Safety couplings in rotary sluices
Loading and unloading large storages of bulk material is more complex as it may seem at first sight. At first it is insignificant whether it is a coal bunker, a grain silo, a storage for alternative fuel (e.g. wood pellets) or a process active storage environment, like e.g. a cement mill.
The challenges are different, for loading as well as for unloading the storage. First it is crucial to be able to continuously transport the medium volumetrically. The bigger the storage, the bigger the pressure which normally weighs on the outlet on the storage medium. Essential, however, is the flow behaviour, which can differ completely depending on the bulk material. Compared to liquids, there is no increase of linear pressure in bulk material within the storage. Here, rotary sluices take over control of the required outflow volume. They are often also known as gravel sluice and have a certain similarity to rotary pumps when it comes to their function. Rotary sluices are normally flanged at the input and output of the material storage and have an input and output themselves. Basically, the function principle is quite simple: a rotor running in a housing is driven electromechanically. On the rotor there are sheet metals attached in regular angular distances parallel to the rotational direction, which form the rotor cells. By means of pressure and gravity, the material drops into the rotor cell from the output of the material storage through the input side of the sluice. Through the rotation of the rotor and the resulting centrifugal force, the material is released from the rotor cell in a controlled manner on the output side of the sluice. The flow rate of bulk material can thus be controlled precisely by the rotational speed and cell size of the rotor. For more complex applications, the rotary sluice simultaneously forms the air seal. Mill chambers, for example, are often conducted in reduced pressure. The rotary sluice takes over this task as well so that as little false air as possible gets into the grinding media.
Depending on the material to be transported, the rotor may lock - due to a clumping or even because of brought in foreign or disturbing matter. Such a blockade inevitably leads to a huge increase of the initiated torque, which is often the reason for damages on the gear or sluice rotor. This does not only mean a loss of production but also unnecessary costs for the replacement of broken drive components. It is unnecessary because the Klingenberg couplings specialist R+W can just prevent such cases perfectly. An appropriate safety coupling for these purposes would be the model of the type "Servomax ® ES2"
The ES2 mainly consists of the two hubs of a classic jaw coupling. In between the hubs there is a flexible compensation element made of valuable synthetic (image 2), which is available in different shore hardness and which is able to compensate process-related misalignments between drive and driven shafts. However, in one of the coupling hubs the function of the safety coupling is additionally integrated, which works backlash free according to the spring-pretensioned ball detent system. With specific disc springs a system of steel balls is braced in the base body of the coupling and hence provides the basis for the flux. As soon as the here adjacent torque reaches a default value, the flux between engine and gear is fully and permanently decoupled in a sheer mechanical way within milliseconds.
The disengagement torque of the coupling is pre-set according to customer specifications and can be readjusted at any time within a given area with just a few hand grips, if requested. The different frame sizes allow a flexible adjustment to the particular assembly situation. The feasible disengagement torques are at between 1 Nm and 1800 Nm, depending on the frame size and version.
Beside the described "full disengagement system" with manual re-engagement, there are further operation principles for each ES2 frame size. A coupling with a “single position automatic re-engagement system” independently engages after the disengagement, as far as the permissible torque reaches the driven side. The re-engagement happens automatically at only one point or after an exact 360° turn, respectively. A coupling with a "multipoint re-engagement system" works the same way. The only difference is that such a coupling does not re-engage after a full turn first but can optionally resume its operation every 30°, 45°, 90° or 120°. With the "load holding version" there is an additional functional system available, in which the components of the drive and driven side only turn a few degrees during an overload, which secures the load.
In principle, this torque is built in a similar way and offers disengagement torques from 200 Nm - 165,000 Nm. In the past, share pin couplings were used. Nowadays, the TORQSET® ST2 operates dependably as controllable emergency shut-down in this area. Here, the advantage also lies in quite low maintenance costs and a simple operation. After having disengaged after a crash, both coupling sides just have to be turned towards each other back to their mark. You strike the modules, which are built in on the boundaries of the coupling, with a plastic hammer and the coupling is operable again in next to no time. Compared to conventional share pin or fluid couplings, additional consumable material like pins, oil or special pumps, are not necessary here. Thus, also non-qualified personnel can quickly and straightforwardly perform a reset or re-engagement.
In addition, the Servomax® ES2 as well as optionally the TORQSET® ST2 provides the possibility of monitoring the condition. When disengaging the coupling, a component is axially moved. This mechanically caused forced movement can be retrieved with the help of an external proximity switch for the further operating process. Despite their compact design, Servomax® ES2 and TORQSET® ST2 are perfectly convenient for harsh environments as well. Optional features like special bearings, additional seals or designs in high-grade steel or for ATEX environments, make this coupling a perfect safety element for almost every drive in every environment.
It cannot be determined in general which operating principle is the correct one and if additional features are recommendable, neither for the application example of the rotary sluice mentioned at the beginning. The experience and the special knowledge of the coupling manufacturer are always a valuable help for choosing the right coupling. The R+W sales representatives are available worldwide and can competently locally assist in selecting the correct application.