roller through the top opening in the respective frame member if each roller has a single turning hole 56 it is preferably located midway between the roller ends each roller may have more than one hole of course once the sheet material 40 is stressed the tensioning rollers 22 must be restrained or locked against reverse rotation i e clockwise rotation in fig3 to release the tension in the material anti rotation means 58 are provided for this purpose the particular anti rotation means illustrated comprises ratchet means including ratchet teeth 60 about the half of each roller diametrically opposite its sheet gripping means 34 i e the lower half of the roller as the latter is viewed in fig3 carried on each frame member 12 and 14 is a generally u shaped spring pawl 62 one leg and the lower bend of the pawl engages a recess 64 in the frame member the other pawl leg is spring biased into engagement with the ratchet teeth 60 on the adjacent roller 22 the pawl and the ratchet teeth preferably extend the full length of each roller it will now be understood that when the sheet material or screen 40 is stressed.