Over-lubrication

Gear couplings When properly installed and maintained, gear-type couplings do not generate a unique forcing function or vibration profile. However, excessive wear, variations in speed or torque, or over-lubrication results in a forcing function. 

Action required Provide splashguards, do not over lubricate or clean belts with solvent. 

An advantage of roller compaction is the increase in bulk density resulting in a reduced tablet size. It can also be used to improve the content uniformity of low-dosage compounds. Characteristics which afifect the tabletability of roller compacted formulas include over-lubrication and precompression of the compact. Typically, the powder is roller compacted between 15 and 40 kN of force prior to tablet compression. 

The finish on the surfaces of the calender rolls is very important they may be highly polished or matt, and these surfaces affect behaviour during processing as well as the appearance of film or sheet produced. Unless it is grossly over-lubricated the hot material will adhere to the hotter of two similar rolls forming a nip, or to the roll running faster—except that irrespective of differences in roll speed or temperature there is a preference to adhere to a matt rather than a polished surface. 

The exposure of lubricants to prolonged shear could reduce their particle size resulting in over-lubrication of the blend that subsequently leads to poorer compression characteristics of the blend and slower rates of dissolution of the dosage form. 

Potential to over-lubricate a blend Generate heat... 

Grease is non-Newtonian in behavior, and, unlike oils, an initial shear stress (yield value) must be applied before it will deform and commence to flow. It is this nonflowing characteristic that enables grease to offer certain advantages over lubricating oils and results in its extensive use for the lubrication of rolling bearings. 

In addition, it cannot be used as a coolant since it has inferior heat transfer properties. As a pure lubricant, however, grease does have many advantages over fluid oil. The major advantages of grease over lubricating oil focus on four basic capabilities. 

C4H8O, CH3COCH2CH3. Colourless liquid with a pleasant odour, b.p. 80°C. It occurs with propanone in the products of the destructive distillation of wood. Manufactured by the liquid or vapour phase dehydrogenation of 2-butanol over a catalyst. Used as a solvent, particularly for vinyl and acrylic resins, and for nitrocellulose and cellulose acetate, also for the dewaxing of lubricating oils. U.S. production 1978 300 000 tonnes. 

The topic of spreading rates is of importance in the technology of the use of mono-layers for evaporation control (see Section IV-6) it is also important, in the opposite sense, in the lubrication of fine bearings, as in watches, where it is necessary that the small drop of oil remain in place and not be dissipated by spreading. Zisman and coworkers have found that spreading rates can be enhanced or reduced by the presence of small amounts of impurities in particular, strongly adsorbed surfactants can form a film over which the oil will not spread [48]. 

It is known that even condensed films must have surface diffusional mobility Rideal and Tadayon [64] found that stearic acid films transferred from one surface to another by a process that seemed to involve surface diffusion to the occasional points of contact between the solids. Such transfer, of course, is observed in actual friction experiments in that an uncoated rider quickly acquires a layer of boundary lubricant from the surface over which it is passed [46]. However, there is little quantitative information available about actual surface diffusion coefficients. One value that may be relevant is that of Ross and Good [65] for butane on Spheron 6, which, for a monolayer, was about 5 x 10 cm /sec. If the average junction is about 10 cm in size, this would also be about the average distance that a film molecule would have to migrate, and the time required would be about 10 sec. This rate of Junctions passing each other corresponds to a sliding speed of 100 cm/sec so that the usual speeds of 0.01 cm/sec should not be too fast for pressurized film formation. See Ref. 62 for a study of another mechanism for surface mobility, that of evaporative hopping. 

Lubricating Oil Extraction. Aromatics are removed from lubricating oils to improve viscosity and chemical stabihty (see Lubrication and lubricants). The solvents used are furfural, phenol, and Hquid sulfur dioxide. The latter two solvents are undesirable owing to concerns over toxicity and the environment and most newer plants are adopting furfural processes (see Furan derivatives). A useful comparison of the various processes is available (219). 

A number of chemical products are derived from Sasol s synthetic fuel operations based on the Fischer-Tropsch synthesis including paraffin waxes from the Arge process and several polar and nonpolar hydrocarbon mixtures from the Synthol process. Products suitable for use as hot melt adhesives, PVC lubricants, cormgated cardboard coating emulsions, and poHshes have been developed from Arge waxes. Wax blends containing medium and hard wax fractions are useful for making candles, and over 20,000 t/yr of wax are sold for this appHcation. 

The most common VI improvers are methacrylate polymers and copolymers, acrylate polymers (see Acrylic ester polymers), olefin polymers and copolymers, and styrene—butadiene copolymers. The degree of VI improvement from these materials is a function of the molecular weight distribution of the polymer. VI improvers are used in engine oils, automatic transmission fluids, multipurpose tractor fluids, hydrautic fluids, and gear lubricants. Their use permits the formulation of products that provide satisfactory lubrication over a much wider temperature range than is possible using mineral oils alone. 

Bonded Solid-Film Lubricants. Although a thin film of soHd lubricant that is burnished onto a wearing surface often is useful for break-in operations, over 95% are resin bonded for improved life and performance (62). Use of adhesive binders permits apphcations of coatings 5—20 p.m thick by spraying, dipping, or bmshing as dispersions in a volatile solvent. Some commonly used bonded lubricant films are Hsted in Table 12 (62) with a more extensive listing in Reference 61. 

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