Computer selection

Belt drives were traditionally selected using a series of pre-engineered tables, but now most belt drive components are computer selected. Selection software is available free from most belt drive component manufacturers. The purpose of this article is to reveal how input data affects results and to interpret output.

Typical input requirements:

1. Service factor
Service factor is a multiplier that is applied to the operating hp to determine a conservative minimum hp rating for belting. Service factors were formerly obtained from charts covering load characteristics such as starting torque, cyclic variation of the driven load, expected hours of use per day, and so forth. Most computer programs assist in the selection of the proper service factor designation.

Helpful hints
• Synchronous belt drives require higher service factors than V-belt drives.

• Service factors that are higher than recommended do not increase belt life unless belt tension is kept at lower than what is normally specified for that belt.

• Excessive service factors may contribute to excessive bearing loads, because tension applied relates to belt capacity.

2. Horsepower
Motor nameplate horsepower is normally referenced when sizing belts. However, the actual horsepower required by the driven device may also be used.

3. Driving rpm
Use full-load rpm ratings of the driver. The driven speed calculations are only as accurate as the input speed entered.

4. Driver shaft diameter
Most programs insert the correct shaft diameter according to National Electrical Manufacturers Association (NEMA) standard publication MG 1 if requested and if within the specified horsepower range. Otherwise, you should enter this dimension if known.

Helpful hints
• Short shaft (TS) motors are designed for direct coupled use only and should not be used with a belt drive.

• There are two frame options for motors with 125 hp at 1,750-rpm, so be sure to verify the shaft diameter.

5. Driven rpm desired
The computer program selects drives with driven speeds within some percentage of the desired rpm. This range can be adjusted in most programs.

6. Driven shaft diameter
Enter this diameter if known, but this is an optional entry on most programs.

7. Center distance desired
Center distance should also be entered if known. Some programs request both minimum and maximum values, but most programs default to some nominal center distance if no value is entered.

Calculations

Most programs let you narrow down the selection process, considering only chosen belt sections. They also allow limits to be set, for example, on pulley diameters, drive widths, and minimum number of belts. Unless you have specific space constraints or other requirements, it is usually best to accept the default values. Additionally, these programs use minimum pulley diameters and drive widths recommended by NEMA unless you override the default settings.

1. Driven speed or ratio
The typical program works with one belt section at a time, first finding all combinations of standard pulley sizes that produce a driven rpm within the desired speed range. The program also makes certain the pulleys will be operating within their rated speeds.

Helpful hints
• When using V-belts, speed calculations are based on the pulley’s “pitch diameter.” With synchronous sprockets, the number of teeth is used to figure speed.

• Traditionally, the pitch diameter was less than the V-belt pulley’s OD. In the case of classical pulleys, the part number was based on the pitch diameter. Due to changes in belt construction over the years, however, the pitch diameter is now located at the pulley OD for most belt sections. The pitch diameter for “B” section pulleys is now greater than the OD.

2. Find belt and center distance
Using either the desired or a default center distance, the program next picks a standard belt length to meet requirements as closely as possible. It then calculates the actual center distance for the belt used.

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