528 MACHINES, WORK, AND ENERGY 



6. Place a piece of cloth in position and stitch a seam for 10 

 seconds by the watch. Let one assistant watch the time while a 

 second assistant counts the number of vibrations of the treadle. 

 How many stitches, then, are taken per minute. (Some manu- 

 facturers claim that as many as 3000 stitches per minute have been 

 taken with their machines.) 



7. Remove the face plate, if removable, and discover exactly how 

 the rotary motion of the pulley is changed to a vibratory motion of 

 the NEEDLE and the TAKE-UP LEVEE. 



8. Note exactly how the presser-foot is raised from the cloth. 

 What is its purpose? 



9. Examine the feeding device and determine how it works. Just 

 how is the length of the stitch regulated? Does the length of the 



FIG. 329. Head of a rotary-hook machine. 



stitch depend upon the speed with which the needle acts or upon the 

 motion of the feed? Be certain that you understand the regulation 

 of the length of the stitch. Do the movements of the feeder tell the 

 direction in which the balance wheel must be turned? 



10. Is the machine studied a chain-stitch or a lock-stitch machine ? 

 Note exactly how the stitch is made. 



11. If the machine is a lock-stitch machine, determine whether it is 

 (a) a rotary -hook machine, (b) an oscillating-hook or oscillating- 

 ehuttle machine, (c) or a vibrating-shuttle machine. 



12. Determine exactly how the knot is produced. Does the loop 

 which forms the knot pass completely around the hook' or shuttle? 

 If so, describe exactly how it does so. Can the hook or shuttle, then, 

 be rigidly attached to any fixed or moving part of the machine? 

 Do not decide this point till you have made a careful study of the 

 way in which the knot is produced. 



