Analyzing the causes of failure in mechanical seals from the perspective of failure modes

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By analyzing the causes of failure, the technical level of applying mechanical seals can be improved. Improvements in structural design stem largely from fault analysis. To analyze faults as accurately as possible, time is sometimes required, and even specialized testing techniques may be needed. I. Principles and methods for seal failure analysis: For each set of mechanical seals, regardless of the cause of failure, a detailed analysis should be conducted, and relevant data should be recorded. When a seal is damaged, it is not sufficient to limit the search for the cause of failure to the damaged component alone. The removed mechanical seals should also be properly collected and cleaned thoroughly ； Place them separately in the stationary and rotating sections, and label them for inspection and recording. The inspection procedure is as follows: first, determine the impact of the damaged seal on the sealing performance, and then carefully examine for signs of wear on the sealing ring, driving components, elastic loading elements, auxiliary sealing rings, anti-rotation mechanism, fastening screws, etc., one by one. A thorough inspection should also be carried out on the accessories, such as gland seals, shaft sleeves, sealing chambers, and sealing systems. In addition, it is also necessary to understand the operating conditions of the equipment, as well as past cases of seal failure. On this basis, through comprehensive analysis, the root cause of the failure can be identified. II. Analysis of the cause of failure based on wear traces Wear traces can reflect the movement pattern and degree of wear of moving parts. Every wear mark can provide useful clues for fault analysis. For example, if the wear marks on the friction pairs are even and normal, and the fit between the various components is good, this indicates that the machine has good coaxiality. If leakage still occurs at the sealed end face, it may not be caused by a problem with the seal itself. For example, if the wear marks on the end faces of a mechanical seal using a metal bellows are even and normal, and the leakage rate remains constant, this indicates that the leakage does not occur between the two end faces; it may instead take place in other areas, such as the static seal used to fix the bellows. When excessive wear occurs on the end face, it indicates that the machine’s coaxiality is very poor. With each rotation of the shaft, the seal must undergo axial displacement and radial movement; obviously, with every rotation, the sealing surfaces tend to separate slightly, resulting in leakage. Taking centrifugal pumps as an example, the reasons for excessive wear generally include: misalignment of the coupling, bent pump shaft, skewed pump shaft, low precision of the shaft, excessive pipeline tension, and vibration. Other causes of vibration include cavitation, surge, water hammer impact, and unbalanced fluid flow. However, in most cases, the vibration is caused by poor coupling alignment and low bearing operating precision.  When installing the couplings, it is necessary to measure the accuracy of the alignment between the centerlines of the two shafts; this is typically done using a dial indicator and feeler gauge. The deviations in the outer diameters of the couplings as well as the gaps at their end faces must be kept within the ranges specified in Table 12-1. For vibrations caused by hydraulic characteristics, an effective remedy is to keep the pump’s displacement below the design value in order to reduce cavitation in the pump. When the width of the wear marks that appear is less than the width of the narrow ring’s surface, it indicates that excessive pressure is acting on the seal, causing the sealing surface to deform. To address this, it should be solved by designing the sealing structure to use one that can withstand high pressure. After the mechanical seal has been in operation for a while, if there are no signs of wear on the friction surfaces, it indicates that the seal was leaking from the very beginning of its use. The leaking fluid oxidizes and deposits around the sealing ring of the compensating ring, thereby preventing the compensating ring from making the necessary compensatory movements. This situation is the cause of the leakage. This situation is likely to occur if high-temperature fluids with high viscosity leak continuously. For rubber bellows-type seals, if there are no signs of wear on the end faces of the friction pairs, this indicates that the sealing end faces may have been pressed together; there is no relative rotation between the friction pairs, rather it is the rubber bellows that rotates relative to the shaft. If this happens, the spring will wear out, as well as the fixing components and the rotating components. Sometimes, the rotating ring does not rotate relative to the stationary ring, but rather rotates relative to the gland of the stationary