Tips for Selecting & Applying Drive Belts: Preventing Vibration, Slippage & Heat Build-up, Exercises of Design

Download Tips for Selecting & Applying Drive Belts: Preventing Vibration, Slippage & Heat Build-up and more Exercises Design in PDF only on Docsity! GatesFacts™ Technical Information Library Gates Compass™ Power Transmission CD-ROM version 1.2 The Gates Rubber Company Denver, Colorado USA ___________________________________________________________________________________________________________________________________ Page 1 TIPS ON SELECTING AND APPLYING DRIVE BELTS Burney Belser Power Transmission Design March, 1993 Want to simplify and improve your belt-drive applications? Having the answers to commonly asked questions will make it easier to select the best drive for the application and get the best drive performance. Though belt-drive applications may seem routine, some areas are often misunderstood, as attested to by the many questions received by belt manufacturers. To put you on the right track, here are the answers to the belt-drive questions most commonly asked. Q. What is the maximum speed that a drive belt can safely handle? A. For most drives, pulley rim speed is the limiting factor, rather than the belt. This limiting speed depends on the pulley material and design, Table 1 and Figure 1. Stock pulleys made of iron are statically balanced for rim speeds up to 6,500 fpm. A pulley running at more than 6,500 fpm may cause vibration, noise, poor bearing life, and high fatigue stresses. Therefore, pulleys that exceed 6,500 fpm should be dynamically balanced as described in Mechanical Power Transmission Association (MPTA) bulletin No. SPB-86. In some cases, depending on pulley size or application requirements, dynamic balancing may be required for pulleys operating at less than 6,500 fpm. For speeds outside of the normal ranges, consult the manufacturer. GatesFacts™ Technical Information Library Gates Compass™ Power Transmission CD-ROM version 1.2 The Gates Rubber Company Denver, Colorado USA ___________________________________________________________________________________________________________________________________ Page 2 Q. What causes belt-drive vibration and how can it be corrected? A. Drive belts experience both vertical and lateral vibrations when their natural frequencies coincide with resonant frequency of connected equipment. Belt tension can affect the amplitude of this vibration. Therefore, to correct the problem, first check for proper tension. A common method to control vertical vibration uses a restraining device (metal rod or idler pulley) placed perpendicular to the belt span and close to or lightly touching the belt. This device should be positioned roughly 1/3 of the span distance from the larger pulley. If this does not work, consider changing other drive parameters to reduce the amplitude of vibration or alter its frequency. Such parameters include span length, belt type, misalignment, inertia of driving or driven machinery, pulley diameter and weight (inertia), speed, and the number of belts. In some cases (where original unit was oversized), it may be possible to downsize the drive by reducing the number of belts or belt width, and increasing the static tension to alter the belt’s natural frequency so it doesn’t coincide with the excitation frequency of the machinery. When it can be done safely, it is preferable to reduce the static tension to keep the operating belt tension below the belt’s natural frequency range. To reduce lateral vibration, increase flexural rigidity in the lateral direction. This can be accomplished by using joined belts, Figure 2, which consist of two or more belts held together with a high-strength band that prevents the belts from bending sideways and keeps them running straight into the pulley grooves even under severe pulsating or shock loads. A wider synchronous belt can increase lateral rigidity, but should be tensioned carefully. Undertensioning may cause a synchronous belt to jump teeth (ratchet). GatesFacts™ Technical Information Library Gates Compass™ Power Transmission CD-ROM version 1.2 The Gates Rubber Company Denver, Colorado USA ___________________________________________________________________________________________________________________________________ Page 5 Q. How is static conductivity of a belt determined and why is it important? A. RMA bulletin IP3-3 explains how to test for static conductivity by passing an electric current of specified voltage through a section of belt while measuring the belt’s resistance to conduct the current. A resistance of 6 MΩ or less prevents measurable static voltage buildup, thus preventing a potentially hazardous static discharge. V-belts are generally manufactured in accordance with this bulletin, but, to be certain, obtain the proper static conductivity from the manufacturer. The RMA bulletin applies to new, clean belts. However, older belts can collect debris or become worn and damaged, which may give a belt infinite resistance. This condition enables a static charge to build up. Thus, in a hazardous environment, additional protection is recommended, such as grounding the entire system to ensure against accidental static spark discharges. Also, a static conductive brush or similar device will bleed off static buildup on the belt. Q. What is the normal shelf life of a drive belt? A. When belts are properly stored, according to RMA bulletin IP3-4, no significant change in performance should be detected for up to 