The rolling friction phenomenon is encountered in a wide range of applications and when two different materials are involved, quantitative characterization is necessary. The parameter to be determined is the coefficient of rolling friction, for whose estimation a methodology is proposed, based on the damped oscillation of a conical pendulum. The pure rolling contact between a sphere and a plane is obtained when a steel ball is the bob of the pendulum, which rolls on an inclined plate made from a second material from the contacting pair. The mathematical model of the motion of a conical pendulum constructed from a revolution body supported on an inclined plane in the presence of the rolling friction is developed. The dynamic equations of the rigid body with fixed point are applied and the differential equation of motion of the pendulum is obtained together with the expressions of the reaction forces in the contact point. For different pairs of materials, tests are performed on a laboratory device. The damped oscillatory motion of the conical pendulum is video-captured for the estimation of the angular amplitude variation. A program for image processing is developed for measuring the values of angular elongations from the analysis of each frame of the video and, finally, the coefficient of rolling friction is obtained. For all the materials tested, a linear decrease in angular amplitude is detected and the slope of angular amplitude can be considered as a characteristic parameter related to the coefficient of rolling friction between the two materials.

The present paper offers a brief description of the testing equipment, experimental methodology and some results regarding the motion resistive effort in automotive ball joints. The local ball joints manufacturers have difficulties in controlling the clamping force of the ball joints in the production process. Due to this, an original experimental device was conceived and built in order to evaluate the magnitude and evolution of internal friction between the joint’s surfaces. The experimental setup’s main advantage is the capacity of controlling and maintaining a constant clamping force and that it ensures a constant motion of the ball stud. The force magnitude was recorded using resistive transducers connected to a data acquisition system. The numerical values were recorded and used to analyse the mobility of the tested ball joints.

Abstract. The present paper describes a simple four balls testing device adapted for a classical drilling machine, using standard methodology meant to evaluate the lubricating properties of lubricants. The advantage of the method consists in the possibility of using any type of balls and a special device adapted to a common drilling machine. The evaluation of anti-wear lubricants properties can be done by comparing the contact scar dimension obtained experimentally, following the procedure described in EN ISO 20623:2003 international standard. The contact scar diameter is measured using an optic microscope equipped with a camera and specialized software, by comparing the scar with a known body dimension. Some experimental results obtained for the contact between four 12.7 mm bearing balls lubricated with grease are presented as well. The contact scar dimension-load dependencies are similar to those presented in literature and therefore validate the experimental setup. IntroductionTribology failures determine about 30 percent of all failures in mechanical systems, [1]. This amount can be minimized by proper selection of lubricant, [2], the key element for proper system running. Knowledge of the anti-wear properties of lubricants is a very important aspect in system running. These properties can be determined by using special test machines and following a well specified procedure which involves specific testing equipment. This can be a real problem for small and medium factories, because it implies additional costs and investments that are unjustified if the low number of tests is considered. In such small manufactories, even to order of the tests from a specialized laboratory that owns the proper testing machine, raises costs and time problems. Usually, only comparing the anti-wear properties of two lubricants is required, in order to adequately choose the best suited one. This comparison is best done using the four balls test.The four balls tests are used to measure the anti-wear and extreme pressure properties of lubricants by monitoring the evolution of wear scars. By comparing the results for different lubricants, it can be determined which is the best lubricant that can be used in certain conditions. The test procedure is described in EN ISO 20623:2003 international standard, [3], and consists in a four balls special tester in which an upper rotating bearing ball is in contact with 3 lower bearing balls, fixed and immersed in lubricant. The upper ball is held in a steel chuck and rotated with 1450-1500 rpm by an electric motor. The lower bearing balls are held against each other in a steel cup. The ensemble is supported by a disc which rests on an axial bearing that allows horizontal displacements and self alignment between the upper rotating ball and the lower balls. The load is applied vertically by a load lever, in order to press the upper ball against the lower balls with a controlled load.

No abstract

The paper aims to contribute to the understanding of scuffing occurrence. The evolution of surface topography during four balls tests using two types of grease was investigated. For testing, a four balls experimental setup was used. The test rig allows loading the contact between balls while maintaining relative motion between surfaces. Constant velocity is maintained for 10 seconds as per the EN ISO 20623:2003 procedure. The wear scar produced during testing was investigated by means of an optical 3D profilometer and the roughness parameters were analysed. Experimental results reveal that the values of arithmetical mean height Sa and root mean square height Sq, have a significant growth and they are correlated with the occurrence of the scuffing phenomenon.