ring; in this case, no wear marks appear on the end faces of the friction pair. The reason may be that the anti-rotation clip has broken, or the hole diameter of the stationary ring gland is smaller than the outer diameter of the seal, resulting in improper installation. The presence of bright spots on the sealed end face without any wear marks indicates that significant warping has occurred in that end face. This is caused by excessive fluid pressure, poor stiffness of the sealing ring, and improper installation, among other reasons. In external mechanical seals, this phenomenon can also occur if the clamped non-compensating ring is secured with only two bolts, the static ring gland does not have sufficient thickness, or the alignment surfaces are uneven. Deeper grooves appear on the end face of the hard ring (ring-shaped patterns, resembling those on a vinyl record). The main reason is poor alignment of the pump’s coupling, or inadequate tracking capability of the seal. When vibration causes the sealing surfaces to separate, larger particles can penetrate between them. If these particles become embedded in the surface of the softer carbon-graphite sealing ring, the soft ring acts like an abrasive wheel, grinding the harder surface and resulting in excessive wear of that surface. If end-face separation is caused by vibration, then transmission components such as drive pins will inevitably show signs of abnormal wear. For mechanical seals operating in particulate media, hard-end face materials are used for both the sealing components; this is an effective way to prevent deep grooves from forming on the sealing surfaces. For example, cemented carbide paired with cemented carbide or with silicon carbide is the best choice. Because the particles cannot be embedded in either end face; instead, they are ground and pass through between the two end faces. The grinding marks on the outer circular surface of the metal bushing may be caused by solid particles that have entered the bushing, and these particles interfere with the sealing element’s ability to adapt ； It could also be shaft skew, caused by a large deviation in the coaxiality between the shaft and the sealing chamber. III. Damage to the end-face material caused by thermal load: The appearance of notches on one or both end faces indicates that the distance between these two end faces is too large; when the end faces are pressed together with force, notches are formed. A common cause of end-face separation is the sudden evaporation of the medium. For example, with water, especially in hot water systems or liquids containing condensed water, water expands when it evaporates, thereby separating the two end surfaces. Cavitation in the pump, combined with blockage of the seals, can also be a cause of notches on the sealing surfaces. In this case, it is not caused by vibration and coupling misalignment, as these are not sufficient to create notches on the end faces. Reducing the end face temperature is a common method to prevent end face damage caused by rapid evaporation of the medium. At the same time, it is also advantageous to use materials with good thermal conductivity for pairing, such as nickel-based cemented carbide paired with copper-impregnated graphite. Furthermore, the use of balanced mechanical seals, or external liquid cooling via special glanding, or direct cooling of the seal inside the chamber, are all very effective in reducing the temperature of the seal surface.   In failed mechanical seals, the end faces of the friction pairs often exhibit very fine radial cracks, or radial cracks accompanied by blister marks, or even cracking. This is caused by overheating of the seal; in particular, ceramic and cemented carbide sealing surfaces are prone to such damage. Poor lubricity of the medium, overload, high operating temperature, high linear velocity, and improper combination of mating materials – any one of these factors, or a combination of several of them – can generate excessive frictional heat. If this frictional heat is not dissipated in time, heat cracks can occur. These fine cracks act like cutting edges, cutting into carbon graphite or other counterpart materials, resulting in excessive wear and high leakage. To address the issue of seal overheating, in addition to changing the end-face balance coefficient and reducing the load, a static seal can be used along with guide sleeves to force the cooling fluid to flow toward the seal surface, or hydrodynamic grooves can be created on the seal end face to solve this problem. There are many small hot spots and isolated discolored areas on the friction surface, indicating that the seal has deformed and twisted under high pressure and thermal effects. For the thermal deformation of the end face, finite element analysis should be employed to improve the design of the sealing ring. Sealing rings with hard materials sprayed on their surface, whether ceramic or cemented carbide, may experience flaking or peeling of their surface layer from the substrate under thermal stress. The occurrence of this phenomenon indicates that dry friction has occurred in the seal. To eliminate this phenomenon, it is first necessary to check whether the sealing is properly lubricated and cooled, whether there are any blockages in the cooling system, and whether the operations are being carried out correctly; appropriate measures should then be taken based on the actual situation. IV. The damage caused by corrosion to seals: Chemical and electrochemical corrosion pose a serious threat to the service life of mechanical seals. The causes of corrosion are complex; here, only the most common forms of corrosion in mechanical seals and the factors that have the greatest impact are analyzed. 