The following links provide easy access to common design considerations:
Drive Alignment
Multi-v 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 and Tensioning
When installing a multi-v 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 multi-v 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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Reinforcement Twist
The reinforcement in a multi-v belt typically contains S and Z twisted fibers. The main reason for twisting the reinforcement is to change its physical characteristics. A heavily twisted reinforcement will have improved flexibility, but may exhibit reduced strength. Conversely, a lightly twisted reinforcement will retain most of its strength, but may exhibit poor flexibility.
There are two types of twisted reinforcement constructions that make up a cord: plain and cabled. Plain construction consists of single filaments twisted together. Cabled construction consists of two or more plain constructions twisted together. The amount of twist and the construction of a reinforcement depends on the desired physical characteristics.
Both the S and Z twist are used in a multi-v belt to aid in tracking. If only one twist is used in a belt, it will track in one direction. Using S and Z twisted reinforcements in a belt stabilizes the tendency to track in one direction. The two twisted reinforcements are placed side-by-side in the belt.
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Reinforcement T.P.I.
Threads per inch (T.P.I.) is the number of twisted cords in a one inch wide belt. Typical Fenner Precision winding pitches are 30 through 72 threads per inch. As cords are wound helically on the mold, filaments can be observed entering and exiting on either side of the belt.
Different winding pitches enable changes in the belt's strength characteristics. A belt with 72 T.P.I. will have a higher break strength than the same belt with 30 T.P.I. containing the same reinforcement type. The reinforcement T.P.I. is application dependent and should be discussed with a Fenner Precision Applications Engineer to determine the optimum value for your drive.
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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.
- For Multi-V belts, the effective length is calculated by adding the effective outside circumference of one of the measuring pulleys to twice the measured center distance between two pulleys.
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