2016 STLE Annual Meeting & Exhibition May 15-19, 2016 Bally’s Las Vegas Hotel and Casino, Las Vegas, Nevada, USA Influences on the Wear Behavior of Small Module Gears lubricated with High Consistency Greases. Hansjörg SCHULTHEISS 1 , Thomas TOBIE 1 , Karsten STAHL 1 1 : Technical University Munich (TUM), Mechanical Engineering, Gear Research Center (FZG), Munich, Germany. Abstract: Small module gears (m n ≤ 1 mm) are increasingly important in drive technology due to ever more rigorous requirements regarding, e.g. performance and weight. They can be found in numerous applications which are subject to very different operating conditions. Often, small module gear applications are grease lubricated. This results in the lubrication supply mechanism (i.e. channeling/circulating) playing an important role in regard to the resultant gear failure mode. In the experimental investigations conducted herein, the focus was on gear sliding wear. The influence of the operating conditions and the grease composition on the lubrication supply mechanism, and thus wear behavior, was investigated using case carburized, small module gears (m n = 1 mm). The operating conditions, especially rotational speed, were shown to have a significant effect on gear wear behavior. Increasing rotational speed did not necessarily lead to lower wear. Also, the grease composition was shown to affect wear behavior. Keywords: Gears: Gears (General), Lubricants: Greases, Wear: Bench Wear Tests 1. INTRODUCTION In drive technology, the requirements of gear boxes in regard to weight, performance and size are increasingly stringent, which has resulted in a clear trend towards smaller gears with a higher power density [3][12]. Previ- ous research at FZG [1][2][5][8] has shown that small module gears show particular characteristics regarding load carrying capacity that deviate from what one would expect from calculation standards such as DIN 3990 [4] or ISO 6336 [9]. The research has shown, for example, that the tooth root as well as pitting load carrying capaci- ties of small module gears (m n = 0.45 - 1.0 mm) can be up to 45% and 35% higher respectively than a gear of refer- ence size according to DIN (m n = 3 - 5 mm). On the other hand, it was also shown that small module gears are more vulnerable to sliding wear than larger gears. According to Plewe [10][11], the minimum lubricant film thickness h min is the primary factor to consider when as- sessing gear wear behavior. Extensive experimental work by Plewe [10][11] has shown that for a pair of gears of similar surface hardness, wear can be expected for a min- imum lubricant film thickness of h min ≤ 0.05 μm. Fur- thermore, for h min ≤ 0.01 μm, high wear rates can be ex- pected [10][11]. According to Braykoff [1], the calculation formula for the minimum lubricant film thickness of a line contact ac- cording to Dowson/Higginson [6] can be simplified to show the effect of module (m n ) and rotational speed (n) on the film thickness more clearly (see (1)). v M represents the kinematic viscosity at bulk temperature. ℎ  ~ 0.7 ∙  ∙  (,   ) (1) This relationship shows that gear size and rotational speed affect the min. lubricant film thickness to different de- grees. Fig. 1 exemplarily shows the calculated minimum lubricant film thicknesses for geometrically similar gears of module 1 mm and module 4.5 mm. From this it be- comes clear that a smaller gear must be operated at a sig- nificantly higher rotational speed than a larger gear to achieve the same lubricant film thickness. This underlines the higher wear risk of small module gears. In field appli- cations with small module gears, the rotational speeds to which the gears are subjected during normal operation are often in a range that leaves them vulnerable to wear. Fig. 1: Exemplary, calculated min. lubricant film thick- ness for different sized gears. A further important factor to consider is the technological limit in the manufacturing of small module gears [1][8]. The surface roughness, for example, cannot simply be scaled down with the reduced module due to the limits of grinding processes. This, in combination with the less favorable hydrodynamic lubrication conditions of small module gears contributes to higher wear vulnerability. Due to certain advantages that grease lubrication bears, small module gear applications are often grease lubricat- ed. In the course of this work, the wear behavior of small module gears (m n = 1 mm) lubricated with oil and grease (NLGI 1) is to be investigated. The focus of the investiga- tions is on the influence of the rotational speed on the lubrication supply mechanism and thus the wear behavior. 2. LUBRICATION SUPPLY MECHANISM The lubrication supply mechanism is of great importance for grease lubricated gears as it determines the amount of lubricant available in the gear mesh as well the degree of heat dissipation [13]. According to Stemplinger [14] there are two main lubrication supply mechanisms for grease lubrication; circulating/churning and channeling. These observations are consistent with earlier observations by 0,00 0,01 0,02 0,03 0,04 0,05 0,06 0 1000 2000 3000 4000 5000 6000 Min. lubricant film thickness in C h C / μm Rotational speed of pinion n 1 / min -1 Test gears m = 1 mm Test gears m = 4,5 mm Danger of wear Danger of high wear Oil: Base oil B Ra = 0,3 μm p C = 1867 N/mm 2 ϑ S = 90°C