The present invention relates to an effort saving flat platform cart device and more particularly pertains to an improved flat platform cart with collapsible casters which is convenient to collapse and use and capable of carrying heavy things. 
At airports, piers, stations and warehouses, when people are required to move heavier luggage or things, they mostly use flat platform carts which are relatively effort saving and convenient.  My China patent application 200710146724.7 has disclosed a flat platform cart with collapsible casters and a caster positioning means thereof which is convenient to collapse and use and capable of carrying heavy things.  It comprises a load supporting platform, a push-pull section and casters, the ends of two supporting bars of the push-pull section are pivotally connected with a supporting frame, the caster sleeves on top of the casters are engaged onto the supporting frame, a caster extending, collapsing and linkage means is disposed between the caster sleeves and the supporting bars.  The caster positioning means comprises a left and a right locking means which lock the caster sleeves on two side arms of the supporting frame with the supporting bars respectively, and a sliding cross bar which jointly moves with the left and the right locking means.  The casters of the flat platform cart can be jointly moved with the push-pull section; when the push-pull section is folded up, the casters can be simultaneously and automatically collapsed to facilitate storage and transportation.  However, as the caster positioning means merely fixes the push-pull section vertically relative to the supporting frame but not horizontally, the push-pull section sways forward and rearward relative to the supporting frame and thereby causing inconvenience in use.