SAFE IN THE FUTURE. MECHANICAL TORQUE LIMITERS IN ELECTRIC VEHICLES
Even if you have an electric car, there may be situations in which you are forced to make an emergency stop. However, the braking energy released in this process imposes high torque on the transmission - more than it can cope with. A safety coupling, which is 60 percent lighter than the standard series, now provides this protection.
Mechanically acting safety couplings protect plants and machines in the event of a collision or incorrect operation. But the couplings can do more - with adjustments they can also be used in electric vehicles. Engineers of the coupling manufacturer R+W Antriebselemente have developed a mechanical safety coupling here for a pilot project. It is located between the two drive shafts on the gearbox and brakes. In the event of emergency braking, the resulting force is distributed differently to the brakes. Since the braking energy imposes a higher torque than the transmission allows, the transmission must be protected against the reaction torque of the brakes. The requirement of the customer was a mechanical safety coupling with a maximum weight of 1.5 kilograms, for an overload of more than 600 Nm at 1,100 revolutions per minute. This could be achieved by the TUV-certified SLN model of R+W's 300 Series. It separates the drive and the driven side within milliseconds. The safety coupling works so that zero backlash is guaranteed. The SL Series is manufactured from alloys in combination with coatings - a new form of couplings technology.
Up to 10,000 disengagements
A coupling for a torque limitation of 600 Nm, as could be used in the electric vehicle, has a net weight of 1.5 kilograms, and a mass moment of inertia of 3 ·10-1 kgm². According to the manufacturer, this ratio of torque limitation to dimensions and weight is currently unique. The coupling series covers four sizes so far. Torque values can be limited safely starting from 10 Nm to 700 Nm. The developers achieved the weight reduction in addition to the choice of materials by compressing the individual components, without any negative impact on accuracy or durability. These safety couplings in lightweight design can achieve in use cycles of up to 10,000 disengagements or more.
Newly developed springs
The use of springs specifically designed for R+W in combination with optimization of the ball detent mechanism leads to an increase in torque of up to 50 percent. The newly developed springs with special characteristics ensure exact torque limitation and short response times. The coupling starts to move only after the set disengagement torque is reached. The balls leave their tapered recesses and the torque flow is interrupted. The spring force is reduced to zero. Back-and-forth movements of the balls in their recesses in the boundary area of the set disengagement torque are not possible. Wear of the coupling in operation below the disengagement torque is thus avoided. The reduced spring force on disengagement and the light switch plate reduce the forces and wear to a minimum. Only the improvements made to the ball detent mechanism in conjunction with new proven high-performance materials have enabled R+W to develop the SL series with a weight reduction of 60 percent compared to its predecessor. Thus a safety coupling with a high power density is available to the market.
For exact positioning
Automatic re-engagement occurs at speeds of up to 3,000 revolutions per minute. This is characterized by the fact that the coupling automatically engages at its defined angle after the overload. Thus exact positioning of the drive side is guaranteed. This principle is made possible by using the spring in the zero range. In this range the spring is shortly before snapping over. It pushes the balls back into the detents with only a very slight axial force. The angle-synchronous design of automatically re-engaging safety couplings facilitates re-engagement after exactly 360 degrees due to specially arranged bores in the body. The multi-position coupling also works on the automatically re-engaging principle. The difference between it and the single position coupling lies in the arrangement of the detents, which are placed at a symmetrical angular distance over the entire body. Re-engagement is usually after 30, 45, 60, 90 or 120 degrees. The multi-position coupling is used in slowly rotating applications or for applications with exclusively swivelling movements (travel of the unit with reversing gear and an angle of 180 degrees for example). Like the single-position version, the multi-position coupling re-engages automatically after the overload is removed. Thus the application is immediately ready for use again.
Author: Bernhard Bremauer, Area Sales Manager South