Measuring and Analyzing the Moment of Friction in Double-Row Ball Bearings Robert Korbut 1,a, Jerzy Nachimowicz 1,b

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1 Solid State Phenomena Online: ISSN: , Vol. 199, pp doi: / Trans Tech Publications, Switzerland Measuring and Analyzing the Moment of Friction in Double-Row Ball Bearings Robert Korbut 1,a, Jerzy Nachimowicz 1,b 1 Bialystok University of Technology, Faculty of Mechanical Engineering, Wiejska St. 45C, Bialystok, Poland a holden77@wp.pl, b nachim@pb.edu.pl Keywords: ball bearing, moment of friction, moment of rolling resistance. Abstract. On the basis of the experiment, the changes in the moment of friction in the double-row ball bearing were estimated. The experimental data indicated the presence of significant differences between the average values of the moment of friction in the whole spectrum of the rotational speeds that were examined. At the same time it was observed that there exist certain characteristic rotational speeds at which the given bearing is informed by a greater working resistance. It may indicate the presence of both optimal and unfavourable working conditions for these bearings; the conditions during which there occur minimal or, respectively, maximal resistances (in a given situation). The results of the examination were analyzed statistically; the after-examination samples were closely observed and this further visual scrutiny proved that during the work of the bearing, the lubricant was displaced. Introduction The construction of rolling bearings is based on the common assumption that the moment of rolling resistance is closely linked with bearings wear and lifespan. The total rolling resistance comprises numerous factors; these factors influence (greatly or slightly, depending on the kind of factor we take into consideration) the total rolling resistance in the bearing. But in such a case it is assumed that the loss of energy in the process of bearing is incomparably smaller than the one in the process of ball spin (lost motion). Nevertheless, during the work of the bearing in standard conditions the following phenomena occur: the rolling friction, the ball spin friction, the bearing seal friction, and the lubricant resistance. Putting aside all the factors that are typically destructive in character (bearing slackness, impurity of lubricant, excessive load on the bearing), resistance in the work of the bearing may originate from: the hysteresis of deformations; the ball spin and the micro ball spin resulting from deformations, the geometry of area contact, the motion caused by the gyroscopic moment, inner friction in the lubricant, the structure of the working surface, and the temperature in the friction pair [1,2,4]. On the basis of the valid, up-to-date publications and catalogues [6] it can be stated that the total moment of rolling resistance comprises the component moments expressed in the equation 1. M = Φish Φ rsm rr + M sl + M seal + M drag (1) where: M - the total moment of rolling resistance (Nmm), M - the moment of rolling friction (Nmm), rr M the moment of ball spin friction (Nmm), sl M the moment of bearing seal friction (Nmm), seal M - the moment of friction resulting from the resistance in the oil environment, the kneading drag of lubricant, splashes, etc. (Nmm), Φ ish - the reducing coefficient which takes into account the truncating of the lubricant in the contact patch (-), Φ rs - the reducing coefficient which takes into account the effect of kinematic refilling/shortage of the lubricant (-). All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-06/03/16,14:47:00)

