Screw shafts

Screw Feeders. Screws are primarily used when feed over a slotted outlet is requited. Screws are a good choice when an enclosed feeder is requited, when space is restricted, when handling dusty or toxic materials, or when attrition (particle breakage) is not a problem. A screw is composed of a series of flights that are wound around a common shaft. The flights have a particular diameter and pitch (the distance between flights). Some screws have constant pitch flights others vary. The screw shaft has to be sized to prevent deflection (12). 

In continuous mixing, the numerous thermocouple wells on the barrel may be used as injection ports for the introduction of separate additive streams (crosslinker, curing agent, etc.) on-line. Figure 19 shows the cross-section of a hollow flighted-screw shaft together with a... 

Fig. 2. Internieshing co-rotating twin-sciew extruder A, motor B, gear box C, feed port D, clam shell barrel E, vent port F, screw shafts G, conveying screws H, kneading paddles I, barrel valve and J, bHster rings. Courtesy of APV Chemical Machinery Inc.

This chapter covers the engineering-related history of the twin-screw, or more precisely the co-rotating twin screw extruder, where both screw shafts rotate in the same direction. In the following, we will simply refer to them as co-rotating extruders (instead of co-rotating, twin-screw extruders). 

Since the - mathematically precise - system is intended to be fully wiping, the central shaft can be a wax blank that is shaped to its corresponding contour by the metal moved screw. The moved screw (Fig. 2.1) with its metal tip x then forms the flank arc y (bold) in the fixed wax shaft. As all mass points of the moved screw describe circles with a radius equal to the centerline distance, including the tip x, the flank arc y of the wax screw must also be an arc with a radius equal to the centerline distance of the two screw shafts an astonishingly simple solution. 

The energy required to melt the polymer components is largely transmitted via the screw shafts the heat flow through the barrel wall serves only to form a melt film on the wall [6]. This melt film is important to create an adhesion of the polymer on the wall, thus generating a shear gradient [11]. To achieve this, the temperature of the barrel wall must be higher than the softening point of the polymer. 

The specific energy input is the most important characteristic value. It is characteristic to each product and applies irrespectively of the size of the extruder. Generally, only measurable value is the specific mechanical energy input Pslx,tif lnCci, via the screw shafts. This is calculated using the following equation ... 

Figure 4.35 Pair of gear elements on screw shaft (EP 0564884 B1)...

Figure 4.37 Ring extruder barrel and screw shafts [231...

Technical requirements may sometimes mean switching between elements with different numbers of threads along an extruder. This always requires transfer elements, spacers in the simplest case, otherwise a slight axial movement of the screw shafts towards one another will cause them to collide. Figure 5.11 shows this for the transition between triple- and single-flighted screw elements. 

The differential pressure within the equation is the difference between the pressure at the tip of the screw (e.g., at the end of the screw shafts) and the pressure at the product intake. The changes in pressure along the extruder, e.g., with further pressure peaks as in Fig. 6.8, are not relevant. Only the state variables at the limits of the extruder are relevant here. The nozzle is deliberately included in the balance because the differential pressure is generally... 

The dimensionless power is ideally formed using the same reference variables as for the pressure number. It can be derived that the power input is proportional to the viscosity of the medium and to the screw section under consideration, as well as to the square of the rotational speed and to the square of the diameter. To this end, the power is derived from the torque M which results from the wall shear stresses xw that draw on the screw shafts. 

Elements are congruent when the geometry of an element remains constant along its axis. Their process effect does not depend on the method of installation on the screw shaft. 

The pitch direction, i. e., the direction of the angle of rotation of the thread. If the thread of the element runs counter to the direction of rotation of the screw shafts, one obtains a conveying element. If the thread runs synchronously with the screw shaft rotation direction, the screw element will have a backward-pumping effect (Fig. 12.4). 

Generally, symmetrical and congruent elements are used, i.e., the elements on each screw shaft are identical. 

The installation is correct with respect to the screw shaft. 

Conveying elements are designed with identically shaped and located face sections. This makes it easier to fit the elements onto the screw shaft to form a continuous profile with no offset angle with respect to the previous element. Double-flighted conveying elements are normally used. Their length is usually equal to a whole pitch or half a pitch, i. e., a complete profile thread or half a profile thread (Fig. 12.11). 

Common to all standard kneading elements is intensive shearing between the kneading discs of the two screw shafts whose faces move past one another in opposite directions with narrow clearances. This has led to the development of the shoulder kneading disc, whose... 

Restrictive discs and blister discs restrict the melt flow over the entire cross-section of the barrel bore. Blister discs (Fig. 12.35) are drilled cylinder discs arranged concentrically around the screw shaft with a defined gap with respect to the barrel wall. A ring is fitted to the counter shaft similar to the TME element. This blocks the entire cross-section of the... 

Reliable torque measurement is indispensable for protecting the extruder. Here, two speed ranges are particularly important. One of them is zero speed, when large torques can stress the screw shafts during start-up if the screw shafts are jammed by solidified product or product residue. The other is the operating situation when nominal extruder torque has been reached and the mechanical safety clutch should be kept from triggering. Case one is... 

When screw shafts are jammed, users often attempt to release the screw shaft by repeatedly switching the main motor on and off. Yet even with accurate torque measurement, torque limitation provides no guarantee against the screw shaft becoming twisted, since the torque limitation cannot absorb the motor s moment of inertia when the motor moves a bit. The same happens when a piece of metal falls into the extruder, abruptly jamming the screw shaft. 

To protect the gearbox and the screw shaft against overload arising on the one hand from the process and on the other hand from the substantial inertia of the motor, the clutch between the motor and the gearbox is designed as a safety clutch. With a properly functioning safety clutch, unavoidable overloads will not lead to significant downtimes for the plant. 

It is particularly important for the clutch to have high, reproducible accuracy, maintainable even after it has been actuated many hundreds of times. The moment of inertia on the gearbox side of the clutch should be as small as possible. Use of a safety clutch is generally recommended, even on small machines. The investment cost is trivial compared with the cost of a damaged gearbox or screw shaft, in terms of both material costs and lost production. 

Power density has increased significantly since the early days of extruder construction. A typical factor, and an important one for scaling up laboratory machines to production machines, is the ratio of screw shaft torque to centerline distance to the power of three. In the 1960s, a value of T/a3=5 Nm/cm3 was typical today values in excess of 13 Nm/cm3 are achieved. 

Finally, Figure 20.7 shows the constitution of the rotary kiln with its screw device after 1800 h operation with different materials. The screw blades and the screw shaft are free from carbonaceous residues due to permanent cleaning by metal spheres and shredded goods. 

Figure 27.4 Screw pyrolyzer [2]. 1 horizontal vessel 2 gearbox 3 screw shaft 4 hopper 5 mouth for balance 6 jacket heating 7 inlet of heating gas 8 outlet of heating gas 9-11 drain pipes 12 outlet of volatile gas 13 char discharge 14 baffle...

There are several types of extruders available in the market some have single and some have twin-screw shafts. The extruder can be considered either wet or dry (3). Today, in the oilseed industry, no one uses twin-screw extruders because of their cost and complexity. Most of the oilseed industry uses either wet or dry (singlescrew) extruders. AU the extmders are basically screw pumps through which material is forced, while in the meantime, subjected to heat, cooking, and shear forces. Several parameters affect the quality of the end product when using extrusion technology. Serrano and Villalbi (4) list the following ... 

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