Cartridges & Mechanical Seals

GENERAL INFORMATION 151 General information Introduction Mechanical seals are elements that ensure the sealing between a rotating shaft and a structural part (usually fixed) of the application as- sembly. This sealing effect is achieved through the rotating movement between two contact surfaces which are extremely flat and the fluid film generated between them, which prevents the passage of the rest of the fluid. Mechanical seals perform much better than other sealing elements for rotating shafts such as lip seals or retaining elements. Since they are made of harder materials, they make the system more solid and durable. They also allow sealing in applications which are especially demanding: chemically aggressive, abrasive, dense or viscous fluids, high temperatures, high pressures, high rotating speeds, etc. Mechanical seal elements 1. Primary ring: This part of the mechanical seal contains the contact surface. It has a spring-operated device which makes it flexible enough to offset small axial or radial displacements or oscillations that may occur during operation. This generally refers to the rotating part of the mechanical seal. 2. Mating ring: In most cases, this part of the mechanical seal is housed in a rigid casing or static flange of the application but it may also be mounted on the shaft. It contains the contact surface. In addition to generating the air tightness with the contact surface of the pri- mary ring, it serves as a guide for ensuring perpendicularity. 3. Secondary seals: These elements generate air tightness between the static parts of the mechanical seal, for instance, between the primary ring and the shaft, the mating ring and the flange or between the di- fferent parts of the primary ring that rotate together. For this pur- pose, elastomers are normally used, with O-rings being the most common element. Bellows seals, square cut seals, collars, wedge seals and in general, any seal that provides air static or linear tightness can also be used. In addition to elastomers, special ma- terials such as PTFE and seals with a graphite base can be used in heavy-duty working conditions. 4. Springs: This element is responsible for maintaining the tension between the contact surfaces when there is no pressure in the system, to ensure sealing in static and depressurised conditions. Different types of spring can be used. The most common ones are: axial cylindrical springs, axial conical springs, multi-spring systems, welded wave springs, “super sinus” wave springs and metal be- llows seals. Other metal parts: It is common to find mechanical seals with construction elements for joining and fixing the above parts. These are usually metal parts or plates, rings, screws, etc. 5. Drive mechanism: This transmits the rotating movement of the shaft to the rotating part of the mechanical seal. This mechanism may consist of flaps, an anti-rotating pin, screws elastomer bellows, etc. Mechanical seal classification Different criteria exist for classifying mechanical seals. They are usually classified based on their hydraulic balance, construction and assembly characteristics. • K Hydraulic balance: Mechanical seals can be classified based on the ratio between the pressure of the fluid to be sealed and the contact pressure between the contact surfaces, i.e., the ratio bet- ween the surface with the hydraulic pressure (A) and the contact surface (a).