Spring assembly for windable mosquito nets, sun blinds, roller shutters or any other screening device windable around a tube, comprising: - a rigid axis, inside and coaxially positioned with respect to a helical torsion spring, whose ends are housed inside a first and a second end assembly; - the first and the second end assembly positioned at the two ends of said rigid axis, each one comprising two portions (131, 132, 141, 142) configured to be able to rotate with respect to each other, whereby each of said end assemblies (13, 14) comprises a first portion (131, 141) of said two portions (131 132, 141, 142) configured to rotate integrally with said rigid axis and a second mobile portion (132, 142) of said two portions (131, 132, 141, 142) being able to rotate with respect to said rigid axis, - said mobile portion of said first end assembly being configured to be fastened to said tube and said first portion of said first end assembly being configured to be fastened to an outer support; - the helical torsion spring, coupled to the mobile portion of the first end assembly and to the first portion of the second end assembly, so that when said mobile portion of the first end assembly rotates with respect to said first portion of the same first end assembly, an end of said spring rotates and the other one remains still, this respective rotation between the fixed portion and the mobile portion of the first end assembly causes an energy accumulation in the spring said spring being configured to exert an axial strength directed towards the centre on said first and second end assembly, whereby said spring assembly is characterized in that said mobile portion and said fixed portion of said first end assembly are axially sliding with respect to each other between a first and a second position, limited by suitable abutting surfaces realized on said fixed portion and on said mobile portion, and in that said spring assembly comprises blocking means provided on both said fixed and mobile portions (131, 132) of said first and assembly, which in said first position allow their respective rotation only in one direction, while in said second position allow their respective rotation in both directions. - a rigid axis, inside and coaxially positioned with respect to a helical torsion spring, whose ends are housed inside a first and a second end assembly; - the first and the second end assembly positioned at the two ends of said rigid axis, each one comprising two portions (131, 132, 141, 142) configured to be able to rotate with respect to each other, whereby each of said end assemblies (13, 14) comprises a first portion (131, 141) of said two portions (131 132, 141, 142) configured to rotate integrally with said rigid axis and a second mobile portion (132, 142) of said two portions (131, 132, 141, 142) being able to rotate with respect to said rigid axis, - said mobile portion of said first end assembly being configured to be fastened to said tube and said first portion of said first end assembly being configured to be fastened to an outer support; - the helical torsion spring, coupled to the mobile portion of the first end assembly and to the first portion of the second end assembly, so that when said mobile portion of the first end assembly rotates with respect to said first portion of the same first end assembly, an end of said spring rotates and the other one remains still, this respective rotation between the fixed portion and the mobile portion of the first end assembly causes an energy accumulation in the spring said spring being configured to exert an axial strength directed towards the centre on said first and second end assembly, whereby said spring assembly is characterized in that said mobile portion and said fixed portion of said first end assembly are axially sliding with respect to each other between a first and a second position, limited by suitable abutting surfaces realized on said fixed portion and on said mobile portion, and in that said spring assembly comprises blocking means provided on both said fixed and mobile portions (131, 132) of said first and assembly, which in said first position allow their respective rotation only in one direction, while in said second position allow their respective rotation in both directions.