2 150 Mechatronic Systems and Materials V In order to better understand the source of rolling resistance, certain processes in the friction pair can be analyzed, namely: the rolling ball crushes the material which, in turn, resists the ball in the contact patch. The normal reaction, which counterbalances the force loading the ball, moves slightly in the direction of the rolling ball from its axis of symmetry. The normal force and the normal reaction form a pair of forces (the moment of resistance in the rolling of the ball). The analysis of the geometry of the co-working elements proves that the curvatures of the surfaces are different; in the condition of elastic deflection the length of the contact area arch (of the both elements) is identical, whereas before the deflection it was different. As a consequence, the deformations within the contact patch are accompanied by a micro ball spin; this, in turn, leads to a remark that as long as the tangential velocities of the ball and the raceway are different, then a classic bearing with ball spin occurs [3,5]. Methodology of the examination The aim of the experiment is to estimate the changes in the moment of friction in the double-row ball bearing TNG-2RS. The tribological examination was carried out by the friction tester SMT-1 (the Amsler machine). The general view of the apparatus and the construction of the friction pair are shown in Picture 1. Fig. 1. The apparatus SMT-1: a) the general view, b) the view of the friction pair, c) the diagram of the pair: 1- the arm applying a load, 2 the bearing, 3 the shaft neck, 4 the clump nut of the strap on the shaft neck Before the tests, in the initial stage, the ball bearing was loaded with the nominal force N= 350 [kg] 3433 [N]. Next, the machine was switched on for the period of time t= 30 [min] with the speed n= 1000 [rotations/min]. After 30 minutes, the speed was increased up to n= 1400 [rotations/min], and once the work has stabilized itself, the moment of friction (M t ) was measured for the period of approx. 15 [s]. Next, the speed was decreased a few times by 200 [rotations/min] each time; as soon as the work of the friction pair was stable the measurement of M t was taken. All the measurements were done by means of the computer equipped with the analogue-digital card PCL-818HG, and working on Genie Light 3.0 software. The bearings analyzed were marked Ł1 and Ł2 respectively. Examination results The results of tribological examination are shown below, in Pictures 2 5. The data collected reveals significant differences between the average values of the moment of friction (M t ) for the whole range of rotational speeds applied (Picture 2). At the same time it can be observed that the highest values of M t were achieved for the speed n = 600 [rotations/min] in the case of the bearing Number 2, and for the speed of n= 1000 [rotations/min] for the bearing Number 1. In addition, the data collected demonstrates that the lower and higher ranges of speeds (in relation to the aforementioned) are characterized by lower values of M t for each particular bearing. It may suggest the existence of both the optimal and the unfavourable working conditions for these bearings, with minimal and, respectively, maximal resistance to motion in a given situation.

3 Solid State Phenomena Vol Fig. 2. The moment of friction (Mt) values in the function of the rotational speed Fig. 3. The relation between the moment of friction and the rotational speed For the sake of better estimation of M t values, a statistical analysis of the results was carried out. The calculations confirm that the middle range of rotational speeds, i.e. n= 600 [rotations/min] (for Ł2) and n=1000 [rotations/min] (for Ł1), is characterized by the highest values of M t (Picture 3). It needs to be mentioned that the visual scrutiny of the examined bearings revealed the fact of proved that during the work of the bearing Ł1, the lubricant was displaced from the friction sphere. Since the measurement was taken first for the highest rotational speeds and then, later, for the lowest rotational speeds, the first traces of the lubricant were spotted at the speed n=1400 [rotations/min]. The significant force outside and the initial work of the bearing (30 min., n= 1000 [rotations/min]) strengthened the tendency. The increase in the rotational speed while the bearing was not leak-tight, intensified the effect; the rising temperature of the bearing was, of course, very important as well. The rise of temperature, the decrease in the viscosity of the lubricant, and its relatively easy leakage from the friction sphere might have led to the increased resistance to motion during the next part of the examination (the data shown in Picture 3 may confirm this); this was so because the examination cycle was carried out non-stop, without separating the friction pair. Because the friction pair was working non-stop, the temperature stabilization took some time (even despite the fact that the rotational speed was lowered); this, in turn, could have resulted in poor lubrication (the lubricant being squeezed out) and a further increase in the value of M t.

