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 manuals.
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.