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Tight control helps to lengthen belt life
Paying more attention to the drive belt – whether in design, selection or maintenance – will help to boost system efficiency, reduce downtime and increase belt life, explains Phil Burge of SKF.
Drive belts are just one example of a vital automation component that is too easily ignored. Like the cam belt on your car, which must be replaced at regular intervals, so drive belts of all varieties – whether V-groove or synchronous – need careful attention if they are to last their full lifetime.
The most common reason for belt failure is poor maintenance. Regular maintenance, whether by visual inspection or more sophisticated means, ensures that belts are properly aligned and have the correct tension. It will also pick up any signs of excessive wear, which are a sure sign that a belt system is running incorrectly. Improper alignment can lead to noise and vibration, and excess wear on the belt. It also drags down the energy efficiency of the whole system. Correct belt tension is vital for system efficiency. If it is too high, the belt is over-stretched and will fail prematurely. If set too low, it can slip – which leads to failure through excess temperature.
Both tensioning and alignment problems can easily be solved using instrumentation. Laser alignment devices can identify problems such as angular misalignment, while belt tension can be checked and adjusted using an electronic tension meter. Rather than being a cause of problems, belt wear is a symptom of incorrect alignment and tension, and can be seen through fraying or other signs of physical degradation.
The solution is to inspect at the correct interval – sometimes as frequently as every fortnight – depending on whether it is a critical drive or a normal drive. These problems show themselves in various ways – such as vibration, noise or physical damage – but the ultimate problem is that the belt fails. No belt is meant to last forever, of course, but attention to maintenance will ensure that it runs efficiently and lasts as long as possible.
Poor design is another factor that cuts belt life short. This is not about the design of the belt itself – although a belt made from more robust material will usually be preferred. Instead, it concerns the design of the automation system. Most commonly, the wrong combination of belt and pulley has been specified, or the belt does not match the speed or load of the equipment.
Redesigning the belt drive system
Many factors within a belt drive system can be redesigned in order to boost efficiency and extend belt life. These include: vibration damping; using a wider belt, or increasing the pulley diameter; and replacing worn pulleys promptly. It is also true to say that some belts are better than others, or more suited to particular applications. The materials used in the construction of belts can overcome common causes of failure and inefficiency. For example, problems of elongation and incorrect tensioning can be addressed by belts with very strong polyester-coated internal tension members. Also, pulley groove wear has been reduced by abrasion-resistant cover fabric, which protects belts from frictional heat if they slip out of position.
By identifying design problems and misalignment, SKF helped multinational dairy company Fonterra Te Rapa save more than £400,000 over the course of a year. Fonterra had found that drive belts on its milk powder line were lasting no more than five weeks. There had also been two costly shutdowns due to belt failure, each of which cost more than £50,000 per hour in lost production.
SKF inspected the failed belts and saw that high running temperatures – of around 60°C – had led the rubber to cure and harden, causing the belts to crack and break. The belt tension was also far higher than it should have been, and the motor was incorrectly aligned – which had caused the high temperature.
The answer was a system redesign, for which SKF trialled two solutions: the first used SKF Xtra Power belts, while the second used SKF 8M timing belt drives. These were run against a ‘control’ using the original belts. After 28 days, the two SKF systems were working well – with tension within specification, and internal running temperatures about 10°C lower – while the control set had hardened and cracked as before. As well as banishing downtime, the new system has also led to extra energy savings due to the belt drives running more efficiently.
Incorrect installation is also a major factor in belt failure. The first thing to get right is to select the correct belt for the application. This is generally a choice between a V-groove and a synchronous (or timing) belt. Before installing a V-belt, pulleys should be properly aligned, otherwise problems such as edge wear will occur. At the same time, all pulley grooves should be the same size. Mixing different brands or belt types on the same drive is not recommended, and nor is the combination of old and new belts – as this may result in uneven load distribution.
For a synchronous belt, pulleys and alignment should be inspected before installing the new belt over the pulleys. The span length needs to be measured, then the centre distance (or idler) can be adjusted to set tension at a specific value, using a variety of methods. Idlers are grooved or flat pulleys that do not transmit any power in a drive system, yet can help in certain designs: those with a long span, for example, in which the idler has a damping effect to solve excessive belt vibration; or in drives with a fixed centre distance, in which case the idler provides the required tension.
Several other factors can help to lengthen belt life. One is remarkably simple: storage. Simply ensuring that belts are kept out of direct sunlight and at room temperature (and certainly no higher than 30°C) will help to keep them in optimum condition. Belts are best stored flat, on a shelf, but if space does not allow this they can be hung on large pegs – taking care that they are not over-coiled.
It may sound small, but keeping belts on the shelf – and applying similar rigour to other factors – can help to ensure long life and efficient production.
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