New materials, surface coatings and biodegradable oils for industrial gears

Ever since gears were first used there has been a constant demand for increasing the power density in gear transmissions, to reduce the power losses, to increase the life time and also to decrease their weight. To satisfy these demands new solutions have been developed, such as the use of new materials, the application of surface coatings, improved surface finishing, different hardening treatments, and the use of new lubricants.;Any new solution presented must have technical performance advantages for its industrial application, and the technical performance must be proved in dedicated tests.;Nowadays some of the main concerns in gear applications are scuffing failures, micropitting fatigue damage (contact fatigue wear phenomenon observed in combined rolling and sliding contacts operating under elastohydrodinamic lubrication (EHL) or mixed EHL/Boundary lubrication conditions) and power loss or efficiency. These were some of the main concerns during this work for the evaluation of new material, surface coatings and lubricants for gears.;The applicability of multilayer composite surface coatings (providing coefficients of friction close to those obtained with solid lubricants like MoS2 and graphite) in gear tooth is discussed in this work, mainly in what concerns to gear efficiency at normal operating conditions and to scuffing load capacity. The average friction coefficient between gear tooth is also discussed and compared with a conventional solution (carburized steel gears).;There is also a growing environmental awareness that leads to an increasing interest in biodegradable non-toxic oils. Of course, higher lubricant performance in what concerns friction, wear, lifetime, etc. has also major impact in the environmental compatibility, because premature wear, high energy needs and low lifetime are also harmful to the environment.;The use of Austempered Ductil Iron (ADI), austempered at 300oC, as a gear material is also discussed, mainly in what concerns fatigue life, both pitting and micropitting, and power loss behaviour.;Two new environmentally friendly lubricants blended for industrial gear applications were investigated in this work. These oils are based on fully or highly saturated esters and are produced from harvestable raw materials. Both oils are biodegradable and have low toxicity and were formulated to replace mineral lubricants in industrial gears, accomplishing CLP specification according to DIN51517.;The overall performance and applicability of the biolubricants was assessed in this work, their tribological performance has been evaluated in combination with the different materials and surface coatings, as well as with the classical carburized steel gears. This will give an overview of the range of tribological systems where ester oils could be used. This research focussed on the scuffing load capacity, micropitting protection and power loss.;The influence of lubricants and material / surface coating combinations on the friction coefficient between gear tooth is discussed and evaluated. A numerical model for the energetic balance of the FZG gearbox has been developed for that purpose, integrating the mechanisms of power loss and heat evacuation. The model also calculates the equilibrium temperature of the gearbox as well as the influence of gearbox components in the overall power loss, providing a better understanding of the variation of the churning and friction losses and their influence in the total power loss.;The ADI material has very interesting properties and presents a behaviour similar to steel, although lower contact pressures must be used.;The surface coatings, specially the MoS2/Ti presented an outstanding increase of scuffing load capacity and also a considerable improvement in the energetic efficiency of gears.;The biodegradable non-toxic lubricants proved to be an advantageous solution for replacing mineral lubricants. They form a better tribological system than the mineral lubricant with all the materials and surface coatings tested.;So, the ester lubricants tested are environmentally compatible not only due to their biodegradability and low-toxicity but also due to the lower power consumption, better load carrying capacity and equal or better micropitting protection.