This invention relates to a filter insert for air filters of internal combustion engines, compressors and other air-aspirating machines comprising axial end plates and a filter ring of pleated filter paper through which air flows radially, and an additional filter element disposed concentric with the filter ring. 
Such air filter inserts must have a great dust-removing ability over their entire period of use and must have a great dust-accumulating capacity.  Another requirement is a very low resistance to the flow of air, but this resistance increases with increasing dust loading, and finally, when a given limit is reached, it necessitates changing of the filter insert. 
To filter the intake air of air-aspirating machines filter inserts are widely used in which the filter medium consists of pleated filter paper.  Such construction of the filter insert permits comparatively large filter areas to be contained in a small amount of space.  Large filter surface areas are particularly needed in filters used in a dusty environment such as occurs, for example, in filters used in work vehicles, earth-moving machines, farm machinery and the like, large filter surface areas are needed. 
Paper filters for dusty environments are commonly used in practice which have a surface area of approximately 4000 cm.sup.2 to 6000 cm.sup.2 with a thickness of 0.3 to 0.6 mm and a density of 0.2 to 0.3 g/cm.sup.3 per cubic meter of aspirated air per minute.  For low-dust operation, for example in passenger automobiles generally driven on paved roads, a paper filter surface area on the order of only 1500 cm.sup.2 per cubic meter of aspirated air per minute is usually provided.  However, an open-pore paper is thereby used. 
To achieve the highest possible dust filtering action, U.S Pat.  No.  3,397,793 discloses a filter insert provided with two filter rings (paper bellows) of pleated filter paper of equal radial depth arranged concentrically.  Between the two filter rings is a supporting ring of perforated sheet metal, and additional supporting rings are disposed on the outer circumference of the outer filter ring and on the inner circumference of the inner filter ring.  According to this patent the filter paper of the inner filter ring is provided with smaller pores than the filter paper of the outer filter ring, in order thus to achieve filtration in stages.  A disadvantage of this arrangement is to be seen first in the fact that a total of three supporting rings are necessary, and each supporting ring of course increases the resistance to air flow. 
It is known that the initial filtering action in the type of filter paper used here is lower than the average filtering action.  The reason for the increase in filtering action over the entire period of operation is to be found in the fact that, while the filter is in operation, a filter cake is formed by the deposition of the filtered particles on the filter paper, which then filters out even very small dust particles. 
However, if, as described in the U.S. Pat.  No.  3,397,793, a filter paper with very small pores is used, there is a danger that these small pores will rapidly become clogged, thereby substantially reducing the filter's useful operating life.  Furthermore, the use of a second filter ring consisting of pleated filter paper requires a considerable amount of space, so that the filtering surface area of the first filter ring is reduced by this second filter, and consequently the required filter surface area can be achieved only by increasing the outside diameter of the filter insert.