Torque limiters for cutterhead motor protection Sep 2020

As TBMs work their way through rock and soil, like moving factories underground, every metre counts. A machine overload, caused through irregularities in the rock, for example, must not stop progress. "Our EAS®-HT torque limiters disengage the drive at a defined and traceable torque, to protect the motor, gears and drive line of the cutterhead from damage through overload," explained Ralf Epple, Product Manager at mayr® power transmission in Mauerstetten, Germany. In this way, long downtimes are avoided and the machines are operated sustainably. "The safety concept must not be susceptible to manipulation," emphasizes Epple. "The torque on our clutches is set on the manufacturer-side and is immediately accessible again after disengagement. The safety aspect is not dependent on recommissioning. The torque limiting clutches work reliably."

Disengaging EAS®-HT torque limiters are reset immediately after rectifying an overload event

Reliable protection at the extreme

"The disengaging EAS®-HT clutches are element clutches that represent the ideal solution for extremely high torques and speeds, or large mass moments of inertia," explained 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® power transmission offers the torque limiters from the EAS®-HT series in a standard version with a torque of up to 440,000Nm. The limits of what is possible, however, have not yet been reached, thanks to the modular construction. The possibilities are actually endless.

Overload protection functionality

On the EAS®-HT torque limiters, the individual overload elements transmit the torque through positive locking during normal operation. In case of overload, they separate the input and output faster than any other control unit. In less than five minutes the limiter switches off the machine with almost no residual torque and with high 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 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,” explained Epple.

Chr. Mayr GmbH + Co. KG
Eichenstraße 1, 87665 Mauerstetten
Tel. +49 08341/804-0
Email: info@mayr.com
www.mayr.com

Non-destructive protection

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 cause of an overload has been removed. The limiters are not damaged and can withstand several hundred overload disengage cases. It is possible also to trace at which torque the EAS®-HT clutches disengaged. "On other systems, the proof, so to speak, is destroyed after the overload occurrence, and uncertainty prevails regarding the disengagement torque,” said Epple. Components destroyed, by systems unable to protect them, have to be replaced after an overload trip event. This generates costs and system downtime, and the replacement components have to be available in the spares inventory on site. Specialist knowledge is then required for recommissioning. “In that manual recommissioning, there is a risk of the torque being incorrectly reset,” said Epple “and an incorrectly adjusted clutch makes no sense - the consequences are simply too high.”

Replaceability, increased productivity

With regard to size and weight, the mayr® power transmission EAS®-HT torque limiters are ahead of the rest. "We have achieved weight savings of up to 50% in the new, compact units that are designed and further developed specifically for the tunnel sector. With this weight, and the compact outer diameters, the EAS®-HT torque limiters are ideally suited as a retrofit solution when existing systems are overhauled," said Epple, "and we have not forgotten to take the price into consideration. In some cases, our clutches are cheaper than other solutions."

EAS®-HT torque limiters for TBM

Prior to shipment from the manufacturing plant, each clutch is extensively tested and set precisely to the required value, calling on decades of experience in development and design. "In the tunnel and mining branch alone, we have put several thousand torque limiters into use and have therefore gathered the appropriate experience with regard to the adjustment values,” said Epple. "In addition, we receive positive feedback from the operators regularly. In particular, if existing systems have to be retrofitted, it is possible to increase their performance using our clutches. We often analyse the drive line together with our customers and, where necessary, can offer torque limiting clutches together with vibration damping systems."

mayr power transmission is a family-run company founded in 1897. It is a leading manufacturer of safety brakes, torque limiters and shaft couplings designed primarily for application in electrically driven machines and systems. The company is active in over 60 branches worldwide and currently has about 700 employees at the headquarters in Mauerstetten. Worldwide, mayr power transmission has more than 1200 employees. With production plants in Poland and China, sales subsidiaries in the USA, in France, UK, Italy, Singapore and in Switzerland as well as representatives in a further 40 countries, the company has a global presence.