4 152 Mechatronic Systems and Materials V Considering the viscosity of the lubricant and the increase of temperature, one is tempted to assume that except for the resistance to motion resulting from the mutual cooperation of the elements of the bearing, one has to take in to account the bigger value of resistant component which is related to the internal friction in the lubricant. However, we need to remember that plastic lubricants used in bearings are considered to be non-newtonian liquids, therefore the answer of their rheological characteristic does not have to be in linear relation to temperature, and a requires meticulous viscosity examination. Two processes were observed: a huge diversification within the value of the moment of friction during the whole examination cycle, and a considerable diversification during singular examinations, i.e. the ones lasting more than 10 seconds, for the chosen rotational speeds. These processes are illustrated by the data in Pictures 4 and 5. Such a strong diversification of the values of the moment of friction for one rotational speed necessitated some further statistical description. The chosen results of the calculations were juxtaposed in a table (Table 1) and in a form of histograms of frequency (Pictures 6 and 7). The results confirm the earlier observations of the values of the average moments of friction. Pictures 6 and 7 show some histograms of frequency. With the assumed level of significance α=0,05, they mark the range within which the searched value of the moment of friction is contained. It was also observed that that these histograms have a normal distribution, which means that all the external factors which could possibly influence the moment of friction, do so to a considerable degree. In other words, there exists no such a factor which could decisively influence the moment of friction in this examination. Fig. 4. The diagram of dispersion (with the confidence intervals marked, n= 1000 [rotations/min]): a) Ł1, b) Ł2 Fig. 5. The diagram of dispersion (with the confidence intervals marked, n= 1400 [rotations/min]): a) Ł1, b) Ł2

5 Solid State Phenomena Vol Tab. 1. The results of statistical calculation for the selected cases n, [rot/min] 1000, [Ł1] 1000, [Ł2] 1400, [Ł1] 1400, [Ł2] Mean Confidence interval -95,000% Confidence interval +95,000% Median Minimum Maximum Variance Standard deviation 0, , , , , , , , , , , , , , , , , , , , , , , , ,2458 0, , , , , , ,06773 Fig. 6. Histogram of frequency for Ł1 with n=1000 [rot/min] Fig. 7. Histogram of frequency for Ł2 with n=1000 [rot/min].

6 154 Mechatronic Systems and Materials V Conclusions On the basis of the examination results and their subsequent analysis we can conclude that the measurements of the moment of friction indicate its diversity as far as the average value is concerned. It can unequivocally be proven that there exist certain ranges within which a maximal value of the moment of friction for the given rotational speed occurs, and that all the measurements below or above this value are characterized by a lower level of M t. Therefore it is justified to claim that there exist certain optimum working conditions for the bearings, and that under these working conditions there occurs some minimal, for the given situation, resistance to motion. The lubrication squeeze in the friction sphere was observed and, in all probability, this had an impact on a range of different factors informing the work of the bearing (the friction sphere temperature, the conditions of lubrication, the intensity of wear, etc). This phenomenon could have influenced the values of M t that were recorded. The total moment of rolling resistance in a bearing comprises first and foremost the friction of the cooperating elements and the internal friction in the lubricant. When M t is measured, the influence of temperature on the viscosity of the lubricant (the internal friction) cannot be ignored; it can be assumed that the change of the lubricant s viscosity is closely correlated with the value of the internal friction in the lubricant. References [1] Dietrich M. (red.): Podstawy konstrukcji maszyn, Wydwnictwa Naukowo - Techniczne Wyd. 3. T.2, Warszawa, 2003, p [2] Hebda M., Wachal A.: Trybologia, Wydawnictwa Naukowo-Techniczne, Warszawa, 1980, p , [3] Nachimowicz J., Jałbrzykowski M.: Methodological Aspects of Evaluating Tribological Properties of Ball Bearings, Solid State Phenomena Volumes, Mechatronic System and Materials III, Trans Tech Publications, Switzerland, p [4] Totten G., Liang H.: Mechanical Tribology, Marcel Dekker, New York 2004, p.64-69, [5] Nachimowicz E., Korbut R.: Friction and wear problems. Scientific and Technical Reports, Vol. 52 (2010), Kiev, pp [6]

7 Mechatronic Systems and Materials V / Measuring and Analyzing the Moment of Friction in Double-Row Ball Bearings /