Heavy Duty Safety Couplings in the Steel Mill
The Optimization of a Chassis Drive Unit
About 43 million tons of crude steel are currently produced annually in Germany. This corresponds to approximately 25% of the total EU output of approximately 174 million tons and barely 5% of the output by the People's Republic of China, which totals 810 million tons. Nevertheless, to successfully compete on the world market, along with the development and production of high quality specialty steels, in-factory optimizations that result in a sustainable increase in productivity in the manufacturing process are also required. Not infrequently, machines and systems of a steel mill have been in operation for several decades. An outstanding approach in this regard is therefore the orientation of individual machines and system components toward the current state of the art. This does not always have to be the large individual projects. The multitude of small improvements in everyday operating procedure that often appear to be insignificant offers enormous savings potential in this area.
One example: A leading steel producer from the western Ruhr district was able to reduce its operating costs for the steel transport vessel by several tens of thousands of euros in the last five years. The steel removal trolley is a rail-bound vehicle that transports a pan beneath the converter that is filled there with molten crude steel. After that, this vehicle transports the pan with the crude steel to the SMP (secondary metallurgical plant, also: pan metallurgy). There, the steel is poured using a crane, e.g. into the vacuum system. The SMP is the device in which the steel receives its quality features through various processes (such as degasification, purity optimization, deoxidation, etc.) before it comes to the continuous casting plant, where it is cast into a solid condition, the slab.
The vehicle has a specific weight of 340 tons. A filled pan weighs around 385 tons. The total weight of around 725 tons is driven by 8 slip ring motors having an individual capacity of 30 kW each. Each of these motors is connected via a simple bolt coupling to a spur gear. After a reduction of i=36.8, the spur gear generates a nominal torque of around 14.85 kNm on its driven shaft. The driven shaft is connected again via a bolt coupling to a mounted shaft at the end of which a pinion is located. This pinion in turn engages in the outer ring gear of the gear set that moves the vehicle over the tracks.
With 24/7 use in which the pan is filled up to 30x per day, the primary importance of this device for the overall process quickly becomes clear, as does what it means for the trolley to fail.
In the past, cracked gears repeatedly resulted in the failure of the vessel. This resulted in the vehicle having to be pulled out of the continuous production process, at considerable expense. This became extremely difficult if the pan was also being filled with the crude steel heated to over 1000°C. Moreover, there was great expense in releasing the latter from the pan after it hardened. This resulted in enormous costs, consisting of the procurement of a new gear and the associated replacement as well as the repair of the steel pan. Not to mention the costs of the general drop in production.
There were various reasons for the gear failures, but they were always of identical origin: stress too high due to excessive torque. These resulted first of all from blockages on the rail path. Negligently operated third party vehicles, incorrectly deposited material or paths that are just heavily soiled are not uncommon in the giant, dimly lit halls despite maximum safety precautions. Another cause is the slag and dirt on the gear teeth. Figure 2 shows a gear after a service time of approximately 6 months. Here, it quickly becomes clear that discharging slag gets deposited on the drive components. Eventually, the slag coating is then so solid that the force acting on the gears is simply too great.
Increase process security – offer individual solutions
At this point, the Klingenberger coupling specialist R+W Antriebselemente was able to help. The most obvious approach to a solution was the integration of an overload or safety coupling. Its purpose is to mechanically cut out torque that is present in the drive train at a defined value in a mechanically controlled manner. However, the great challenges in this were:
- the extremely restricted installation space relative to the power density of the coupling
- the extreme environmental conditions
- the cut-off torque, which can be defined only with difficulty
- easy operation or fast restoration in the event of damage
For the extremely restricted installation space, R+W therefore developed a special variant of the TORQSET ST4 Heavy Duty Safety Coupling (Figure 3) that has proven itself over the years. This replaces the bolt coupling located between gear and drive pinion shaft. The ST4 has not only the function of overload protection, but also takes on the function of an offset-compensating shaft-to-shaft connection. Nevertheless, R+W decided to use a low-backlash tooth coupling instead of an elastic bolt coupling. This had the advantage of being able to transmit higher torques with a reduced size. In this way, open installation space was gained for the operating area of the safety coupling. Due to the low rotational speed of not even 20 rpm, it was possible to eliminate an elastic damping element.
However, there was yet a further challenge: At which torque level is the coupling to cut out? A machine that has been in use this long, on which already generations have provided proof of their technical capabilities in day-to-day operation simply does not have the required documentation to permit exact calculations. Based on rough calculations and empirical values, a value of 42000 Nm was ultimately settled upon. Then, as soon as such a torque was present in the drive unit, the coupling disconnected the flow of driving force within the span of a few milliseconds. Indeed, already at the initial startup with a filled pan, the coupling released to the surprise of everyone involved. The reason: The startup torque under full load was evidently substantially greater than everyone assumed. The simple and advantageous handling of the ST series revealed itself here. With a face wrench and a few moves of the hand, it was possible to adjust the pre-set disengagement torque. With steps in the 1 kNm range, it was ultimately possible to precisely approach the required target mark. Today, the coupling is set to a disengagement torque of 57000 Nm and switches only in the event of genuine crash. Once this happens, an employee corrects the fault in the travel path or manually clears the gear of its coating of slag. Then, he just has to re-engage the coupling and already production can resume.
Here, an additional advantage of the ST model series from the R+W house becomes clear. No special tools at all are required for the restoration of operation. Unlike as it was with the shearing pin couplings that were standard in the past, no special replacement parts have to be additionally stockpiled. The coupling operates according to the backlash-free ball-detent principle, which has proven itself over the years. The two halves of the ST coupling are connected to each other in a positive-lock arrangement by a defined number of switching elements. In the interior of each switching element, a ball is axially pre-tensioned via disk spring assembly. One half of the ball shows from the element and sits on the second coupling half in a form-fitting detent. The torque is backlash-free and reliably transmitted via this force/form fitting. As soon as the tangential force generated by the applied torque and acting on the balls is great enough that the spring assemblies can no longer pretension the balls, the ball slips completely back into the interior of the switching element within the span of milliseconds. The ball is guided inside the switching element by a bolt that is correspondingly moved axially during the switching operation. To place the coupling back in operation, the two coupling halves just need to be turned into the correct position, and with the force of a soft-faced mallet on the bolt of each element, the ball is then pressed back into its detent recess in order to restore the flow of force. However, R+W had a few more advantageous features for the customer in the described scenario for the steel trolley. For easier turning of the coupling halves after a switching operation, the customer can also insert a lever into extra radially placed holes that are provided for this purpose. In addition, a re-engagement groove was placed on the hub of the coupling half with the switching elements. This is because, due to the scarce installation space because of the shortened hub on the safety coupling side, re-engagement using the soft-faced mallet is not possible. However, with the aid of the groove, the customer can insert a mounting iron there and push back the ball using the lever force created with it.
According to the testimony of the employees in the steel mill, the coupling has already reliably switched multiple times. Since then, there have not been any failures as described at the outset. Accordingly, the coupling has already paid for itself several times over.
The couplings of the ST series, like all R+W products, are governed by a modular design principle. In conjunction with individual adaptations, R+W can in this way satisfy nearly any customer requirement. Regardless of size and quantity.
Especially in the last ten years, R+W has impressively shown that high-precision couplings in conjunction with customer-specific special requests are not just reserved for servo drive technology. In that time, R+W has also fully established itself in the area of heavy duty couplings.
Author: Christopher Monka