The results o f special experiments on the size effect in contact fatigue are presented. It is established that under constant contact loading conditions the durability is higher, the larger is the diameter o f a tested element. The methods fo r estimation o f contact fatigue resistance o f gear wheels, which is based on the statistical model fo r a deformable solid body having a critical volume, are proposed. The limiting stresses o f a gear wheel are estimated using a regulated base fo r this machine parts. K e y w o r d s : size effect, contact fatigue, gear wheels.The durability and fatigue resistance o f com ponents operating under cyclic loading by bending, tension, com pression, twisting, etc. appear to be the lower, the larger are the com ponent dim ensions [1,2] Since the phenom ena o f contact fatigue are based on the same processes as those occurring under other types o f fatigue, it is natural to expect that increasing the absolute dim ensions o f a com ponent w ould decrease its contact fatigue limit. However, by analyzing the available results o f studies on the size effect in contact fatigue it is im possible to get a definite opinion on this problem [2][3][4][5][6][7][8][9][10][11][12][13].Some researchers state that the size effect in contact fatigue undergoes inversion, i.e., as the diam eter o f contacting parts is increased, the durability grows [4][5][6]. However, others assert that increase in a com ponent size leads to reduction o f bending and contact fatigue lim its [7,10].For the basic regularities o f the size effect in contact fatigue to be elucidated, special experim ental studies have been m ade. The test schem e is shown in Fig. 1. C ylindrical sam ple 2 serves as a tooth o f a gear wheel. A counterbody -roller 1 -is pressed to the surface o f sam ple 2 by a contact load F N in the contact zone x. R oller 1 serves as a tooth o f the second gear w heel that transm its the contact load F n to sam ple-m odel 2 .Sam ple 2, w hich is fastened in spindel 3, is rotated w ith an angular velocity « 1. C ounterbody 1 is rotated w ith an angular velocity « 2 , its rotation axis being parallel to that o f sam ple 2. R egulating the ratio o f the velocities «1 and « 2 allows one to obtain the required slip coefficient, im itating the slip in gearing. The contact load F N provides a sim ultaneous excitation both o f contact and bending stresses in the corresponding zones, w hereas the distance betw een these zones is chosen tobe equal to that betw een the pitch point and the tooth root.U sing the counterbody (roller) w ith a constant diam eter D =100 m m and sam ple-m odels o f various diam eters d (Fig. 2) m akes it possible to change two m ain curvatures and to obtain the size ratio o f the contact area (a /b ) w ithin the range 0.4-0.8, w hich is satisfactory for practical purposes.
