Phil,
I have attached a pic of the explanation from McMaster Carr. It says:
Starts — The number of independent threads on a screw. Multiple-start screws provide faster nut travel than single-start screws. For example, a single-start, 10-turns-per-inch, 1/2" dia. Acme lead screw with 10 threads per inch requires 10 revolutions for the nut to travel one inch. A multiple-start, 2-turns-per-inch, 1/2" dia. Acme lead screw with 10 threads per inch and five starts only requires two revolutions for the nut to travel one inch. Most single-start screws are self-locking and help resist backdrive so a braking device is not necessary. Multiple-start screws are not self-locking and may require some sort of braking device to hold them in position.
To Determine Number of Starts— Place a pencil tip in your thread's groove and turn the thread one revolution (see diagram, left). If the end of your pencil mark is in the next groove, you have a single-start screw. If there's a thread groove between the beginning and end of your pencil mark, you have a two-start screw. If there are two thread grooves between the beginning and end of your pencil mark, you have a three-start screw, and so on. Another way to determine the number of starts is to look at the end of your screw (see diagram, left). The end of a single-start screw is an offset circle. The end of a two-start screw is shaped like a football. The end of a four-start screw has four corners. You can easily count the starts on screws with five or more starts.
Marv |