Experimental methods for the evaluation of lubrication conditions in gear contacts
|Kustantaja||Tampere University of Technology|
|Tila||Julkaistu - 2010|
|Nimi||Tmpereen teknillinen yliopisto. Julkaisu|
The constant pressure to build lighter, more heavily loaded, more efficient and extremely reliable gearboxes defines the main requirements for gear design. In a typical elastohydrodynamic lubrication (EHL) contact, which occurs in gear contacts, the lubrication pressure rises to several GPa above ambient levels for times of 200 – 400 μs and the protecting film thickness is usually below the 1 μm. Operating under such conditions, a gearbox is required to last for more than 20 years, which sets strict requirements for the gears and for the lubrication itself. The standard calculation methods provide the safety factor against failure, but they give no detailed information on what is really happening in the gear contacts. To have a fuller understanding of lubricated contacts designed to minimize friction, heating and failure rates in modern gearboxes, it is important to analyze gear contact in more sophisticated ways.
The objective of this thesis is to increase the understanding of lubrication conditions in gear contacts. This involved the development of test devices and methods for determination of lubrication conditions and high pressure properties of lubricants in gear contacts as well as the evaluation of lubrication conditions in controlled elliptical contacts and in real gear contacts.
A high pressure twin-disc test device was developed where the grinding of discs has been done perpendicular to the rolling direction, which corresponds to real gear surfaces. This allowed the study of lubricant high-pressure properties and lubrication conditions by simulating the gear contact along the line of action. The test device was equipped to measure the mean contact resistance, the bulk temperature and the frictional force.
A method for determination of the limiting shear stress and actual viscosity of lubricants was developed using a numerical traction model based on elliptical EHL contact and traction curves measured over a wide range of temperatures and pressures with a twin-disc test device.
The transversely ground elliptical contact was studied under mixed lubrication conditions using a twin-disc test device. The calculated thermal Λ-values of real gears and the measured mean contact resistance correspond well. This kind of simulation gives more local information about the friction coefficients, lubrication conditions and temperatures along the line of action than can be obtained from real gear measurements. The simulation can be also used to provide reference data for testing of mixed lubrication models.
The contact resistance and bulk temperature measurement were applied to a modified FZG gear test device to detect on-line transient lubrication conditions in real transient gear contacts under mixed lubrication conditions. The trend in the curves of the measured mean contact resistance and the calculated steady-state based film thicknesses correspond well with different operating parameters such as load, pitch line velocity and oil inlet temperature. Some deviations were observed, which were explained as being the result of nonsteady- state lubrication conditions.