How to Choose Compatible Lead Screws and Nuts

When selecting or replacing lead screws and nuts, make sure all components have the same thread size, thread direction, thread fit, and number of thread starts.

Thread Size

Thread size is the actual measured diameter of the threads and the threads per inch or thread pitch.

Thread size is listed in the following format:

Threads per inch versus metric thread pitch

Thread Direction

    Thread direction determines which direction a nut will travel when the lead screw rotates.

  • Right-hand threaded lead screws must be used with right-hand threaded nuts.

    Right-hand threaded screw and nut

    When the lead screw rotates clockwise, the nut travels towards you.

  • Left-hand threaded lead screws must be used with left-hand threaded nuts.

    Left-hand threaded screw and nut

    When the lead screw rotates clockwise, the nut travels away from you.

Thread Fit

For acme lead screws and nuts, thread fit indicates the amount of clearance between the lead screw and nut.

Application Products Thread Fit Compatible Thread Fit
Heavy duty lifting, lowering, and clamping Acme Lead Screws and Nuts
Metric Acme Lead Screws and Nuts		 2G
7E, 7H		 2G
7E, 7H
High-cycle automated movement Precision Acme Lead Screws and Nuts
Metric Acme Precision Lead Screws and Nuts		 2C
8E		 2C
8E

Number of Thread Starts

The number of thread starts is closely related to speed ratio and travel distance per turn. They all describe how fast components travel on a lead screw.

A thread start is the channel at the end of the lead screw or nut where the thread begins. Most lead screws, like most nuts and bolts, have only one thread start. However, some lead screws and nuts may have up to 15 thread starts. Lead screws with more than one thread start, also known as fast-travel lead screws, create faster linear travel than single-start lead screws and nuts. To determine the number of thread starts on your lead screw or nut, look at the end of the lead screw or nut and count the number of channels.

Speed ratio indicates how fast a nut will travel on the lead screw. The higher the number of thread starts, the faster the nut will travel on the lead screw.

One thread start Five thread starts Eight thread starts
No. of Thread Starts 1 5 8
Speed Ratio 1:1 5:1 8:1

Travel distance per turn, also known as screw lead, is the distance a nut moves with one revolution of the lead screw. The higher the number of thread starts, the farther the nut will travel on the lead screw. For example, a nut with a 3/4"-8 thread and one thread start will travel 1/8" per turn. A nut with the same thread size but with eight thread starts will travel 1" per turn.

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