The capacity of an inclined screw falls off progressively according to how the material slips on the face of the screw flight. Very crudely, you could say that at 30 degree to the horizontal it will handle approx/ 30% of its horizontal capcity. Some mitigation can be secured by the use of reduced pitch flights, but any slope above 20 degrees to the horizontal should be treated with great care. Measurement of contact friction is a prime design parameter but the 'state' of the material should also be taken into account. A product that fluidises easily will run back through the clearance between the flight and the casing and adopt a low surface repose angle to spill back over the centre shaft at a low cross sectional loading.

The mechanics of a screw elevator and am inclined screw feeder differs from the 'gravity mode' of operation of a screw conveyor in that the casing is circular and the 'dynamic mode' of an elevator exploits centrifugal force to create a vortex of rotation and the 'flood mode' of a screw feeder fills the cross section to inhibit fallback. The latter two machines, and the recently developed 'static screw elevator', will move material at inclinations up to 90 degrees but require more technical evaluation for assessing capacity.

With regards to power, the requirements of a screw conveyor is sensitive to the friction, shear strength and potential jamming charactistice of the bulk material handled. A crude assessment can be made on the basis of potential energy required for the material to slide down a chute at an inclination of the helix angle of the screw flight plus an allowance for the weight of the mass shearing over the dead layer in the casing clearance. Again, you will see that the power is material dependent. Whilst crude approximations of handling rates can be made on the basis of volumetric capacity this is an example of the danger of using data bases and standard charts for assessing sensitive issues.

It is far better to employ fundamental principles and quantified values of material properties for rate and power calculations. My book, 'Guide to the design, selection and application of screw feeders' sets out the background in some detail but the subject is rather awkward to accurately condense to simple rules of thumb. ■

Hi Lyn,

In our process we are using Inclined Shaftless Srew Conveyors to convey powder (starch) . With an inclination around 35-40 degree, we always encounter breakage on the screw. What are the items to consider? to check ? and to improve?

Hoping to help me regarding my equipment.

regards,

jared ■

Originally Posted by jaredelijah16

Hi Lyn,

In our process we are using Inclined Shaftless Srew Conveyors to convey powder (starch) . With an inclination around 35-40 degree, we always encounter breakage on the screw. What are the items to consider? to check ? and to improve?

Hoping to help me regarding my equipment.

regards,

jared

Without details of the equipment it is difficult to assess potential failure conditions. Points to examine are whether the screw suffers an overload due to blocked outlet, failure to convey due to ‘fall-back’ at this inclination, jamming of foreign bodies, hard packing of residue binding the flight tips or other evidence at breakdown. The nature of the failure surface will normally provide clues as to the mechanical reason that it occurred. The failure point should be examined for signs of weld failure, stress fracture or fatigue, i.e. conchoidal marking, shiny fracture surface, progressive cracks. Are breakages at common site or of prior repair? The supplier should also be invited to explain why his equipment is failing to provide the service for which it was supplied. ■