OPTICAL SYSTEMS

Even when the target tension is known, the traditional force-deflection method of installing the belt and setting the tension can be a challenge.

In contrast, optical belt frequency meters directly measure belt vibration. Using solid-state infrared technology (the source signal, the non-coherent infrared output of a small low-level diode) the sensor actually sees the belt surface — and observes any displacement of that surface. In this way, the sensor is free of bias from events or stray signals, and only measures the target belt's vibration.

Both acoustic and optical meters monitor sensor output and report the frequency directly in Hertz. Most meters also include software programmed to provide some rudimentary calculations. Once the operator selects a measurement system and inputs some basic drive constants (such as span length and belt mass, and weight per unit length) the meter can report tensioning results in standard force units such as pounds-force or Newtons. These results are then compared to target values computed for the application, or to more generic target values based upon belt properties. With this comparison, drive center distance is either increased or decreased (if tension is found too high) in successive iterations until the measured tension matches the target tension.

But again, optical measurement of belt vibration frequency is inherently more accurate and reliable than acoustic readings. The application of string theory provides a direct correlation between belt tension and natural belt strand vibration. Optical belt frequency meters are durable and error free, and most belt-drive analysis programs are already programmed to simultaneously report target belt vibration frequency in Hz and traditional tension force values. All this streamlines installation and subsequent maintenance inspections.

In fact, manufacturer drive analysis programs are usually available at no cost from their power transmission distributors or website. Most can even run the program for a customer. The customer service department only needs the application's basic parameters to provide tensioning information, including belt vibration frequency.

Some tips: A copy of the results should be kept in a permanent application file. The installer should be equipped with the frequency target and a state-of-the-art optical belt frequency meter. A good belt frequency meter measures the vibration frequency of all style belts, even from different manufacturers. When tensioning a set of multiple belts, the installer should focus the sensor on a single belt toward the array's center.

The full benefits of optical measurement are realized during maintenance checks, because technicians need only to shut down the drive, aim the sensor, thump the belt, and read the results.

Another tip here: Hand rotating the system and taking multiple readings at different belt orientations may help identify problems with other drive components. Tension excursions are indicative of component problems such as bent shafts, poorly mounted sprockets or pulleys, or irregular pulley grooves.

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The traditional approach to measuring belt tension doesn't make use of vibrations at all. The target tension of a belt, expressed in units of force, is measured mechanically. By using specific application geometry, the target tension is broken into two values — deflection force and distance. Armed with these two target numbers, a straight edge, force gauge, and a deflection scale, plus the requisite tools, a designer can install a belt properly. But even under the best conditions, it is a balancing act.

Compared to this traditional force-and-deflection method of determining belt tension, strumming the belt and measuring resultant vibration frequency is vastly more efficient.

For more information, visit Veyance Technologies, the manufacturer and marketer of Goodyear Engineered Products, at goodyearep.com or click on Motion Component Zones on the motionsystemdesign.com homepage.