Motion System Design

Belt drives

There are many different types of belts and pulleys, but sizing considerations are similar for most of these drives. Many of the product specifications included in this article are published jointly by the Rubber Manufacturer’s Assn. (RMA) and the Mechanical Power Transmission Assn.

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.


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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• The calculated center distance is the center distance setting required for a new drive with a properly tensioned belt. A lesser center distance is required to install belts, and a greater one is needed as the pulleys and belts wear.

• You must allow for center distance adjustment or else use an idler to adjust tension, even with synchronous drives. Do not use spring-loaded tensioners on synchronous drives.

3. Calculate horsepower per belt or per unit of belt width
Horsepower formulas are complex. They usually include a basic rating plus corrections for speed ratio and belt length. The formula considers three main components: the transmitted torque, losses due to bending fatigue, and losses due to centrifugal or speed effects. Horsepower formulas used by various belt manufacturers frequently give higher ratings than those published by RMA/MTPA. 5VX.

4. Determine number of belts or belt width
Dividing the design horsepower by the rating per belt determines the number of belts (or width of belt) required.

5. Verify drive components
The program verifies that pulleys with the required number of grooves are available as standard items and that these pulleys will fit on the required shaft diameters. The program typically proceeds to determine the price of the drive.

6. Calculate specifications and installation data
Now that a valid drive has been found, the program calculates all the drive output details. The data is then stored in an array, and the program repeats the entire procedure until all possible drives have been identified. When finished, the program usually displays a sorted summary of the drives found. The user can pick his preference from the list and display the details for that drive.

Output data

Output data formats vary greatly, but the content is fairly consistent. In addition to component part numbers, weights, and prices, most programs detail the following:
1. Actual service factor and/or drive horsepower rating
2. Driven rpm
3. Actual center distance
4. Belt installation information

Helpful hints
• There is no standard method of calculating force deflection tensioning values, therefore these values and the corresponding hub load values may be based on differing assumptions. Even though various programs may produce different values for the same drives, the actual forces required to do the job are identical regardless of who makes the components. You may want to inquire as to how the numbers you use were derived.

• Belt slippage occurs when tension is insufficient to transmit a load. Even if the drive is undersized, you can eliminate slippage by applying more tension. Excessive tension decreases the life of the belt, and may overload the shaft bearings.

• Installation allowance reflects the center distance adjustment required to install the belts without removing the pulleys.

• Take-up refers to the total increase in center distance expected over the life of the drive.

5. Hub loads
Hub loads reflect the vector sum of the tight and loose belt strand pull on the shaft. This number does not include pulley weight, as its contribution can be altered by the angle of belt pull, which is not known.

6. Special balance recommendations
Standard pulleys are given a one-plane balance. If either pulley runs at a speed that falls into the area where a two-plane balance is recommended, this should be noted on the output. MPTA standard B2- 1998 is used to make this determination. Information for this article was provided by Lewis E. Baer, Manager of Mechanical Engineering, TB Wood’s Inc., Chambersburg, Pa.

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