COUPLINGS IN RAIL TRAFFIC
In Europe local and long-distance rail, apart from the passenger car, is and remains the main mode of transport for commuters and travellers. However, the constantly increasing number of users confronts the relevant service providers with infrastructural and procedural challenges.
Investments in completely new rail vehicles are held back by both private and public operators for cost reasons. Instead, existing vehicles are brought up to the latest state of the art. Who is not familiar with the "n-cars" of the Deutsche Bahn, which due to its unpainted stainless steel outer skin were known as "pieces of silver". Over 5000 of this type were built between 1959 and 1980. Some of these are even still in use and were given different paint finishes and technical improvements in the course of the years . Here as well the advance of servo drive technology did not stop at the platform edge. As an example here is the seemingly simple process of opening and closing the entrance doors of passenger carriages. The currently developed door opening concept using compressed air had the disadvantage that at least one guard had to be present on the platform. In addition to checking ticket validity, the guard had the task of inspecting whether all the doors had been closed properly. The train could not leave as long as all doors were not safely locked. A time-consuming procedure in the first place, which caused some delays.
In modernised carriages, control today is centrally from the driver's cab with the aid of servo drive technology. The train driver is responsible for monitoring, control and safety. The installation space provided for the drive in the upper area of the door of a passenger carriage is limited to a minimum in all European models. A compact DC motor including a connected planetary gear with high speed reduction to about 30 rpm must be accommodated in it. The problem of the installation space: In many carriages curvature of the convex roof construction already begins after less than 10cm above the door. Because of the overall length of a compact gear motor available on the market, this can be installed only horizontally. This in turn has the consequence that space for a bevel gear is required in the drive train to transmit the rotational force to the vertical drive shaft of the door. Of course, the entire gear combination should not be self-blocking, so that the doors can still be moved manually in the case of an emergency. An average force of around 30Nm is required for the safe movement of a hinged folding door.
The problem of vandalism: Every railway company has an unchanging group of vandals against whom video surveillance and electronic controls have no effect. Frequently only pure mechanics helps to protect the drive in question against everyday vandalism. This is where a special solution by R+W plays its part. The above-mentioned force for moving the door is increased many times by kicks and blows. But the drive shaft of the door withstands only a comparatively small load force. To prevent breakage of the shaft due to such overload peaks, the coupling manufacturer based in Klingenberg am Main was awarded the contract to develop a suitable torque limiter. In a similar way to the principle of a cordless screwdriver, the coupling should disengage when 30Nm are exceeded and thus decouple the power flow between input and output. As soon as the applied torque is in a range below the preset 30Nm, the power flow should be restored automatically. A requirement which first of all could be solved with the proven and patented principle of the ESL coupling (Fig 1). This coupling works according to the spring loaded ball-detent principle. With the help of disc springs an axial force is exerted on two ball tracks lying above one another. In this way there is backlash-free engagement which transmits the defined torque precisely. In the event of a torque peak, the applied tangential force is greater than the spring force. The two vertically arranged ball tracks can disengage for the duration of the overload. When the applied torque drops after the overload, the ball tracks are pressed into one another again and thus the power flow is restored. This detent element is installed in the body of an elastomer coupling. Thus, in addition to overload protection, the ESL coupling also fulfils the classical task of a shaft-to-shaft connection.
Back to the application in question. Special requirements on the installation situation prevail here: As the distance between the carriage wall and door drive shaft is only 52mm, the overall outside diameter of the coupling may not exceed a dimension of 49mm. In addition, the coupling cannot be used as shaft to shaft connection. The restrictive installation situation results in a parallel misalignment between the motor shaft and door drive shaft of just on 2 cm A problem that cannot be solved with a standard coupling.
The R+W solution: The coupling consists essentially of an elastomer coupling with integrated multi-position detents according to the proven ESL principle. The hub for the connection of the door drive shaft is provided with a cylindrical bore and two keyways offset by 180°. The hub on the motor side, however, comes with external gearing according to DIN 867. There is also a pinion gear located on the gear motor shaft. In a similar way to a spur gear, the force is thus transmitted through these two geared components and the parallel misalignment between input and output is overcome. For the high disengagement force compared to the size of the coupling , R+W together with its suppliers have developed a special disc spring with the necessary spring properties. The advantage of this torque limiter lies in its compact design. The R+W principle works in all weather conditions free of wear and with precise repeatability. As with all R+W torque limiters, the TÜV-Süd (Technical Inspection Association), as independent testing and inspection authority, monitors design as well as production.
Conclusion: An individual solution, once again underlining the technological leadership of R+W in the development of compact precision torque limiters, was developed here.
Author: Christopher Monka, R+W Area Sales Manager North West