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There are a wide variety of mechanical seal designs. These designs differ in the way they are driven, the choice of secondary seals, and the location of the springs. Mechanical seals have all been designed to meet certain criteria in terms of application, cost, fit, ease of installation, and capability to seal. There are, however, a few basic characteristics that are common to any good mechanical seal design.
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Many early mechanical seal designs placed the spring inside the process fluid. Most products (process fluids) that are sealed are not very clean. When the spring mechanism of the mechanical seal is immersed in this unclean fluid, dirt collects between the springs. This situation eventually impacts the spring's ability to respond to movements and vibrations, and the ability to keep the seal faces closed. Over time, clogging of the springs will cause premature seal failure.
The ideal design offers springs on the atmospheric side of the mechanical seals. The springs will be protected from the process fluid and their ability to work will not be impeded.
The pressure from both the seal springs (Ps) and the hydraulic pressure of the liquid in the pump (Pp) provide a compression force that keeps the seal faces closed. Balanced seals reduce the seal ring area (Ah) on which the hydraulic pressure of the liquid in the pump (Pp) acts.
By reducing the area, the net closing force is reduced. This allows for better lubrication that results in lower heat generation, face wear, and power consumption. Balanced seals typically have higher pressure ratings than unbalanced seals.
The balance ratio (B) of a mechanical seal is given by the ratio of the hydraulically loaded area (Ah) and the sliding surface area (As):
Mechanical seals can be designed with inserted seal faces or with monolithic seal faces. In both cases, the sacrificial seal face is often made from carbon/graphite. This material offers good running properties but is relatively weaker from a mechanical standpoint than other options. Inserted face designs use a metal rotary holder to transmit the shaft torque to the seal face.
The disadvantage of this inserted face design is that the face and holder material have different coefficients of thermal expansion. This changes the net interference force between both parts when they are exposed to heat from the process fluid or face friction. The seal face deforms, which results in leakage and accelerated wear.
More modern seals are equipped with monolithic seal faces that are made out of only the seal face material itself. The torque transmission is applied directly to the seal face. This is possible if the geometry of the seal face is designed in a particular shape to give it the strength to handle the torque through its geometric design. These monolithic seal face designs have been made possible through the use of Finite Element Analysis (computer modeling).
Monolithic seal faces provide a more stable fluid film between the faces, and they do not deform in operation compared to inserted faces (or to a much lesser degree). Therefore, they are more commonly used nowadays when reliability and low emissions are vital.
All mechanical seal designs have at least one secondary seal that interacts with the dynamic movement of the flexible mounted face. This secondary seal moves with the springs to keep the seal faces closed and is defined as the dynamic secondary seal. During operation of a rotary design, springs will keep the seal faces closed. They adjust with each rotation for any misalignment from installation and parts tolerances. As the springs compensate, the dynamic secondary seal moves back and forth, twice per revolution. This rapid movement prevents the protective chrome oxide layer (the layer that protects the metal) from forming. Erosion of this unprotected area under the dynamic secondary seal will cause a groove to develop. Eventually this groove becomes so deep that O-Ring compression is lost and the seal leaks. In most cases, fretted shafts must be replaced to achieve an effective seal.
Non-fretting seals are designed so that the dynamic secondary seal rides on a non-metallic surface, usually the sacrificial face.
A rotary mechanical seal has the spring mechanism in the rotating section (A) of the mechanical seal.
With rotary mechanical seals, it is important that the stuffing box face is perpendicular to the shaft for the faces to stay closed. There will always be some resulting misalignment from installation and parts tolerances. The springs must adjust with each rotation to keep the seal faces closed. This adjustment becomes more difficult at higher speeds.
In contrast, a stationary seal is a mechanical seal designed in such a way that the springs do not rotate with the pump shaft; they remain stationary. Because the springs do not rotate, they are unaffected by rotational speed. The springs do not need to correct or adjust with each rotation; they adjust for misalignment only once when installed.
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Rotary seals are simple in design which makes them inexpensive. They are suitable for lower speeds only. Stationary seals are more complicated to design but are suitable for all speed ranges. Because of design complexity, stationary seals are more commonly configured as cartridge seals rather than component seals.
In the mechanical world, where machinery and equipment make the earth move and gears rotate, the oil seal is an important component. Oil seals, or shaft seals, are a crucial part of various industrial equipment and applications, ensuring that lubricants don't escape and contaminants don't enter. While they may seem simple, their construction, design, and application are anything but. This in-depth guide aims to help you understand the essential role of oil seals, their construction, the various designs available, and key factors to consider when selecting one for your application.
An oil seal serves three crucial purposes within any machinery. First, it prevents the leakage of lubricants or fluids outside the seal, even under high pressure. This function ensures the effective operation of equipment, as sufficient lubrication is a key requirement for the smooth functioning of machinery. Second, it retains the lubricating oil within the machinery. This retention function reduces the need for constant maintenance or re-lubrication, saving time and resources. Third, the oil seal acts as a barrier against contaminants. It prevents dirt, dust, and other potential contaminants from entering the machinery, protecting sensitive parts from damage or wear.
The construction of an oil seal is a testament to meticulous engineering. Each oil seal primarily comprises two core components: the sealing element and the metal case. The collaboration of these parts brings about the seal's functionality and effectiveness. A garter spring may also be included as an available feature, providing an extra layer of operational support.
The sealing element, also known as the sealing lip, forms the interior of the oil seal. Various materials can make up the lip depending on the application's specific needs. Below are some commonly used materials:
The metal case serves as the oil seal's exterior or frame, providing rigidity and strength to the seal. The case material selection depends on the environment in which the seal will operate. Often, the same rubber material used in the seal element covers the case to help seal the exterior of the oil seal in the housing bore.
Oil seals with outer metal cases may include finishes or treatments applied to the outer edge to aid in rust protection, identification, and sealing of scratches or imperfections in the housing bore. Common finishes applied to the outside edge of metal O.D. oil seals include plain (a bonding agent of usually a yellowish-green color), a color-painted edge, and a grinded-polished edge.
When included, the garter spring applies pressure to the sealing lip against the shaft, ensuring a tight seal. The choice of material, like that of the case, largely depends on the environment of use.
Garter springs are generally used when the lubricant is oil, as it provides the necessary downward force to maintain a tight seal. However, when grease is the lubricant, garter springs can often be eliminated. Due to its low viscosity, grease doesn't require as much downward force to maintain an effective seal.
Oil seals come with various lip designs, each serving a unique purpose and suitable for different applications. Let's discuss the most common industry-standard lip designs:
Beyond the variety of lip designs, oil seals also come in various case designs, each serving a unique role. Here are some of the most common ones:
Selecting the right oil seal involves comprehensively evaluating your application's needs and conditions. Below are the key factors to consider when choosing an oil seal:
It is crucial to understand that oil seals, like any other mechanical component, are subject to failure over time. The key to minimizing downtime and enhancing operational efficiency is recognizing the signs of oil seal failure and understanding its reasons. Here are some common failure modes:
Proper maintenance and regular inspection are vital for prolonging the service life of oil seals and preventing unplanned downtime. Here are some tips:
Oil seals are integral components in a range of machinery and equipment, playing a vital role in keeping lubricants in, contaminants out, and machinery operating efficiently. Understanding the design, materials, and selection factors of oil seals can help you make an informed choice regarding your industrial needs. The reliability, longevity, and efficiency the right oil seal can bring to your machinery is priceless.
Contact us to discuss your requirements of NBR OIL SEAL. Our experienced sales team can help you identify the options that best suit your needs.
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