Bed assembly

A bed assembly comprising:

• a bed (1) having a head end (11), a foot end (12) and a frame (13) which is connected between the head end (11) and the foot end (12);
• a water-proof pad (2) located between the head end (11) and the foot end (12), the water-proof pad (2) being tightened, loosened and movable up and down,
• a bed board unit (4) pivotably connected to two sides of the frame (13), multiple inflatable balls being located on the bed board unit (4), and
• an operational panel (6) connected to the foot end (12) and controlling a transmission device (7) in the foot end (12),
• wherein the transmission device (7) includes a top link (71), a bottom link (72) and a gear reduction motor (74), the water-proof pad (2) is wrapped to the top link (71), the top link (71) includes a top gear (711) connected to each of two ends thereof, the bottom link (72) includes a bottom gear (721) connected to each of two ends thereof, a chain (73) connected to the top gear (711) and the bottom gear (721) of each of the two ends of the top link (71) and the bottom link (72), each of the top gears (711) includes an electro-magnetic valve key (712), a passive gear (722) is co-axially connected to the bottom link (72), the bottom link (72) includes a threaded section (723) and a slide (724), the slide (724) has a rack (725) which is threadedly engage with the threaded section (723) so that the slide (724) is movable along the threaded section (723), the slide (724) has a pin (726) extending therefrom to restrict rotation of the bottom gear (721), the gear reduction motor (74) directly drives a first bevel gear (741), the first bevel gear (741) is engaged with the passive gear (722) to control rotation of the bottom link (72) which then rotates the top link (71) via the chains (73).