Slip ring

from Wikipedia, the free encyclopedia

Mechanical and counter rings are the two main components of a mechanical seal . The axially movable sealing ring of a mechanical seal is called a sliding ring. It is pressed against the counter ring by a spring. O-rings are used for secondary sealing. One of the two rings sits rigidly in the stationary part / housing (usually the counter ring), the other is attached to the rotating shaft (usually the sliding ring). The sliding ring and mating ring are in constant contact, but do not touch, but slide over one another. They are also known as opposing pairs. They can consist of the same material, but often different materials also form the opposing pairs.

Materials for sliding and counter rings

A number of different materials can be used as the sliding ring or counter ring, which can be divided into four main groups:

carbon
Carbon-graphite, electrographite, synthetic resin-bonded carbon
Ceramic materials
Silicon carbide , aluminum oxide , tungsten carbide
metallic materials
Stainless steel , cast iron, cast chrome
plastic
PTFE

Carbon materials

Sliding rings made of carbon-graphite materials are mostly used as the softer, wearing partner in hard-soft pairings. Depending on the type of material and the application, the advantages here include: chemical resistance, high temperature resistance, food approval or dry-running capability. In combination with the suitable sliding partner, the carbon-graphite material has the ability to optimally adapt to the partner with a certain initial wear. It also adapts to varying pressure and temperature conditions.

A distinction is made between three main classes of carbon and graphite materials:

Carbon graphite

This material (also known as hard coal) is made from a mixture of petroleum coke and pitch coke, carbon black and graphite as fillers and a thermoplastic binder in the form of coal-tar pitch or synthetic resin. Annealing - the so-called carbonization - at temperatures of up to 1200 ° C causes pyrolysis, i.e. the decomposition of the binder into volatile components and coke. Subsequent impregnation ensures gas tightness. Synthetic resins, carbon and various metals such as antimony are used as impregnating agents. The composition of the fillers can be used to modify the hardness, thermal conductivity, bulk density, porosity, flexural strength and coefficient of friction of the materials for almost all conceivable requirements. An abundance of different impregnating agents, which also have a strong influence on the properties, multiplies the variety.

Properties / areas of application: Impregnated carbon graphites in particular are suitable for the highest sliding speeds and pressures. Depending on the type of impregnation, it can be used as a sealing ring in dry running, through multi-part sealing rings or as a highly stressed sliding ring, even at high temperatures and chemically aggressive media.

Electrographite

The starting material is carbon graphite. A graphitizing process, a temperature treatment up to 3000 ° C, gives the carbon the graphitic properties required for many purposes.

Properties / areas of application: Good sliding and lubricating properties, high electrical and thermal conductivity, high chemical resistance, high thermal shock resistance and, compared to non-graphitized materials, improved corrosion resistance. Their area of ​​application extends (also depending on possible impregnation) from dry running, highly stressed carbon slide valves (carbon lamellas for vane pumps) to carbon bearings and slide rings in wet and dry running, especially in aggressive media and in high temperature applications. The electrical conductivity enables them to be used, for example, as electrodes and carbon brushes .

Resin-bonded carbon materials

Synthetic resin as the base material (binder) is filled with natural or electrographite plus additives.

Properties / areas of application: Wet running, low sliding speeds and pressures, low requirements for chemical resistance.

Ceramic materials

Ceramic materials, as hard partners in mechanical seals, have largely replaced metallic materials such as cast chrome, cast chrome-molybdenum and steel-chrome-nickel-molybdenum due to their better tribological properties. In addition to aluminum oxide and tungsten carbide, silicon carbide in particular has achieved great importance due to its excellent properties. It is light (3 g / cm³), extremely hard (1500–3000 HV) and therefore very wear-resistant, very good thermal conductivity and has a modulus of elasticity that is about twice as large as chrome steel. Due to its importance, a number of other special ceramics based on silicon carbide have now been developed. The following are the most important:

SSiC: Direct sintered SSiC is made from a mixture of α- or β-SiC powder at 1900 to 2200 ° C in the presence of traces of aluminum, beryllium or boron as a sintering aid. Direct sintered SiC is very fine-grained and extremely corrosion-resistant.

