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In search of the ‘green belt’

28 November 2011

As industry strives to find new ways to conserve energy and maximise system effectiveness, a growing number of plant and factory operators are focusing their attention on poorly specified belt drives. We take a closer look at this often neglected, but nonetheless significant aspect of industrial power transmission

Motors with belt drives are found in approximately one third of all applications in the industrial and commercial sectors so, considering the volume of energy used in running these motors, it would be reasonable to assume that their performance is closely managed. Surprisingly, this does not seem to be the case and belt drives are responsible for a large amount of lost energy.

Fortunately, attitudes are changing; rising energy costs and the growing need to make cost savings have sharpened our wits regarding energy efficiency and the fact that there are now financial penalties for companies who do not take steps to manage their carbon emissions has honed them even further.

At last, thanks to some striking facts and figures regarding energy losses, word is spreading that more attention needs to be paid to the specification, installation and maintenance of belt drives.

The most common factory equipment – fans, pumps, blowers, conveyors – is usually belt-driven and, naturally, there are efficiency features already in place. Technically then, belt drives can be energy efficient, with the additional benefits of being relatively simple to maintain and low in cost. Unfortunately, such benefits can also bring problems.

For example, the fact that belt drives are such a familiar feature in the workplace means that they can fall foul of sloppy practices. Poor system design, incorrect installation and insufficient or inefficient maintenance combine to create conditions where motor driven systems operate at a level far below their optimum efficiency.

Installation is key to belt drive performance and the factors that most reduce the effectiveness of belt drives are creep, hysteresis, friction energy loss, pulley misalignment, pulley groove wear and incorrect tensioning. Let’s take each of these in turn:

Creep describes the movement of the surface of a belt over a pulley, caused by a change in length between the tight and slack sides of the belt, which occurs as torque tension in the belt changes when the drive is set in motion.

Hysteresis and frictional energy losses occur when energy has been converted into heat as a result of the continual flexing of the belt during operation (particularly at the point where the belt enters or exits a pulley groove). As the belt bends and straightens, hysteresis energy is converted into heat because of the finite damping energy ratio of the materials from which it is made.

Pulley misalignment needs little explanation but it does result in energy loss. Pulley groove wear, meanwhile, requires a little more clarification. When friction belt drives are incorrectly tensioned, belts can slip, resulting in heat generation due to friction. This heat will actually cause the belt to go through a secondary curing process, which makes the parent rubber material extremely hard. The result is that the belt wears the vee-form groove out of shape, and a replacement belt will sit in a groove that is now ‘U’ shaped. Consequently, the belt only has point contact with the groove, meaning it does not transmit its rated power at the quoted efficiency.

An optimised belt drive system can provide long lasting, efficient and cost-effective power transmission. For example, a ‘V’ or wedge belt offers an efficiency of around 93% at installation, while Cogged Raw Edge (CRE) belts offer 94%. A synchronous belt drive such as Fenner’s Torque Drive Plus 3 (TDP3) is even more efficient, delivering something in the region of 96%, and maintaining it over a wide load range.

For particularly high-power requirements, the high tooth shear resistance and exceptional tensile strength of the Synchrochain synchronous drive belt delivers reliable power transmission at high speeds and under high dynamic load at up to 40m/sec with a peak efficiency of 96%. The robustness of these belts also makes them ideal for the tough conditions found in quarrying and mining, and on construction sites, where high torque needs to be transmitted in harsh operating environments.

While the most powerful belt drives such as Synchrochain need no lubrication and are low maintenance, most belt drives can benefit from some simple steps that can save a lot of energy waste and unnecessary wear. For example, under-tensioning leads to slippage and over-heating of the belt, which can not only reduce the effectiveness of the belt drive but damage both the belt and the pulley and bring about their premature replacement.

The use of a simple-to-use belt tension gauge can side step that problem, whilst a pulley groove gauge and drive alignment laser can ensure perfect pulley alignment, as well as detecting any troublesome groove wear.

If your current belt-drive components are beyond maintenance and you need to correctly specify and install new equipment, you may need an expert assessment from a specialist. A typical survey will include the key drives on your site, an analysis of their effectiveness, calculation of the increase in efficiency achievable through the use of new belts and an estimation of the possible cost savings.

Installing an improved or better specified type of belt drive correctly during periods of scheduled maintenance, can result in a typical drive lifespan of 25,000 hours for friction based systems and 20,000 hours for synchronous. Indeed, correctly specified and installed belts can deliver efficiency gains equivalent to upgrading from an IE1 to an IE2 electric motor, while substantial savings can also be made with the use of a variable speed drive.

It is important to remember that energy can only truly be saved by switching plant off or by slowing it down and, as we have attempted to show in this article, there are inherent losses with belt drives, whether friction or synchronous.

However well specified, installed and maintained, belts will always consume energy and real savings can only be achieved by using a variable speed drive in conjunction with a belt drive. Variable speed drives will not only save energy but also reduce shock loading on belts with features such as ‘soft start’, minimising both maintenance and operating costs.

With thanks to ERIKS’ Drives and Power Transmissions Core Competence Centre for the information contained in this article

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