Belt drives are often considered the fuse of the power transmission drive path, which can be an advantage for belt drives. In that line of thinking, not much thought is given to the impact on overall equipment reliability with respect to the belt drives connecting driver and machine.

Belt drive design, belt installation and belt maintenance affect more than just the belts, but the driver and machine as well. Machine components, such as shafts, bearings and seals, are some of the elements that will suffer from poor belt reliability.

Chronic belt failures and short life can increase valuable maintenance manpower, which is already under constant pressure in today’s environment. Unhealthy belt drives also typically increase equipment vibration, which can negatively impact a component’s life, as well as processes. Poorly operating belt drives are also energy wasters, as low power transmission efficiencies come along with poor reliability. Lastly, unreliable belt drives carry with them a potential safety risk. Constantly working on belt drives with multiple pinch points puts maintenance technicians in the line of fire for potential accidents.

A belt drive must be designed correctly to have a chance at a full life in a 24/7 industrial application. Many belt drive reliability issues have been traced back to an initial design problem. There are many belt drive power transmission suppliers that offer online software to aid in belt drive selection, although some are just designer’s choice, such as what type of belt to use. However, there are several design factors that must be accurate because even good maintenance won’t correct them. For example, having enough belts to transmit the connected horsepower (hp). This article concentrates on belt drive maintenance, so it’s assumed the belt drive design has no deficiencies.

Storage and Handling

Since equipment reliability begins with material storage and handling, drive belts should be stored and handled in a way where they are ready to deliver good power transmission. Belts should not be stored in tight radii smaller than minimum recommended sheave diameters. Many manufacturers have a number of coils and loops for a certain belt cross section and belt length. Also, do not coil in reverse bend directions.

Optimum belt storage conditions are less than 85°F and 70 percent relative humidity. Belts stored around machine areas where excess heat exists will have a much shorter life. Belts also should not be stored in direct sunlight or near chemicals that can break down the rubber compounds. Most belt labels should have a manufacture date. While they usually will still run after being in storage for six years, full life may be reduced after long storage times.

Best Practices

When installing belts on used drive sheaves, the sheave wear should be checked. The small sheave (typically the motor sheave) especially should be checked as this is the one most likely to have the most wear. Sheave templates, such as the one shown in Figure 1, can help determine if wear is over 1/32” and needs to be replaced.

Guarding may not always play a hand in reliability, but it can have negative effects in certain conditions. Besides protecting employees from injury, guarding is a noise suppressor and keeps contaminants out of the belt drive. In general, guarding should be vented to allow cooling because elevated belt temperature will cause decreased performance and belt life.

While belt and sheave alignment is not as critical as a direct drive, it will have negative effects on belt drive reliability. The maximum allowed misalignment for V-belts is approximately .5 degrees (~.100”/ft of center distance). Synchronous belts require better alignment, with a maximum allowed .25 degrees (.062”/ft of center distance). Synchronous belts usually squeal with significant misalignment as the belt rubs the side flange on a sprocket. There are several acceptable methods for aligning belt drives, such as a straight edge, string, or a laser tool.

All three methods have a potential for error. The straight edge and string need to have two points of contact on one sheave. Some lasers with sights on the outside of the sheave may need to account for sheave flange thickness differences, while lasers with sights on sheave grooves may have to account for sheave wear.

Another best practice is to install sheaves with the least amount of overhang as possible. This reduces the resultant bearing reaction loads, hence improving bearing life.

When installing belts, be sure to loosen the adjustable base so the belts are not rolled on. Not only is rolling belts on a safety issue, it can cause damage to the belts and result in high shaft and bearing loads. If individual belts are used, do not mix new and old belts. This ensures belt length and stretch differences do not affect power transmission. It is also not advisable to mix belts from different manufacturers because length tolerances can vary.

Setting Belt Tension

Since V-belts transmit power by friction, the most important step in belt drive reliability is setting belt tension. The best belt tension is just enough to drive the load and not slip. Knowing the type of operating conditions (e.g., shock loading or steady loading) is a key factor in determining the correct belt tension. V-belts work off friction between the belt sidewall to the sheave, so belt tension is what develops that friction force to drive the load.

Several methods can be used to obtain a belt’s tension targets. Belt manufacturers have tables that give target belt tension information (e.g., force, frequency) depending on the belt’s maximum cross section size, speed and minimum sheave diameter. Since the tension given is based on the belt’s cross section, most applications may be over tensioned for the drive power to be transmitted. A more precision method for getting tension information is to calculate the tension based on the actual drive’s horsepower to be transmitted. This can be done manually or by using a sizing program.

This tension targets the actual drive power to be transmitted instead of horsepower capacity of the belts on the drive. The impact to bearing reaction loads and bearing life can be significant. Double the bearing load and the bearing life is reduced by a factor of 10. For example, in a standard table shown in Table 1, a 250 hp belt drive on a fan using 8/8V3000 belts calls for up to 41 lbs per strand of belt tension. At this tension, the belt pull force is nearly 6,000 lbs. From engineered belt tension calculations for a 250 hp drive, the target belt tension is 18 to 22 lbs. The result is a belt pull force of 2,000 lbs. In this example, bearing life is reduced by 97 percent by using the maximum value in the belt tension table instead of the specific engineered belt drive tension from the connected drive’s horsepower.

There are several methods to set belt tension. The more common ones are the belt deflection force method, sonic tension measurement and the powerband multiplier method. The belt deflection force method, shown in Figure 2, uses a simple spring tension tool that shows the deflection distance and deflection force. The single-barrel type tools are typically good for belts up to 33 lbs deflection force. For powerbands, a double-barrel tester may be used, which is good for up to 66 lbs deflection force.

Figure 2: Belt deflection force tension method (Source: Gates Preventive Maintenance Manual, pg. 7)

The other thing to remember about powerbands is that the tension is given per strand, so the target belt tension must be multiplied by the number of strands in the powerband for the powerband’s target belt tension. For wide, large, cross section powerbands, the belt deflection force method is generally not a feasible method due to very high belt deflection forces. When this happens, the powerband multiplier method may be used. The untensioned belt length (circumference) is measured with a Pi tape and multiplied by a multiplier to obtain a target circumference, hence stretch and belt tension.

The newest method for setting belt tension is the sonic tension method. The belts are hit or strung to vibration naturally. A sonic measurement tool is used to measure the frequency, typically in hertz. The belt tension is adjusted and the process is repeated until the actual belt’s natural frequency matches the target belt frequency. The only limitations for this method are very low frequencies, typically less than 10 Hz, and noisy areas, which may pose a challenge in measuring belt frequencies.

One last, somewhat obvious reminder, but not always followed, is to never tighten belts on the run. The first concern is personal safety, as hold down bolts must be loosened. Equipment that is running is under load, so any temporary looseness will likely cause belts to slip. The other big issue is there is no way to measure belt tension on the run, so it becomes reliability roulette.

Conclusion

While performing maintenance on a belt drive may appear routine, to achieve best reliability requires precision practices and attention to detail. The journey to improved belt drive reliability begins with knowing and executing the basics. 

​Randy Riddell

Randy Riddell, CMRP, PSAP, CLS, is the Reliability Manager for Essity at the Barton Mill in Alabama. Randy has over 30 years of industrial experience with a career focused on equipment reliability. www.essity.com

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