The invention applies to the mattress construc- tion of the pneumatic mattress track system disclosed in the international PCT Publication No. WO 85/01710. For this publication we will use henceforward the shortname WO. The ability of the mattress construction presented in WO, to withstand pulling forces is restricted to 15-20 % of the mattress bearing capacity, because only the sidewalls of the bags can take up the shear forces in the mattress. The ability of the mattress to withstand pulling forces can be signifi¬ cantly improved by equipping the mattress bags with internal diagonal sheets or straps that connect the upper part of a bag wall, that is perpendicular to the direction of travel, with the lower part of the opposite wall or the part of the bag bottom nearest to the opposite wall. The characteristics of the invention are defined in patent claim 1.

Another shortcoming of the mattress construction in WO is that the use of the hull facing wall of the mattress as a pulling element requires rollers, the same width as the mattress. Together with their shafting the rollers would be rather heavy. By utilizing one or more narrow pulling elements to which bars have been fastened transversely on the outside, and by fastening the mattress bags to the bars, one may use correspondingly narrow wheels instead of the full width rollers. This construction is practical only if one uses diagonal supports inside the bags which is clarified in the specification.

The mattress construction according to the present invention makes it possible to use narrow propulsion wheels with a light shafting. Moreover one

can use optimal pulling elements for the propulsion. The mattresses of large vehicles can be manufactured more easily because the pulling part of the mattress consists of smaller parts. The overhaul of the mattress becomes easier because the bags are attached only to the bars. In the original construction in WO, the replacement bags are open at their tops and would have to be fastened also to the sides of the band.

Below the invention is explained in detail by means of an embodiment example illustrated in the accompanying drawings, wherein

Figure 1 is a side view of one end of a mattress track system, where the side plates and a side support have been left out to show the mattress construction more clearly.

Figure 2 is a horizontal section A-A of figure 1 through the drive wheel and the mattress. The inner side of the mattress facing the hull can be seen. Figure 3 is a cross section B-B of figure 1 through the mattress.

Figure 4 is a cross section of a transverse bar and neighbouring bag parts showing the attachment of the mattress bags to the bars.

Figure 5 is a top view of the end of a bar and the bag parts next to it.

Figure 1 shows the hull (1) of the pneumatic mattress track system, the drive wheel (2) at the end of the hull, the mattress in the form of discrete bags (5) running around these, the hull facing wall of which in the following is called "band (3)", and the air cushion space (4) between the hull (1) and the band (3) . Figure 2 shows the orifices (7) in the band (3) through which cushion air can fill the mattress. The above is technics in accordance with WO. Figures 1 and 3 show the flexible diagonal

supports (9) of the bags (5) which effectively can resist horizontal shear forces developing in the mattress when driving up or down hill or when pulling a load. Fiqures 2 and 3 show best a mattress construction that can be used when the bags (5) are equipped with diagonal supports (9) . The mattress is built up of, a pulling element (6) , in this case a flat belt, bars (8) , fastened transversely to the outside of the belt, and bags (5) , fastened to the bars (8) . Figure 3 shows a recess (10) made in the bottom of the hull (1) for the pulling element (6) in order to permit the use of a thin air cushion (4) .

The pulling element (6) stretches under strain causing a change in the spacing of bars (8). In order to avoid stresses in the bags (5) due to this, one may use the bag fastening method shown in figures 4 and 5. The fastening of the bags is on a certain distance from the pulling element permitting the upper wall (3) of the bags to adjust itself according to the changing bar spacing. In order to avoid pulling strains in the bag end walls when the bags roll over wheels (2) , due to the larger turning radius of the bag fastening points in relation to the wheel radius, the fastening of the bags to the bars has to end inward of the bag end walls, as shown in figure 5. This leads to that the end walls cannot effectively take up longitudinal shear forces in the mattress. Thus the diagonal supports (9) are essential. In the embodiment example a flat belt has been used a pulling element. One could equally well use toothed belts, V-belts, metal bands, wires or different kinds of chains, all belonging to the consept of a pulling element. In the embodiment example only one pulling element has been used in the mattress but in

accordance with the invention one may use several pulling elements in parallel which may be situated at different distances from the mattress center line, for example at the ends of the bars.