PowerGripÃ‚Â® GTÃ‚Â® Belt Drives

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Polyflex ® JB ® and Micro-V ® Belts – Engineering I. Operating Characteristics A. High Speed Drives One major advantage of Gates Polyflex JB and Micro-V belts is the ability to operate smoothly and efficiently at high belt and shaft speeds. For example, shaft speeds of 30,000 rpm and higher have been achieved with smaller Polyflex JB cross sections. Polyflex JB and Micro-V belts both have the ability to transmit power to shafts turning at high speed mainly because the self-imposed centrifugal force acting on it is low. Centrifugal tension occurs in a belt because of centrifugal force – the belt is rotating around the drive. Two things govern the magnitude of centrifugal tension acting in a belt drive: (1.) The size of the belt (cross-sectional mass). (2.) The speed the belt is traveling. The relationship of these items is expressed in the following formula: Tc = MV 2 Where: Tc = Centrifugal tension, (lb) M = A constant depending on mass of belt V = Belt speed, (ft/min) The formula shows, as the size (mass) of the belt increases, centrifugal tension increases. But, more influential is speed, which increases exponentially. Because a V-belt is basically a tension carrying member, its horsepower rating increases with speed until a maximum value is reached, as illustrated in Fig. 21. As belt speed increases, the increase in centrifugal tension creates a need to increase the installation tension so the desired running tension is obtained. Beyond this point, centrifugal tension increases to such a level it starts to reduce the available working tension, causing the horsepower rating to drop off. (See Fig. 21). Figure 21 Polyflex JB belts have proportionally higher horsepower ratings than any other conventional type V-belt. This is due mainly to the belt cross-sectional shape and special polyurethane material, as previously described. For high speed drives, this higher horsepower rating is further extended because the rotating mass of Polyflex JB belts is reduced ? smaller cross section, lighter material. Because of these unique characteristics, Polyflex JB belts may be applied on high speed drives where other belt types can not be used. B.Smooth Running Gates Polyflex JB and Micro-V belts are the smoothest running V-belt drives available. For most applications, conventional V-belts transmit power from the driveR to driveN shafts very smoothly, with a minimum of vibration. Some high speed, high precision equipment may demand better performance than a conventional V-belt which is designed mainly for heavy duty, industrial power transmission. Smooth running characteristics of a belt drive are determined by measuring the variation in center distance as the belt slowly rotates around sheaves on a special measuring machine. The machine uses one shaft on a movable base and holds the belt at constant measuring tension. There are no industry standards prescribing levels of smooth running performance. When smooth running is specified as a special requirement, most manufacturers will only supply these belts on a made-to-order basis. Conventional V-belts are usually manufactured by plying up layers of various rubber stocks, fabric and tensile cord. In most applications, the layers and overlaps do not disturb the drive operation. But, as speeds increase and sheave diameters decrease, or the machinery supporting the drive becomes more sensitive or compliant, the small discontinuities in the layers, coupled with overlaps in fabric, become much more apparent with respect to smooth running. Due to the special patented manufacturing process, Polyflex JB belts have exceptionally smooth operating characteristics. Belts are cast in polyurethane and precision ground to assure the best possible cross sectional uniformity. The result is low center distance variation and minimal vibration. Gates Micro-V belts also have excellent smooth running characteristics. Micro-V and Polyflex JB drives have been used extensively on applications such as precision grinders, medical equipment, office equipment, business machines, etc. where smooth running is an essential performance requirement. C. Noise Polyflex JB and Micro-V drives generate minimal noise levels when installed and operating properly. They can, however, generate noise in adverse conditions such as misalignment, improper tension, under design, etc. Since Polyflex JB belts have a polyurethane body with a high coefficient of friction, they may react on a steel surface causing noise, particularly on a misaligned drive. When a drive is misaligned the V- belt will not enter the sheave groove properly. It will stick and rub heavily on the side on the sheave causing noise. (See Fig. 22). Low belt tension can also be a source of noise problems. (See Section III. Drive Alignment and Section IV. Belt Tensioning, page 78.) Figure 22 Ribs molded laterally across the top of Polyflex JB belts help to reduce bending stresses and increase torsional stiffness. Because of the ribbed overcord, a backside idler may generate objectionable noise. The sound level produced is usually inversely proportional to the pulley diameter. So, the larger the pulley diameter, the lower the noise level. Backside idlers should be as large as possible. See Table 42, Page 81, Minimum Sheave Diameter for more information on backside idler recommendations. 76 The World’s Most Trusted Name in Belts, Hose & Hydraulics.
