The operating force on the roller shutter strap should be not more than 30N according to DIN EN 13659 (class 1). The max. roller shutter weight depends on the strap thickness, the guide angle and power loss on the strap guide rollers (very low for needle bearing strap guide rollers), strap roller diameter, shaft diameter, roller shutter slat thickness, slat shape (camber), weight per m² and the height to width ratio.
More force is required when the winding diameter on the strap roller decreases.
Roller shutters down = small bale diameter and high residual weight.
Roller shutters up = large bale diameter and low residual weight.
The gear coiler puts a lot of force on the crank at the beginning (small winding diameter) and afterwards a small force on the belt using a constant force effort.
These different behaviours counteract each other to some extent, thus the operating force stays almost the same during the complete movement.