SiSiC: Reaction-sintered silicon carbide SiSiC is produced by pressing a mixture of α-SiC powder, graphite and organic binders, which coke at 1,000 ° C. Subsequently, liquid or gaseous silicon is infiltrated into the blanks, which when heated to 1500-2200 ° C partially forms secondary SiC with the graphite. Excess SiC remains in the pores with a volumetric proportion of approx. 10–15%. Reaction sintered silicon carbide has excellent tribological properties. Such ceramics retain the high hardness, thermal conductivity, chemical resistance and corrosion resistance of silicon carbide, while the silicon embedded in the pores improves the oxidation resistance.

SiC30: The material SiC30 has a special position in the field of silicon carbide materials, as it combines the properties of silicon and graphite. SiC30 is produced by impregnating a highly porous electrographite with molten silicon. With the penetration of silicon into the pores at the prevailing temperatures, silicon carbide is formed through a chemical reaction between carbon and silicon. The process continues until the pores are completely filled and only a small amount of free silicon (approx. 3%) remains.

Properties: SiC30 is completely resistant to aqueous salt solutions, organic reagents, strong acids such as HF, HCl or HNO 3 as well as hot inert gases. The material can be used to a limited extent in air and oxidizing gases, metal melts or strongly alkaline media.

The main areas of application are slip rings made of SiC30 as problem solvers, especially in poorly lubricating media. Compared to other materials, SiC30 is particularly impressive in hard-hard combinations due to its emergency running properties and dry running capability. The combination of the properties of graphite (good dry running properties and high thermal shock resistance) combined with the hardness, strength and abrasion resistance of silicon carbide allow the design of special mechanical seals that are not possible with other materials.

Material pairings

The key factor in the design of mechanical seals is the selection of materials. The type of pairing depends heavily on the operating conditions. Hard-soft combinations are recommended for sealing gaseous media, for example. A practical example of this are pumps that run dry. They require excellent self-lubricating properties of the carbon material. Different materials are suitable for this, depending on the operating temperature, the corrosiveness of the media or the speed of rotation. In such cases, for example, carbon materials stand out, which are superior to other materials in terms of dry-running suitability, chemical resistance, temperature resistance and thermal conductivity. In turn, wear-resistant hard-hard combinations are required in abrasive media. For this purpose, ceramic materials modified in the direction of sliding properties are particularly suitable, such as the material SiC30, which particularly proves its unique properties such as emergency running capabilities when paired SiC30 against SiC30.

Overview of particularly suitable material pairings
Materials According to DIN 24960 B. A. Y S. U2 U3 U4 U5 U6 V
Carbon graphite synthetic resin impr. B. - 0 0 0 0 0 0 0 0 0
Carbon graphite antimony impr. A. - - 0 0 0 0 0 0 0 0
PTFE Y - - - 0 0 0 0 0 0 0
Cast chrome S. # # - - 0 0 0 0 0 0
Tungsten carbide nickel-plated U2 #, × * - - ×,> 0 0 0 0 0
Silicon carbide pressure sintered. U3 #, × - - - ×,> - 0 0 0 0
Silicon carbide reactive U4 #, × * - - ×,> × ×,> 0 0 0
Carbon graphite surface silicon. U5 #, × - - - #, ×,> #, ×,> #, ×,> ×, ×,> 0 0
Carbon graphite silicon impr. U6 #, × * - - #, ×,> #, ×,> #, ×,> #, ×,> * 0
Alumina V #, × - × - ×,> ×,> ×,> × #, × -

Legend: # Conditional emergency running property; × corrosion resistant; > wear-resistant; * extremely pressure and temperature resistant; - unusual pairing
table from: "ABC of mechanical seals"

The selection of the suitable material pairing should be made in direct cooperation and only after careful consultation with the manufacturer of the mechanical seal or the manufacturer of the mechanical seal.

Manufacturer

The most important manufacturers of sliding materials worldwide include:

See also

Individual evidence

  1. ↑ Jürgen Feßmann and Helmut Orth Inorganic, non-metallic materials . In: Applied chemistry and environmental technology for engineers: manual for study and operational practice. Ecomed-Sicherheit, Hüthe Jehlig Rehn Publishing Group. Heidelberg, 2002. Page 57. ISBN 978-36096-8352-2 .