Bearings applications

Materials which reduce the friction of mouldings and other finished products when these are rubbed against adjacent materials which may or may not be of the same composition. The most well-known examples here are graphite and molybdenum disulphide used in quantities of the order of 1-2% in nylons and other thermoplastics used in gear and bearing applications. 

Self-lubricating grades are of particular value in some gear and bearing applications. One commercial nylon compound incorporates 0.20% molybdenum disulphide and 1 % of graphite whilst many other commercial compounds contain only one of these two lubricants. 

In order to reduce the coefficient of friction in bearing applications small amounts of molybdenum disulphide are incorporated although in quantity this material may cause problems through acidic impurities. Blends of polyacetal and FIFE (20-25% P l FE) have a very low coefficient of friction (as low as 0.02)... 

Welded or seamless pipe used for structural or load-bearing applications in aboveground installations. Fabricated in nominal wall thicknesses and sizes to ASTM specifications in round, square, rectangular and other cross-sectional shapes. 

As indicated above, dissimilar metals with low frictional characteristics are most suitable for plain bearing applications. With steel shafts, plain bearings made of bronze or Babbitt are commonly used. Bronze is one of the harder bearing materials and is generally used for low speeds and heavy loads. 

The ferritic steels rapidly lose strength at elevated temperature as shown in Fig. 7.26 and are of little value for load-bearing applications. 

Intergranular Oxidation Intergranular penetration of oxide can be a serious problem particularly when thin-walled components are used in load-bearing applications such as cooled turbine blades. Mass change data often do not adequately reflect the extent of this type of attack, and examination... 

The pace of development has increased with the commercialization of more engineering plastics and high performance plastics that were developed for load-bearing applications, functional products, and products with tailored property distributions. Polycarbonate compact discs, for example, are molded into a very simple shape, but upon characterization reveal a distribution of highly complex optical properties requiring extremely tight dimension and tolerance controls (3,223). 

Engineering-grade PET finds widespread use in numerous load-bearing applications, as demonstrated in Table 14.15. 

The three most important properties of concrete used in calculations for load-bearing applications are the compressive strength, the tensile strength and the modulus. However, for certain applications, e.g. water-retaining structures, the permeability or porosity of the concrete will be a relevant design criterion and this is also considered here. 

Calcium and Phosphate Salts Bloactive Glass Granules Bloactive Glass-Ceramic Granules Orthopedic Load-Bearing Applications... 

Activators. Activators are chemicals that increase the rate of vulcanization by reacting first with the accelerators to form mbber soluble complexes. These complexes then react with the sulfur to achieve vulcanization. The most common activators are combinations of zinc oxide and stearic acid. Other metal oxides have been used for specific purposes, ie, lead, cadmium, etc, and other fatty acids used include lauric, oleic, and propionic acids. Soluble zinc salts of fatty acid such as zinc 2-ethylhexanoate are also used, and these mbber-soluble activators are effective in natural mbber to produce low set, low creep compounds used in load-bearing applications. Weak amines and amino alcohols have also been used as activators in combination with the metal oxides. 

When used in load-bearing applications, isotropic polymers may also fail because of low modulus. The moduli that must be considered in the design of functional polymers are Young s modulus E, shear modulus G, and bulk modulus K. Poisson s ratio P should also be considered. 

Plastics. Almost all commercial plastics find some use both dry and lubricated for sliding at low speeds and light loads the most commonly used thermoplastics are nylon, acetal resins, and polytetrafluoroethylene (PTFE). Typical thermosetting resins for bearing applications are phenolics, polyesters, and polyimides. Table 8 compares the characteristics of plastic bearing materials with those of graphite, wood, and rubber which find use in somewhat similar applications. 

A variety of barium sulfonates have found use in antifriction lubricants for high speed bearing applications. Calcium and sodium salts of sulfonated olefins, esters, or oils are used for the enhancement of extreme pressure properties of grease and gear lubricants,... 

Also, materials with small amounts of TiN additions show lower friction in ball bearing applications leading to higher loading capacities of the bearings [525, 583],... 

FIGURE 3.8 Journal bearing application. Outer cylinder (stainless steel) is bonded to inner cylinder (polyamide-imide) with an epoxy adhesive. Exposure to low temperatures causes significant stress on bond due to differences in coefficient of thermal expansion. 

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