Cold rollers for cool calculators

Description

Tests carried out by Chilean conveyor technology specialist Tecnología en Transporte (TTM) have shown that precision rollers from German manufacturer Rulmeca are much better suited for heavy-duty conveyors in the bulk materials handling industry than simple rollers from the local competitor.

At first glance, there appears to be nothing remarkable. All rollers for bulk material conveyor belts are round, made of steel and mostly painted red in the heavy-duty sector. The shaft axis offset is also standardized to a large extent. So, judging from external appearance, one roller looks very much like the other. One may have a superior paint finish, the other may be delivered with better packaging. But for many users, a conveyor belt roller is just a conveyor belt roller. Any metal engineer will be able to handle the technical assemble and the required investment is minor, considering the unit price. As rollers are often priced by weight, the current steel price frequently plays a greater role in price negotiations than the quality, which is the actual key to ensuring that bulk material conveyors operate reliably and with minimum maintenance costs for many years.

The differences in quality are immense, as conveyor technology specialist TTM revealed through extensive tests on a belt system used by a Chilean company that conveys crushed limestone for cement production. Featuring an impressive 11,385 rollers to allow freest possible conveyor belt movement, the test object provided a lot of measurement points. Over a period of 188 days, the temperatures of almost 15% of the used rollers were analyzed, comparing rollers from different manufacturers with identical physical footprint.

Throughout this extensive test period, all Rulmeca’s cold rollers in use measured below 50 °C and on average 30 °C, which means they were in a satisfactory condition at all times. Over the same period, 3.2% of the local competitor’s rollers needed replacing, with 286 reaching alarming temperatures between 80°C and 130°C, and 50 in a dangerous state with temperatures above 130°C. Over an entire year, the projected failure rate would have been 6.06%. Bearing in mind that each of these rollers represents a high to extremely high fire risk, it would have been statistically necessary to change almost two rollers every day (exactly 1.78 per day). Quality rollers such as those from Rulmeca, on the other hand, would have required neither replacement nor maintenance costs.

Since these tests, the use of Rulmeca rollers has become mandatory for the staff maintaining the conveyor system. And with good reason. In view of the complexity of the plant, and the fact that it fulfils business-critical functions which require high availability, running the comparatively high risk of a roller failure and the associated fire hazard was no longer acceptable.

What’s the manufacturing secret?

The high availability, long service life and lower failure rates of precision rollers are achieved through the use of significantly higher quality bearings, precision steel tubes for the shafts and – depending on the required reliability and smoothness – dynamically balanced rollers. Such high-quality rollers for bulk material conveyor systems run much more smoothly and with less wear. They continue to operate with high reliability even under the most extreme environmental conditions also thanks to their hermetically sealed design, which uses carefully selected materials. The sealing system that protects the roller bearings is of particularly great importance in this context. Just how significant this is can easily be seen from the immense dust deposits on the rollers of the Chilean conveyor.

The technical secret behind the imperviousness of the Rulmeca rollers lies in the use of multiple labyrinth seals. In such seals, the path that ingress has to travel before it can seep in is extended, which considerably increases resistance. This extension is usually achieved by interlocking tooth-shaped elements on the rotary shaft with those on the stationary housing, a process also known as ‘crimping’. Simple rubber seals, on the other hand, are not acceptable in dusty environments, as they wear out quickly and lose their sealing effect.

But the bearing behind the seal must also be optimally protected. Finding the ideal lubrication is not easy and requires experience: Too little lubrication causes heat and wear, while too much lubrication leads to resistance, heat and wear, which can damage the seals. However, in case of the competitive solution, it was impossible to provide the optimum level of lubrication during the tests because the competitor's roller design would not have supported more intensive lubrication.

Another common mistake is using metal for the bearing cages. When bearings are faulty, this can get hot and eventually cause sparks that can lead to a fire. To avoid this, Rulmeca uses mostly polyamide cages; they simply disintegrate when damage occurs, offering protection against overheating and fire by default. Ultimately, roller concentricity is also a key parameter. The concentricity error should not exceed 0.2 mm TIR/FIM (Total Indicator Reading /Full Indicator Movement) and should be balanced dynamically, which requires precision-made tubes as well as balanced rollers.

Such high-quality production methods naturally come at a cost, which is reflected in the purchase price. However, what ultimately counts is not the purchase price but the total cost of ownership (CAPEX vs. OPEX). If high availability is business-critical because any downtime costs serious money, the decision for quality is certainly easy. But even if system failures are not so critical to the company, the short MTBF of the simple rollers quickly contributes to the amortization of an investment in quality rollers since complex conveyor technology must provide a long service life. When the higher maintenance costs, as well as energy savings from lower resistances, are added, an investment in high-quality rollers on average pays for itself after just 8 months. Since such generic calculations invite contradiction, Rulmeca helps customers work out the comparative calculations for their specific project. If necessary, Rulmeca is even offering in-house load testing so users and system manufacturers can be absolutely sure that they are making the right investment decision. After all, the quality of a conveyor roller is measurable - and this applies also to smaller installations than the huge one in Chile.

Author:* Jens Hagenau, Product and Export Manager BULK at Rulmeca Germany GmbH