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Reliable torque limitation

26 February 2018

When torques have to be transmitted accurately and then limited reliably in the food sector – for example in filling or packaging systems – the compact ROBA-contitorque continuous slip clutch by mayr power transmission offers a solution.

Every minute, an average 200 finished and individually wrapped chocolate bars drop out of the machine. After this, 24 of these bars are packed into each cardboard box and prepared for shipping. In its packaging systems, the manufacturer of chocolate bars and biscuit specialities uses accumulating conveyors to transport the products through the machines. In case of malfunctions, for example if the work flow starts to stagnate during one of the packaging steps, the transport plates of the accumulating conveyor must remain in one position for a prolonged period of time, holding the biscuits or bars, in order to prevent collision and consequential damage to the products. In order to bring the transport plates to a standstill, continuous slip clutches are used. It was important to the operator that the clutches are designed for a duty cycle of 100 %, feature high-precision torque adjustment and are wear-free and low-maintenance. This is why the manufacturer chose the reliable ROBA-contitorque hysteresis clutch by mayr power transmission.

Contactless torque transmission

This magnetic clutch is located in the drive of the accumulating conveyor and transmits, in malfunction-free operation, the set torque synchronously from the input side to the output side. The torque is developed through magnetic forces which are generated by permanent magnets and which magnetise hysteresis material. The torque transmission takes place contactlessly. In case of malfunctions during which the set limit torque is exceeded, the clutch slips through. This means that the input and output sides rotate at relative speed, the so-called slip speed, towards each other. Here the hysteresis material is constantly magnetically reversed; the torque transmission takes place asynchronously. The torque on the ROBA contitorque thus remains at a constant value independent of the slip speed. Only after the cause of the malfunction has been removed does the relative speed of the clutch return to zero and the torque is once again synchronously transmitted between the input and output sides; after which the accumulating conveyor bearing the products starts to move again.

Wear-free and precise closing

On the ROBA contitorque clutches, the hysteresis technology ensures contactless and therefore wear-free torque transmission: No contamination takes place due to abrasion – which makes it ideal for applications in the food industry. The clutches work with high levels of precision and are characterised by their high torque repeat accuracy of +/- 2%. Thanks to their constant and impact-free torque, they are for example also ideal for the application of screw closures made of plastic with pre-moulded threads onto plastic bottles. In addition to hysteresis clutches, permanent magnet clutches are also used for this purpose in filling systems. However, in slip operation these generate a pulsing torque, which leads to rattling on the lid and which, in part, “hammers down” the screw connections onto the PET bottles, making it difficult to open the bottles again later.

Furthermore, the ROBA contitorque also provides decisive advantages for filling hot liquids in comparison to permanent magnet technology. For example, a soup concentrate manufacturer for industrial kitchens repeatedly had problems with the closing characteristics of their permanent magnetic clutch: The torque impacts from the permanent clutch led to deformation of the bottle neck, and therefore to a high number of rejects, because the plastic in the threaded neck of the plastic container lost strength during the filling of the hot soup. Now, a ROBA contitorque closing clutch size 4 with an adjustable torque range from 0.5 to 6 Nm has provided a remedy for the problem. With the high precision and uniform closing characteristics of the hysteresis clutch, damage to the containers resulting from torque impacts is prevented, meaning that the reject rate has been significantly reduced. ROBA contitorque can easily be integrated into existing systems through the use of simple adapters. The upper section of the existing closing head and the lid adapter have been maintained. 

On the ROBA contitorque hysteresis clutches, the limit torque can be easily and steplessly adjusted and can be read off directly on a scale. The clutch is available in a rustproof stainless steel version, which, due to the special design, provides reliable protection against contamination. Cleaning agents and other liquids cannot penetrate into the clutch or drain off immediately. Deep groove ball bearings made of stainless steel with foodstuff-compatible lubricant, encapsulated permanent magnets and coated hysteresis material provide further protection against corrosion – the optimum prerequisites for use in the foodstuffs industry.