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.