1. General corrosion and local corrosion: General corrosion refers to uniform corrosion that occurs on the surface of a component in contact with a medium. Its characteristic is a reduction in the weight of the component; in severe cases, the component can be completely corroded, losing its strength and hardness. This phenomenon occurs when multi-spring assemblies made of 1Cr18Ni9Ti stainless steel are used in dilute sulfuric acid. Local corrosion can be simply identified by spots and holes of erosion on the parts. Local corrosion causes the surface layer of a part to become soft and porous, prone to flaking off, and to lose its wear resistance. Local corrosion is a type of corrosion in which a certain phase in a multiphase alloy or a certain element in a single-phase solid solution is selectively dissolved by the surrounding medium. For example, when cobalt-based cemented carbides are used in high-temperature strong alkalis, the bonding phase metal cobalt is prone to corrosion, and the tungsten carbide skeleton of the hard phase loses its strength, leading to grain flaking under mechanical stress. For example, in the reaction sintering of silicon carbide, the surface develops pitting due to the corrosion of free silicon (when pH > 10). Corrosion has a significant impact on the performance of seals. Since seals are smaller and more precise than the components of the main unit, materials that are more corrosion-resistant than those used in the main unit are usually chosen. For seals that are in direct contact with the medium, although appropriate materials can be selected by referring to the data in corrosion manuals, such data may not correspond to the operating conditions in mechanical sealing systems, as they are mostly corrosion data for a single medium under static conditions, whereas the medium in the process flow is a mixture of multiple substances. Experience shows that pressure, temperature, and sliding speed can all accelerate corrosion. The corrosion rate of seals increases exponentially with rising temperature. When handling highly corrosive fluids, the use of external or double-end seals can minimize the impact of corrosion on the seals, as it reduces the number of components that come into contact with the process fluid. This is also one of the most important principles for selecting a sealing structure under highly corrosive conditions. 2. Stress corrosion: Stress corrosion is a corrosion phenomenon that occurs in metal materials when they are under stress and exposed to a corrosive environment. Whether it is external loads or residual stresses, corrosion will be exacerbated. Materials prone to stress corrosion include austenitic stainless steels, copper alloys, etc. The process of stress corrosion generally involves the formation of selective corrosion grooves on the metal surface, leading to continuous localized corrosion; ultimately, under the influence of stress, cracks develop at the bottom of these grooves. A typical example is the drive sleeve of the type 104 mechanical seal, whose material is 1Cr18Ni9Ti. When used in ammonia water pumps, the drive lugs of this sleeve are most prone to stress corrosion cracks, which leads to damage to those lugs. To this end, changing its concave earrings to solid protruding lugs can prevent such stress corrosion from occurring. 3. The high-speed motion between the abrasive seal and the fluid causes microscopic irregularities on the contact surface. When the fluid is a corrosive medium, it accelerates the chemical reactions on the sealing contact surfaces; such reactions can be beneficial at times and harmful at other times. If the formed oxide layer is damaged, corrosion occurs. The destruction of materials caused by the alternating action of wear and abrasion is called abradion. Generally, the damage caused by abrasion to the non-primary components of mechanical seals such as spring seats, thrust rings, and ring seats does not immediately lead to changes in the sealing performance; however, it is one of the main forms of failure in the friction pair. To this end, in highly corrosive media, the friction pair should be made of materials with good corrosion resistance, such as 99.5% high-purity alumina ceramics, or hot-pressed sintered silicon carbide free of free silicon. 