Screw Technology

Acme Screws

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 x 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)

Ball Screws


Ball screws offer an efficient means of converting rotary motion to linear motion. A ball screw is an improvement over an acme screw just as an antifriction ball bearing is an improvement over a plain bushing.

Ball screw assemblies have a number of bearing balls that transfer the load between the nut and screw. The thread form in which the bearing balls ride is an ogival shape formed from two arcs of the same radius with offset centers. This form is also referred to as a gothic arch. (See Image 2 in Figure 1 Below)

NRS™ Planetary Roller Screws

Application and Design

Nook Planetary Roller Screws (NRS), a member of the lead screw family, are remarkable devices designed to convert rotary motion into axial force or vice versa.

The NRS design offers multiple advantages and reliability for the most demanding applications when compared with other lead screw types due to its rolling motion. NRS offers high efficiency even in relatively shallow lead designs. The multitude of contact points can carry large loads and provide very high resolution (small axial movement) when using very shallow leads. NRS produces high rotational speeds with faster acceleration without adverse effects.

NRS planetary roller screws shown in the following pages cover a large spectrum of possibilities and application demands. Nook engineers are at your disposal to suggest the suitable product for your application requirements. (See Image 3 in Figure 1 Below)

NRS Planetary Roller Screws are used in:

  • Aerospace & Outer Space Applications
  • Machine Tools
  • Measuring Equipment
  • Positioning Systems
  • Optical Equipment
  • Photography Equipment
  • Ordnance
  • High Force Actuators
  • Plastic Machinery
  • Transpiration (Train Tilt Mechanisms)
Figure 1