The term “Datum” was first adopted by the International Standards Organization
(ISO 1081-1980) and recently by the Rubber Manufacturers’ Association
Engineering Standard f or Classical V-belt and Sheaves (IP-20-1988, Gates Form
# 14495-B). Classical sheaves were specified by pitch diameters until 1988,
when the Datum System was adopted by the USA. This change was necessary because
the nominal pitch diameter of a sheave no longer corresponded with the actual
pitch line of the modern V-belt as it passes through the sheave groove.
Over several decades, construction improvements enhanced the performance
of V-belts in many ways. New, advanced cord materials allowed the move from
multiple unit tensile belts to high performance single unit tensile
constructions which dramatically improved the horsepower capacity of V-belts.
For example, a B-Section belt in 7.0 inch sheaves was rated at 4.2 HP (1750
RPM) by 1945 RMA standards. Today, a Gates Hi-Power II belt is rated at over 11
HP under the same conditions. This increased capacity is due in part to the
move of the center of the tensile cord line to a location higher in the V-belt.
In general, the center of the tensile cord is associated with the pitch
line. In the new higher position, the load carrying tensile has a greater torque
carrying moment arm and more undercord support through which to transmit normal
force to the sheave walls. In addition, manufacturers have determined that the
optimum position for the tensile cord is very close to the outside diameter o f
a standard depth sheave. So the diameter through which the pitch line passes is
nearly equal to the outside diameter for most belts.
By definition, the diameter through which the pitch line passes should be
the pitch diameter. This is precisely what the Datum System accomplishes.
Figure No. 1 illustrates the construction change and its effect on the location
of the pitch line.
Originally, machining standards for classical sheaves were established with
the pitch diameter as a basis. The system is built around the notion of constant
"pitch width" as the basis for machining standards. The pitch width
sheave specification is tabulated f or each V-belt cross-section. Because
V-belt cross-sections distort more as they bend around smaller sheaves, sheave
groove angle is varied with sheave diameter.
In classical sheaves, the groove angle is pivoted about the old pitch width
at the old pitch diameter. Figure 2 illustrates the old pitch system and the new
Datum System as related to sheave angle. Note that Datum diameter/width
directly replaces pitch diameter width as the “base” dimensions about which the
machining dimensions are derived.
Because of the shortcomings of the old system, Datum diameters have been
adopted by the industry as the means of designating sheave size. Datum
diameters are now used to place an order for Classical sheaves. An old pitch
diameter (PD) designated sheave is directly replaced by the new Datum diameter
(DD) designation (i.e., old 8 .0 inch Pitch Sheave = 8.0 inch Datum Sheave.)
To simplify, modern pitch diameters are equivalent to outside diameters (OD)
for standard depth sheaves for most belts. An exception is A-section belts or
AX-section belts in A/B Combination Sheaves. Conversion values for PD to OD for
these exceptions and DD to OD values are tabulated in manufacturers’ design
Essentially, the Datum System removes complexity and inaccuracy from the
V -belt drive design process. The challenge for power transmission
professionals is using a new name for an old term.
|Figure 1 Figure 2|