Screw conveyor

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Principle of the screw conveyor
Horizontal screw conveyor for transporting grapes in a winery

Screw conveyors are conveying systems for bulk goods based on the functional principle of the Archimedean screw . The screw conveyor lying in a trough transports - usually motor-driven - the corresponding goods over several meters, with the help of intermediate storage up to 60 meters.

functionality

Rotation of the goods with the screw is prevented by the force of gravity and the friction of the transported goods on the trough walls . It is thus continuously transported. If the friction on the wall is low when the filling level is high, the product only rotates transversely to the screw axis and is not transported. During vertical transport, wall friction must therefore be generated by centrifugal force , which requires high speeds of 250–400 min −1 .

It is possible to transport the goods horizontally, diagonally and vertically. Processing steps such as mixing, dewatering or compression can also be carried out during transport, and cooling or drying during transport is also possible.

construction

In general, a screw conveyor consists of the following structural elements:

  • the screw trough, depending on the design with trough cover
  • the screw conveyor , consisting of
    • a worm thread
    • the worm shaft with drive shaft, end bearing shaft and seals
  • a drive unit consisting of
  • the storage of the worm shaft
  • an inlet and outlet

Screw trough

The U-shaped screw trough accommodates the screw conveyor over its full length and prevents the material being conveyed to the side and downwards. It must therefore be closely matched to the screw conveyor. In the case of screw troughs made of metal, a trough wall bevelled at the top can increase the stability. If goods are to be conveyed which cause heavy wear , exchangeable wear plates can also be used on the bottom of the trough. In addition, strips are installed in the conveying direction, which promote the formation of a bulk material coating on the trough wall. This reduces wear and tear and increases the friction on the trough wall, which improves the conveying capacity.

The trough cover usually consists of flat cover segments that rest on the trough edge and, if necessary, on trough crossbars. To increase the dust tightness, seals are inserted between the lid and the trough. In the case of free-flowing bulk goods in ascending screw conveyors, an almost horizontal product surface can arise. In this case, the trough cover is equipped with displacement bodies, creating a round cross-section. This can prevent the product from flowing back.

The tubular trough is a completely closed screw trough with a round cross-section. The options for inspection and cleaning of the interior are limited or not available with this design. This trough shape is mainly chosen for extraction screws and for ascending screw conveyors. In special cases, such as for pumping water, the pipe trough can also be firmly connected to the screw conveyor.

A screw trough with a double jacket is used when the bulk material is to be cooled or heated during the conveying process. For this purpose, the cooling or heating medium is passed through the spaces.

Auger

Conveyor screw of a decanter centrifuge

The conveying screw is defined as a shaft to one or more helically winding passages in the form of flat sheets, rubber cloth or wood carving coiled be. Screw conveyors can be designed as rigid or flexible (flexible) screws. With flexible screws, the shaft must also be flexible. The shaft can also be dispensed with (soulless / shaftless screw or spiral), whereby the limits rotate around their own curved axis. In this case, the stabilizing effect of the rigid shaft is achieved by profiles that are worked into the spiral.

Worm thread

In modern designs, metal is usually used for the worm thread, other materials are also used in special cases. The worm thread is either firmly connected to the shaft (welded) or manufactured in one part with the shaft. The following designs can be distinguished, which are used depending on the task and the goods to be conveyed:

  • A full-blade screw consists either of a continuous thread or of individual screw wing segments that are butt-connected to one another. Whole leaf screws are used for powdery, fine-grained and non-baking bulk goods.
  • In the case of a tape thread or "shaftless worm", the shaft is missing, there is a cavity. The thread consists of a spiral band. This design is used for conveying lumpy and sticky or sensitive bulk materials.
    A design with a space between the thread itself and a central shaft, in which the thread is connected to the shaft via webs, is rare.
  • The paddle worm does not have a continuous thread, it consists of individual segments that are connected to the shaft. This design is suitable for baking goods.

For small versions, the worm thread is turned from the solid material (e.g. from a piece of round steel) or made as a cast part , as is often seen in household meat grinders. However, it is also possible to take a shaft made of round steel and weld bent flat or strip steel to it. The dimensions and shapes are specially designed for their application conditions in the process. There are numerous ready-made solutions for standard applications. For large systems, the worm thread is welded to the drive shaft. A full- blade screw consists either of a continuous sheet metal sheet or of individual screw wing segments which are butt- welded together. The "tape thread" is welded to the worm shaft by webs, leaving a free space between the thread and the shaft.

Worm shaft

Hollow shafts are usually used as worm shafts , because they have a significantly lower weight than solid shafts with a similarly high strength . The bearing journals are welded in at both ends.

