There are a number of design considerations common to the StretchBand™, Semi-Stretch fabric reinforced belts, Stable Yarn Reinforced belts and Stable Fabric Reinforced belts supplied by Fenner Precision. These considerations include:
Drive Alignment
Flat belts are sensitive to misalignment and should not be used where misalignment is inherent in the drive. Any degree of misalignment will result in some reduction in belt life, inconsistent wear, and unequal loading conditions. This effect has the potential to lead to improper belt operation or premature belt failure.
It is recommended that the drive have one adjustable component so installation is easier and the belt can be tensioned correctly. There are several ways to incorporate an adjustment into a drive system, including adjustable lock-downs or spring-tensioned pulleys/idlers. If your application is currently designed with a fixed center distance, contact a Fenner Precision Applications Engineer to discuss how we can improve the overall performance of your drive.
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Idlers
Idlers are commonly used to take up belt slack, apply installation tension, or clear obstructions within a system.
In unidirectional drives, idlers should be located on the slack side of the drive. Backside (or outside) idlers should be located as close as possible to the driveR pulley in the system. Backside idlers should be flat and flanges are recommended. Diameters of backside idlers should not be smaller than 1.3 times the smallest loaded pulley in the system. Inside idlers should be located as close as possible to the pulley with the most teeth in mesh and should not be smaller in diameter than the smallest loaded pulley in the system. Inside idlers larger than an equivalent 40 groove pulley may be flat.
Spring loaded idlers must be designed to prevent the belt from ratcheting, or “jumping teeth”, under the highest loading conditions of the drive; this includes starting torque or any shock loading which may occur during normal operation.
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Belt Installation & Tensioning
When installing a flat belt, be sure that all adjustable components in the system are loose. Do not force the belt over a flange as this will cause internal damage to the belt tensile member, and may result in premature belt failure.
There are several methods used to install a flat belt and adjust its tension. Two of the more common methods, adjustable or spring-loaded idlers and the adjust and lock-down method, are described below.
Adjustable or spring-loaded pulleys/idlers can be used to tension the belt in a drive system. When using adjustable pulleys/idlers, be sure to do a vector analysis of the forces to ensure the proper installed tension in the belt. In a spring loaded system, be sure that the k-value for the spring and spring extension are properly determined during installation. If the belt tension from the applied load is too large, damage to drive components such as the motor, the belt, or bearings may occur. If the belt tension is not sufficient, ratcheting may result.
Once the drive has been set, the sonic tension method is a common way to determine belt tension. This method uses the sound waves generated by “plucking” a single span of the belt. A microphone is held just above the belt in the middle of the plucked span to measure frequency. As installed tension changes, the frequency changes. Through applying known installed loads to the belt, a graph is developed correlating frequency to tension. Once the frequency values are determined, belt tension can be adjusted to the proper value.
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Belt Length Determination
- Install the belt over the measuring pulleys and apply the belt tensioning force smoothly to prevent shock loading.
- Rotate the pulleys at least two revolutions in order to seat the belt properly into the pulley grooves and divide the tension equally between the two spans of the belt.
- Read the tolerance from the measuring scale. The reading should be added/subtracted to the nominal center distance.
- Remove the belt immediately after the reading is taken.
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Pulley Tracking
Circular crowns should always be used. Angular crowns concentrate stresses and wear. If angular crowns must be used, full life testing is necessary. Between .01” and .03”/1 belt width crown is suggested, the higher number for long center distance drives or where significant pulley misalignment is present.
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