The square thread form is a common thread form for leadscrew

Leadscrew

A leadscrew , also known as a power screw or translation screw, is a screw designed to translate turning motion into linear motion...

s. It gets its name from the square cross-section of the thread.

Advantages & disadvantages

The greatest advantage of square threads is that they have a much higher intrinsic efficiency than acme

Acme thread form

Trapezoidal thread forms are screw thread profiles with trapezoidal outlines. They are the most common forms used for leadscrews . They offer high strength and ease of manufacture. They are typically found where large loads or high accuracy are required, as in a vise or the leadscrew of a lathe...

 threads. Due to the lack of a thread angle

Thread angle

The thread angle of a screw is the angle between the threads. This is a defining factor for the shape of a screw thread. Standard values include:...

 there is no radial pressure, or bursting pressure, on the nut. This also increases the nut life.

The greatest disadvantage is the difficulty in machining such a thread. The single-point cutting tools

Tool bit

The term tool bit generally refers to a non-rotary cutting tool used in metal lathes, shapers, and planers. Such cutters are also often referred to by the set-phrase name of single-point cutting tool. The cutting edge is ground to suit a particular machining operation and may be resharpened or...

 or taps and dies used to cut the thread cannot have efficient rake and relief angles (because of the square form), which makes the cutting slow and difficult. Square threads also cannot carry as much load as a trapezoidal thread, because the root

Root

In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...

 of the square thread is smaller. Also, there is no way to compensate for wear on the nut, so it must be replaced when worn out.

Classification

Square threads are defined as follows by ISO standards:


where Sq designates a square thread, 60 is the nominal diameter in millimeters, and 9 is the pitch in millimeters. When there is no suffix it is a single start thread. If there is a suffix then the value after the multiplication sign is the lead

Lead (engineering)

Lead is the axial advance of a helix or screw during one complete turn The lead for a screw thread is the axial travel for a single revolution....

and the value in the parentheses is the pitch. For example:


would denoted two starts, as the lead divided by the pitch is two. The "LH" denotes a left hand thread.

Standard pitches for metric diameters

Nominal diameter [mm]	Pitch [mm]
22, 24, 26, 28	5
30, 32, 36	6
40, 44	7
48, 50, 52	8
55, 60	9
65, 70, 75, 80	10
85, 90, 95, 100	12