Only the distance which it moved stores energy in a spring and it returns itself.Carrying capacity and travel lenght can be selected.Because it is a wheel drive, pit and pulling wires are not needed, so that the shop floor will keep safer. Even if the cart hits to operator, the power more than a spring drive doesnot produce, so it is safe.It can be drive source for the KARAKURI device.Also it can be used in many ways as a drive unit.