Micropitting is a topic that has generated a lot of interest, discussion, research, and many technical papers in the last few years, but it remains a failure mode where much is unknown. There are many individual items that affect and are related to micropitting, such as oil film thickness, temperature, and the slide-to-roll ratio, but lubrication and surface finish are of critical importance in micropitting.
Despite the fact that it is one of the significant modes of gear failure, there are no generally accepted methods available to predict the relative risk of micropitting or design guidelines for preventing it.
Micropitting appears as a dull, gray etched finish and is difficult to see under regular lighting. It is best observed with intense directional lighting. It is a surface fatigue phenomenon that causes destructive wear. The wear is usually uneven in nature, and the involute tooth form is changed, causing non-conjugate gear tooth rolling action and further damage. This can lead to additional noise, vibration, and misalignment.
It is more common in carburized gears. It is sometimes difficult to distinguish from other modes of failure; it also can lead to other modes of failure, for example, macropitting and bending fatigue failures.
Role of Lubricants
Let’s examine the role of lubricants in this process. For this purpose, I will be referring to Mobil Oil Corporation data, documents, and theory, including EHL theory (from the Mobil EHL Guidebook, Fourth Edition).
EHL (elastohydrodynamic lubrication) describes the type of lubrication regime that provides protection for rolling bearings, gears, and cams. Both rolling and sliding motions can take place and with differing speeds relative to the different surfaces. Typically, in the EHL contacting surfaces, there is an inlet area and a pressure distribution area where elastic deformation takes place locally and then an outlet. The pressure distribution area is semi-elliptical in shape, and its maximum value depends on the load and elastic properties of the surfaces. The thickness of the EHL film depends on and is a function of speed, geometry of the contact zone, and lubricant parameter (LP number), but is relatively insensitive to load.
One important property for EHL is that the viscosity of the lubricant increases significantly with pressure. This viscosity increase is almost exponential in the pressure range for the inlet to the EHL contact for mineral and synthetic lubricants. For calculating EHL film thickness, both the viscosity and the pressure-viscosity properties of lubricants are combined into a single value by Mobil, which is called the lubricant parameter (LP). These LP values have also been plotted against temperature for most of Mobil’s oils and greases. Worksheets and spreadsheets can be requested to give lubricant selection for gears, rolling bearings, and cams based on simplified EHL theory.
From the worksheets or spreadsheets, a method for determining either the thickness of the EHL film with a given lubricant or the LP number needed to generate an appropriate film thickness can be calculated.
In the worksheet, the inputs are some of the gear-related items: center distance, pressure angle, helix angle, gear ratio, face width, speed of the slower gear, power applied, gear tooth contact temperature (this is the temperature of the gear tooth itself, not the oil sump temperature), and composite surface roughness. A target specific film thickness (λ) can also be specified.
The outputs from the worksheet are the lubricant EHL film thickness and specific film thickness, if a lubricant and LP number was already specified, or the LP number required to achieve the lubricant EHL film thickness and specific film thickness desired.
The worksheet and spreadsheet are interactive so that one can make changes to one or more of the parameters and immediately see the effect that this has on the resulting output parameters. They are simple, easy to use, and one of the few tools available for this purpose — evaluating the relative risk of micropitting, particularly related to lubrication.
I have found this to be a powerful tool to evaluate the relative micropitting risk of a set of gears in a gearbox. The LP numbers of various different lubricants can and do vary greatly from one to the other. Sometimes, a simple change in the choice of oil to be used in a gearbox can have a profound effect on the oil film thickness developed and the risk of micropitting. I evaluate nearly every new gearbox design using these methods to determine the EHL film thickness and either the minimum required LP number for the lubricant that should be used or the film thickness for the LP number of the lubricant if I have already chosen one.
Anything that can be done to evaluate and quantify the relative risk of micropitting is beneficial, and this is one way to help avoid the pitfalls of micropitting.