There is no rule of thumb because many factors influence the prospect of jamming in the flight tip clearance or locking in the swept volume. Particle shape, hardness and surface condition have to be taken into account in relation the the conveyor or feeder size, tip clearance any other equipment features. Various techniques are available to minimise the risk of blockages so, in any questionable situation it pays to deal with a specialist supplier. Quite large items can be handled in some cases, whereas the lamination or interlocking characteristics of other shapes can be awkward. See the notes on jamming in hopper outlets in the forum 'Solid Sense' section for a review of jamming aspects. ■

The allowable size of a lump in a screw conveyor is a function of the radial clearance between the outside diameter of the central pipe and the radius of the inside of the screw trough, as well as the proportion of the lumps in the mixture. The lump size of the material affects the selection of screw diameter which should be at least 12

times larger than the lump size of a sized material and four times larger than the largest lumps of an unsized material. ■