The users are placing increasing value on a secure and, if possible, maintenance-free sealing of the shaft openings. Sealing systems with integrated bearings are used here. Sealing takes place with sealing lips made of modified PTFE .

In the case of flexible screws, there is no shaft (soulless screw or spiral). The enveloping trough or the pipe can also be omitted if the screw helix is ​​not designed as a conveying surface but as a coiled trough.

Drive unit

Gear motors in foot, flange or slip-on design are mainly used as drives. Depending on the given installation situation, the drive can be installed either on the inlet or on the outlet side. It is important that the fixed bearing of the shaft bearing is provided on the drive side. Both elastic couplings , FlexCons and chain drives or belt drives are used for power transmission . In the case of chain drives, the speed can be reset by changing the gear ratio. However, it is often desirable to be able to make adjustments to the screw speed during operation. In the past, mechanical variable speed gears were used for this purpose; today, frequency converters are increasingly used.

Executions

The delivery rate depends on the speed, the inside and outside diameter, the gradient, the filling level and the friction of the medium on the screw. A maximum delivery rate is achieved when the friction on the screw conveyor is small compared to the friction on the trough wall. If liquids are transported, the delivery rate decreases with increasing wear, because the chambers are no longer completely separated from each other and a backflow occurs.

When designing a screw, the probable degree of filling over the process length is important. It is significantly influenced by the pitch of the screw and the flight depth . As the slope increases, the degree of filling continues to increase.

Depending on the task and product properties, single-screw and twin-screw screw systems are built, the latter mainly for the continuous mixing and crushing of product agglomerates. Screw conveyors can be arranged horizontally, inclined and vertically, curved conveying paths can be implemented with spiral conveyors . With an appropriate design, they are also suitable for heavy use and continuous operation. Due to their compact design and small cross-sections, they are particularly suitable for difficult installation conditions. The conveying direction depends on the type of thread (right -hand or left-hand thread ) and the direction of rotation of the worm shaft. For example, with a combination of right-hand and left-hand threads on a worm shaft, two external inlets can feed to a central outlet.

As a rule, the conveyor lengths are up to 6 m. In principle, any conveying lengths can be achieved through the use of intermediate storage facilities; in practical applications, conveying lengths of 60 m are rarely exceeded. The conveying speeds are usually below 0.6 m / s, depending on the material being conveyed. If a high speed is not permitted , for example when used in potentially explosive areas in accordance with Directive 94/9 / EC ( ATEX ), a lower speed can be selected without affecting the function itself.

In extruders and meat grinders , the geometry of the helix is ​​designed in such a way that the cross section narrows towards the exit. This generates high pressures in order to press plastic or tough conveyed goods through mold templates or into casting molds.

application

Disassembled meat grinder with screw conveyor

The selection of goods that can be conveyed with screw systems is very large. In practice, dusty, granular, semi-moist and fibrous substances are transported. Screw conveyors are used for liquids , powders or granulates . Application examples are grain silos , wood chip and pellet heating systems , plastic machines ( extruders ) or in meat grinders . The delivery rates range from a few cm³ / h to a few hundred m³ / h.

Sticky, heavily baking, lumpy goods and goods that are sensitive to comminution are less suitable for conveying with screw conveyors . Solid or tough masses are conveyed with extruders in bakeries or with plastic injection machines .

Conveyor screws can act with high forces on the conveyed goods and, depending on their properties, generate high pressure. They are therefore not suitable for the transport of fragile goods, such as those found in the food industry (fruit gums, shaped chocolate parts, loose fillings for muesli bars, etc.).

Because the material to be transported is always mixed by the screw conveyor, it is possible to use the screw conveyor in cement production plants to mix concrete at the same time , with dry raw material, water and additives in the desired ratio being added to the screw conveyor at the same time and to each other at the other end come out mixed up.

In sewage treatment plants , screw pump systems for transporting waste water and sludge achieve very long lifetimes (up to 50 years), almost unrivaled levels of efficiency and can also convey coarse materials.

Screw conveyors are also used in mechanical process engineering. They are the heart of a decanter centrifuge (industrial centrifuge, solid bowl screw centrifuge , decanter centrifuge ). In a decanter centrifuge, solid particles are separated from a liquid with the aid of sedimentation. The screw conveys the solid particles in the direction of the discharge. The clarified liquid runs down the conveyor screw in the opposite direction.

In some weapons of the type Gatling screw conveyors are used for beltless ammunition feed ( linkless ammunition feed system used) to very high rates of fire to achieve.

See also

Web links

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