What is a multi-start thread screw?

In general, there is one ball groove in one screw shaft. This is called a “single start thread” screw.When two or more ball grooves are involved, such screws are collectively called "multi-start thread" screws.

As the lead becomes larger, the spaces between ball grooves become wider. In addition, as the lead becomes larger, it is more difficult to maintain the number of effective turns of balls. (The nut cannot be made longer because of some limitations associated with the manufacturing reasons.) In order to have the maximum number of effective turns, we have to increase the number of ball grooves. By adding extra ball grooves to the inter-groove spaces (a more effective utilization), you can have more effective turns of balls. This is the reason why the multi start screws are used for high helix lead ball screws.