The first rule of thumb when choosing rollers that will last is to consider your environment, the application in which they will be used, the belt speed and the size of your material load. These factors, as well as the size of your rollers and the number of rollers you need, should be a good starting point when choosing your rollers.
How long does a roller last?
Most rollers are selected on the basis of theoretical bearing life calculations, based mainly on load and speed.
Idler life is determined by many factors, such as seals, bearings, shell thickness, belt speed, lump size/material density, maintenance, environment, temperature and the idler design that handles the maximum calculated idler load. And while all the variables listed above can affect the life of the roller, the only variable for which laboratory tests have provided standard value is the bearing rating.
In an ideal situation rollers would last as long as the bearings last. Eventually, the bearings would just wear out due to metal fatigue. However, each application presents environments and characteristics that simply can’t be taken into account during laboratory testing. This is referred to as “design life vs actual life”, or the time the roller is supposed to last based on testing in laboratory conditions versus the time the roller will actually last in day to day working conditions.
Modes of roller failure
There are many different causes of roller failure, but the most common are abrasion, corrosion, seal failure and end design failure.
Regular surface wear, or abrasion, from belt contact can limit the roller life. The increasing use of polymers (such as engineered nylon and high density polyethylene, or HDPE), instead of steel, can be useful in combating this type of roll failure. In this case, corrosion is directly related to abrasion, in the sense that the more corrosion you have, the more the rate of abrasion accelerates as the surface continues to weaken.
Seal design and effectiveness are key to the success of the bearing and, in turn, to the success of the roller. A seal design that is resistant to moisture and other fines protects the roller from contamination and internal corrosion. It is estimated that 43 per cent of bearing failures is due to moisture and other contaminants.
One way to prevent this is by opting for a roller with a quality “centrifugal flinger seal”. This feature offers up to nine times more centrifugal force, to help “spin out” fines, water and other contaminants. The design of the flinger seal discourages the entry of contaminants and protects the bearing inside the roller.
End discs may be made of durable materials, but the method used to join the end disc to the core can affect the life of the roller. Every roller has a core and roller end piece(s), and if they come apart the result can be disastrous for the belt. In the case of steel, the end disc separating from the core can act like a sharp knife, cutting the belt. With many composite rollers the end disc can work loose, causing premature roll failure.
Is roller weight an issue?
When choosing a roller for your belt conveyor system, compatibility, availability and assurance of superior performance should certainly be at the top of the list, but should roller weight also factor in the selection process? When making decisions about replacement items for your system, serviceability and ergonomics need to be considered.
On the topic of roller weight, some might think a heavier roller has a thicker wall and thus will last longer. As long as you don’t have to sacrifice performance, less is definitely more. Consider that:
- Half of work-related injuries or illnesses in Australia were sustained mostly by lifting, pushing or pulling objects (27 per cent) or by hitting or being hit or cut by an object (25 per cent), according to the Australian Bureau of Statistics.
- Muscular stress (due to manual handling or repetitive movement) accounted for 32 per cent of claims in Australia, according to Safe Work Australia’s workers’ compensation claims database.
- More than 60 per cent of those who experienced a work-related injury in Australia received some sort of financial assistance, and of those who received financial assistance more than half (59 per cent) received workers’ compensation. More than 55 per cent of those who experienced a work-related injury had some time off. If those three statistics don’t convince you roller weight should be a factor, then consider that it often takes only a single reportable injury at your workplace to raise your workers’ compensation insurance premiums, lower productivity, increase absenteeism and possibly increase your legal costs fighting an injury suit.
Weight is a safety issue when it comes to your workers. Workers injure themselves in several ways, including lifting cumbersome, heavy rollers.
Traditionally, rollers have been constructed from steel; the heavier the application, the heavier the roller. For example, a single 1800mm roller can weigh up to 45kg, making the roller difficult to carry and install. The longer the roll, the more concern arises about weight and the risk of back injury. On incline conveyors or conveyors not accessible by maintenance vehicles especially, the weight of a roller can become a major drain on productivity and a source of injury risk.
