Yarn strength is very dependent on the wrapping twist (Fig. 41). For optimal yarn strength, the wrapping twist should be in the range of 140 to 160 αm. With lower or higher wrapping twist, yarn strength is reduced. The strength/ wrapping twist ratio is thus very similar to the strength/ twist curve of a ring-spun yarn. For optimal yarn strength, however, air-jet spun yarns need a somewhat higher twist than ring-spun yarns.
The strength of air-jet spun yarns depends to a lesser degree on the percentage of wrapping fibers. It is known from experience that good strength values are achieved with a wrapping fiber percentage of about 15% or more. If the percentage drops much below 15%, the yarn is no longer fully covered by wrapping fibers, the yarn axis becomes distorted into a corkscrew shape, and yarn strength is reduced.
Within the product range of air-jet spinning, yarn strength is situated between the strength of ring-spun and rotorspun yarns, the strength of air-jet spun yarns being nearer to rotor-spun yarns for shorter staples, and nearer to ringspun yarns for longer staples (Fig. 42 and Fig. 43).