8 years. Proper storage, as described in this bulletin, means the belt should be protected from moisture, temperature extremes, direct sunlight, and high ozone levels. The belt should be stored in it original package, avoiding sharp bends or crimping that could damage the belt. Also, belts should not be bent or hung on anything with a diameter less than the smallest recommended pulley diameter for that cross section. Machines using belt drives sometimes stand idle for prolonged periods (6 mo. or longer). The tension on such belts should be relaxed during idle periods and the equipment should be stored in an environment that is consistent with belt-storage guidelines. If this is not possible, such as with equipment stored outdoors in a cold environment, the belts should be removed and stored separately. GatesFacts™ Technical Information Library Gates Compass™ Power Transmission CD-ROM version 1.2 The Gates Rubber Company Denver, Colorado USA ___________________________________________________________________________________________________________________________________ Page 6 Q. What is the acceptable temperature range for most belt drives? A. There is no well defined temperature limit that ensures satisfactory performance. However, a properly applied belt generally yields acceptable service within an ambient temperature range of -30 to 140 F as specified in RMA bulletin IP 3-1. When a rubber belt operates with excessively high internal temperatures, the adhesion between belt components breaks down, causing premature failure. Internal belt temperature is affected by ambient temperature, time of exposure, and ventilation, as well as drive design. Tests indicate that for every 36 F increase in ambient temperature, V-belt service life is cut in half. And, for every 2 F increase in ambient temperature, there is a 1 F increase in the belt’s internal running temperature. Thus, each 18 F increase in internal belt temperature cuts the belt life in half. For drives operating in ambient temperatures above 140 F, consult the manufacturer for recommendations. Specially compounded belts can be used in high or low ambient temperatures. But, if the manufacturer uses a compound to raise the upper temperature limit, the lower limit rises as well, and vice versa. Belts that contain polyurethane compounds perform satisfactorily in ambient temperatures between -65 and 185 F. Q. What are the primary causes of heat buildup in belt drives? A. Both internal and external heat is generated when a belt drive operates. Internal heat (within the belt) is caused by belt flexing as it moves around the pulleys. External heat is created by slippage between for example, a V-belt and sheave. Parameters that affect belt operating temperature include pulley diameter, load, belt flexing, belt type, maintenance, ambient temperature, and air cooling. To counteract adverse effects on belt temperature, apply these guidelines: • Use the largest pulley diameter possible. This reduces internal heat buildup due to small-radius bending. Plus, it reduces belt tension and bearing loads, increases air flow, and increases belt contact area, all of which minimize belt slip and heat buildup. GatesFacts™ Technical Information Library Gates Compass™ Power Transmission CD-ROM version 1.2 The Gates Rubber Company Denver, Colorado USA ___________________________________________________________________________________________________________________________________ Page 7 • Follow proper installation procedures. V-belts require a run-in period and re-tensioning to ensure proper seating and prevent slippage, which can generate external heat. • Install a belt guard that allows good ventilation. If additional measures are necessary, use forced ventilation or finned pulleys to dissipate heat and reduce heat buildup. • Select a flexible belt type to reduce heat buildup. Synchronous, V-ribbed, and molded notched V-belts may provide good solutions to temperature buildup, particularly for small-diameter pulleys. • If heat is a suspected problem, use pulleys made of steel and similar materials that conduct heat away from the belt. Avoid plastic materials, which do not conduct heat. Q. What determines if a set of belts match? A. Horsepower requirements in many applications call for a multibelt drive. RMA standards IP-20 and IP-22 specify permissible length variations, Table 2, for a set of classical or narrow industrial V-belts. For example, all belts up to 63 in. long in a set must not vary more than 0.15 in. from the longest to the shortest belt. If they exceed this limit, the load will not be evenly distributed and belts will wear out faster. Industry leaders have improved their manufacturing processes in the past 10 to 12 years so that classical, narrow, and molded notched belts now have tighter than RMA tolerances. Thus, for example, any Gates V80 belt of a given length designation will run with any other V80 belt of the same cross-section and construction. Some belt types are still grouped by the old match-number system, in which numbers are printed on individual belts; each number represents a measured belt-length range. These numbers are grouped in sequential order for matching according to length. Table 2 - RMA standard for matching V-belt lengths Belt length, in. RMA tolerance, in. Tolerance as a ratio of maximum length Up to 63 0.15 0.0024 63 - 150 0.30 0.0020 151 - 250 0.45 0.0018 251 - 375 0.60 0.0016 376 - 500 0.75 0.0015 501 - 660 0.90 0.0014 Q. How can belt-drive efficiency be improved?