In addition, many workplaces set maximum lifting requirements that limit the weight one person can carry to 25kg or less. A roller made of lighter materials would require only one individual to lift, carry and place, especially while a steel roller of the same size would require two workers. Opting for a polymer-based roller, weighing up to 40 per cent less than steel, is a wise choice. With a lighter option, you not only have less chance of a workplace back injury, but increased productivity.
Product performance can also be a factor when the rollers are heavier. Routine roller maintenance duties may be neglected because the rollers are large and cumbersome to replace. This situation can affect both safety and productivity, as some rolls may be left on the line for too long, resulting in catastrophic belt failure.
Selection vs consumption
It costs money to operate a business – whether you’re buying materials, paying employees or simply moving material. Any time you can save money, it is a victory. Depending on your workload, your conveyor systems may be operating 24 hours per day, seven days per week, so making sure your rollers are running as efficiently as possible can only add dollars to your bottom line. While the power needed to rotate a single idler roller is small, it accumulates along the length of a conveyor.
Two factors contribute to the energy consumption required to operate a large number of rollers on a system.
The first is breakaway energy – the power required to get the conveyor and all the rollers supporting the belt started. Breakaway energy is a function of the rolling resistance generated by seals and bearings and overcoming the moment of inertia, which is a function of the weight and geometry of the rollers. Conveyor start-up can utilise as much as 13 per cent of available power. Electric motors are typically designed to operate at peak power, three times as much as their operating rating, specifically to provide the needed energy at start-up.
The power required to get a belt conveyor up and running is largely dependent on the effort (or energy) required to get the system running. The design of the rollers plays a big part in this calculation, because of the large number of rollers in the system and the rolling resistance they have in a stationary state.
Two things contribute to rolling resistance. First is the seal design. A roller using a very tight seal to prevent ingress of water and fine materials will have much greater rolling resistance than a roller using a non-contact labyrinth or centrifugal seal. Labyrinth seals rely on a complex path and often a grease chamber to make dirt and water ingress more difficult. Centrifugal seals use the churning effect created by the rotating motion of the roller to constantly move materials away from the bearing, to prevent internal corrosion. Using a seal with lower rolling resistance can significantly reduce the energy required to get the roller moving and to keep it moving.
Second is the moment of inertia. Moment of inertia is used to determine how difficult it is to change the motion of a rotating object around an axis. In this case, it is used to determine how difficult is it to get a roller to turn around the shaft. With rollers, this is almost entirely a function of the weight of the shell since the shaft remains stationary.
Rollers made of lightweight materials will have a significantly reduced moment of inertia, which can translate to large savings in the energy required to get the conveyor system running. As long as the roller does not lack performance in other areas, going with a lightweight material can be an effective way to significantly reduce energy consumption.
Rollers with composite sleeves over a steel core will often weigh nearly as much as steel rollers and will not provide the reduced moment of inertia that can be achieved with a roller made with a full composite shell.
In addition to requiring enormous amounts of energy to begin rotating, steel rollers also generate substantial friction. Once the belt is moving, significant energy is required to keep the rollers running. This is a function of the friction caused by the bearings and seals, in addition to the “running friction” of the rollers on the belt. Steel rollers tend to have much greater running friction than composite rollers, making them more costly to operate in terms of power consumption.
When it comes to energy savings, engineered nylon or HDPE rollers are a smart choice. They can weigh 40 to 50 per cent less than steel rollers and many designs have lower running friction values, which, depending on the application, can decrease power bills significantly. Installing rollers with non-contact seals is also wise if you are looking to save money on power because they require less breakaway energy. However, not all non-contact seals are the same. A centrifugal flinger seal will deter moisture, fines and other contaminants from entering the inside of the shell, while protecting the bearing. Other non-contact seals are packed with grease to create a barrier, which contributes to low breakaway energy and running friction numbers.
