Lawn mower blades

 

— blades intended for use in lawn mowers are predominantly of the rotary type. The rotary cutting system is a double-sided blade that rotates on a vertical axis like a propeller. Accordingly, a replaceable knife for such devices usually looks like a symmetrical bar of a special shape, with cutting edges on both sides and an axle mount in the middle. Such knives are "consumables", often their materials and design are such that when dull it is easier to buy a new knife than to restore the old one to working capacity.