This section is from the book "The Manufacture Of Boots And Shoes: Being A Modern Treatise Of All The Processes Of Making And Manufacturing Footgear", by F. Y. Golding. Also available from Amazon: The Manufacture Of Boots And Shoes.
The tensions or tautness placed upon the threads may be either upper or under. The upper tension is arranged by the thread passing between two discs known as tension discs (B, Fig. 212). The lower is regulated by a spring in the shuttle, actuated by a small screw, A, in Fig. 210. A perfectly uniform tension of the two threads is essential, so that the lock may take place in the centre of the material stitched. In high-speed machines the under tension is kept as constant as possible, the bulk of the adjustment being performed with the top tension.
Pressers may be either a foot or wheel variety, or may be vibratory. The spring pressure keeps the work upon the feed, and it can be readily understood by an examination of Fig. 211, where the face plate is shown removed, thus exposing the spring, etc.