Is roller noise an issue?
It is surprising how much of the noise from bulk material handling systems is generated by rollers.
After all, shouldn’t the belt be moving smoothly across the rollers with little noise or friction? In a perfect world, yes, but heavy material loads, friction between the belt and rollers, and splices reverberating over the rollers can affect the sound produced by conveyor systems.
Combine that with the noise rollers generate due to resonance in the hollow cavity, and a system can be loud enough to be problematic.
In many countries, local jurisdictions and municipalities have their own noise regulations, and heavy fines can be levied when noise exceeds allowable levels. Depending where you are located, you may find certain decibel levels are allowed during the day and lowered at night. If operating near a residential area, these limits are even lower.
Work health and safety regulation legislation states the exposure standard for noise in relation to a person cannot exceed 85dB(A) for more than eight hours. This level is tightened by many mine sites, in particular sites in or near residential areas. The penalties in New South Wales are (a) $6000, in the case of an individual, or (b) $30,000, in the case of a body corporate. Violations can cut into both profit and production.
Community relations, safety
Even if local ordinances don’t regulate the noise coming from your operation, it’s important to be a good neighbour to residences and other businesses within the area.
Many operations install sound barriers for this exact reason – to combat noise pollution on crushers, blast zones and impact beds where material is being dropped and transferred. If your operation isn’t located near a residential area or within a jurisdiction that has stated noise ordinances, you’re safe from a governmental point of view, but how safe are your workers? Continuous exposure to elevated noise levels can be dangerous for workers in these environments. A single roller may not be dangerous to workers, but the combination of hundreds of rollers and other machinery can add up to hazardous noise levels.
Why corrosion matters
The rollers on your conveyor system are often an afterthought, viewed as simply a disposable commodity that is easily replaceable.
In fact, rollers are integral system components that can positively or negatively affect your productivity, and that means they should be carefully selected and matched to your application for maximum longevity and performance. If you think about it from a numbers perspective, there are more rollers than any other component on a conveyor.
Other than the belt, they usually represent the highest maintenance cost item in the system. So a failed or seized roller is no small matter. One of the main causes of premature roller failure is corrosion.
To better understand the causes of roller corrosion, one must understand the different types of corrosion.
External corrosion affecting the outside of the roll can be caused by many environmental factors. For example, being near a saltwater coast or applications that feature salt or chemicals can affect the rollers. Corrosion accelerates the process of abrasion, changing the properties of the surface and stripping away layers of roller material.
Internal corrosion is usually caused by moisture and fines making their way inside the roller and either damaging the bearing or causing build-up that seizes the internal components and stops the roller turning.
Since environment, temperature and the type of material being conveyed can’t be changed, many people write off corrosion as an inevitable occurrence. However, that doesn’t have to be the case. Both the material the roller is constructed from and the seal design that protects the bearing can have a huge impact on the life of the roller.
Metal can rust, corrode and scale. Plastic is more resistant to corrosion than metal. Rollers that match the ratings of steel are now available in polymers such as engineered nylon and HDPE. These materials are resistant to corrosion from chemicals, salt and moisture.
Steel rollers can fall victim to several environmental factors. When exposed to moisture, nylon actually gains toughness, while virtually eliminating the risk of premature failure due to surface corrosion.
Seal design is also paramount to the success of the roller. Many rollers don’t reach their design life because of moisture and contamination reaching the bearing. In fact, studies show 43 per cent of bearings fail prematurely for this reason.
The best way to ensure the bearing in a conveyor roller reaches its full life is to prevent moisture and fines from reaching it. One way to accomplish this is by again opting for a roller with a quality centrifugal flinger seal.
This feature offers up to nine times more centrifugal force, to help “spin out” fines, water and other contaminants. The design of a flinger seal, combined with a rock shield, discourages the entry of contaminants and protects the bearing.