Electrically driven roll-up rear window shades are known in the prior art. These prior art roll-up window shades a wind-up shaft that is supported rotatably underneath the rear-window shelf and to which one edge of the shade element. The other edge of the shade element is connected to a pull rod that is guided on its ends in guide rails. The guide rails run next to the side edges of the rear window, starting from the rear-window shelf or underneath the shelf up to the vicinity of the top edge of the window. To bias the shade element, a spring motor that biases the wind-up shaft in a wind-up direction of the shade element is usually arranged in or next to the wind-up shaft.
The unwinding or tensioning of the shade element is performed with the help of linear push elements that are guided in a buckle-free manner in the groove chamber of the guide rails. For driving the push elements, a common geared motor is provided. The geared motor lies next to the wind-up shaft approximately at the height of its center. To connect the geared motor to the bottom end of the guide rails, guide tubes are provided that end on the gear housing of the geared motor. With the help of these guide tubes, the push elements are guided in a buckle-free manner between the drive motor and the guide rails so that they can perform the pushing function.
Because the motor sits relatively closely next to the wind-up shaft due to space considerations, the guide tubes extend approximately parallel to the wind-up shaft in the vicinity of the geared motor and must be deflected in a direction perpendicular to the wind-up shaft for the guide rails. In turn, the radius of curvature of the guide tubes is comparatively very narrow next to the opening into the guide rails also because of space reasons.
Practice has shown that with such roll-up shades the predominant part of the driving force delivered by the geared motor is absorbed by the push elements in the guide tubes. Only a comparatively small portion of the force delivered by the motor is actually needed for extending the roll-up shade.
The guide tubes have, in general, a relatively complicated, three-dimensional shape. Thus, their production and adaptation to the vehicle conditions is complicated. In addition, the push elements lying behind the motor must also be protected. The length of this part of the push element extending past the motor depends on the distance the roll-up shade extends. The push element extends the least when the roll-up shade is extended, while the projection is the greatest when the roll-up shade is completely retracted. Because the amount of travel is typically larger than half the width of the wind-up shaft, the storage tube holding the excess part must also be adapted in a complicated three-dimensional way to the space requirements of the vehicle. Thus, the roll-up window shade is not only complicated to manufacture, it is also difficult to install in the vehicle.
The problems with the known designs have been described in connection with a roll-up rear-window shade. Similar problems are with roll-up sunroof shades that are driven in a comparable way.
The high friction losses of the push elements into the guide and storage tubes also make the design and dimensioning of an electric based pinch protection system, which is based on the measurement of the motor current, considerably more difficult. Depending on the magnitude of the friction losses, more or less power is available for a given turn-off current to clamp or squeeze body parts.