A New Scuffing Test using Contra-rotation
The mode of lubricant failure known as scuffing provides a significant design constraint in high sliding gears, cams and metal cutting and forming processes. It is therefore important to have an effective test method to measure the scuffing resistance of lubricant formulations. In most existing scuffing bench tests, a moving surface is rubbed against a stationary one at a fixed sliding speed and the load at which scuffing occurs is determined. This approach has two disadvantages. One is that wear of the stationary surface can lead to a large decrease in effective contact pressure during a test. The second is that viscous lubricants often generate significant elastohydrodynamic films at the sliding speeds employed. This means that the scuffing tests measure a complex combination of the influence of the fluid and boundary film-forming properties of the lubricant on scuffing rather than reflecting solely the influence of lubricant formulation. This paper describes a new scuffing test method in which the two metal surfaces are rubbed together in mixed rolling–sliding with the two surfaces moving in opposite directions with respect to the contact, i.e. in contra-rotation. This enables the sliding speed to be decoupled from the entrainment speed so that the scuffing properties of a lubricant can be determined in boundary lubrication conditions over a wide range of sliding speeds. Also, because both surfaces move relative to the contact, wear is distributed and this minimises changes in contact pressure during a test.