Making light work of belt tensioning
It's universally recognized that proper synchronous belt tension is key to efficient drive operation. And where newer-generation belts drive loads, proper tensioning is critical.
First-generation synchronous belts, which date to the 1940s, usually include a flat plane trapezoidal shape. The tensile member is polyester, fiberglass filament, or even steel. Second-generation synchronous belts were introduced in the early 1970s; they have a curvilinear tooth profile, a tooth design loosely based on an involute tooth gear model. The exact shape varies in small ways between manufacturers, but for all of them, the sprocket-to-belt fit is much more precise than with first-generation belts. Second-generation belts also incorporate a high-modulus tensile member engineered of fiberglass, aramid, or some blend of the same.
Currently, there is some debate as to whether the newest designs constitute a third generation of synchronous belting. Many improved second-generation offerings exist, but purists say the next generation should be a quantum leap in belting technology, and not just a refinement. When the leap happens, these new belts will put much greater demand on bearings and shafts, as power density and load carrying capability will undoubtedly exceed today's standards.
In any case, proper tensioning is paramount in newer-generation belt operation. Why? Power transmission drives incorporating synchronous belts are essentially precision gear trains, with one gear component (the belt) made of elastomer materials. Their requirements are similar to those of traditional all-metal gear trains: A proper mesh between components is critical to optimizing performance and service life. More specifically, the pitch and operating tension of a belt are directly related, so tension must be well controlled for proper pitch. This is increasingly important as belt operating speed increases.
The good news is that the latest synchronous belts include very stable, high-modulus tensile members (cords) that maintain proper pitch dimensions over wide load ranges. Too, modern manufacturing processes yield precision, close-tolerance belts. However, setting tension too high or low does prevent belts from meshing, because their pitch goes out of sync with their pulleys.
In contrast: V belt tension
V belts are not as sensitive to fit; their issue is general efficiency. Why? V belts drive by friction between the driving faces of the belt and the pulley's sidewalls. Tensioning conditions directly affect power transmission as well as abrasion wear of both belts and pulleys.
Applied tension must stay above a minimum level to overcome slip between the belt and pulley. Trouble is, tension in a V-belt system usually decreases during routine use.
Belt tension is still a critical component, but average tension in a V configured belt can be 30% more than that of a comparable synchronous belt. The generally accepted best practice is to check and reset their tension on a regular schedule.
Synchronous belt tension
A synchronous belt operates on tooth engagement rather than pure friction. Therefore, the tension required to successfully operate a synchronous drive can be considerably lower. If proper installation procedures are followed, generally a synchronous belt should not require periodic tension adjustments.
Manufacturers provide tensioning target values based upon specific application conditions — values that are a function of the required belt pull, in turn, a function of required torque and pulley diameter. The effects of centrifugal forces and the pecific belt configuration's frictional properties correct it. But even when the target tension is known, the traditional force-deflection method of installing the belt and setting the tension can be a challenge.
Verifying tension through vibration
Professional musicians know that the vibration frequency of a plucked string is determined by the tension applied to that string. Laboratory tests show that power transmission belts react in a similar manner. There is a direct relationship between belt tension and a belt's natural frequency of vibration: In effect, a strummed belt can indicate the belt tension setting, if the technician is properly equipped to receive and translate the message.
There are at least two ways to detect belt vibration. Appropriately sensitive instruments hear or see these vibrations.
The vibration itself — a rapid back-and-forth dislocation of the belt strand — is a motion that produces a disturbance in the air immediately surrounding the belt. While such a cyclic disturbance presents itself as a waveform at or slightly below the audible spectrum, the specialized microphones of acoustic-type sensors can ‘hear’ the pressure differentials of the air disturbance. These sensors then provide the input signal for a belt frequency meter. In this way, acoustical meters provide indirect measurement of belt vibration by sensing air disturbances in the immediate vicinity of the belt.
A major challenge in gathering an acoustic measurement of belt strand vibration is that in the average industrial setting there can be a lot of low-frequency air disturbance.
Separating actual belt noise from the ambient noise (which is often in the immediate vicinity of the belt) is difficult. Belt tension should be based on recommended parameters, not, for example, on parameters supplemented by the blade rotation of a shop-ventilating fan. As a result, most acoustical meters feature a gain adjustment or internal circuitry to tune out a portion of the ambient noise, with variable success.
OPTICAL SYSTEMS
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.
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.