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T–72 Center  Distance  Designer:  Provides  computerized  Drive  Ratio  and  Center  Distance calculations. The Center Distance Designer program, on the web, computes belt lengths for various center distances and checks the number of teeth in mesh for both pulleys. It calculates pulley drive ratios and the minimal center distance for a designated pulley pair. The  Center  Distance  Designer  searches  and  retrieves  all  pulleys  and  belts  shown  in  the handbook that fits within the customer criteria. Once the design is completed, the part numbers can be instantly retrieved from the database. Each part number is then linked to an electronic catalog page, which is viewable and can be printed. The user can design a drive in a most efficient manner, since the program described above presents  available  alternatives,  as  well  as  a  direct  reference  to  catalog  page  numbers  and  part numbers involved. It is assumed, however, that not all users of this Handbook have access to a computer.  Therefore, the Drive Ratio and Center Distance Tables are presented in this Handbook in printed format. SECTION 21    DRIVE RATIO TABLES In the design of belt drives, we usually know the speed ratio (transmission ratio) and we need to determine pulley sizes, center distance and belt length.  These quantities are shown in Figure 39, for an open (uncrossed) belt. The  Drive  Ratio  Tables  (Table  41,  starting  on  page  T-74)  are  designed  to  facilitate  the determination of these quantities.  They list the following information: N1/N2 = the transmission ratio obtained when the larger pulley (N1 teeth) is the input and smaller pulley (N2 teeth) is the output.  Given to 3 decimal places. N2/N1 = the transmission ratio obtained when the larger pulley (N1 teeth) is the output and the smaller pulley (N2 teeth) is the input.  Given to 3 decimal places. (Note that N1/N2 is the reciprocal of N2/N1) N1 = number of teeth on larger pulley. N2 = number of teeth on smaller pulley. N1 – N2 = difference between number of teeth on larger and smaller pulleys.  This number is useful in center-distance determination. C MIN = The minimum center distance between pulleys for a belt of unit pitch.  If the pitch is denoted by p, the actual minimum center distance is a product of C MIN and p. The minimum center distance is determined from the condition that at the minimum center distance, the pitch circles of the pulleys can be assumed to touch.  This will generally give a satisfactory approximation to the practical minimum center distance. The table is based on the equation:            N1 + N2 C MIN = –––––––– x Belt Pitch     (21-1)                 2p At the beginning of the table, a list of standard pulley sizes is shown.  The smallest pulley has 10 teeth and the largest, 156 teeth.  A standard size will be the most economical.  If a nonstandard size is needed, however, please contact Stock Drive Products for assistance.