Window shade and actuating system thereof

An actuating system for a window shade, comprising:

• a sleeve having an inner surface;
• a braking part operable to apply a braking force for preventing rotation of the sleeve around a rotation axis;
• a brake releasing part connected with the braking part, the brake releasing part being operable to cause the braking part to release the braking force for rotation of the sleeve;
• an axle coupler disposed through an interior of the sleeve, the axle coupler being rotatable around the rotation axis for raising and lowering a movable rail of a window shade;
• and
• at least an engaging part disposed between the axle coupler and the inner surface of the sleeve, the engaging part being adapted to be in rolling contact with the axle coupler and the inner surface of the sleeve;
• wherein the engaging part has a coupling position with respect to the axle coupler where the sleeve and the axle coupler are locked to each other in a first direction of rotation, and the engaging part is movable relative to the axle coupler away from the coupling position for rotation of the axle coupler relative to the sleeve in a second direction of rotation opposite to the first direction of rotation;
• and
• wherein the sleeve and the axle coupler are rotatable in unison in the first direction with the engaging part in the coupling position as the braking part releases the sleeve by operation of the brake releasing part.