Acme Screw Thread Form Terms

Thread Types

The acme thread form, established over 100 years ago, replaced square thread screws, which had straight sided flanks and were difficult to manufacture.

There are three main classes of Acme thread forms: General Purpose (G), Centralizing (C), and Stub Acme. The General Purpose and Centralizing thread forms have a nominal depth of thread of 0.50 × pitch and have a 29° included thread angle. Some Nook sizes have 40° included angle. Trapezoidal thread forms have a 30° included thread angle.

When compared to general-purpose thread forms, centralizing threads are manufactured with tighter tolerances and reduced clearance on the major diameter.

Stub Acme threads follow the same basic design, but have a thread depth less than one half the pitch.

If an acme nut is side loaded with a radial load, a "G" class will "wedge" when the nut thread flanks come in contact with the screw thread flanks. To prevent wedging, less clearance and tighter tolerances are allowed between the major diameter of the nut and the major diameter of the screw.

CAUTION - Although a side load will not cause a centralizing thread to wedge, the nut is not designed to operate with a side load such as a pulley, drive belt, etc. See "Load Definition" section for further information. (See Image 1 in Figure 1 Below)

Land (Major) Diameter

The outside diameter of the screw.

Pitch Diameter

On an acme screw, this diameter is approximately halfway between the land diameter and the root diameter. It is the diameter at which the thread thickness is equal to the space between threads.

Root (Minor) Diameter

The diameter of the screw measured at the bottom of the thread.

Pitch

The axial distance between threads. Pitch is equal to the lead in a single start screw.

Lead

The axial distance the nut advances in one revolution of the screw. The lead is equal to the pitch times the number of starts.

PITCH × STARTS = LEAD

NOTE: Nook Industries acme screw designations reference major diameter and effective turns per inch. For example: ¼" -4 RH requires four turns for one inch of travel. A ¼" -4 RH has 4 starts and a 0.062" pitch.

0.062" PITCH × FOUR STARTS = 0.250" LEAD

Screw Starts

The number of independent threads on the screw shaft; example one, two or four. (See Image 2 in Figure 1 Below)

Lead Accuracy

Lead accuracy is the difference between the actual distance traveled versus the theoretical distance traveled based on lead. For example: A screw with a 0.5 inch lead and 0.004 inch per foot lead accuracy rotated 24 times theoretically moves the nut 12 inches.

(24 Revolutions × .500 inches per revolution = 12.000 inches of travel)

With a Lead accuracy of .0003"/inch, actual travel could be from 11.996 to 12.004 inches.

Refer to the listings in the design guide for the lead accuracy of a particular screw.

Figure 1