = (0.8) > 70% with a 5% reference line surface finish parameters. The thickness of the coating is 0.25 mm and hardness to be 64 HRC. The surface finish process is through plunge grinding. This sleeve was acquired with the support of my industrial partner (acknowledgements).
Tungsten Carbide (TC): Commonly known as ‘hardide’ coatings, they are gaining popularity, as they do not polish in over-time, maintaining their initial surface parameters, including roughness. These coatings provide improved wear and fatigue in contrast to other coatings. The specified standard Ra and Rmr values: Ra = 0.2/0.4 and Rmr = (0.8) >70% with a 5% reference line surface finish parameters. The outer diameter is case hardened to give a case depth of 0.5 mm and hardness to be >55 HRC. The surface finish process is through plunge grinding. The manufacturer for this coating is Hardide, it was acquired with the support of my industrial partner (acknowledgements).
Stainless Steel (SS): This sleeve represents an uncoated stainless steel shaft. The purpose of testing with this sleeve is to draw a comparison between the sleeves made to the required specifications and a regular stainless steel sleeve concerning the contact temperatures. It is also to derive results to demonstrate the need for specific surface coatings for optimum seal performance. A university verified local manufacturer manufactured this sleeve.
The seals tested in this experiment are radial lip PTFE-lined oil seals and three varying spring loads are used i.e., 8.5, 12, and 14 oz. ‘PTFE-lined’ means that this is essentially an elastomeric seal, with a PTFE coating on the lip to improve the tribological properties and lubrication at the sealing interface. The PTFE coating is a filled polymer matrix and the sealing lip is pre-loaded with an extension garter spring pressing down on the lip to ensure seal-sleeve contact to aid in static and dynamic sealing. The seal design includes a garter spring to maintain and provide the radial force of the lip on the surface of the sleeve. Due to various environmental conditions, particularly heat, the elastomer will age. However, to ensure that it will remain stable and unchanged during the service, the spring is heat-treated. The garter spring is tightly wound thus carries an initial tension. The sum of the force required to overcome the initial tension and the force due to the spring rate equals the total force that the spring exerts. Over the lifespan of the garter spring, even as the seal wears, it is designed to provide nearly a constant force. The calculation of applied circumferential and radial load are in 4.1.
below demonstrates a cross sectional schematic of the seal used in this experiment. This seal design is the current state of the art with the sealing lip, the garter spring, metal insert and dust lip (ISO 6194) [3
]. As mentioned above, the sealing lip has a PTFE-coating bonded to the elastomer (visible in Figure 2
below); this PTFE coating is filled with certain materials to improve the mechanical properties of the sealing lip. While PTFE provides better lubrication at the interface due to its own properties, it is extremely soft and reinforcement with fillers elevates its’ strength.
In the case of these dynamic seals, a thin film of fluid separates the seal and the sleeve surface (2–5 microns). Leakage will depend on the application, and extent of technical tightness desired for that case (DIN 3761) [10
]. In certain systems, some amount of leakage is acceptable to ensure that the seal does not run dry. The lubricant used in this study is Aeroshell 555, approved for the US military standard DOD-PRF-85734 [24
]. The seal material is also approved for compatibility with this standard [25
], confirming that oil and component compatibility will not arise as a problem. Table 1
below outlines the parameters of this gearbox transmissions oil used in this experiment.
Developed with high temperature and load carrying performance in mind, their viscosity at 100 °C classifies synthetic gas turbine oils like Aeroshell 555. This oil is a 5 cs oil, which is the most commonly used in engines for commercial aviation. 5 cs oils are limited by a specification to a maximum of 130,000 cs at −40 °C and are called ‘type 2’ oils. This is a reference to their viscosity grade.
This study considers three important parameters to investigate their effect on the contact temperatures at the sealing interface: seal spring loads, surface coatings and surface roughness.
The total load of the sealing system is a combination of the load provided by the elastomer and the garter spring. The garter spring is designed to maintain a constant radial force on the seal lip, even as the elastomer ages and the load decreases. An increasing radial load results in frictional forces to increase at the interface [31
], which would cause temperatures to rise. However, it has been shown that increasing temperatures at the interface causes contact forces to subsequently decrease [32
], which would likely result in a decrease in the rate of change of temperature rise. Thus, it is not as straight forward as higher radial load will result in higher temperatures and vice versa. In this study, an optimum seal of 12 oz exhibits controlled temperatures at the contact interface. A lower seal spring of 8 oz results in slightly higher temperatures, as does a 14 oz seal spring. This is unexpected as a lower radial load would decrease friction at the interface and thus, presumably cause temperatures to decrease. However, for this application, findings show that a lower seal spring of 8 oz does not cause a reduction in contact temperatures. This may be due to an improper formation of the dynamic seal gap with this applied force, resulting in increased friction at the interface. It is difficult to predict with certainty why a 12 oz seal spring exhibited optimum performance. However, it can be concluded that for such applications, a 12 oz seal spring reaches optimum temperatures at the contact interface—higher and lower values result in increase of temperature.