4. Intergranular corrosion occurs when a medium is present between metal surfaces or between a metal and a non-metallic component, creating small gaps; due to the stagnant flow of this medium, it leads to accelerated corrosion of the metal within those gaps. This type of corrosion is known as intergranular corrosion. Typical examples are the grooves or erosion spots that occur between the mechanical seal spring seat and the shaft, or between the compensating ring and the sealing ring (of course, micro-wear also exists in these areas). The reason for this is that the medium within the crack is in a state of stagnant flow, which makes it difficult for substances involved in the corrosion reaction to reach the inside of the crack. At the same time, the corrosion products formed within the crack have difficulty spreading outward. As a result, as corrosion progresses, the composition and pH value of the medium within the crack increasingly differ significantly from those of the overall medium, leading to intensified corrosion of the metal surface inside the crack. Gap corrosion poses a serious threat to sealing performance; grooves form at the seal ring and the shaft, which prevents the compensating ring from moving axially, causing it to lose its ability to follow movements and resulting in separation of the end faces and leakage. For intergranular corrosion, it can usually be mitigated through proper material selection and reasonable structural design. If materials with good resistance to intergranular corrosion are used, the structural design should aim to avoid the formation of gaps and areas where fluid can accumulate ； Circulation is carried out through self-flushing, keeping the medium in the sealed chamber in a state of continuous replacement and flow to prevent changes in the concentration of the medium components. For pump units that are not in use for extended periods, any accumulated liquid should be drained promptly. It is impossible to completely eliminate gaps in the structure; therefore, protective shaft sleeves are generally used, and corrosion-resistant materials can be sprayed at the locations where the sealing rings are installed to provide protection. 5. Electrochemical corrosion is, in fact, one of the various forms of corrosion that occur in mechanical seals, and all of them are more or less related to electrochemical corrosion. As for mechanical seal friction pairs, they are often subjected to the threat of electrochemical corrosion. Since the components of such friction pairs are typically made of different materials, when they are in an electrolyte solution, the difference in their inherent electrical potentials leads to an electrochemical cell effect between the two materials; this results in one material’s corrosion being accelerated while the corrosion of the other material is suppressed. For example, when copper is paired with nickel-chromium steel and used in oxidizing media, the nickel-chromium steel undergoes electrolytic decomposition. Saltwater, seawater, dilute hydrochloric acid, dilute sulfuric acid, etc., are all typical electrolyte solutions. Seals are prone to electrochemical corrosion; therefore, it is best to choose materials with similar potentials or to pair ceramics with glass fiber-reinforced polytetrafluoroethylene. V. Failure of rubber sealing rings: Auxiliary sealing rings used in mechanical seals often consist of synthetic rubber O-rings. About 30% of mechanical seal failures are caused by the failure of the O-ring. Its failure modes are as follows. 1. Aging, high temperatures, and chemical corrosion are usually the main causes of hardening and cracking in rubber products. Rubber aging is characterized by the hardening of the rubber, a decrease in its strength and elasticity; in severe cases, cracking may occur, resulting in a loss of sealing performance. During storage, rubber can age when exposed to sunlight for long periods, when in contact with ozone, or if stored for too long. Excessive heat can cause the rubber components to decompose or even carbonize. In high-temperature fluids, rubber is at risk of further vulcanization, eventually losing its elasticity and causing leaks. Therefore, it is necessary to understand the safe operating temperature for each type of synthetic rubber. 2. The permanent deformation of rubber seals is usually more severe than that of other materials. For example, rubber O-rings become square in use. If the sealing ring is exposed to high temperatures for an extended period, it will take on a cross-sectional shape similar to that of the groove; even when the temperature remains constant, it can still perform a sealing function ； However, when the temperature drops, the sealing ring contracts rapidly, creating a leakage path that leads to leaks. Therefore, attention should be paid to the temperature limits for various types of adhesives; prolonged use at these limit temperatures should be avoided. If it is not possible to change the sealing operating conditions, structural improvements must be made to mitigate the adverse effects of temperature on rubber materials. For example, choose rubber