Infinite or stepless variation has an extensive industrial application with in general categories.<br>1. Synchronizing the speeds of parts of the same machine or operations carried out in series to provide for uniform flow.<br>2. Proving rapid, manual or automatic, flexibility in the operations of a machine for increased capacity, lower production costs, and higher precision standards. This subsection will treat the mechanical methods of stepless speed variation. Mechanical systems have the advantages of being low in the first cost, simple in construction and maintenance, and comparatively efficient in operation. Stepless speed variation under may be obtained through the use of one or more of the following five mechanical principles:<br>A. Flat belt type.<br>B. Friction disk or cone type.<br>C. Ratchet or interrupted motion type.<br>D. V drives with adjustable conical pulley or pulleys.<br>1. Vari-speed pulley type.<br>2. Interlocking dual pulleys, countershaft type.<br>3. Adjustable cone pulleys, parallel shaft type.<br>a. Belt drive.<br>b. Chain drive.<br>c. Steel ring drive.<br>4. Compound adjustable pullys and interlocking dual pulleys.<br>E. Controlled differential gear system<br>This paper deals with the special V belt used for this D-3-a type, and consists of the following four parts:<br>1. In order to get a general conception on these belts and apparatus, the auther states about sizes and contents of belts, and mechanism of these machinery.<br>2. The auther calculates the tension of belts for two cases. One is the case when it has the different radii at the tension and sluck sides, and the other is the case when it has the some radius.<br>3. The auther tells about the maximum stress on the sectional area of V velt. He sometimes encounterd the initial cracking in the core of V belt tested, and this will be explained by the fact that maximum stress is originated in the center of cross section of belt.<br>4. The belt is given compression stress from its both sides by sheaves, in the part which is laid between the sheaves. According to these compression from both sides, belt receives more tensile stress in the longitudinal direction than in other directions. The auther calculated the amount of these tension. The reason why the initial wrecking takes place usualy at both side of belt, is explained by the fact that at both sides belt receives more tensile stress than others.