Basic Screw Geometry

The same procedure can be used to construct the helical geometry of the flight along the O.D. of the screw as shown in the left-hand side of Fig. 7.1. 

It should be noted that the helix angle at the O.D. of the screw cp is different from the helix angle at the root of the screw cp. The screw geometry is usually represented by straight flights, as shown in Fig. 7.2. 

However, in reality the flights are S-shaped, as seen in Fig. 7.1. If a cross-section is made perpendicular to the flights, it can be seen that the screw channel is not a true rectangle. The bottom and top surface of the screw channel are curved and the flight flanks diverge. Thus, the channel width is larger at the screw O.D. than at the root of the screw. 

Important geometrical relationships will be given next. The pitch of the screw equals the sum of the axial channel width and the axial flight width. 

The lead L is the pitch times the number (p) of parallel flights. L = p(B + b) 

---

In order to simulate an extrusion process or design a screw, the mathematical description of the screw geometry must be understood. This section provides the basic details that describe a screw and the complex mathematics that describe the channels. 

Most hardfacing materials do not plate well, i.e., a wavy line occurs at the intersection of the welded material. This problem can be eliminated by the use of an inlay, as shown in Fig. 11.14. Here, two basic hardfacing geometries are shown one that has the hardfacing applied to the full width of the flight and the other that has an inlay with hardfacing. Inlays can only be applied to new extruder screws. 

Of particular importance to carbon nanotube physics are the many possible symmetries or geometries that can be realized on a cylindrical surface in carbon nanotubes without the introduction of strain. For ID systems on a cylindrical surface, translational symmetry with a screw axis could affect the electronic structure and related properties. The exotic electronic properties of ID carbon nanotubes are seen to arise predominately from intralayer interactions, rather than from interlayer interactions between multilayers within a single carbon nanotube or between two different nanotubes. Since the symmetry of a single nanotube is essential for understanding the basic physics of carbon nanotubes, most of this article focuses on the symmetry properties of single layer nanotubes, with a brief discussion also provided for two-layer nanotubes and an ordered array of similar nanotubes. 

A. Geberg supplemented his investigations by determining the basic geometries of screws in practical applications with varied parameters number of threads and channel depth and their dependent variables, tip angle (Fig. 2.4), and free cross-sectional area that can be filled with product (Fig. 2.5). 

The application of the basic geometry by computer for a screw described in this way (see Section 2.2.1) results in the calculation of the profile curves in cross-section and in longitudinal section and, for example output for manufacture. Process engineering target values, such as the usable product volume and all surfaces, are also determined. 

The second example shows a pressure build-up zone which follows a left-rotating element (Fig. 8.16). The same basic geometry was used for the screw elements as in the first example. Only the pitch, the length, and the direction of rotation vary from element to element ... 

The conventional multi-flighted extruder screw has a number of advantages and disadvantages. The basic geometry is shown in Fig. 8.52. 

---