The application of thin and hard coatings to metals with lower wear resistance is a practice that has improved the tribological interaction of contacting bodies and surfaces. Components operating in high temperature environments like gas turbine engines would experience premature wear, damage and failure without protective hard coatings [33
The CP sleeve experienced the highest contact temperatures from the three coated sleeves, albeit not as high as the uncoated SS sleeve. The SS sleeve, in contrast, experienced the highest contact temperatures from all the four sleeves tested. The SS sleeve was uncoated and its surface roughness was not within the desired ranges as required by the ISO, and DIN standards. The CP sleeve fulfilled the surface roughness requirements, so its failure in terms of contact temperature falls on the nature of the coating. The CP coating is one that has become seldom used in the aerospace industry and due to the implementations of EU directives such as 1999/13/EC (VOC), 2011/65/EU (RoHS), and 2012/19/EU (WEEE) [20
], alternatives to such a coating is encouraged. Additionally, it has a nature of polishing in easily overtime [28
]. This polishing in effect will result in a smoother surface than it initially was as the roughness peaks are removed by this phenomenon. This can eventually also result in wear of the sleeve surface. Smooth surfaces are considered undesirable for seal counter surfaces as they will result in more friction at the interface, and as a result, higher contact temperatures.
This is why this CP sleeve experienced higher contact temperatures than TC and CO. However, the SS sleeve has a higher roughness and is uncoated, both of which contribute to the higher friction, resulting in excessively higher temperatures at the interface in all test cases.
Micro roughness, groove textures and other laser-textured surfaces are known to reduce the friction at contacting interfaces [37
]. Table 5
presents the surface roughness range for sleeves as recommended by the standards. These standards also specify that a plunge ground surface finish is recommended, as the micro roughness patterns on the surface are a critical aspect of achieving an effective sealing system. Lead on the sleeve surface is also undesirable and ISO 6194 and DIN 3761 [3
] recommend zero lead on the sleeve surface. Any lead, coupled with the direction of shaft rotation may result in pumping of oil toward either side [38
] which can starve the interface of lubricant. Thus, the surface roughness is critical in controlling friction and temperatures at the sealing interface.
An uncoated SS shaft or sleeve is not finished to the desired specifications, and as we observe in this study, does not perform up to the required standards in terms of contact temperatures at the sealing interface. In this study, the three coated sleeves (TC, CO, CP) are surface finished to the desired standard, while the SS sleeve is generically manufactured without any surface finishing process. While this results in a flawed comparison from one aspect, the test is designed this way to draw a comparison between sleeves that are coated and surface finished as opposed to using a plain SS sleeve/shaft that has not undergone any surface treatment.
The first finding is that a microscopically rougher shaft will result in increased temperatures at the sealing interface. The flaw in the study is that it is difficult to pinpoint whether the temperature rise is attributed to the sleeve being rougher or because it is uncoated, as an uncoated SS sleeve with the correct roughness has not been studied here. However, previous studies have established that a surface that is ‘too rough’ is undesirable, just as a perfectly smooth (super-finished) surface is. A shaft surface ‘rougher’ than required can result in excessive and undesirable wear, thus decreasing seal life. Consequently, a shaft surface ‘too smooth’ can also be detrimental as the seal will not wear in as required, potentially resulting in high leakage levels. Horve concluded that this phenomenon is a critical aspect necessary to achieve reliable seal performance [39
]. Thus, it is not presumptuous to conclude that the microscopically rougher surface of the SS sleeve contributes to higher temperatures at the sealing interface.
This experimental study reveals the maximum temperatures at the sealing interface for a number of seal-sleeve combinations at sliding velocities of 5–25 ms−1. The results demonstrate that a 12 oz spring load is an optimum load, as higher spring loads of 14 oz result in undesirable and higher temperatures at the sealing interface. Consequently, a lower spring load of 8.5 oz does not result in lower temperatures than observed in the 12 oz spring as one might expect. Depending on the requirement of the application, a 14 oz spring may be optimal if controlled leakage is more important than controlled contact temperature. In contrast, an 8.5 oz spring may be optimal if wear is a more poignant concern for that application. However, for applications where excess temperature at the sealing interface is a high priority concern, 12 oz seal spring provides an optimal sealing environment for controlled temperature increase.
The seal-running surface, the sleeve, is of great importance in achieving a successful sealing environment. The presence of state-of-the-art seal specification standards (described in 4.3 depict the importance of the surface roughness of the sleeve. The microscopic roughness of the SS is higher than the other three sleeves, mainly because it has not been finished to a desired standard. The results of this sleeve reveal higher contact temperatures with all seal springs, which is due to this microscopically rough surface. It will result in higher friction at the interface, resulting in higher temperatures at the sealing interface. Additionally, the SS does not have a surface coating. Evidenced by the disparity of temperature increase for this sleeve in particular in contrast to others, the significance of an appropriate surface coating on the sleeve is re-enforced.
The results from the SS clearly demonstrate the need for surface coating and its role in contact temperature at the sealing interface. The CP sleeve also exhibits higher contact temperatures over the TC and CO sleeve, albeit lower contact temperatures than the SS sleeve. CP sleeves are not as highly recommended as seal-mating surfaces anymore as they have been in the past. They are undesirable as they polish in over time; resulting in ‘smoother’ surfaces (smoother than the recommended roughness guidelines) and cause wear in the system. This study concludes that another reason for its poor performance is the tendency to have higher contact temperatures at the sealing interface, which will only aggravate wear of the seal lip through elastomer aging and other mechanisms.
Results from the TC and CO sleeves consistently exhibit excess temperature of 40 K or less at the sealing interface, indicating that they are ideal sleeve surface coatings in terms of temperature control. However, other factors that may contribute to the increase of contact temperatures include long-term wear, vibration in the system and shaft eccentricity. Evaluation of these factors was outside the scope of this work.
Future work can benefit from a deeper insight into the surface micro roughness of the sleeves, including the texture and lead. This may provide more insight into specific micro patterns, grooves or textures that are causing contact temperatures to increase and thus, detrimental to the overall success of the sealing system.