O-rings with a larger cross-section as much as possible; keep the O-rings away from the end faces of the friction pairs, increase the hardness of the O-rings appropriately, use a grooved assembly structure (avoid push-ring compression structures and prevent spring force from acting on the O-rings), and so on. 3. Swollen and deformed synthetic rubber can experience phenomena such as expansion, stickiness, or dissolution in certain media. Therefore, the appropriate material should be selected using the charts in relevant materials based on the properties of the working medium. If the composition of the working medium to be transported is not well known, immersion tests should be conducted to guide the proper selection of materials. Some mixed solutions may cloud various synthetic rubbers; in such cases, polytetrafluoroethylene must be used as the sealing ring. 4. The rubber O-ring in the rectangular groove of the distortion and extrusion damage compensation ring undergoes twisting during assembly or use. The reasons include: the O-ring having low hardness and an excessively small cross-sectional diameter, or uneven cross-sectional diameter; fluctuations in operating pressure; shock vibrations; as well as low internal pressure and poor lubrication, all of which can cause the O-ring to twist. The areas where distortion occurs are mostly in the middle of the O-ring. When the distortion is severe, the cross-section at that location becomes narrower, simultaneously leading to an increase in leakage and friction. The methods to prevent the O-ring from twisting are as follows. ① Before installation, the O-ring should be coated with grease in the groove, and the rotating shaft must be smooth to ensure that the O-ring can roll freely. ② The compression amount should be set to an appropriate value, and the width of the groove should be increased slightly to allow the O-ring to roll within it. ③ When several cross-sections are available, an O-ring with a larger cross-section should be preferred. ④ Use other seals that do not cause distortion, such as those with an X-shaped cross-section. Rubber O-rings are always in a compressed state under static and moving conditions; therefore, they tend to be squeezed into gaps under high-pressure conditions. O-ring extrusion occurs when an O-ring under high pressure experiences stress concentration at the gap; once this stress reaches a certain level, a burr forms on the O-ring and gets embedded in the gap, leading to wear or damage of the O-ring and causing the seal to fail prematurely, resulting in leakage of the medium from the seal area. Obviously, the causes of extrusion are mainly related to pressure and the gap at the sealing area, as well as the hardness of the O-ring material. Reducing the gap can prevent extrusion, but it reduces the floating follow-up characteristic of the sealing ring. So, under high-pressure conditions. A measure to prevent the extrusion of rubber O-rings is to install a retaining ring in the O-ring groove. Especially for O-rings with small cross-sections, retaining rings made of polytetrafluoroethylene or polyimide must be added. VI. Failure of springs or bellows: In use, the possible forms of failure for the springs or metal bellows in mechanical seals include permanent deformation, fracture, corrosion, creep, or relaxation. Among them, the factors affecting permanent deformation and fracture failure of metal bellows are the most complex. Cylindrical compression coil springs are commonly used as the loading elastic elements in mechanical seals. Therefore, this mainly analyzes the failure of cylindrical compression coil springs, and in principle it is also applicable to other springs or metal bellows. 1. Permanent deformation of springs is one of the main causes of spring failure; when a spring undergoes permanent deformation that exceeds the allowable limit, it will affect the proper functioning of the seal. The permanent deformation of the spring, that is, the reduction in its free height, results in a decrease in the working load when the working height remains constant. Permanent deformation is caused by design flaws in the spring and defects in the manufacturing process, which lead to failure. It is related to the following factors. ① Under the given conditions, the main factor affecting the permanent deformation of the spring is the working stress. Under different working load conditions, the permanent deformation of the spring also varies. Foreign sources suggest that the working stress of a spring should not exceed 0.3δb (tensile strength) of its material. ② The permanent deformation of a spring is related to its diameter. Seal designers often focus on adjusting the spring diameter to meet load requirements, while paying little attention to the effect of the spring diameter on permanent deformation; as a result, they may end up addressing one issue at the expense of another. Reducing the spring diameter can minimize permanent deformation. ③ The smaller the free height of the designed spring, the greater the relative