Abstract
It is very important to choose the right lubricant for gearboxes and with that to minimise the friction and loss of power. The most important parameter of the lubrication is oil viscosity. Different lubricants have different viscosity, which also change with temperature and applied pressure. The thickness of the generated lubricant film also suffers from the same causes. At first glance it appears that more viscous lubricants would give better performance, due to thicker generated films, and better separation of two contact surfaces would be achieved. This is not always the case since more viscous oils require more power to be sheared. Consequently the power losses are higher and more heat is generated resulting in a substantial increase in the temperature of contacting surfaces, which may lead to the failure of the component. For engineering applications the oil viscosity is usually chosen to give optimum performance at the required temperature. Knowing the temperature at which the oil is expected to operate is critical, as the oil viscosity is extremely temperature dependent. The viscosity of different oils varies at different rates with temperature. The velocities of the operating surfaces (shear rates) can also affect the viscosity. The knowledge of the viscosity characteristics of a lubricant is therefore very important in the design and in the prediction of the behaviour of a lubricated gear system.
A computer system was set up to determine the optimum viscosity and quality of the oil lubricants. Optimum oil viscosity is estimated in regard to the minimal lubricant film thickness, proposed by Dowson and Higginson EHD theory, temperature according to DIN 3990, and surface roughness. The oil viscosity and surface roughness are iterative determined considering also geometrical and operational data of gears. The method provides the relative lubricant film thickness, which is satisfactory for engineering practice. The oil quality is also a very important parameter and is determined in regard to the FZG test results taken from DIN 3990. By using the program for oil viscosity and quality optimisation, one can easily estimate the optimum viscosity in regard to the minimal lubricant film thickness, temperature and surface roughness, and which load case should the oil carry by FZG test