tailieunhanh - Engineering Tribology 2E Episode 2

Tham khảo tài liệu 'engineering tribology 2e episode 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 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 12 14 . table 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 - Heavy machine oil 153 34 Heavy machine oil 250 Cylinder oil 810 135 34 28 22 Medium VI oils Spindle oil 20 16 13 Light machine oil 45 12 28 20 16 Medicinal white oil 107 Heavy machine oil 122 Heavy machine oil 171 31 28 23 18 Low VI oils Spindle oil Heavy machine oil 165 Heavy machine oil 310 Cylinder oil 2000 180 24 Other fluids and lubricants Water 0 0 0 Ethylene oxide- propylene oxide copolymer 204 Castor oil 360 80 Di 2-ethylhexyl phthalate Glycerol glycerine 535 73 Polypropylene glycol 750 - - - - Polypropylene glycol 1500 177 - - - - Tri-arylphosphate ester - - - - 22 Engineering Tribology 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 . their viscosity is proportional to shear rate as shown in Figure .