Ten Key Steps for Reliable Screw Conveyor Performance

by Lyn Bates

1. Make sure that the discharge ports cannot block, or if they can that the equipment is fail safe by means of an interlocked pressure switch, level detector, or that an safety coupling, robust stall condition or an emergency overflow are incorporated. A screw can develop extremely high direct compacting forces, but bulk material is not be easily pushed around corners unless in a saturated condition.

2. Do not overload the casing cross section, particularly if hanger bearings are fitted. If is possible for the supply feed to surge, as with dust collectors, serving plate filters, and some process equipment, then incorporate a reduced capacity feed control section, such as reduced pitch flights, stepped screw diameter or a separate feeder, to prevent the in-feed overfilling the screw section.

3. If the size range of the bulk material can trap in the standard flight tip clearance, use a larger casing size or smaller screw to give more working clearance. Take special care with products such as wood chips, scrap paper, and hard, flat, flakes that can laminate to form a firm and incompressible bed. Use larger than usual clearances, off-set the screw in the casing to give an increasing clearance as the screw rotates, and have plenty of power with strong screw construction. (Not over-thick flights, which increase the area of confinement and take longer to clear of a rotating obstruction). Beware of materials that 'cake', as these set in the casing clearance and exert very high tip loadings on the screw flights. Flexible troughs made from conveyor belting allow the crust to be periodically broken as the casing deforms. Alternatively, a narrow bead weld of hard wearing material can be deposited on the flight tips.

4. If it is essential that material does not pass an open intermediate outlet then use an ‘expanded’ outlet port and a discontinuous flight arrangement over the outlet. I.e. where the continuous flights stop, position the continuing flight is 1800 offset on the centre shaft to allow the material to fall from the blade into space. When the outlet is closed, a bed of material forms on which the material is carried. Product is then picked up by the continuing screw blade, without a gap in the direction of movement.

5. Use ribbon flights for damp, sticky and cohesive bulk products, but not for ‘stringy’ materials. Ajax has developed a special ‘LynFlowTM’ form of ribbon flight for maximum resistance to build up around the shaft. ‘Coreless’ type screws are especially resistant to clogging, except when the drive is at the discharge end and a shaft is necessary to transmit the power. Stringy material hanging over the flight will be captured at the shaft junction.

6. Do not incline a screw conveyor at more than 30° to the horizontal, preferably not more than 200 without severe de-rating of capacity. ‘Fall-back’ of material into prior pitch space dramatically increases the cross sectional loading of the conveyor when the inclination of the outer tip helix angle of the screw to the horizontal reduces below the contact friction angle of material. Short pitch flights can give a limited advantage, but for inclinations that have to be greater than 30°, get help.

7. There are many types of screw flight forms for mixing, pre-breaking, compacting and other process functions. It is normally best to consult a specialist for non-standard duties.

8. Include access/inspection ports over outlets and intermediate bearings. These are extremely useful for servicing and confirming reliable performance.

9. The drive can be at either end of the conveyor, but it is good practice for the outlet end to carry the thrust on the screw to avoid buckling loads on slender screw shafts. Hollow shaft geared motors are very useful for compact drive assemblies, as they dispense with the need for a separate drive end bearing. However, they lack the flexibility of a chain drive for retrofit speed adjustment and they also have to be carefully installed to avoid placing a side load on any gland fitted to the casing end plate.

10. Off-the-shelf standard screw conveyor components are relatively economical in unit price, but devoid of performance responsibility if bought in sections. They are also limited in the lengths available, whereas a custom built unit can incorporate a large diameter centre shaft for longer spans and be made with construction and features to suit the application. When considering purchase, weigh the capital cost against the utility of reliable performance. ‘What period of production down-time would equate to the cost difference of quotations from alternative suppliers?’ is a good measure of the value of a buyer’s confidence in a supplier’s competence and support.

NEVER confuse the function of a Screw Conveyor with those of a Screw Feeder or a Screw Elevator. These three classes of machine work on different operating principles although they can be combined in special designs.

A Screw Conveyor Works by a 'Gravity Mode' of conveying. I.e. The material moves by sliding down the surface of the rotating flight. It advances by one pitch for each rotation of the shaft. Once the material level increases to spill over the centre height of the shaft, it falls back into the proceeding pitch space and the increase in trough sectional loading is not reflected by a corresponding increase in transport capacity. A conveyor screw can operate at up to 15 degrees inclination with only a small reduction in transfer capacity. Short pitch construction can enable steeper angles to be used but output rapidly falls off above a slope of 20 degrees, to about 30% of horizontal capacity at 30 degrees inclination and more rapidly thereafter.

A Screw Feeder works by a 'Flood Mode' of conveying. The material totally fills the casing cross section and advances axially in a spiral manner according to the screw flight pitch angle and the frictional reaction on the face of the screw flight. After the initial cross section is filled, no further material can enter except by virtue of an increased screw transfer capacity due to geometrical changes. For a fixed outer diameter the effective 'live' extraction length of a screw is limited to about six times its diameter. For practical purposes extraction is never even along the screw length, depending as it does on incremental transfer capacity based upon contact frictional values.

A Screw Elevator works in a 'Dynamic Mode'. The material behaves as a fluid vortex, spirally advancing because rotation with the screw is impeded by contact friction with the inner wall of the casing. The wall friction determines the amount of power absorbed in conveying, not the helical direction of motion. This depends only on the flight face angle and surface friction. 'Leakage' and 'Fallback' reduce the amount of material transferred. A critical rotational speed is necessary to provide notional 'self clearance' of the casing contents, except for the inlet region that cannot clear because the material is flung out into the inlet chute to re-circulate over the length of the inlet port.



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For any problem not covered, contact Ajax at:
http://www.ajax.co.uk/


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