Couplings for tunnel boring machines
The safety concept of the new generation of ST torque limiting couplings (switching element couplings) from R+W reduces long and costly downtimes of tunnel boring machines.
Whether it’s the construction of the world’s longest underwater tunnel in the fjords of Norway or a drainage tunnel in Hong Kong – the demands on tunnel boring machines are high. They depend, among other things, on the geological conditions, for example whether drilling is being carried out in solid rock or in unconsolidated rock, or whether groundwater-bearing soils are involved. Each machine is a special design and is constructed according to the local conditions. The components of the machines must be particularly robust and reliable under these conditions.
Tunnel boring machines often have a diameter of up to 20 meters. The total length of the machine, including auxiliary equipment and conveyors, can be up to 200 meters. The most important component of a tunnel boring machine is the boring head. Tunneling projects in hard rock usually involve the use of a rotating drill head that is pressed against the face with gigantic driving force. The drill head, which is driven by the large gear wheel, is subjected to high resistance during the drilling process. Irregularities in the rock can cause blockages in the machines. If the load on the machine is too high, there is a risk of damage to the motor, planetary gear or elements of the drive train. To avoid this, for example, a mechanical clutch based on the safety clutch with switching elements and key connection model ST1 from R+W can be used.
Protection of the drive components
In the event of an overload, R+W industrial safety couplings disengage at a set torque. The disengagement torque required by the customer is set at the factory on the test bench. All switching segments are manufactured and tested for the corresponding disengagement torque. In the case of an overload, there is no longer any torque transmission. The tunnel boring machine stops without damaging any machine elements.
Due to the high torsional moments and mass moments of inertia, ST1 torque limiting clutches from R+W, for example, are ideally suited for tunnel construction projects. In a past project during the construction of a supply tunnel in the Asian region, the connection was solved as follows: On the drive side of the ST1 torque limiting coupling, the connection between the torque limiting coupling and the motor was made via a motor shaft with splines to DIN 5480. On the output side, a gear wheel was also connected via a toothed shaft, which drives the drill head via the toothed bearing inner ring of the tunnel boring machine.
Functionality of the safety coupling
The ST series safety couplings are designed for high torques (0.2 – 165 kNm). This is made possible by the robust shift segments, which are evenly distributed around the circumference and operate as spring-loaded positive clutches. The transmittable torques are determined by the number and pitch circle diameter of the shift segments. For torque adjustment, the adjusting nut is adjusted here by several turns after loosening the clamping screws. The setting is made at max. limited by a fixed stop. In the event of an overload, the balls, which are preloaded within the switching segments, move axially out of the spherical caps and cause permanent disengagement of the input and output sides within milliseconds. Re-engagement is achieved simply by applying axial pressure to the shift plunger. For large couplings with high axial forces, re-engagement can be performed by an engagement and disengagement device. With this device, the clutch can also be disengaged manually when not in use. If required, the preset disengagement torque can be adjusted subsequently and in just a few steps.
For the tunnel boring machine application, this means that when the machine is put back into operation after the overload has been eliminated, the coupling can be re-engaged in exactly the same position. Re-engagement of the clutch is simple and is achieved by applying axial pressure to the shift plunger.
The advantage of R+W safety couplings is that the preset disengagement torque – unlike many other safety solutions – is difficult to manipulate. A factory locking device can completely prevent tampering with the coupling. This is not possible with hydraulic couplings, for example. In this case, the disengagement torque must be set again after the overload by refilling the hollow bushing or the hydraulic cushion and mounting the shear valve. In the process, it can happen that errors creep in due to the user during filling. “With the R+W safety coupling, the disengagement torque is immediately available again,” says Rogg.
Another plus of the safety clutch with switching elements from R+W is that it is fully operational again – once the cause of the overload has been eliminated. This is not always the case with hydraulic couplings. When using hydraulic couplings, surrounding machine elements may become contaminated with oil or damaged by the shear valve in the event of an overload. “The quick return to service of the R+W safety coupling can avoid long and costly downtimes at the construction site,” says Rogg. In addition, the safety coupling is virtually maintenance-free. Damaged or worn coupling elements can be easily replaced.
High rigidity, compact design and absolute zero backlash
With its various series of mechanical safety couplings, R+W Antriebselemente GmbH offers the right solutions for a wide range of applications. The torque-limited couplings are available in a wide variety of model series, sizes and variants for direct and indirect drives and can be obtained, for example, with keyway, flange or cone clamp connections, as torsionally stiff or vibration-damping shaft-shaft connections. They are basically characterized by their high precision, the high rigidity of the components, the compact design and absolute freedom from backlash. The maximum permissible torque always results from the specifications of the drive machine. Industrial safety couplings of the ST series can be obtained as standard for torques up to 250,000 Nm – in special designs also far beyond 1,000,000 Nm.
Due to their reliability and robustness, safety couplings are used in a wide variety of applications across all industries. Typical applications include excavators, shredders, rolling mills, steel mills, wind turbines, extruders, industrial conveyors and tunnel boring machines around the world.