be made of the same rubber elastic material are situated at the end faces of window tube 2 and in the completely assembled rubber cartridge spring they are axially compressed between the ends of window tube 2 and flanges 4 projecting radially inwardly from outer tube 1 both ends of window tube 2 are beveled outwardly in mirror image symmetry to one another so that with increasing distance from the axis of the rubber cartridge spring the distance between the two end faces diminishes sealing rings 3 also made of rubber elastic material thereby undergo during their axial compression between the metal surfaces of outer tube 1 and those of window tube 2 a displacement into gap 5 that is to be sealed the latter is thereby reliably and lastingly sealed fig2 to 7 show enlarged sections of various embodiments of area a of fig1 in the embodiment of fig2 the end face of window tube 2 is beveled only in the area of its outer circumference the axial load bearing capacity of window tube 2 with respect to outer tube 1 is thereby improved in the embodiment shown in fig2 and 3 the sealing ring 3 has the cross sectional shape of a parallelogram which during axial compression against flange 4 comes into contact with it first in the area of its inside circumference in this case as well continued compression results in.