permanent deformation. Tests have shown that increasing the free height of the spring can reduce its permanent deformation. It should also be noted that an excessively large free height may also cause bending and instability (in small-diameter springs). ④ The permanent deformation of the spring is related to the pitch. When the free height of the spring remains unchanged, increasing the pitch of the spring and reducing the number of active turns makes permanent deformation of the spring more likely to occur. ⑤ The permanent deformation of a spring is related to factors such as the material properties of the spring, the manufacturing process, and the choice of heat treatment method. For spring manufacturers, it is necessary to strengthen the management of material properties and processing quality. Firstly, it is necessary to strengthen the quality inspection and proper management of materials entering the factory, and strictly prohibit substandard materials from reaching the production site. When selecting the machining and heat treatment processes for springs, it is necessary to follow general principles as well as take into account the effects of permanent deformation in order to improve the quality of mechanical seal springs. In addition to the factors mentioned above, the permanent deformation of springs and metal bellows is also related to the operating temperature, which must remain within the range specified by the material.  2. Broken springs are also one of the main forms of spring failure. Depending on the loading characteristics of the spring, its operating environment, and the mode of failure, there are fatigue fracture, stress corrosion fracture, and overload fracture, among others. The causes of fatigue fracture in springs or metal bellows are mostly due to factors such as improper design, material defects, poor manufacturing, and harsh operating conditions, which lead to the propagation of fatigue cracks. Fatigue cracks often originate in high-stress areas. If a crack appears on the inner surface of a compression spring, it often extends at an angle of 45° to the axis of the spring material before breaking through to the outer surface; in the case of metal bellows, fractures usually occur at the troughs of the bellows. In welded metal bellows, if there are manufacturing defects such as uneven spacing between the corrugations, higher stresses may occur in certain of these corrugations, leading to their premature failure. The so-called manufacturing defects refer to uneven waveplate spacing, unequal wave depths, and inconsistent plate thicknesses, among others. When installing a stationary metal bellows mechanical seal, defects may occur due to an inclined connection between the gland and the support points. This defect also generates stress within the waveplate, leading to fracture. In many cases, resonance can occur when the frequency of the periodic expansion and contraction movement of the welded metal bellows is equal to the natural frequency of the sealing device, resulting in high stresses that lead to premature fatigue fracture. Two types of vibration can occur within welded metal bellows sealing devices: axial vibration and torsional vibration. Axial vibration is caused by the axial movement of the shaft, while torsional vibration is usually generated by the friction force between the friction pairs. Friction tends to tighten around the bellows until the friction becomes less than the tightening force within the bellows. Thereafter, the force is released on its own. This repeats in a self-cycling manner. This torsional vibration transforms into axial vibration on its own. When two adjacent waveplate solder balls collide with each other, the vibration weakens and its amplitude decreases, and this process repeats in a cyclic manner. To prevent resonance, the natural frequency of the seal should be designed to be higher than the main vibration frequency (by changing the material, sheet thickness, number of sheets, spacing, and installation length), or asymmetric waveforms can be used along with shift forks to transmit torque. Furthermore, various damping methods can also be used to eliminate vibrations; for example, a damping pad can be placed around the bellows to generate a slight elastic load, thereby ensuring contact with the bellows and reducing vibrations before their amplitude increases. The damping pad then dissipates the kinetic energy of the bellows. Under the effects of medium erosion and material stress, springs and metal bellows can experience fracture, a phenomenon known as stress corrosion fracture. Austenitic steel springs are prone to stress corrosion caused by oxides under alternating stress; therefore, Hastelloy is recommended for use. Springs and bellows operating in corrosive media are first corroded in the stress areas of their cross-sections, as corrosion and stress act together on certain weak points of the components, forming crack nuclei. Over time, these cracks gradually propagate at a subcritical rate. When the crack reaches a critical size, its