Special coupling solution in rail transport

In Europe, local and long-distance rail transport is and remains the most important mode of transport for commuters and travelers alongside the private car. However, the steadily increasing number of users presents the respective service providers with infrastructural and procedural challenges.

Investments in completely new rail vehicles are usually held back by both private and public operators for cost reasons. Instead, existing vehicles are being upgraded to the latest technology. For example, the DB’s “n cars”, which became known as “Silberlinge” because of their unpainted Nirosta outer skin. Over 5000 of this type of wagon were built between 1959 and 1980. Some of them are even still in use and have undergone different paint schemes and technical optimizations over the years. The introduction of servo drive technology has not stopped at the edge of the platform. As an example, consider the seemingly simple process of opening and closing the access doors of a passenger car. The air-operated door opening concepts developed at the time had the disadvantage that at least one train attendant had to be present on the platform. In addition to checking tickets, one of the tasks of the ticket inspector was to make sure that all doors were closed properly. Until all doors were securely locked, the train could not depart.

In modernized railcars, control is now performed centrally from the driver’s cab with the aid of servo drive technology. The train driver is thus responsible for monitoring, controlling and securing. The installation space provided for the drive in the upper door area of a passenger car is limited to a minimum in all European models. This must accommodate a compact DC motor incl. downstream planetary gear with high reduction of the speed to about 30 rpm. In many wagons, the curvature of the convex roof structure already begins after less than 10cm above the door. Due to the overall length of a compact gearmotor available on the market, these can thus only be installed horizontally. This in turn means that an additional angular gearbox is required in the drive train to transmit the rotational force to the vertical drive shaft of the door. The entire transmission combination must not be self-locking in order to still be able to move the doors manually in an emergency. An average torque of around 30 Nm is required to safely move a hinged folding door. In addition, the aim is to protect the aforementioned drive from consequential damage due to vandalism. A mechanical overload protection provides a remedy here.

This is where a special solution from R+W comes into play. The aforementioned force to move the door is increased many times by kicks and bumps. However, the door’s drive shaft can only withstand a comparatively low load. To prevent the shaft from breaking before such overload peaks, the coupling manufacturer from Wörth am Main was commissioned to develop a suitable safety coupling. Similar to the principle of a cordless screwdriver, the clutch is designed to disengage when 30 Nm is exceeded, thus decoupling the power flow between the input and output sides. As soon as the applied torque returns to a range below the preset 30 Nm, the power flow should be automatically restored. A requirement that can be solved with the proven and patented principle of the ESL coupling (Fig. 1). This coupling operates according to the spring-loaded ball detent principle (Fig. 2). Disc springs are used to exert an axial force on two superimposed ball tracks. This achieves a backlash-free form fit that precisely transmits the defined torque. In the case of a torque peak, the applied circumferential force acts greater than the spring force. For the duration of the overload, both ball tracks (Fig. 3) can then snap through one above the other. When the applied torque drops after the overload, the ball tracks are pressed into each other again, thus restoring the force flow. This system, integrated in an elastomer coupling, offers not only overload protection but also a shaft-to-shaft connection.

Special requirements had to be met for the application described: The total outer diameter of the coupling must not exceed 49 mm, as the distance between the wagon wall and the door drive shaft was only 52 mm. In addition, the coupling cannot be used as a shaft-to-shaft connection. The restrictive installation situation forms a parallel offset between the motor shaft and the door drive shaft of just under 2cm. With a standard coupling, therefore, a problem that cannot be solved.

The coupling essentially consists of an elastomer coupling with integrated snap-through protection according to the proven ESL principle. The hub for connecting the door drive shaft is provided with a cylindrical bore and two keyways offset by 180° to each other. The hub on the motor side, on the other hand, comes with external teeth according to DIN 867. There is also a pinion on the gearmotor shaft. Similar to a pinion gear, the power is thus transmitted via these two toothed components and the parallel offset between the input and output sides is overcome. For the high disengagement torque compared to the clutch size, R+W has developed a special diaphragm spring with the required spring characteristics together with its suppliers. The advantage of this overload clutch is its compact design. The R+W principle works in all weather conditions without wear and with precise repeatability. As with all safety couplings from R+W, this coupling has also been tested by TÜV-Süd.

Here, an individual solution was created that once again underlines R+W’s technological leadership in the development of compact precision safety couplings.