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Tham khảo tài liệu 'engineering tribology 2011 part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Physical Properties of Lubricants 21 where a is the pressure-viscosity coefficient m2 N p is given by the following expression 12 13 p __ - 9 z ln n 9.67 1 5.1 X 10 p Ỉ s 1 00 - 138 The above formula appears to be more comprehensive than the others since it takes into account the simultaneous effects of temperature and pressure. The a values and dynamic viscosity n0 for some commonly used lubricants are given in Table 2.3 12 14 . table 2.3 Dynamic viscosity and pressure-viscosity coefficients of some commonly used lubricants adapted from 12 . Lubricants Dynamic viscosity n0 measured at atmospheric pressure X10 3 Pas Pressure-viscosity coefficient a 10-9 m2 N 30 C 60 C 100 C 30 C 60 C 100 C High VI oils Light machine oil 38 12.1 5.3 - 18.4 13.4 Heavy machine oil 153 34 9.1 23.7 20.5 15.8 Heavy machine oil 250 50.5 12.6 25.0 21.3 17.6 Cylinder oil 810 135 26.8 34 28 22 Medium VI oils Spindle oil 18.6 6.3 2.4 20 16 13 Light machine oil 45 12 3.9 28 20 16 Medicinal white oil 107 23.3 6.4 29.6 22.8 17.8 Heavy machine oil 122 26.3 7.3 27.0 21.6 17.5 Heavy machine oil 171 31 7.5 28 23 18 Low VI oils Spindle oil 30.7 8.6 3.1 25.7 20.3 15.4 Heavy machine oil 165 30.0 6.8 33.0 23.8 16.0 Heavy machine oil 310 44.2 9.4 34.6 26.3 19.5 Cylinder oil 2000 180 24 41.5 29.4 25.0 Other fluids and lubricants Water 0.80 0.47 0.28 0 0 0 Ethylene oxide- propylene oxide copolymer 204 62.5 22.5 17.6 14.3 12.2 Castor oil 360 80 18.0 15.9 14.4 12.3 Di 2-ethylhexyl phthalate 43.5 11.6 4.05 20.8 16.6 13.5 Glycerol glycerine 535 73 13.9 5.9 5.5 3.6 Polypropylene glycol 750 82.3 - - 17.8 - - Polypropylene glycol 1500 177 - - 17.4 - - Tri-arylphosphate ester 25.5 - - 31.6 - - 22 Engineering Tribology 2.6 VISCOSITY-SHEAR RATE RELATIONSHIP From the engineering view point it is essential to know the value of the lubricant viscosity at a specific shear rate. For simplicity it is usually assumed that the fluids are Newtonian i.e. their viscosity is proportional to shear rate as shown in Figure 2.5.