elastic component suddenly breaks. Stress corrosion fracture is closely related to the working medium; when the medium contains chlorine, bromine, or fluorine, metal elastic components are prone to stress corrosion fracture. Stress corrosion cracking is, in terms of mechanism, an anodic reaction, whereas hydrogen embrittlement cracking is primarily a cathodic reaction. In most cases, the spring suffers from hydrogen-induced brittle fracture; hydrogen atoms penetrate into the grain boundaries of the spring material and combine to form hydrogen molecules, thereby generating high stresses that cause the spring to fracture brittlely even under low-stress loads. Hydrogen embrittlement fracture typically occurs within a bending angle range of 45° to 90°. If the hardened spring coil is clamped in a vise, the protruding part is tightened with pliers and bent forcefully, the spring can be easily broken into two or three sections. If the fracture is caused by other reasons, it will be found that the spring material still retains sufficient toughness. In seawater, sulfides, sulfuric acid, sulfates, caustic alkalis, liquid ammonia, and hydrogen-containing media, brittle fracture occurs as a result of the hydrogen gas generated by chemical reactions being absorbed by the spring material. In addition to the factors mentioned above, the fracture failure of springs or bellows is also caused by the following reasons. ① Heat treatment defects. Internal defects are inherent in the material due to improper heat treatment processes. If heat treatment causes the grains in the spring material to become coarse, although the desired hardness is achieved, the spring deforms rapidly during use and eventually breaks. ② Scars caused by tools. During the spring manufacturing process, especially with regard to the bends of hooked springs, improper manufacturing techniques often result in scars that create stress concentration areas, leading to the breakage of those bends. It is evident that, in addition to selecting appropriate materials and determining suitable stress levels during design based on the operating conditions of the spring, it is also essential to adopt appropriate manufacturing processes during production to prevent fracture failure. VII. Wear, fracture, or corrosion of the sealing drive elements: Drive pins, drive screws, flanges, shift forks, and even individual large springs can all be used to transmit torque and drive the seal to rotate. Vibration or misalignment in the installation position, as well as lack of alignment, can all cause the transmission components to wear out, bend, or even get damaged. The set screws used in mechanical seals cannot be made of hardened materials. When checking for wear, the transmission connection points should be inspected first; signs of wear can be found on pins, notches, flanges, and pull forks. The wear of the drive pin or drive groove is caused by adhesion-sliding action. If the two end faces stick together in an instant, the uneven rotation of the rotating ring will cause fluctuations during rotation, and the drive pin will be subjected to high stress. When starting and stopping frequently or under excessive stress, the drive pin is also prone to breaking, causing the seal to fail suddenly. Poor lubrication can also lead to adhesion – sliding. Other reasons for the breakage of the drive pin include: excessive spring force ； High medium pressure, combined with the use of unbalanced seals or seal fluids with poor lubricating properties, results in high torque ； Tilt in drive pin assembly ； Single-force application ； Only the corrosion resistance of the friction pair materials was considered during selection, without taking into account the mating performance ； Cavitation in pumps, etc. VIII. Damage caused by frictional heat Abnormal frictional heat damage is also one of the causes of mechanical seal failure. The shaft (or bushing), gland, sealing chamber, and seals can all be damaged due to abnormal overheating. Frictional heat damage can be determined from friction marks and color. As the temperature rises, metals change color; for example, the color of stainless steel changes: light yellow at around 370°C, blue at around 590°C, and dark gray at around 648°C. In some pumps, abnormal overheating can be caused by excessive shaft deflection, which leads to friction between the pump’s throat and the shaft; friction between a gland that lacks proper positioning and guidance and the pump shaft (or bushing); friction resulting from loose fixing screws and the sealing chamber; and sliding contact between the gland gasket and the rotating ring. The large amount of heat generated by abnormal friction is sufficient to melt the PTFE V-ring or cause the rubber O-ring to carbonize. Other causes of abnormal frictional heating include: the gland lacking proper positioning and guidance coming into contact with the pump shaft (or shaft sleeve) ； The stationary ring rotates ； Aggregated dirt inside the sealed chamber ； The sealing chamber is not aligned with the shaft, etc.