Reliable cutterhead protection for tunnel boring machines

Tunnel boring machines work their way through the mountain like moving factories underground. And every single metre counts. An overload case for example – caused through irregularities in the rock – must not stop the gigantic drills.

“If our EAS-HT torque limiters disengage on the construction site, then at a defined and traceable torque and only in order to protect the motor, gears and drive line of the cutterhead from damage through overload,” explains Ralf Epple, product manager at Mayr Power Transmission in Mauerstetten.

In this way, long downtimes can be avoided and the machines can be sustainably operated. "The safety concept must not be susceptible to manipulation", emphasizes Epple. "This means that the torque on our clutches is set manufacturer-side and is immediately accessible again after disengagement. The safety aspect is not dependent on recommissioning. The torque limiting clutches work reliably."

“The disengaging EAS-HT clutches are element clutches which represent the ideal solution for extremely high torques and speeds or large mass moments of inertia,” explains Epple. These clutches are based on individual overload elements which are integrated into the flanges. The achievable torque results from the pre-tension force of the elements multiplied by the number of elements and the radius at which the individual elements are arranged.

Due to the modular concept, not only clutches tailored to the respective application case can be built, but also far higher torques can be managed than with a central disengagement mechanism. Mayr offers the torque limiters from the EAS-HT construction series in a standard version with a torque of up to 440,000 Nm. However, the limits of what is possible have not yet been reached: Thanks to the modular construction, the possibilities are endless.

On the EAS-HT torque limiters, the individual overload elements transmit the torque through positive locking during functional operation. In case of overload, they separate the input and output faster than any control unit in less than 5 ms almost residual torque-free with high switch-off and repetitive accuracies. Only the friction of the high-quality pressure flange bearing takes effect.

The kinetic energy of the rotating mass stored in the system can slow down freely. The element clutches can be re-engaged simply and quickly, only using a hammer. Additional tools are not required. Overload clutches can cope with long slow-down times for the drive following disengagement. Only the robust clutch bearing is to be considered when defining the permitted slow-down time.

“Our clutches are designed so that the bore stroke can, if necessary, be traversed to the end with the clutch disengaged,” explains Ralf Epple.

In contrast to other overload systems which work for example with pressurized hydraulic oil or shear pins, the EAS-HT torque limiters are immediately ready for operation after the overload cause has been removed. They are not damaged and can withstand several hundred overload cases. In addition, it is possible to trace at which torque the EAS-HT clutches have disengaged.

“On other systems, the ‘proof’ so to speak is destroyed after the overload occurrence, and uncertainty prevails regarding the disengagement torque,” says Epple. “Moreover, the destroyed components have to be replaced after overload. This generates costs and system downtime. The required components have to be available, and in addition, specialist knowledge is required for recommissioning. There is a risk of the torque being incorrectly adjusted. However, an incorrectly adjusted clutch does not make sense. The consequences are simply too high - not only in the field of tunnelling and mining.”

Also with regard to size and weight, the EAS-HT torque limiters by Mayr are ahead of the rest. “We have achieved weight savings of up to 50 percent in the new, compact designs, which have been specially designed and further developed for the tunnel sector. With this weight and the compact outer diameters, they are ideally suited as a retrofit solution when existing systems are overhauled", adds Ralf Epple. “And we have not forgotten to take the price into consideration; our clutches are in some cases even cheaper than other solutions.”

Prior to leaving the Mayr plant in Mauerstetten, each clutch is extensively tested and set exactly to the required value. For this purpose, the company has diverse, modern testing equipment; it can also call on decades of experience in development and design. “In the tunnel and mining branch alone, we have in the meantime put several thousand torque limiters into use and have therefore gathered the appropriate experience with regard to the adjustment values,” adds Ralf Epple.

“In addition, we regularly receive positive feedback from the operators. In particular if existing systems have to be retrofitted, it is possible to decisively increase their performance using our clutches. We often analyse the drive line together with our customers and, where necessary, can also offer torque limiting clutches together with vibration damping systems.”