Polyflex ® JB ® and Micro-V ® Belts – Engineering I. Operating Characteristics–continued Micro-V belts can be prone to slippage when installed with less than adequate tension, or when operating on a drive system with flexing centers. This potential slippage can result in noise. See Section IV. Belt Tensioning on Page 78. In many cases the belt drive system is not the primary cause of noise. Undersized, poorly lubricated, worn or misaligned bearings can cause significant noise levels. Rotating parts of a total system can create air movement patterns that generate noise. A weak structure could flex under the load and cause misalignment and affect components in the drive system, thereby creating noise. Also, make sure that the total system has not been designed into an echo chamber, amplifying an otherwise insignificant noise. D. Static Conductivity Polyflex JB belts do not meet the static-conductive requirements specified in RMA Bulletin IP-3-3. This bulletin describes those belt characteristics necessary for safe operation in potentially explosive or flammable environments. However, laboratory testing confirms that under dynamic operating conditions, Polyflex JB belts are nonstatic generating. Under normal operating conditions, Polyflex JB belts do not build up significant static charge. When Polyflex JB belts are used in explosive or flammable environments, additional steps must be taken to protect against accidental static spark discharges: (1) The entire system must be properly grounded. (2) A static conductive brush or similar device is recommended to bleed off any static buildup on the belt if it should occur. The nonstatic generating characteristics apply to new, clean belts. It is the user’s responsibility to establish an effective preventive maintenance program to monitor equipment and to replace or repair components, as needed, for continued safe operation. Micro-V belts do meet the static conductive requirements specified in RMA Bulletin IP-3-3 describing the characteristics necessary for safe operation in potentially explosive or flammable environments. This applies to new clean belts. Belt conductivity properties can be influenced by age and environmental debris. E. Operating Environments Caution should be used when installing Polyflex JB and Micro-V belt drives where debris is present. Debris can damage belts by becoming wedged between the belt’s strands, causing damage or separation. Debris can also cause belts to become unstable and turn over. If the drive must operate in this type of environment, be sure to provide adequate shielding. Since Polyflex JB belts use a 60° angle system, the coefficient of friction between the belt and sheave is extremely important. Any substance which decreases the coefficient of friction can cause belt slip. If the belt slips excessively, enough heat may be generated to cause catastrophic failure. Environmental contamination can also cause Micro- V belts to slip. Polyflex JB belt performance is generally unaffected in ambient temperature environments between -65°F and +185°F (-54°C and +85°C). Micro-V belts are capable of operating between -30°F and +180°F (-34°C and +82°C). Contact Gates Application Engineering if the drive must operate in temperatures that exceed these limits. II. Calculating Speed Ratio Nominal pitch is satisfactory for use in speed calculations on most drives. Most machines do not require exact driveN speeds to operate efficiently, and speeds vary because the speed of common driveR machines usually varies by several percent. For example, the speed of an ordinary induction type electric motor varies with load and line voltage. Speed ratio may be determined by the following formula: DriveN Pitch Diameter Speed Ratio = DriveR Pitch Diameter Find the pitch diameters of the sheaves by adding the appropriate factor listed in Table 32 on Page 71 to the sheave outside diameters. The World’s Most Trusted Name in Belts, Hose & Hydraulics. 77
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PowerGripÃ‚Â® GTÃ‚Â® Belt Drives

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