unit 12 is provided with the device shown in fig2 and 3 a widening roller such as the roller engaging the fabric t issuing from below consists of two co axial half rollers 60 and 62 having oppositely coiled helical ribs the operation of such widening rollers having been described in the above described prior u s and british patent publications each adapted to exert a transverse pull on the fabric the half rollers are individually driven by separate motors 64 and 66 respectively assuming that these half rollers are concurrently driven at the same speed they will jointly act to operate as a widening roller at a proper location such as downstream of a pair of squeezing rollers 56 and of a tensioning dancer roller 58 such devices are known and therefore are not described in detail herein sensor means 68 and 70 are each located on opposite sides of one of the edges of the fabric t in fig3 the sensor means 68 and 70 have their opposing sensor positions on the right side of the fabric although they can equivalently be provided on the left side of the fabric each sensor means can comprise for example a photocell sensor portion and a source of light sensor portion which is adapted for exciting the photocell.